DC Motor

A DC motor like we all know is a device that deals in the conversion of electrical energy to mechanical energy and this is essentially brought about by two major parts required for the construction of dc motor, namely.

1) Stator – The static part that houses the field windings and receives the supply and,

2) Rotor – The rotating part that brings about the mechanical rotations.

Other than that there are several subsidiary parts namely the

3) Yoke of dc motor.

4) Poles of dc motor.

5) Field winding of dc motor.

6) Armature winding of dc motor.

7) Commutator of dc motor.

8) Brushes of dc motor.

All these parts put together configures the totalconstruction of a dc motor.

Now let’s do a detailed discussion about all the essential parts of dc motor.

Essential Parts of DC Machine

Yoke of DC Motor

The magnetic frame or theyoke of dc motor made up of cast iron or steel and forms an integral part of the stator or the static part of the motor. Its main function is to form a protective covering over the inner sophisticated parts of the motor and provide support to the armature. It also supports the field system by housing the magnetic poles and field winding of the dc motor.

Poles of DC Motor

The magnetic poles of DC motor are structures fitted onto the inner wall of the yoke with screws. The construction of magnetic poles basically comprises of two parts namely, the pole core and the pole shoe stacked together under hydraulic pressure and then attached to the yoke. These two structures are assigned for different purposes, the pole core is of small cross sectional area and its function is to just hold the pole shoe over the yoke, whereas the pole shoe having a relatively larger cross-sectional area spreads the flux produced over the air gap between the stator and rotor to reduce the loss due to reluctance. The pole shoe also carries slots for the field windings that produce the field flux.

Field Winding of DC Motor

The field winding of dc motor are made with field coils (copper wire) wound over the slots of the pole shoes in such a manner that when field current flows through it, then adjacent poles have opposite polarity are produced. The field winding basically form an electromagnet, that produces field flux within which the rotor armature of the dc motor rotates, and results in the effective flux cutting.

Armature Winding of DC Motor

The armature winding of dc motor is attached to the rotor, or the rotating part of the machine, and as a result is subjected to altering magnetic field in the path of its rotation which directly results in magnetic losses. For this reason the rotor is made of armature core, that’s made with several low-hysteresis silicon steel lamination, to reduce the magnetic losses like hysteresis and eddy current loss respectively. These laminated steel sheets are stacked together to form the cylindrical structure of the armature core.

The armature core are provided with slots made of the same material as the core to which the armature winding made with several turns of copper wire distributed uniformly over the entire periphery of the core. The slot openings a shut with fibrous wedges to prevent the conductor from plying out due to the high centrifugal force produced during the rotation of the armature, in presence of supply current and field.

The construction ofarmature winding of dc motor can be of two types:-

Lap Winding

In this case the number of parallel paths between conductors A is equal to the number of poles P.

i.e A = P
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***An easy way of remembering it is by remembering the word LAP-----→ L A=P

Wave Winding

Here in this case, the number of parallel paths between conductors A is always equal to 2 irrespective of the number of poles. Hence the machine designs are made accordingly.

Commutator of DC Motor

The commutator of dc motor is a cylindrical structure made up of copper segments stacked together, but insulated from each other by mica. Its main function as far as the dc motor is concerned is to commute or relay the supply current from the mains to the armature winding housed over a rotating structure through the brushes of dc motor.

Brushes of DC Motor

The brushes of dc motor are made with carbon or graphite structures, making sliding contact over the rotating commutator. The brushes are used to relay the current from external circuit to the rotating commutator form where it flows into the armature winding. So, the commutator and brush unit of the dc motor is concerned with transmitting the power from the static electrical circuit to the mechanically rotating region or the rotor.

The direct current motor or the DC motor has a lot of application in today’s field of engineering and technology. Starting from an electric shaver to parts of automobiles, in all small or medium sized motoring applications DC motors come handy. And because of its wide range of application different functional types of dc motor are available in the market for specific requirements.
The types of DC motor can be listed as follows
•DC motor
  • Permanent Magnet DC Motor
  • Separately Excited DC Motor
  • Self Excited DC Motor
       • Shunt Wound DC Motor
       • Series Wound DC Motor
       • Compound Wound DC Motor
              • Cumulative compound DC motor
                      • Short shunt DC Motor
                      • Long shunt DC Motor
              • Differential Compound DC Motor
                      • Short Shunt DC Motor
                      • Long Shunt DC Motor

Now let’s do a detailed discussion about all the essential types of dc motor.

Separately Excited DC Motor

As the name suggests, in case of a separately excited DC motor the supply is given separately to the field and armature windings. The main distinguishing fact in these types of dc motor is that, the armature current does not flow through the field windings, as the field winding is energized from a separate external source of dc current as shown in the figure beside.

From the torque equation of dc motor we know Tg = Ka φ IaSo the torque in this case can be varied by varying field flux φ, independent of the armature current Ia.

Permanent Magnet DC Motor

The permanent magnet DC motor consists of an armature winding as in case of an usual motor, but does not necessarily contain the field windings. The construction of these types of DC motor are such that, radially magnetized permanent magnets are mounted on the inner periphery of the stator core to produce the field flux. The rotor on the other hand has a conventional dc armature with commutator segments and brushes. The diagrammatic representation of a permanent magnet dc motor is given below.

The torque equation of dc motor suggests Tg = Ka φ Ia. Here φ is always constant, as permanent magnets of required flux density are chosen at the time of construction and can’t be changed there after.

For a permanent magnet dc motor Tg = Ka1Ia

Where Ka1 = Ka.φ which is another constant. In this case thetorque of DC Motor can only be changed by controlling armature supply.

Self Excited DC Motor

In case of self excited dc motor, the field winding is connected either in series or in parallel or partly in series, partly in parallel to the armature winding, and on this basis its further classified as:-

Shunt wound DC motor.
Series wound DC motor.
Compound wound DC motor.

Let’s now go into the details of these types of self excited dc motor.

Shunt Wound DC Motor

In case of a shunt wound dc motor or more specificallyshunt wound self excited dc motor, the field windings are exposed to the entire terminal voltage as they are connected in parallel to the armature winding as shown in the figure below.

