Electrical Machines - I Lab Manual12 Brake test on DC shunt motor 13 Retardation test on DC shunt motor 14 Separation of core losses in DC shunt motor . Electrical Machines-I lab manual

Electrical Machines-I lab manual

Anurag College of Engineering, Aushapur Department of EEE

Electrical Machines - I

Lab Manual

ELECTRICAL AND ELECTRONICS ENGINEERING

ANURAG COLLEGE OF ENGINEERING

Aushapur (V), Ghatkesar (M) - 501301



INDEX

S. No. List of Experiments

1 Identifying the terminals of DC Motors

2 To study the parts of 3 Point, 4 Point Starter and Drum controller starter

3 Magnetization characteristic of DC shunt generator

4 Load test on DC shunt generator

5 Load test on DC series generator

6 Load test on DC compound generator

7 Hopkinson’s test on DC shunt machines

8 Field’s test on DC series machines

9 Speed control of DC shunt motor

10 Swinburne’s test on DC shunt motor

11 Brake test on DC compound motor

12 Brake test on DC shunt motor

13 Retardation test on DC shunt motor

14 Separation of core losses in DC shunt motor



IDENTIFY THE TERMINALS OF FOLLOWING DC MACHINES

(a) DC Shunt Wound Motor:

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 current and 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 the voltage and current being supplied from the electrical terminal to the

motor be given by E and Itotal respectively. This supply current in case of the shunt wound DC

motor is split up into 2 parts. Ia, flowing through the armature winding of resistance Ra and Ish

flowing through the field winding of resistance Rsh. The voltage across both windings remains

the same. From there we can write

Thus we put this value of armature current Ia to get general voltage equation of a DC shunt

motor.

Now in general practice, when the motor is in its running condition, and supply voltage is

constant the shunt field current given by,

https://www.electrical4u.com/armature-winding-pole-pitch-coil-span-commutator-pitch/

https://www.electrical4u.com/voltage-or-electric-potential-difference/

https://www.electrical4u.com/electric-current-and-theory-of-electricity/


https://www.electrical4u.com/electrical-resistance-and-laws-of-resistance/



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.

(b) Series Wound DC Motor or DC Series Motor:

A series wound DC motor like in the case of shunt wound DC motor or compound

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 is exposed to the entire armature current unlike in the case of a

shunt motor.

Construction of Series DC Motor:

Construction wise a 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.

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 supply voltage and current given to the electrical port of the motor be given by E and Itotal

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

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.

https://www.electrical4u.com/shunt-wound-dc-motor-dc-shunt-motor/

https://www.electrical4u.com/compound-wound-dc-motor-or-dc-compound-motor/





https://www.electrical4u.com/types-of-dc-motor-separately-excited-shunt-series-compound-dc-motor/

https://www.electrical4u.com/dc-motor-or-direct-current-motor/



Where, Eb is the back emf. Rse is the series coil resistance and Ra is the armature resistance.

Since Ise = Ia, we can write,

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.

But since here and

i.e. the field flux is proportional to the entire armature current 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.

(c) Compound Wound DC Motor or DC Compound Motor:

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 motor is bestowed with an extremely efficient speed

regulation characteristic, whereas the DC series motor has high starting torque.

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:

1. 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.

https://www.electrical4u.com/dc-motor-or-direct-current-motor/


https://www.electrical4u.com/what-is-flux-types-of-flux/


https://www.electrical4u.com/magnetic-flux/

https://www.electrical4u.com/shunt-wound-dc-motor-dc-shunt-motor/

https://www.electrical4u.com/series-wound-dc-motor-or-dc-series-motor/

https://www.electrical4u.com/series-wound-dc-motor-or-dc-series-motor/



Voltage and Current Equation of Long Shunt Compound Wound DC Motor

Let E and Itotal be the total supply voltage and current supplied to the input terminals of the

motor. And Ia, Ise , Ish be the values of current flowing through armature resistance Ra, series

winding resistance Rse and shunt winding resistance Rsh respectively. Now we know in shunt

motor, And in series motor

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

and its voltage equation is,

2. 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 and current supplied to the input terminals of

the motor. And Ia, Ise, Ish be the values of current flowing through armature resistance Ra, series

winding resistance Rse and shunt winding resistance Rsh respectively. But from the diagram

above we can see,

Since the entire supply current flows through the series field winding. And like in the case of a

DC shunt motor,

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

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

But since

Thus the final voltage equation can be written as,

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.

