LOAD CURVE QUESTION AND ANSWER

Q:What do you mean by Economics of power generation? 
The art of determining the per unit i.e. one KWh cost of production of electrical energy is 
known as Economics of power generation. 

2) Explain the term depreciation. 
The decrease in the value of the power plant equipment and building due to constant
use is known as depreciation. In practice, every power station has a useful life ranging from fifteen to thirty years. From the time the power station is installed, its equipment steadily deteriorates due to wear and tear so that there is a gradual reduction in the value of the plant. This reduction in the value of plant every year is known as annual depreciation. 

3) Define load factor. 
Load factor is the ratio of average demand to the maximum demand during a certain 
period of time and is applicable to both generating equipment and receiving equipment.
Load factor = Average demand/Maximum demand 

4) What is load curve? 
The curve showing the variation of load on the power station with reference to time is 
known as a load curve. 

5) Define diversity factor. 
Diversity factor is defined as the state of being dissimilar to one another. It is defined as 
the ratio of sum of the maximum demands of individual consuming units in a group during a specified period to the maximum demand of the whole group during the same 
period. 
Diversity factor = Sum of individual demands of different units in a group / Maximum 
demand of the entire group 
The value of diversity factor is always greater than one. If the diversity factor is higher, 
the cost per unit of generation will be lesser. 

6) What do you mean by utilization factor? 
It is a measure of the utility of the power plant capacity and is the ratio of maximum 
demand to the rated capacity of the power plant. It is always less than unity. 
Utilization factor = Maximum Demand on the power station / Rated capacity of the power 
station 
A low value of utilization factor indicates that the plant has been installed much in 
advance of need. A high value indicates that the plant is probably most efficient in the 
system. If its value exceeds unity, it means that the load has been carried in excess of 
rated capacity of the plant. 

7) Write short note on load duration curve? 
When the load elements of a load curve are arranged in the order of descending 
magnitudes, the curve thus obtained is called load duration curve. 
The load curve is obtained from the same data as the load curve but the ordinates are 
arranged in the order of descending magnitudes. In other words, the maximum load is 
represented to the left and decreasing loads are represented to the right in the 
descending order. Hence the area under the load duration curve and the load curve are 
equal. 
Load factor and diversity factor play a vital role in the cost of the supply of electrical 
energy. Higher the values of load factor and diversity factor, lower will be the overall cost 
per unit generated. 

8) Write the significance of load factor? 
Higher load factor means greater average load, resulting in greater number of units 
generated for a given maximum demand. Thus, the standing charges, which are 
proportional to maximum demand and independent of number of units generated, can be
distributed over a large number of units supplied and therefore overall cost per unit of 
electrical energy generated will be reduced. 

9) What is mean by base load?
The unvarying load which occurs almost the whole day 
on the station is known as base load.

 10) What are the methods for determining depreciation charges? 
The decrease in the value of the power plant equipment and building due to constant
use is known as depreciation. The cost of depreciation will depend on the size and type 
of equipment and on its estimated life. 
The reduction in the value of the plant every year is known as annual depreciation. Due 
to depreciation the plant has to be replaced by new one after its useful life. Therefore a 
suitable amount must be set aside every year, so that by the time the plant retires the 
collected amount by way of depreciation equals the cost of equipment. 
The methods commonly used for determination of annual depreciation charges are

11) What are the objectives of tariff? 
Electrical energy is sold at such a rate so that it not only returns the cost but also earns 
reasonable profit. Therefore, a tariff must cover the following items: 
 transmitting and distributing 
equipment. 
 equipment. 
costs etc. 

12) Define energy audit. 
Energy Audit means studying the energy consumption pattern in the utilities or
equipments by obtaining necessary data analyse the same to identify the areas where 
wastages or loses occur and suggest methods to avoid wastages or loss and also other 
consumption measures to ensure efficient use of energy.

13) What are the causes of low power factor? 
The following are the causes of low power factor 
otors are of induction type which have low lagging power factor. 
These motors work at a power factor which is extremely small on light load 0.2 to 0.3
and rises to 0.8 to 0.9 at full load. 
 heating furnaces operate at low 
lagging power factor. 
 during morning and evening and 
low at other times. During low load period, supply voltage is increased which increases 
the magnetization current. This results in the decreased power factor. 14) Define the term connected load factor. 
It is the sum of the continuous rating in KW of all electrical devices installed at the 
consumer’s premises and connected to the supply system. 

15) What are the important points to be taken into consideration while selecting the size 
and number of units? 
 variable demands from time to 
time. The nature of these demands can be seen from the load curve. The load variation 
is greater with a poorer load factor. 
units is per decided from the annual load 
curve of the station. The number and size of the units are selected in such a way that 
they correctly fit the station load curve. 
The capacity of the plant should be made 15% to 20% more than the maximum demand 
to meet the future load requirements. There should be a spare generating unit so that 
repairs and overhauling of the working units can be carried out. 