To understand the characteristic of these types of DC motor, lets consider the basic voltage equation given by,

$E;=;E_b;+;I_aR_a;;cdotscdotscdotscdotscdotscdots(;1;)$
[Where E, Eb, Ia, Ra are the supply voltage, back emf, armature current and armature resistance respectively]
$Now,;E_b;=;k_aphi omega ;;cdotscdotscdotscdotscdotscdots(;2;)$
[since back emf increases with flux φ and angular speed ωω]

Now substituting Eb from equation (2) to equation (1) we get,
$E;=;k_aphi omega ;+;I_aR_a$

The torque equation of a dc motor resembles,
$T_g;=;K_aphi I_a;;cdotscdotscdotscdotscdotscdotscdots(;4;)$

This is similar to the equation of a straight line, and we can graphically representing the torque speed characteristic of a shunt wound self excited dc motor as

The shunt wound dc motor is a constant speed motor, as the speed does not vary here with the variation of mechanical load on the output.

Series Wound DC Motor

In case of a series wound self excited dc motor or simplyseries wound dc motor, the entire armature current flows through the field winding as its connected in series to the armature winding. The series wound self excited dc motor is diagrammatically represented below for clear understanding.

Now to determint the torque speed characteristic of these types of DC motor, lets get to the torque speed equation.

From the circuit diagram we can see that the voltageequation gets modified to

$E;=;E_b;+;I_a(;R_a;+;R_s;)cdotscdotscdotscdotscdotscdots(;5;)$

Where as back emf remains Eb = kaφω

Neglecting saturation we get,
$phi ;=;K_1I_f;=;K_1I_a$
[ since field current = armature current]
$Therefore,;E_b;=;k_aK_1I_aomega ;=;K_sI_aomega ;;cdotscdotscdotscdotscdotscdots(;6;)$

From equation (5) & (6)

From this equation we obtain the torque speed characteristic as

In a series wound dc motor, the speed varies with load. And operation wise this is its main difference from a shunt wound dc motor.

Compound Wound DC Motor

The compound excitation characteristic in a dc motor can be obtained by combining the operational characteristic of both the shunt and series excited dc motor. The compound wound self excited dc motor or simply compound wound dc motor essentially contains the field winding connected both in series and in parallel to the armature winding as shown in the figure below:

The excitation of compound wound dc motor can be of two types depending on the nature of compounding.

Cumulative Compound DC Motor

When the shunt field flux assists the main field flux, produced by the main field connected in series to the armature winding then its called cumulative compound dc motor.

$phi _{total};=;phi _{series};+;phi {shunt};$

Differential Compound DC Motor

In case of a differentially compounded self excited dc motor i.e. differential compound dc motor, the arrangement of shunt and series winding is such that the field flux produced by the shunt field winding diminishes the effect of flux by the main series field winding.

$phi _{total};=;phi _{series};-;phi _{shunt}$
The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application.

The compounding characteristic of the self excited dc motor is shown in the figure below.

Both the cumulative compound and differential compound dc motor can either be of short shunt or long shunt type depending on the nature of arrangement.

Short Shunt DC Motor

If the shunt field winding is only parallel to the armature winding and not the series field winding then its known as short shunt dc motor or more specifically short shunt type compound wound dc motor.

Long Shunt DC Motor

If the shunt field winding is parallel to both the armature winding and the series field winding then it’s known as long shunt type compounded wound dc motor or simply long shunt dc motor.

Short shunt and long shunt type motors have been shown in the diagram below.

A series wound dc motor like in the case ofshunt wound dc motor orcompound wound dc motor falls under the category of self-excited dc motors, and it gets its name from the fact that the field winding in this case is connected internally in series to the armature winding. Thus the field winding are exposed to the entire armature current unlike in the case of a shunt motor.

Construction of Series DC Motor

Construction wise a this motor is similar to any other types of dc motors in almost all aspects. It consists of all the fundamental components like the stator housing the field winding or the rotor carrying the armature conductors, and the other vital parts like the commutator or the brush segments all attached in the proper sequence as in the case of a generic DC motor.

Yet if we are to take a close look into the wiring of the field and armature coils of this dc motor, its clearly distinguishable from the other members of this type. To understand that let us revert back into the above mentioned basic fact, that the this motor has field coil connected in series to the armature winding. For this reason relatively higher current flows through the field coils, and its designed accordingly as mentioned below.

i) The field coils of dc series motor are wound with relatively fewer turns as the current through the field is its armaturecurrent and hence for required mmf less numbers of turns are required.

ii) The wire is heavier, as the diameter is considerable increased to provide minimum electrical resistance to the flow of full armature current.

In spite of the above mentioned differences, about having fewer coil turns the running of this dc motor remains unaffected, as the current through the field is reasonably high to produce a field strong enough for generating the required amount of torque. To understand that better lets look into the voltage and current equation of dc series motor.

Voltage and Current Equation of Series DC Motor

The electrical layout of a typical series wound dc motor is shown in the diagram below.

Let the supplyvoltage andcurrent given to the electrical port of the motor be given by E and Itotalrespectively.

Since the entire supply current flows through both the armature and field conductor.

$Therefore,;;I_{total};=;I_{se};=;I_a$

Where Ise is the series current in the field coil and Ia is the armature current.

Now form the basic voltage equation of the dc motor.

$E;=;E_b;+;I_{se}R_{se};+;I_aR_{se};)$

Where Eb is the back emf.

Rse is the series coil resistance & Ra is the armatureresistance.

Since Ise = Ia, we can write,

$E;=;E_b;+;I_a(;R_a;+;R_{se};)$

This is the basic voltage equation of a series wound dc motor.

Another interesting fact about the dc series motor worth noting is that, the field flux like in the case of any other dc motor is proportional to field current.

$I_{se};propto ;phi$

But since here Ise = Ia = Itotal

$phi ;propto ;I_{se};propto ;I_a$

i.e. the field flux is proportional to the entire armaturecurrent or the total supply current. And for this reason, the flux produced in this motor is strong enough to produce sufficient torque, even with the bare minimum number of turns it has in the field coil.

Speed & Torque of Series DC Motor

A series wound motors has linear relationship existing between the field current and the amount of torque produced. i.e. torque is directly proportional to current over the entire range of the graph. As in this case relatively highercurrent flows through the heavy series field winding with thicker diameter, the electromagnetic torque produced here is much higher than normal. This high electromagnetic torque produces motor speed, strong enough to lift heavy load overcoming its initial inertial of rest. And for this particular reason the motor becomes extremely essential as starter motors for most industrial applications dealing in heavy mechanical load like huge cranes or large metal chunks etc. Series motors are generally operated for a very small duration, about only a few seconds, just for the purpose of starting. Because if its run for too long, the high series current might burn out the series field coils thus leaving the motor useless.