Differential Compounding of DC Motor:

Similarly a compound wound DC motor is said to be deferentially 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.



https://www.electrical4u.com/kirchhoff-current-law-and-kirchhoff-voltage-law/


https://www.electrical4u.com/what-is-flux-types-of-flux/



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.



DC 3-POINT STARTER

A 3 point starter in simple words is a device that helps in the starting and running of a

shunt wound DC motor or compound wound DC motor. Now the question is why these types 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 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 starting

current 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 variable resistance, 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 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 starting resistance, 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 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 field current

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.

Drawbacks of a Three Point Starter

The 3 point starter suffers from a serious drawback for motors with large variation of

speed by adjustment of the field rheostat. To increase the speed of the motor field resistance can

be increased. Therefore current through shunt field is reduced. Field current becomes very low

which results in holding electromagnet too weak to overcome the force exerted by the spring.

The holding magnet may release the arm of the starter during the normal operation of the motor



and thus disconnect the motor from the line. This is not desirable. A four point starter is thus

used.



DC 4-POINTER 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 starting current 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. This naturally brings

about some difference in its functionality, though the basic operational characteristic remains the

same. The basic difference in circuit of 4 point starter as compared to 3 point starter is that the

holding coil is removed from the shunt field current and is connected directly across the line with

current limiting resistance in series. Now to go into the details of operation of 4 point starter,

let’s 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.)

4. Like in the case of the 3 point starter, and in addition to it there is, A 4th point N. (Connected

to the No Voltage Coil NVC)

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.

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

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

3. And a 3rd part flowing through the no voltage coil in series with the protective resistance 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 no voltage 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 current to a shunt wound

DC motor or compound wound DC motor, and thus act as a protective device.

https://www.electrical4u.com/3-point-starter-working-principle-and-construction-of-three-point-starter/





Working principle of 4-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

starting current produced in the absence of back emf as 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. This naturally brings about some difference in its

functionality, though the basic operational characteristic remains the same. The basic difference

in circuit of 4-point starter as compared to 3-point starter is that the holding coil is removed from

the shunt field current and is connected directly across the line with current limiting resistance in

series. Now to go into the details of operation of 4-point starter, have a view of constructional

diagram and figure out its point of difference with a 3-point starter.



DRUM CONTROLLER STARTER

For winch and crane motors where frequent starting, stopping, reversing and speed

variations are necessary, drum type controllers are used. They are called controllers because they

can be left in the circuit for any length of time. In addition to serving their normal function of

starters, they also used as speed controllers.

Series and cumulative compound motors are often used on cranes, elevators, machine tools, and

other devices where the motor is under the direct control of an operator and where frequent

starting, varying speed, stopping, and reversing are necessary. A manually operated controller

that is more rugged than a starting rheostat is used in these applications. This starting rheostat is

called a drum controller.

A typical drum controller is illustrated in Figure 22–10. Inside the switch is a series of contacts

mounted on a movable cylinder. These contacts, insulated from the cylinder and from each other,

are the movable contacts. There is another series of contacts, located inside the controller, called

stationary contacts. These contacts are arranged to make contact with the movable contacts as the

cylinder is rotated. On top of the drum controller is a handle that is keyed to the shaft for the

movable cylinder and contacts. This handle can be moved in either a clockwise or a

counterclockwise direction, providing a range of speed control in either direction or rotation.