16) Define capacity factor. 
It is the ratio of average demand to plant capacity 

17) What is plant use factor? 
It is defined as the ratio of kwh generated to the product of plant capacity and number of 
hours for which the plant is in operation. 

18) What are the important types of power factor tariff? 
(i) kVA maximum demand tariff 
(ii) sliding rate tariff 
(iii) kWh and kVAR tariff 

19) What is three part tariff? 
The total charge comprised of fixed charge made during billing period, charge per kW of 
maximum demand and charge per kWh of energy consumed. 

20) What is the difference between base load and peak load? 
The unvarying load which occurs almost the whole day is known as base load. 
The various demands of load over and above the base load is the peak load. 

INDUCTION MOTOR QUESTION AND ANSWER

Q: State the types of Induction motor ?

1) Squirrel cage Induction motor

2) Wound or slip ring Induction motor.

Q: Why skewing is done in squirrel cage Induction motor?

i)    For more uniform torque

ii)   Reduction in noise

iii) To decrease magnetic locking tendency

Q:What is the purpose of connecting external resistors to slip ring Induction motor?

It increases starting torque and decreases starting current.

Q.Can we add extra resistance in series with squirrel cage rotor? State the reason? 

We cannot add extra resistance in series with the rotor because all the copper bars of the rotor are short circuited in both the sides by copper end rings to have a closed circuit.

Q:State the difference between squirrel cage Induction motor and slip ring Induction motor.

Parameters			Squirrel cage IM	Slip ring IM
Rotor winding			Copper bars are short circuited at both ends	Three phase winding
End ring			present	Absent
Efficiency			high	low
External resistance in rotor			Not possible	Possible
Starting torque			Moderate	High
Construction			Simple & robust	Complicated
Maintenance			less	High
Industrial use			95%	5%
Speed control from rotor side			Not possible	Possible
cost			Economical	Expensive
Applications			Lathes, drilling machine, water pumps, blowers, fans, printing machine	Lifts, compressors, hoists, cranes, elevators, belt, conveyor

Q:  Why wound rotor construction is adopted?

The wound rotor has the facility of increasing the rotor resistance through slip rings. High value of rotor resistance is needed during starting to get a high value of starting torque.

Q:What are the advantages of slip ring motor over squirrel cage motor?

A:1.  The starting torque can be varied by adding resistance to rotor.

2.  The speed of the machine can be varied by injecting an emf through slip

rings to the rotor.

Q.What are slip rings? 

The slip rings  are made of copper alloys and  are fixed around the shaft insulating it. Through these slip rings and brushes the rotor winding can be connected to external circuits. 

Q:  What are the materials used to manufacture the brushes for slip rings of an  induction motor?

A:The slip rings are made of brass and phosphor bronze. The brushes are made of metal graphite which is an alloy of copper and carbon.

Q:Name the materials used to insulate the laminations of the core of

induction motor.

The materials used to insulate the laminations are kaolin and varnish.

Q: State the principle of 3 phase IM?

When three phase supply is given to three phase winding of stator, a rotating magnetic field is produced. The rotor conductors cut the rotating magnetic field and emf is induced in it which results in flow of current. According to Lenz’ law flux is produced and it interacts with rotating magnetic field. This produces torque and hence the rotor rotates.

Q:  How the slip ring motor is started?

The slip ring motor is started by using rotor resistance starter. The starter consists of star connected resistance to slip rings. While starting the full resistance is included in the rotor circuit to get high starting torque. Once the rotor starts rotating the resistance is gradually reduced in steps. At running condition the slip rings are shorted and so it is equivalent to squirrel cage rotor.

Q:Why Induction motor is called rotating transformer?

The principle of operation of induction motor is similar to that a transformer. The stator winding is equivalent to primary of the transformer and the rotor winding is equivalent to short circuited secondary of a transformer. In transformer the secondary is fixed but in induction motor it is allowed to rotate. Hence the induction motor also called rotating transformer.

Q: What is Synchronous speed?

The speed at which rotating magnetic field of three Phase Induction motor rotates is called synchronous speed.

Ns=120f/p

Q. Why the induction motor is called asynchronous motor?

Since the induction motor runs always at a speed lesser than synchronous speed, it is called asynchronous motor.

Q:What is slip speed?

The difference between synchronous speed and rotor speed is called slip speed.

Q: What is Slip?

The difference between synchronous speed and rotor speed with respect to synchronous speed is called slip.

S=Ns-N/Ns*100

Q. What would happen if synchronous speed is equal to rotor speed? 

The rotor never obtained the synchronous speed because at that speed there would be no relative speed between rotor conductors and stator revolving field and induction phenomenon is not possible. Its speed is always less than synchronous speed. 

Q  How the magnitude of rotor emf is related to the slip in an I M?

Rotor circuit emf per phase E2’=SE2

Q  How the frequency of rotor emf is related to the slip in an I M?

Frequency of rotor emf/current fr=Sf   

Q   What is the normal value of slip of an I M operating at full load?

3 -   5%

Q: Give formula for starting torque.