Speed Regulation of Series Wound DC Motor

Unlike in the case of a DC shunt motor, the dc series motor has very poor speed regulation. i.e. the series motor is unable to maintain its speed on addition of external load to the shaft. Let us see why?

When mechanical load is added to the shaft at any instance, the speed automatically reduces whatever be the type of motor. But the term speed regulation refers to the ability of the motor to bring back the reduced speed to its original previous value within reasonable amount of time. But this motor is highly incapable of doing that as with reduction in speed N on addition of load, the back emf given by,

Also reduces as Eb ∝ N

This decrease in back Emf Eb, increases the net voltage E- Eb, and consequently the series fieldcurrent increases,

$I_{se};=;E;-;frac{E_b}{R_a;+;R_{se}}$

The value of series current through the field coil becomes so high that it tends to saturate of the magnetic core of the field. As a result the magnetic flux linking the coils increases at a much slower rate compared to the increase in currentbeyond the saturation region as shown in the figure below.

The weak magnetic field produced as a consequence is unable to provide for the necessary amount of force to bring back the speed at its previous value before application of load.

So keeping all the above mentioned facts in mind, a series wound dc motor is most applicable as a starting motor for industrial applications.

The shunt wound dc motor falls under the category of self excited dc motors, where the field windings are shunted to, or are connected in parallel to the armature winding of the motor, as its name is suggestive of. And for this reason both the armature winding and the field winding are exposed to the same supply voltage, though there are separate branches for the flow of armature currentand the field current as shown in the figure of dc shunt motor below.

Voltage and Current Equation of a Shunt Wound DC Motor

Let us now consider thevoltage and current being supplied from the electrical terminal to the motor be given by E and Itotal respectively. This supply current in case of theshunt wound dc motor is split up into 2 parts. Ia, flowing through the armature winding of resistance Ra and Ishflowing through the field winding of resistance Rsh. Thevoltage across both windings remains the same.

From there we can write Itotal = Ia + Ish

Thus we put this value of armature current Ia to get generalvoltage equation of a dc shunt motor.

$E;=;E_b;+;I_aR_a$

$Or;;E;=;E_b;+;(;I_{total};-;I_{sh};)R_a$

Now in general practice, when the motor is in its running condition, and supply voltage is constant the shunt fieldcurrent given by,

But we know Ish ∝ Φ

i.e. field flux Φ is proportional to filed current Ish

Thus the field flux remains more or less constant and for this reason a shunt wound dc motor is called a constant flux motor.

Construction of a Shunt Wound DC Motor

The construction of a dc shunt motor is pretty similar to other types of dc motor, as shown in the figure below.

Essential Parts of DC Machine

Just that there is one distinguishable feature in its designing which can be explained by taking into consideration, the torque generated by the motor. To produce a high torque,

i) The armature winding must be exposed to an amount ofcurrent that’s much higher than the field windings current, as the torque is proportional to the armature current.

ii) The field winding must be wound with many turns to increase the flux linkage, as flux linkage between the field and armature winding is also proportional to the torque.

Keeping these two above mentioned criterion in mind a dc shunt motor has been designed in a way, that the field winding possess much higher number of turns to increase net flux linkage and are lesser in diameter of conductor to increase resistance(reduce current flow) compared to the armature winding of the dc motor. And this is how a shunt wound dc motor is visibly distinguishable in static condition from the dc series motor (having thicker field coils) of the self excited type motor’s category.

Self-Speed Regulation of a Shunt Wound DC Motor

A very important and interesting fact about the dc shunt motor, is in its ability to self regulate its speed on application of load to the shaft of the rotor terminals. This essentially means that on switching the motor running condition from no load to loaded, surprisingly there is no considerable change in speed of running, as would be expected in the absence of any speed regulating modifications from outside. Let us see how?

Let us do a step-wise analysis to understand it better.

1) Initially considering the motor to be running under no load or lightly loaded condition at a speed of N rpm.

2) On adding a load to the shaft, the motor does slow down initially, but this is where the concept of self regulation comes into the picture.

3) At the very onset of load introduction to a shunt wound dc motor, the speed definitely reduces, and along with speed also reduces the back emf, Eb. Since Eb ∝ N, given by,

This can be graphically explained below.

4) This reduction in the counter emf or the back emf Ebresults in the increase of the net voltage. As net voltage Enet = E − Eb. Since supply voltage E remains constant.

5) As a result of this increased amount of net voltage, the armature current increases and consequently the torque increases.

Since, Ia ∝ Τ given by

The change in armature current and torque on supplying load is graphically shown below.

6) This increase in the amount of torque increases the speed and thus compensating for the speed loss on loading. Thus the final speed characteristic of a dc shunt motor, looks like.

From there we can well understand this special ability of the shunt wound dc motor to regulate its speed by itself on loading and thus its rightly called the constant flux or constant speed motor. Because of which it finds wide spread industrial application where ever constant speed operation is required.

A compound wound dc motor or rather a dc compound motor falls under the category of self excited motors, and is made up of both series the field coils S1 S2 and shunt field coils F1 F2 connected to the armature winding as shown in the figure below.

Both the field coils provide for the required amount of magnetic flux, that links with the armature coil and brings about the torque necessary to facilitate rotation at desired speed.

As we can understand, a compound wound dc motor is basically formed by the amalgamation of a shunt wound dc motor and series wound dc motor to achieve the better off properties of both these types. Like a shunt wound dc motoris bestowed with an extremely efficient speed regulation characteristic, whereas the dc series motor has high starting torque.

So the compound wound dc motor reaches a compromise in terms of both this features and has a good combination of proper speed regulation and high starting toque. Though its staring torque is not as high as in case of dc motor, nor is its speed regulation as good as a shunt dc motor. Overall characteristics of dc shunt motorfalls somewhere in between these 2 extreme limits.