Once set, a roller and notched- wheel arrangement keeps the cylinder and movable contacts

stationary until the handle is turned by the operator.



A schematic of a drum controller having two steps of resistance is shown in Figure 22–11. In this

wiring diagram, the contacts are shown in a flat position to make it easier to trace connections.

For operating in the forward direction, the movable contacts on the right connect with the center

stationary contacts. For operation in the reverse direction, the movable contacts on the left touch

the stationary contacts in the center.

There are three forward positions and three reverse positions in which the controller handle can

be set. In the first forward position, all resistance is in series with the armature. The circuit for

the first forward position is traced as follows:

1. Movable fingers A, B, C, and D contact the stationary contacts 7, 5, 4, and 3.

2. The current path is from 7 to A, from A to B, from B to 5, and then to armature terminal A1.

3. From A1, the current path is through the armature winding to terminal A2, then to stationary

contact 6, and then to stationary contact 4.

4. From contact 4, the current path is to contact C, to D, and then to 3.



5. From 3, the current path is through the entire armature resistor, through the series field, and

then back to the line.

In the second forward position, part of the resistance is cut out by the connection from D to E.

The third forward position bypasses all resistance and puts the armature circuit directly across

the source voltage.

In the first reverse position, all resistance is again inserted in series with the armature. Figure 22–

12 illustrates the first position of the controller for the reverse direction.

The current in the armature circuit is reversed. However, the current direction in the shunt and

series fields is the same as for the forward direction. As shown earlier, changing the direction of

the current in only the armature changes the direction of rotation. In the second position, part of

the resistance circuit is cut out. The third reverse position cuts out all resistance and puts the

armature circuit directly across line voltage.

There are more elaborate drum controllers with more positions and a greater control of speed.

However, they all use practically the same circuit arrangement.



MAGNETIZATION CHARACTERISTICS OF DC SHUNT GENERATOR

Aim:

To draw the magnetization characteristics or open circuit characteristics curve of a dc

shunt generator and to determine its critical field resistance.

Name plate details:

Details Motor Generator

Rating

Voltage

Current

Speed

Apparatus:

S. No Apparatus Range Type Quantity

1. Ammeter 0-1/2 A MC 02

2. Voltmeter 0-300 V MC 01

3. Rheostat 0-350 Ohms / 2A WW 02

4. Tachometer 0-1000 rpm Digital 01

5. SPST - - 01

6. Connecting wire - - required

Procedure:

1. Make the connections as per the circuit diagram

2. Initially keep the motor field rheostat in minimum position and generator field rheostat in

maximum position

3. Close the DPST switch and start motor slowly with the help of starter.

4. Adjust the motor field rheostat till rated speed of motor is obtained.

5. Note down the voltmeter and ammeter reading and close the SPST.

6. Now decrease generator field resistance in steps of field current and note down the

corresponding voltmeter reading till the generated emf is 120% of rated value.

7. Increase the generator field resistance in steps of field current and note down the

corresponding voltmeter reading till the field current zero.

8. Bring back rheostats to their initial position and open DPST.



Circuit Diagram:

Tabular column:

S. NO Field current

If (Amperes)

Generated EMF in Volts

Average Eg

in Volts

(If Increasing) (If Decreasing)

Model Graph:



Precautions:

1. Before opening the DPST switch ensure that rheostats are in their original position.

2. Loose connections are to be avoided.

3. Avoid parallax error.

Result:

Conclusion:

Viva questions:

1. Define critical field resistance.

2. How do you get the maximum voltage to which the generator builds up from OCC?

3. What does the flat portion of OCC indicate?

4. Why OCC does not start from origin?

5. Why is Rf > Ra in dc shunt machine?

6. How do you create residual magnetism if it is wiped out?

7. Why does the OCC differ for decreasing and increasing values of field current?

8. Under what conditions does the DC shunt generator fail to self - excite?

9. OCC is also known as magnetization characteristic, why?

10. How do you check the continuity of field winding and armature winding?

11. How do you make out that the generator is DC generator without observing the name plate?

12. Does the OCC change with speed?



LOAD TEST ON DC SHUNT GENERATOR

Aim:

To perform on load test on DC Shunt generator and to draw its performance

characteristics

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A, 0-1/2 A MC 02




5. Load rheostat 4 KW - 01


Procedure:


2. Initially keep the motor field rheostat in minimum position and generator field

rheostat in maximum position



5. Now decrease generator field resistance till voltmeter decreases rated EMF.

6. Note down readings of meters at no load.

7. Increase load in steps and note down voltmeter and ammeter readings till generator

generates rated current.

8. Switch off load bring back rheostats to their initial position and open DPST.



Circuit Diagram:

Tabular column:

S. No. Voltage V in

volts

Load current

IL in Amps

Field current

ISH in Amps

Armature

current IA in

Amps

Generated

EMF EG in

volts



Model Graph:

Precautions:




Result:

Conclusion:

Viva questions:

1. Specify the applications of DC shunt generators.

2. Differentiate between DC shunt Motor and DC shunt generator.

3. Which method is suitable for testing of high rating DC generator?

4. Why the terminal voltage decreases when load is increased on the generator?

5. Why is the generated EMF not constant even though the field circuit resistance is kept

unaltered?

6. Find out the voltage drop due to full load armature reaction?

7. State the conditions required to put the DC shunt generator on load.

8. How do you compensate for the armature reaction?

9. What happens if shunt field connections is reversed in the generator?

10. The EMF induced in armature conductors of DC shut generator is AC or DC?



LOAD TEST ON DC SERIES GENERATOR

Aim:

To perform on load test on DC Series generator and to draw its performance

characteristics

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A MC 01






Procedure:


2. Initially keep the motor field rheostat in minimum position and switch on load and

apply minimum load on generator



5. Note down readings of volt meter and ammeter.

6. Increase load in steps and note down voltmeter and ammeter readings till rated

current.

7. Reduce the load to initial position, bring back the rheostats to their initial position and

open DPST.



Circuit Diagram:

Tabular column:

S. No. Voltage V in

volts

Load current

IL in Amps

Generated

EMF EG in

volts



Model Graph:

Precautions:

1. Minimum load should be applied generator before switching ON OFF the supply




Result:

Conclusion:

Viva questions:

1. In what way does the series generator differ fundamentally from shunt generator?

2. Why does a series generator have rising characteristics?

3. Why the series generators will only built up when load switch is on?

4. Why the series generator used as voltage booster in transmission system? 5. What are the applications of DC series generator?

6. To conduct the test on DC series generator, can we use any other prime mover other than DC

shunt motor?

7. Why DC series motor should not start without any load?

8. State the applications of the series generator.

9. State voltage builds up conditions of a series generator.



LOAD TEST ON DC COMPOUND GENERATOR

Aim:

To perform on load test on DC compound generator and to draw its performance

characteristics

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A, 0-1/2 A MC 02






Procedure:






5. Now decrease generator field resistance till voltmeter decreases rated emf.


7. Increase load in steps and note down voltmeter and ammeter readings till generator

generates rated current.

8. Switch off load bring back rheostats to their initial position and open DPST.

9. Connect the generator for differential compounding by interchanging the series field

terminals and repeat the above procedure.



Circuit Diagram:

Tabular column:

S. No. Voltage V in

volts

Load current

IL in Amps

Field current

ISH in Amps

Armature

current IA in

Amps

Generated

EMF EG in

volts

Model Graph:



Precautions:




Result:

Conclusion:

Viva questions:

1. What do you understand from load curves?

2. Which causes the drop between internal & external characteristics?

3. A cumulative compound generator is generating full load, what will happen if its series field

winding gets short – circuited?