Types of Compound Wound DC Motor

The compound wound dc motor can further be subdivided into 2 major types on the basis of its field winding connection with respect to the armature winding, and they are:

Long Shunt Compound Wound DC Motor

In case of long shunt compound wound dc motor, the shunt field winding is connected in parallel across the series combination of both the armature and series field coil, as shown in the diagram below.

Voltage and Current Equation of Long Shunt Compound Wound DC Motor

Let E and Itotal be the total supply voltage and currentsupplied to the input terminals of the motor. And Ia , Ise , Ishbe the values of current flowing through armature resistanceRa, series winding resistance Rse and shunt windingresistance Rsh respectively.

Now we know in shunt motor, Itotal = Ia + Ish

And in series motor Ia = Ise

Therefore, the current equation of a compound wound dc motor is given by

$I_{total};=;I_{se};+;I_{sh};;cdotscdotscdotscdotscdotscdotscdots(;1;)$

And its voltage equation is,

$E;=;E_b;+;I_a(;R_a;+;R_{se};)$

Short Shunt Compound Wound DC Motor

In case of short shunt compound wound dc motor, the shunt field winding is connected in parallel across the armature winding only. And series field coil is exposed to the entire supply current, before being split up into armature and shunt field current as shown in the diagram below.

Voltage and Current Equation of Short Shunt Compound Wound DC Motor

Here also let, E and Itotal be the total supply voltage andcurrent supplied to the input terminals of the motor. And Ia , Ise , Ish be the values of current flowing through armatureresistance Ra , series winding resistance Rse and shunt winding resistance Rsh respectively.

But from the diagram above we can see,

$I_{total};=;I_{se};;cdotscdotscdotscdotscdotscdotscdots(;2;)$

Since the entire supply current flows through the series field winding.

And like in the case of a dc shunt motor,

$I_{total};=;I_a;+;I_{sh};;cdotscdotscdotscdotscdotscdotscdots(;3;)$

Equation (2) and (3) gives the current equation of a short shunt compound wound dc motor.

Now for equating the voltage equation, we apply Kirchoff’s law to the circuit and get,

$E;=;E_b;+;I_aR_a;+;I_{se}R_{se}$

But since Ise = Itotal

Thus the final voltage equation can be written as,

$E;=;E_b;+;I_aR_a;+;I_{total}R_{se}$

Apart from the above mentioned classification, a compound wound dc motor can further be sub divided into 2 types depending upon excitation or the nature of compounding. i.e.

Cumulative Compounding of DC Motor

A compound wound dc motor is said to be cumulatively compounded when the shunt field flux produced by the shunt winding assists or enhances the effect of main field flux, produced by the series winding.

$phi _{total};=;phi _{series};+;phi _{shunt}$

Differential Compounding of DC Motor

Similarly a compound wound dc motor is said to be differentially compounded when the flux due to the shunt field winding diminishes the effect of the main series winding. This particular trait is not really desirable, and hence does not find much of a practical application.

$phi _{total};=;phi _{series};-;phi _{shunt}$

The net flux produced in this case is lesser than the original flux and hence does not find much of a practical application.

The compounding characteristic of the self excited dc motor is shown in the figure below.

Starting of DC Motor

The starting of DC motor is somewhat different from the starting of all other types of electrical motors. This difference is credited to the fact that a dc motor unlike other types of motor has a very high starting current that has the potential of damaging the internal circuit of the armature winding of dc motor if not restricted to some limited value. This limitation to thestarting current of dc motor is brought about by means of the starter. Thus the distinguishing fact about the starting methods of dc motor is that it is facilitated by means of a starter. Or rather a device containing a variable resistanceconnected in series to the armature winding so as to limit the starting current of dc motor to a desired optimum value taking into consideration the safety aspect of the motor.

Now the immediate question in why the DC motor has such high starting current ?

To give an explanation to the above mentioned question let us take into consideration the basic operational voltageequation of the dc motor given by,

$E;=;E_b;+;I_aR_a$

Where E is the supply voltage, Ia is the armature current, Ra is the armature resistance. And the back emf is given by Eb.

Now the back emf, in case of a dc motor, is very similar to the generated emf of a dc generator as it’s produced by the rotational motion of the current carrying armature conductor in presence of the field. This back emf of dc motor is given by

and has a major role to play in case of the starting of dc motor.

From this equation we can see that Eb is directly proportional to the speed N of the motor. Now since at starting N = 0, Eb is also zero, and under this circumstance the voltage equation is modified to

$E;=;0;+;E_bR_a;$

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For all practical practices to obtain optimum operation of the motor the armature resistance is kept very small usually of the order of 0.5 Ω and the bare minimum supply voltagebeing 220 volts. Even under these circumstance the starting current, Ia is as high as 220/0.5 amp = 440 amp.

Such high starting current of dc motor creates two major problems.

1) Firstly, current of the order of 400 A has the potential of damaging the internal circuit of the armature winding of dc motor at the very onset.

2) Secondly, since the torque equation of dc motor is given by

Very high electromagnetic starting torque of DC motor is produced by virtue of the high starting current, which has the potential of producing huge centrifugal force capable of flying off the rotor winding from the slots.

Starting Methods of DC Motor

As a direct consequence of the two above mentioned facts i.e high starting current and high starting torque of DC motor, the entire motoring system can undergo a total disarray and lead towards into an engineering massacre and non-functionality. To prevent such an incidence from occurring several starting methods of dc motor has been adopted. The main principal of this being the addition of external electrical resistance Rext to the armature winding, so as to increase the effective resistance to Ra + Rext, thus limiting the armaturecurrent to the rated value. The new value of starting armature current is desirably low and is given by.

Now as the motor continues to run and gather speed, the back emf successively develops and increases, countering the supply voltage, resulting in the decrease of the net working voltage. Thus now,

At this moment to maintain the armature current to its rated value, Rext is progressively decreased unless its made zero, when the back emf produced is at its maximum. This regulation of the external electrical resistance in case of the starting of dc motor is facilitated by means of the starter.

Starters can be of several types and requires a great deal of explanation and some intricate level understanding. But on a brief over-view the main types of starters used in the industry today can be illustrated as:-

1) 3 point starter.

2) 4 point starter.

Used for the starting of shunt wound DC motor andcompound wound DC motor.

3) Series wound DC motor's starter using no load release coil.

All of these play a very significant role in limiting starting current of DC motor for proper starting and running of the DC motor, and are described vividly under their respective sub-headings.