4. Explain the difference between cumulative and differential compound generators. 5. Where you can use DC Compound Generator?

6. Comment on the shape of load current Vs speed curve of the differential compounded generator.

7. How do you reverse the terminal voltage of an over compounded short shunt generator without effecting the over compounding?

8. Mention the applications of differential compound generator.

9. Mention the applications of over compound generator.



HOPKINSON’S TEST ON DC SHUNT MACHINES

Aim:

To perform on Hopkinson’s test on dc shunt generator and to determine the efficiency of

both motor and generator.

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A, 0-1/2 A MC 04






Procedure:






5. Now decrease generator field resistance till voltmeter decreases rated emf.

6. If the voltmeter reads zero then close parallel switch

7. Load the generator in steps of load current either by decreasing field resistance of

generator or by increasing field resistance of motor

8. Note down the reading of meters for each load and measure the corresponding speed

9. Reduce the excitation of generator by increasing the field resistance of generator and

open switch



10. Bring back the rheostat to its initial position and open the DPST switch

Armature field resistance / field resistance

i. Connect the circuit as shown

ii. Note down the readings of voltmeter and ammeter by varying

resistance.

Circuit Diagram:



Tabular column: G

ener

ato

r

effi

cien

cy

Mo

tor

Eff

icie

ncy

Str

ay l

oad

loss

es W

Gen

erat

or

fiel

d

curr

ent

If

Gen

erat

or

curr

ent

Ig

Gen

erat

or

Vo

ltag

e V

Mo

tor

fiel

d

curr

ent

Ish

m

Mo

tor

curr

ent

Ia

Vo

ltag

e V

S.

No



Model Graph:

Precautions:


2. Take excessive care while closing the parallel switch the voltmeter must read zero for

switch to be closed.



Result:

Conclusion:

Viva questions:

1. Hopkinson’s test is a ………..test.

2. What are the disadvantages of this test?

3. What are heat run tests? 4. What are the advantages of the test?

5. Can you explain this test be applied to compound machines?

6. When two DC machines are paralleled as is done in this test, which machine acts as generator and

which machine acts as motor?

7. Hopkinson’s test on DC machines is conducted at ….load.



FIELD TEST ON DC SERIES MACHINES

Aim:

To determine the efficiency of two given dc series machines which are mechanically

coupled.

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A MC 02




Procedure:

1. Make the connections as per the circuit diagram.

2. Apply minimum load on generator

3. Close DPST switch and start motor slowly with the help of starter.

4. Increase load in steps and note down corresponding voltmeter and ammeter reading

till rated current.

5. Decrease load in steps put minimum load on generator and open DPST.



Circuit Diagram:



Tabular column: M

ote

r

effi

ecie

n

y

Ou

tpu

t

po

wer

Inp

ut

po

wer

Gen

erat

or

effi

ecie

nc

y

Ou

tpu

t

po

wer

Inp

ut

po

wer

Co

nst

ant

loss

es

Lo

ad

curr

ent

IL

Lo

ad

vo

ltag

e

VL

Cu

rren

t

I1

Vo

ltag

e

V1

S.

No

.



Model Graph:

Precautions:

4. Minimum load should be applied on generator before switching ON/OFF the supply.



Result:

Conclusion:

Viva questions:

1. Why the field of generator connected to motor?

2. What are the applications of D.C series generator?

3. Why the series generator used as voltage booster in transmission system? 4. Why Series motor should not start at no load?

5. What is the main advantage of this test?

6. Is it possible to conduct Field test on any another DC machine



SPEED CONTROL OF DC SHUNT MOTOR

Aim:

To draw the speed characteristics of dc shunt motor by field control method and armature

control.

Name plate details:

Details Motor

Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-1/2 A MC 02





Procedure:


2. Initially keep the motor field rheostat in minimum position.


4. For the filed control method adjust the armature rheostat in position so that voltmeter

reads a selected value and keep it constant

5. Increase the field resistance thereby decreasing the field current in steps and note down

the readings of field current and speed for each step

6. For armature control method adjust field current at selected value keep it constant.

7. Decrease the armature resistance in steps of armature voltage and note down the

voltmeter reading and speed for each step

8. Bring back rheostat to its initial position and open DPST switch.



Circuit Diagram:

Tabular column:

Field control method

Eb

Eb

Field current (A) speed in rpm Field current speed in rpm



Armature control method

If

If

Eb(V) N(rpm) Eb(V) N (rpm)

Model Graph:

Precautions:






Result:

Conclusion:

Viva questions:

1. Explain why the graph of armature speed control of motor is linear?

2. Comment on the efficiency calculated by this method.

3. Why do you need a starter in a dc motor?

4. What is meant by rated speed?

5. Can we start the dc shunt motor and series motor without load?

6. What is meant by speed regulation?

7. Can we operate a dc motor an ac supply?

8. What are the other methods of controlling the speed of dc shunt motor?

9. How do you change the direction of rotation of a D.C. motor?

10. What is the disadvantage of using armature control of speed on load?

11. What are the limitations of shunt field control?

12. Can we conduct continuity test on ac supply?

13. While running if the field winding gets disconnected, what will happen?

14. What is the shape of the curve of field control of method motor speed? Explain why is it so?



SWINBURNE’S TEST ON DC SHUNT MOTOR

Aim:

To perform on Swinburne’s test on a given DC Shunt machine and to predetermine its

efficiency at any desired load both as motor and generator.

Name plate details:

Details Motor

Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A, 0-1/2 A MC 02





Procedure:






6. Bring back rheostat to its initial position and open DPST.



Circuit Diagram:

Tabular column:

S. No. Voltage V in

volts

Line current

IL in Amps

Field current

ISH in Amps



Model Graph:

Precautions:




Result:

Conclusion:

Viva questions:

1. Will the values deduced from the Swinburne’s method exactly coincide with the values realized

by direct loading on the machine? Why?

2. Why are the constant losses calculated by this method less than the actual losses?

3. Can we conduct Swinburne’s test on dc series motor?

4. What are the drawbacks of Swinburne’s test?

5. Why Swinburne’s is used to find efficiency of high rating motors?

6. How you can say that the wattmeter reading in the experiment is constant losses?

7. Why constant losses are constant irrespective of load?

8. Advantage of this test.



BRAKE TEST ON DC COMPOUND GENERATOR

Aim:

To perform brake test on DC compound motor and to draw its performance

characteristics

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A MC 01





Procedure:



3. Check whether the belt on pulley is free so that there is no load on the motor.



6. Note down readings of ammeter, voltmeter and speed at no load

7. Apply the load on pulley gradually in steps of input current till the rated current of

motor by tightening the belt around it.

8. Note down the voltmeter, ammeter and spring balance readings and measure the

speed for each step.

9. Remove load bring back rheostat to its initial position and open DPST switch.



Circuit Diragram:

Tabular column:

S. No. Voltage V in

volts

Load current

IL in Amps

Field current

ISH in Amps

Armature

current IA in

Amps

Generated

EMF EG in

volts



Model Graph:

Precautions:

1. Cool the pulley while the experiment is performed.


3. While measuring radius of pulley, effective radius must be considered.



Result:

Conclusion:

Viva questions:

1. Differentially compounded after reversal?

2. Mention the applications of the cumulative compounded motor?

3. Which type of DC starter is used to start the compound motor?

4. Why differentially compounded motors are not in common use?

5. What is the speed regulation of DC motor?

6. What is Difference between Shunt and compound motors?



BRAKE TEST ON DC SHUNT MOTOR

Aim:

To perform on brake test on DC Shunt motor and to draw its performance characteristics

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20 A MC 01





Procedure:




3. Check whether the belt on pulley is free so that there is no load on the motor



6. Note down readings of ammeter and voltmeter at no load.

7. Apply the load on pulley gradually in steps of input current till the rated current of

motor is obtained by tightening the belt around it

8. Note down the voltmeter, ammeter and spring balance readings and measure the

speed for each step.