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DC Generator

AC Generator (Alternator)

Transformer

An electric motor is a machine which converts electrical energy into mechanical energy.

Principle:

It is based on the principle that when a current-carrying conductor is placed in a magnetic field, it experiences a mechanical force whose direction is given by Fleming's Left-hand rule and whose magnitude is given by

Force, F = B I l newton

Where B is the magnetic field in weber/m².

I is the current in amperes and

l is the length of the coil in meter.

The force, current and the magnetic field are all in different directions.

If an Electric current flows through two copper wires that are between the poles of a magnet, an upward force will move one wire up and a downward force will move the other wire down.


Figure 1: Force in DC Motor	Figure 2 : Magnetic Field in DC Motor

Figure 3 : Torque in DC Motor	Figure 4 : Current Flow in DC Motor

The loop can be made to spin by fixing a half circle of copper which is known as commutator, to each end of the loop. Current is passed into and out of the loop by brushes that press onto the strips. The brushes do not go round so the wire do not get twisted. This arrangement also makes sure that the current always passes down on the right and back on the left so that the rotation continues. This is how a simple Electric motor is made.

A 3 point starter in simple words is a device that helps in the starting and running of a shunt wound DC motoror compound wound DC motor. Now the question is why thesetypes of DC motors require the assistance of the starter in the first case. The only explanation to that is given by the presence of back emf Eb, which plays a critical role in governing the operation of the motor. The back emf, develops as the motor armature starts to rotate in presence of the magnetic field, by generating action and counters the supply voltage. This also essentially means, that the back emf at the starting is zero, and develops gradually as the motor gathers speed.

The general motor emf equation E = Eb + Ia.Ra,
at starting is modified to E = Ia.Ra as at starting Eb = 0.

Thus we can well understand from the above equation that the current will be dangerously high at starting (as armature resistance Ra is small) and hence its important that we make use of a device like the 3 point starter to limit the startingcurrent to an allowable lower value.

Let us now look into the construction and working of three point starter to understand how the starting current is restricted to the desired value. For that let’s consider the diagram given below showing all essential parts of the three point starter.

Construction of 3 Point Starter

Construction wise a starter is a variableresistance, integrated into number of sections as shown in the figure beside. The contact points of these sections are called studs and are shown separately as OFF, 1, 2,3,4,5, RUN. Other than that there are 3 main points, referred to as

1. 'L' Line terminal. (Connected to positive of supply.)

2. 'A' Armature terminal. (Connected to the armature winding.)

3. 'F' Field terminal. (Connected to the field winding.)

And from there it gets the name 3 point starter.

Now studying the construction of 3 point starter in further details reveals that, the point 'L' is connected to an electromagnet called overload release (OLR) as shown in the figure. The other end of 'OLR' is connected to the lower end of conducting lever of starter handle where a spring is also attached with it and the starter handle contains also a soft iron piece housed on it. This handle is free to move to the other side RUN against the force of the spring. This spring brings back the handle to its original OFF position under the influence of its own force. Another parallel path is derived from the stud '1', given to the another electromagnet called No Volt Coil (NVC) which is further connected to terminal 'F'. The starting resistance at starting is entirely in series with the armature. The OLR and NVC acts as the two protecting devices of the starter.

Working of Three Point Starter

Having studied its construction, let us now go into the working of the 3 point starter. To start with the handle is in the OFF position when the supply to the DC motor is switched on. Then handle is slowly moved against the spring force to make a contact with stud No. 1. At this point, field winding of the shunt or the compound motor gets supply through the parallel path provided to startingresistance, through No Voltage Coil. While entire starting resistance comes in series with the armature. The high starting armature current thus gets limited as the current equation at this stage becomes Ia = E/(Ra+Rst). As the handle is moved further, it goes on making contact with studs 2, 3, 4 etc., thus gradually cutting off the series resistance from the armature circuit as the motor gathers speed. Finally when the starter handle is in 'RUN' position, the entire starting resistance is eliminated and the motor runs with normal speed.

This is because back emf is developed consequently with speed to counter the supply voltage and reduce the armature current. So the external electrical resistance is not required anymore, and is removed for optimum operation. The handle is moved manually from OFF to the RUN position with development of speed. Now the obvious question is once the handle is taken to the RUN position how is it supposed to stay there, as long as motor is running ?

To find the answer to this question let us look into the working of No Voltage Coil.

Working of No Voltage Coil of 3 Point Starter

The supply to the field winding is derived through no voltage coil. So when fieldcurrent flows, the NVC is magnetized. Now when the handle is in the 'RUN' position, soft iron piece connected to the handle and gets attracted by the magnetic force produced by NVC, because of flow of current through it. The NVC is designed in such a way that it holds the handle in 'RUN' position against the force of the spring as long as supply is given to the motor. Thus NVC holds the handle in the 'RUN' position and hence also called hold on coil.

Now when there is any kind of supply failure, the current flow through NVC is affected and it immediately looses its magnetic property and is unable to keep the soft iron piece on the handle, attracted. At this point under the action of the spring force, the handle comes back to OFF position, opening the circuit and thus switching off the motor. So due to the combination of NVC and the spring, the starter handle always comes back to OFF position whenever there is any supply problems. Thus it also acts as a protective device safeguarding the motor from any kind of abnormality.

Working Principle of Four Point Starter

The 4 point starter like in the case of a 3 point starter also acts as a protective device that helps in safeguarding the armature of the shunt or compound excited dc motor against the high startingcurrent produced in the absence of back emf at starting.

The 4 point starter has a lot of constructional and functional similarity to a three point starter, but this special device has an additional point and a coil in its construction, which naturally brings about some difference in its functionality, though the basic operational characteristic remains the same.

Now to go into the details of operation of 4 point starter, lets have a look at its constructional diagram, and figure out its point of difference with a 3 point starter.

Construction and Operation of Four Point Starter

A 4 point starter as the name suggests has 4 main operational points, namely

1. 'L' Line terminal. (Connected to positive of supply.)

2. 'A' Armature terminal. (Connected to the armature winding.)

3. 'F' Field terminal. (Connected to the field winding.)