9. Remove load bring back rheostat to its initial position and open DPST switch.



Circuit Diagram:

Tabular column:

Voltage

V

Current

IL

LOAD

S1 S2

SPEED

IN RPM

TORQUE INPUT

POWER

O/P POWER EFFICIEN

CY



Model Graph:

Precautions:

1. Cool the pulley while the experiment is performed.




Result:

Conclusion:

Viva questions:

1. Why did you use a 3-point starter for starting DC shunt motor?

2. What is the efficiency range of DC motor?

3. Where can you use the DC shunt motor?

4. What is the starting torque?

5. If starter is not available, how can you start DC motor?

6. Why is it considered as a constant speed motor?

7. Why brake test is used to find the efficiency of DC motor?



RETARDATION TEST DC SHUNT MOTOR

Aim:

To separate the mechanical and iron losses of the given dc shunt machine.

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20A, 0-1/2 A MC 02




5. SPST - - 01


Procedure:


2. Keep the switches SW1 and SW2 in ON position and keep switch SW3 in OFF

position.

3. Gradually raise the motor speed up to 100 rpm above the rated speed.

4. Switch OFF SW1 and press stop watch simultaneously. Note down the time taken

until the motor speed falls to 100 rpm below the rated speed.

5. Stop the motor.

6. Switch ON SW1 and SW2 switches and keep SW3 in OFF position.

7. Start the motor again and raise the speed up to 100 rpm above the rated speed.

8. Switch OFF SW2 switch and press stop watch simultaneously. Note down the time

taken till the motor speed falls to 100 rpm below the rated speed.

9. Stop the motor.

10. Switch on SW1 and SW2 switches and keep SW3 in OFF position.



11. Start the motor again and raise the speed up to 100 rpm above the rated speed.

12. Note down Va and Ia readings

13. Switch off SW2 and switch on SW3 switch and press stop watch simultaneously.

14. Note down the time taken till the motor speed falls to 100 rpm below the rated speed.

15. Note down Va and Ia readings at both the speeds.

16. Stop the motor

Circuit Diagram:

Tabular column:

S1 CLOSE, S2 OPEN

S.NO V (VOLTS) If (A) TIME



S1 OPEN, S2 CLOSE

S.NO V (VOLTS) If (A) TIME

Precautions:




Result:

Conclusion:

Viva questions:

1. What is another name for Retardation test?

2. What is the difference between Retardation and Swinburne test?

3. The values obtained from this test are Pessimistic or Optimistic?

4. Is it possible to conduct Retardation test on DC series machines?



SEPERATION OF LOSSES IN DC SHUNT MOTOR

Aim:

To obtain separately hysteresis, eddy current, friction and windage losses of a given

motor.

Name plate details:


Rating

Voltage

Current

Speed

Apparatus:


1. Ammeter 0-10/20A, 0-1/2 A MC 02





Procedure:



3. Increase the armature voltage till the speed is about 200 rpm more than the rated

value

4. Now, gradually decrease the armature voltage, and note down the values of armature

voltage, armature current and speed.