Like in the case of the 3 point starter, and in addition to it there is,

4. A 4th point N. (Connected to the No Voltage Coil)

The remarkable difference in case of a 4 point starter is that the No Voltage Coil is connected independently across the supply through the fourth terminal called 'N' in addition to the 'L', 'F' and 'A'. As a direct consequence of that, any change in the field supply current does not bring about any difference in the performance of the NVC. Thus it must be ensured that no voltage coil always produce a force which is strong enough to hold the handle in its 'RUN' position, against force of the spring, under all the operational conditions. Such a current is adjusted through No Voltage Coil with the help of fixed resistance R connected in series with the NVC using fourth point 'N' as shown in the figure above.

Apart from this above mentioned fact, the 4 point and 3 point starters are similar in all other ways like possessing is a variable resistance, integrated into number of sections as shown in the figure above. The contact points of these sections are called studs and are shown separately as OFF, 1, 2, 3, 4, 5, RUN, over which the handle is free to be maneuvered manually to regulate the starting current with gathering speed.

Now to understand its way of operating lets have a closer look at the diagram given above. Considering that supply is given and the handle is taken stud No.1, then the circuit is complete and line current that starts flowing through the starter. In this situation we can see that the current will be divided into 3 parts, flowing through 3 different points.

i) 1 part flows through the starting resistance (R1+ R2+ R3…..) and then to the armature.

ii) A 2nd part flowing through the field winding F.

iii) And a 3rd part flowing through the no voltage coil in series with the protectiveresistance R.

So the point to be noted here is that with this particular arrangement any change in the shunt field circuit does not bring about any change in the no voltage coil as the two circuits are independent of each other. This essentially means that the electromagnet pull subjected upon the soft iron bar of the handle by the novoltage coil at all points of time should be high enough to keep the handle at its RUN position, or rather prevent the spring force from restoring the handle at its original OFF position, irrespective of how the field rheostat is adjusted.

This marks the operational difference between a 4 point starter and a 3 point starter. As otherwise both are almost similar and are used for limiting the starting currentto a shunt wound DC motor or compound wound DC motor, and thus act as a protective device.

On application of load the speed of a dc motor decreases gradually. This is not at all desirable. So the difference between no load and full load speed should be very less. The motor capable of maintaining a nearly constant speed for varying load is said to have good speed regulation i.e the difference between no load and full load speed is quite less. The speed regulation of a permanent magnet DC motor is good ranging from 10 - 15% whereas for dc shunt motor it is somewhat less than 10 %. DC series motor has poor value of regulation. In case of compound DC motor the speed regulation for dc cumulative compound is around 25 % while differential compound has its excellent value of 5 %.

Speed of a DC Motor

The emf equation of DC motor is given by

$E; =;frac{N P phi Z}{60A}$

Here N = speed of rotation in rpm.

P = number of poles.

A = number of parallel paths.

Z = total no. conductors in armature.

$Thus,; speed; of; rotation;N; =; frac{60A}{PZ}Xfrac{E}{phi}$

$Rightarrow ; N; =; frac{E}{kphi};;;Where;k;=;frac{PZ}{60A}; is;a;constant$

Hence, speed of a DC motor is directly proportional to emf of rotation (E) and inversely proportional to flux per pole (φ).

Speed Regulation of a DC Motor

The speed regulation is defined as the change in speed from no load to full load, expressed as a fraction or percentage of full load speed.

Therefore, as per definition per unit (p.u) speed regulation of DC motor is given as,

$Speed; Regulation_{pu}; =; frac{N_{no;load} -N_{full;load}}{N_{full;load}}$

Similarly, percentage (%) speed regulation is given as,

$Speed; Regulation(%); =; frac{N_{no;load} -N_{full;load}}{N_{full;load}}X100%$

Where Nno load = no load speed and Nfull lod = full load speed of DC motor.

Therefore, Percent speed regulation = Per unit (p.u) speed regulation X 100 %.

A motor which has nearly constant speed at all load below full rated load, have good speed regulation.

Speed control means intentional change of the drive speed to a value required for performing the specific work process. Speed control is a different concept from speed regulation where there is natural change in speed due change in load on the shaft. Speed control is either done manually by the operator or by means of some automatic control device.

One of the important features of dc motor is that its speed can be controlled with relative ease. We know that the expression of speed control dc motor is given as,

$N; =; K V;- ;frac{ I_a (R_a; +; R)}{phi}$

Therefore speed (N ) of 3 types of dc motor – SERIES, SHUNT AND COMPOUND can be controlled by changing the quantities on RHS of the expression. So speed can be varied by changing

(i) terminal voltage of the armature V ,

(ii) external resistance in armature circuit R and

(iii) flux per pole φ .

The first two cases involve change that affects armature circuit and the third one involves change in magnetic field. Therefore speed control of dc motor is classified as

1) armature control methods and

2) field control methods.

Speed Control of DC Series Motor

Speed control of dc series motor can be done either by armature control or by field control.

Armature Control of DC Series Motor

Speed adjustment of dc series motor by armature control may be done by any one of the methods that follow,

1. Armature resistance control method: This is the most common method employed. Here the controlling resistance is connected directly in series with the supply to the motor as shown in the fig.
diagram

The power loss in the control resistance of dc series motor can be neglected because this control method is utilized for a large portion of time for reducing the speed under light load condition. This method of speed control is most economical for constant torque. This method of speed control is employed for dc series motor driving cranes, hoists, trains etc.

2. Shunted armature control: The combination of a rheostat shunting the armature and a rheostat in series with the armature is involved in this method of speed control. The voltage applied to the armature is varies by varying series rheostat R1. The exciting current can be varied by varying the armature shunting resistanceR2. This method of speed control is not economical due to considerable power losses in speed controlling resistances. Here speed control is obtained over wide range but below normal speed.
Diagram :

3. Armature terminal voltage control: The speed control of dc series motor can be accomplished by supplying the power to the motor from a separate variablevoltage supply. This method involves high cost so it rarely used.

Field Control of DC Series Motor

The speed of dc motor can be controlled by this method by any one of the following ways –

1. Field diverter method: This method uses a diverter. Here the field flux can be reduced by shunting a portion of motor current around the series field. Lesser the diverter resistance less is the field current, less flux therefore more speed. This method gives speed above normal and the method is used in electric drives in which speed should rise sharply as soon as load is decreased.diagram

2. Tapped Field control: This is another method of increasing the speed by reducing the flux and it is done by lowering number of turns of field winding through whichcurrent flows. In this method a number of tapping from field winding are brought outside . This method is employed in electric traction.