5. Repeat the experiment at some other field current.

6. Measure the armature resistance separately, after disconnecting the circuit.



Circuit Diagram:

Tabular column:

Full excitation: Field Current=0.9A

S. No Armature

Voltage

Armature

current

Speed W=VIa-Ia2Ra



Reduced excitation: Field Current=0.6A

S. No Armature

Voltage

Armature

current

Speed W=VIa-Ia2Ra

Model Graph:

Precautions:




Result:

Conclusion:



Viva questions:

1. Where are eddy current losses occurring in a D.C. Machine?

2. How are the magnetic losses minimized?

3. How is brush contact resistance loss taken into consideration in practice?

4. Give the expression for hysteresis loss?

5. Differentiate MNA and GNA?

6. Which test gives us stray losses?

7. How Hysteresis losses occur in a D.C. Machine?

8. What is the effect of armature reaction?

9. How do you minimize cross magnetizing effect of armature reaction?



Viva questions

1. Why, the magnetic losses calculated by Swinburne’s test are different from the actual value?

2. Comment on the accuracy of Swinburne’s test over other methods.

3. What is the condition required for maximum efficiency?

4. State the applications of a D.C. shunt generator.

5. Explain the armature reaction.

6. Explain the principle of a D.C. motor.

7. For the same Ia, why the generator is having more efficiency than the motor?

8. Which winding has low resistance Shunt/Armature?

9. Give the reasons, why the voltage of D.C. generator drops as the load is applied?

10. Define the armature reaction. Explain its effect on the internal

11. What do you understand from the external characteristic of D.C. shunt generator?

12. How, the objectionable drooping characteristics of D.C. shunt generator can be improved?

13. If you take the demagnetizing effect of the armature reaction into consideration show the

effects of speed and degree of saturation on the characteristic of a D.C. shunt generator.

14. Shunt field plays the dominating role in a compound generator. Explain.

15. What are the differences between cumulative and differential compound generators?

16. Differentiate between the under-compound, flat-compound, and over-compounded

generators.

17. A cumulative compound generator is delivering at full load, if its shunt field gets opened,

what will happen? If its series field gets short circuited, what will happen?



18. Two D.C. generators (one cumulatively and other differentially compounded) are delivering

the same load ampere at same voltage level. How will the terminal voltage change when the

above loads are switched off?

19. What is the effect of speed on the degree of compounding; if the no load voltage is same in

each case?

20. What are the effects of commutating poles on the characteristic of both types of generators?

21. How, the number of turns for a desired degree of compounding may be determined

experimentally?

22. Discuss the performance of a D.C. compound generator (both cumulative and differential)

using one field winding at a time.

23. Why the speed falls as the load increases for a D.C. shunt motor?

24. When is the efficiency of the motor, maximum?

25. What will happen when a D.C. shunt motor is started with load?

26. What is back electro motive force in a D.C. motor?

27. What is the effect on speed, if part of the field winding is shorted?

28. When will the motor be able to draw maximum power?

29. Explain the process of commutation.

30. While running a D.C. shunt motor, if the field winding is disconnected, what will happen?

31. Why, the speed is maintained constant during the experiment on a generator?

32. What is residual magnetism?

33. Explain hysterisis phenomena.

34. Define coercive force.

35. Explain magnetization curve.



36. Define critical field resistance.

37. Define critical speed.

38. Explain the buildup of voltage in a self excited D.C. shunt generator.

39. What are the conditions required to build up the voltage in a D.C. generator?

40. What will happen if the armature rheostat is set to zero (i.e., maximum Va) and field

circuit resistance kept at high (i.e., minimum If) at the time of starting?

41. What will happen if A.C. supply is given to a D.C. motor?

42. Why, a starter is used for starting a D.C. motor?

43. How do you change the direction of rotation of a D.C. shunt motor?

44. What would be the direction of rotation if both the field windings and armature connections

are reversed?

45. What are the limitations for shunt field control method?

46. Why, a series generator characteristic is called a raising characteristic?

47. List some applications of it applications.

48. Draw speed ~ torque characteristics of a series motor and hence try to co-relate with the load

characteristics of the generator.

49. How does the speed of a prime mover affect the generator characteristics?

50. Why, the retardation test is more appropriate for large machines?

Documents

Electrical Machines - I Lab Manual12 Brake test on DC shunt motor 13 Retardation test on DC shunt motor 14 Separation of core losses in DC shunt motor . Electrical Machines-I lab manual