Diagram

Speed Control of DC Shunt Motor

Speed of dc shunt motor is controlled by the factors stated below

Field Control of DC Shunt Motor

By this method speed control is obtained by any one of the following means –

1. Field rheostat control of DC Shunt Motor: In this method , speed variation is accomplished by means of a variable resistance inserted in series with the shunt field . An increase in controlling resistances reduces the field current with a reduction in flux and an increase in speed. This method of speed control is independent of load on the motor. Power wasted in controlling resistance is very less as field current is a small value. This method of speed control is also used in DC compound motor.

Limitations of this method of speed control:

A. Creeping speeds cannot be obtained.

B. Top speeds only obtained at reduced torque

C. The speed is maximum at minimum value of flux, which is governed by the demagnetizing effect of armature reaction on the field.

2. Field voltage control: This method requires a variable voltage supply for the field circuit which is separated from the main power supply to which the armature is connected. Such a variable supply can be obtained by an electronic rectifier.

Armature Control of DC Shunt Motor

Speed control by this method involves two ways . These are :

1. Armature resistance control : In this method armature circuit is provided with a variable resistance. Field is directly connected across the supply so flux is not changed due to variation of series resistance. This is applied for dc shunt motor. This method is used in printing press, cranes, hoists where speeds lower than rated is used for a short period only.

2. Armature voltage control: This method of speed control needs a variable source of voltage separated from the source supplying the field current. This method avoids disadvantages of poor speed regulation and low efficiency of armature-resistance control methods. The basic adjustable armature voltage control method of speed d control is accomplished by means of an adjustable voltagegenerator is called Ward Leonard system. This method involves using a motor –generator (M-G) set. This method is best suited for steel rolling mills, paper machines, elevators, mine hoists, etc.

Advantages of this method –

A. Very fine speed control over whole range in both directions

B. Uniform acceleration is obtained

C. Good speed regulation

Disadvantages –

A. Costly arrangement is needed , floor space required is more

B. Low efficiency at light loads

Armature windings are mainly of two types – lap winding and wave winding. Here we are going to discuss about lap winding.

Lap winding is the winding in which successive coils overlap each other. It is named "Lap" winding because it doubles or laps back with its succeeding coils.

In this winding the finishing end of one coil is connected to one commutator segment and the starting end of the next coil situated under the same pole and connected with same commutator segment.

Here we can see in picture, the finishing end of coil - 1 and starting end of coil - 2 are both connected to the commutator segment - 2 and both coils are under the same magnetic pole that is N pole here.

Simplex Lap Winding

A winding in which the number of parallel path between the brushes is equal to the number of poles is called simplex lap winding.

Duplex Lap Winding

A winding in which the number of parallel path between the brushes is twice the number of poles is called duplex lap winding.

Some important points to remember while designing the Lap winding:
If, Z = the number conductors
P = number of poles
YB = Back pitch
YF = Front pitch
YC = Commutator pitch
YA = Average pole pitch
YP = Pole pitch
YR = Resultant pitch

Then, the back and front pitches are of opposite sign and they cannot be equal.
YB = YF ± 2m
m = multiplicity of the winding.
m = 1 for Simplex Lap winding
m = 2 for Duplex Lap winding
When,
YB > YF, it is called progressive winding.
YB < YF , it is called retrogressive winding.
Back pitch and front pitch must be odd.
Resultant pitch (YR) = YB - YF = 2m
YR is even because it is the difference between two odd numbers.
$Average; pitch; (Y_A) = frac{Y_B + Y_F}{2}; =; polepitch; (Y_P );=;frac{Z}{P}$
$Back; pitch ;(Y_B);approx ;frac{Z}{P}$
Commutator pitch (YC) = ±m
Number of parallel path in the Lap winding = mP

Let us start from 1st conductor,

BACK CONNECTIONS	FRONT CONNECTIONS
1 to (1+YB)=(1+5)=6	6 to (6-YF)=(6-3)=3
3 to (3+5)=8	8 to (8-3)=5
5to (5+5)=10	10 to (10-3)=7
7 to (7+5)=12	12 t0 (12-3)=9
9 to (9+5)=14	14 to (14-3)=11
11 to (11+5))=16	16 to (16-3)=13
13 to (13+5)=18=(18-16)=2	2 to (18-3)=15
15 to (15+5)=20=(20-16)=4	4 to(20-3)=17=(17-16)=1

Advantages of Lap Winding

1. This winding is necessarily required for large current application because it has more parallel paths.
2. It is suitable for low voltage and high current generators.

Disadvantages of Lap Winding

1. It gives less emf compared to wave winding. This winding is required more no. of conductors for giving the same emf, it results high winding cost.

In a dc motor, an armature rotates inside a magnetic field. Basic working principle of DC motor is based on the fact that whenever a current carrying conductor is placed inside a magnetic field, there will be mechanical force experienced by that conductor. All kinds of DC motors work in this principle only. Hence for constructing a dc motor it is essential to establish a magnetic field. The magnetic field is obviously established by means of magnet. The magnet can by any types i.e. it may be electromagnet or it can be permanent magnet. When permanent magnet is used to create magnetic field in a DC motor, the motor is referred aspermanent magnet dc motor orPMDC motor. Have you ever uncovered any battery operated toy, if you did, you had obviously found abattery operated motor inside it. Thisbattery operated motor is nothing but a permanent magnet dc motor or PMDC motor. These types of motor are essentially simple in construction. These motors are commonly used as starter motor in automobiles, windshield wipers, washer, for blowers used in heaters and air conditioners, to raise and lower windows, it also extensively used in toys. As the magnetic field strength of a permanent magnet is fixed it cannot be controlled externally, field control of this type of dc motor cannot be possible. Thus permanent magnet dc motor is used where there is no need of speed control of motor by means of controlling its field. Small fractional and sub fractional kW motors now constructed with permanent magnet.

Construction of Permanent Magnet DC Motor or PMDC Motor

Stator of Permanent Magnet DC Motor

As it is indicated in name of permanent magnet dc motor, the field poles of this motor are essentially made of permanent magnet. A PMDC motor mainly consists of two parts. A stator and an armature. Here the stator which is a steel cylinder. The magnets are mounted in the inner periphery of this cylinder. The permanent magnets are mounted in such a way that the N – pole and S – pole of each magnet are alternatively faced towards armature as shown in the figure below. That means, if N – pole of one magnet is faced towards armature then S – pole of very next magnet is faced towards armature.

In addition to holding the magnet on its inner periphery, the steel cylindrical stator also serves as low reluctance return path for the magnetic flux. Although field coil is not required in permanent magnet dc motor but still it is sometimes found that they are used along with permanent magnet. This is because if permanent magnets lose their strength, these lost magnetic strengths can be compensated by field excitation through these field coils. Generally, rare earth hard magnetic materials are used for these permanent magnet.

Rotor : The rotor of pmdc motor is similar to other DC motor. The rotor or armature of permanent magnet dc motor also consists of core, windings and commutator. Armature core is made of number of varnish insulated, slotted circular lamination of steel sheets. By fixing these circular steel sheets one by one, a cylindrical shaped slotted armature core is formed. The varnish insulated laminated steel sheets are used to reduce eddy current loss in armature of permanent magnet dc motor. These slots on the outer periphery of the armature core are used for housing armature conductors in them. The armature conductors are connected in a suitable manner which gives rise to armature winding. The end terminals of the winding are connected to the commutator segments placed on the motor shaft. Like other dc motor, carbon or graphite brushes are placed with spring pressure on the commutator segments to supply current to the armature.

Working Principle of Permanent Magnet DC Motor or PMDC Motor

As we said earlier the working principle of PMDC motor is just similar to the general working principle of DC motor. That is when a carrying conductor comes inside a magnetic field, a mechanical force will be experienced by the conductor and the direction of this force is governed by Fleming’s left hand rule. As in a permanent magnet dc motor, the armature is placed inside the magnetic field of permanent magnet; the armature rotates in the direction of the generated force. Here each conductor of the armature experiences the mechanical force F = B.I.L Newton where B is the magnetic field strength in Tesla (weber / m2), I is thecurrent in Ampere flowing through that conductor and L is length of the conductor in metre comes under the magnetic field. Each conductor of the armature experiences a force and the compilation of those forces produces a torque, which tends to rotate the armature.

Equivalent Circuit of Permanent Magnet DC Motor or PMDC Motor

As in PMDC motor the field is produced by permanent magnet, there is no need of drawing field coils in the equivalent circuit of permanent magnet dc motor. The supply voltage to the armature will have armatureresistance drop and rest of the supply voltage is countered by back emf of the motor. Hence voltageequation of the motor is given by,
$V ;=; I R;+; E_b$
Where I, is armature current and R is armature resistance of the motor.
Eb is the back emf and V is the supply voltage.

Advantages of Permanent Magnet DC Motor or PMDC Motor

PMDC motor have some advantages over other types of dc motors. They are :

No need of field excitation arrangement.
No input power in consumed for excitation which improve efficiency of dc motor.
No field coil hence space for field coil is saved which reduces the overall size of the motor.
Cheaper and economical for fractional kW rated applications.

Disadvantages of Permanent Magnet DC Motor or PMDC Motor

In this case, the armature reaction of DC motor cannot be compensated hence the magnetic strength of the field may get weak due to demagnetizing effect armature reaction.
There is also a chance of getting the poles permanently demagnetized (partial) due to excessive armature current during starting, reversal and overloading condition of the motor.
Another major disadvantage of PMDC motor is that, the field in the air gap is fixed and limited and it cannot be controlled externally. Therefore, very efficient speed control of DC motor in this type of motor is difficult.

Applications of Permanent Magnet DC Motor or PMDC Motor

PMDC motor is extensively used where small dc motors are required and also very effective control is not required, such as in automobiles starter, toys, wipers, washers, hot blowers, air conditioners, computer disc drives and in many more.

Ward Leonard control system is introduced by Henry Ward Leonard in 1891. Ward Leonard method of speed control is used for controlling the speed of a DC motor. It is a basic armature control method. This control system is consisting of a dc motor M_1 and powered by a DC generator G. In this method the speed of the dc motor (M_1) is controlled by applying variable voltage across its armature. This variable voltage is obtained using a motor-generator setwhich consists of a motor M_2(either ac or dc motor) directly coupled with the generator G. It is a very widely used method of speed control of DC motor.

Principle of Ward Leonard Method

Basic connection diagram of the Ward Leonard speed control system is shown in the figure below.

The speed of motor M1 is to be controlled which is powered by the generator G. The shunt field of the motor M1 is connected across the dc supply lines. Now, generator G is driven by the motor M2. The speed of the motor M2 is constant. When the output voltage of the generator is fed to the motor M1 then the motor starts to rotate. When the output voltage of the generator varies then the speed of the motor also varies. Now controlling the output voltageof the generator the speed of motor can also be controlled. For this purpose of controlling the output voltage, a field regulator is connected across the generator with the dc supply lines to control the field excitation. The direction of rotation of the motor M1 can be reversed by excitation current of the generator and it can be done with the help of the reversing switch R.S. But the motor-generator set must run in the same direction.

Advantages of Ward Leonard System

It is a very smooth speed control system over a very wide range (from zero to normal speed of the motor).
The speed can be controlled in both the direction of rotation of the motor easily.
The motor can run with a uniform acceleration.
Speed regulation of DC motor in this ward Leonard system is very good.

Disadvantages of Ward Leonard System

The system is very costly because two extra machines (motor-generator set) are required.
Overall efficiency of the system is not sufficient especially it is lightly loaded.

Application of Ward Leonard System

This Ward Leonard method of speed control system is used where a very wide and very sensitive speed control is of a DC motor in both the direction of rotation is required. This speed control system is mainly used in colliery winders, cranes, electric excavators, mine hoists, elevators, steel rolling mills and paper machines etc.

DC motor

Brushless

Uncommutated

Brush

Permanent magnet stators

Wound stators

Series connection

Shunt connection

Compound connection

Brushed DC electric motor

Permanent-magnet motors

Axial field motors

Speed control

Series-parallel

Field weakening

Simple two-pole DC motor

What is DC Motor ?

Principle of DC Motor

Detailed Description of a DC Motor

Working Principle of DC Motor-Video

Construction of DC Motor | Yoke Poles Armature Field Winding Commutator Brushes of DC Motor