Ts = k ФE2R2/ (R2²+X2²)

Q: State the condition for maximum starting torque?

R2=X2

That is rotor resistance equals rotor standstill reactance

Q: Give formula for maximum starting torque.

Ts = k ФE2/2R2

Q: State the condition for maximum running torque?

R2=sX2

Q:  List the undesirable effects produced by certain combination of rotor and stator slots.

1.  The motor may refuse to start (cogging)

2.  The motor may run at sub synchronous speed (Crawling)

3.  Severe vibrations may develop and the noise will be excessive.

Q:  What is crawling ?

The tendency of squirrel cage Induction motor to run at 1/7 times of its synchronous speed is called crawling.

Q: What is Cogging?

Sometimes at low voltage the squirrel cage motor fails to start. This is due to magnetic locking between stator and rotor as the number of stator slot is equal to number of rotor slot. This phenomena is cogging.

Q: What is the necessity of starter?

Its function is to reduce starting current  and also provide protection against over load and over voltage.

Q:What are the types of starters?

Direct on line

  Autotransformer

 Star delta

Rotor resistance starter

Soft starter or Electronic starter

Q. Name the tests to be conducted for predetermining the performance of 3-phase induction machine.

(a) No load test

(b) Blocked rotor test

Q. What are the informations obtained from no-load test in a 3-phase I M?

(i) No – load input current per phase, Io

(ii) No load power factor and hence no load phase angle

(iii) Iron and mechanical losses together

(iv) elements of equivalent circuit shunt branch

Q:What are the informations obtained from blocked rotor test in a 3-phase I M?

(i)Blocked rotor input current per phase at normal voltage

(ii) Blocked rotor power factor and hence phase angle

(iii) Total resistance  and leakage reactance per phase of the motor as

refered to the stator

Q What is circle diagram of an I M?

When an I M operates on constant voltage and constant frequency source, the loci of stator current phasor is found to fall on a circle. This circle diagram is used to predict the performance of the machine at different loading conditions as well as mode of operation.

Q  What are the advantages and disadvantages of circle diagram method of predetermining the performance of 3 – phase I M?

The prediction can be carried out when any of the following information

is available

The input line current., the input power factor, The active power input, The

reactive power input, The apparent power input, The output power , The slip

of operation, The torque developed, The equivalent rotor current per phase,

Maximum output power, Maximum torque developed.

The only disadvantage is, being a geometrical solution, errors made during

measurements will affect the accuracy of the result.

Q  What are the advantages and disadvantages of direct load test for 3 – phase I M?

Advantages

Direct measurement of input and output parameters yield accurate results Aside from the usual performance other performances like mechanical vibration, noise etc can be studied.

By operating the motor at full load for a continuous period, the final steady temperature can be measured.

Disadvantages

Testing involves large amount of power and the input energy and the entire energy delivered is wasted Loading arrangement cannot be provided for motors of large power rating

Q: Explain why the no load current of an induction motor is much higher than that of an equivalent transformer.

In induction motor, due to the presence of the air gap, the magnetizing current that is required to set up the flux is much higher. The working component of the current has to meet the hysteresis loss, eddy current loss, friction and windage losses. Hence the no load current of induction motor is higher.

Q: List out the methods of speed control of cage type 3-phase induction motor?

a) By changing supply frequency

b) By changing the number of poles

c) By operating two motors in cascade

Q: Mention different types of speed control of slip ring induction motor?

a) By changing supply frequency

b) By changing the number of stator poles

c) By rotor rheostat control

d) By operating two motors in cascade

Q: What are the advantages of 3-phase induction motor?

a) It was very simple and extremely rugged, almost unbreakable construction b) Its cost is very low and it is very reliable

c) It has been suffiently high efficiency .No brushes are needed and hence

frictional losses are reduced

d) It requires minimum of maintenance.

Q: What are the different types of stator winding in induction motor?

The different types of stator windings are mush winding, lap winding and wave winding.

Q: Where mush windings are used?

The mush windings are used in small induction motors of ratings below 5 HP.

Q: What types of slots are preferred for the induction motor?

Semi enclosed slots are preferred for induction motor. It results in less air gap contraction factor giving a small value of magnetizing currents, low tooth pulsation loss and much quieter operation(less noise)

Q:  Which part of induction motor has the maximum flux density? What is the maximum flux density in that part?

The teeth of the stator and rotor core will have maximum flux density. The

maximum value of flux density in the teeth is 1.7 wb/m

Q: What are the advantages and disadvantages of large air gap length in induction motor?

Advantage: A large air gap length results in higher overload capacity, better cooling, reduction in noise and reduction in unbalanced magnetic pull.

Disadvantages: The disadvantage of large air gap length is that it results in high value of magnetizing current.

Q:  What happens if the air gap length is doubled?

If the air gap of an induction motor is doubled then the mmf and magnetizing current approximately doubles. Also increase in air gap length increases the overload capacity, offers better cooling, reduces noise and reduces unbalanced magnetic pull.

Q: List out the methods to improve the power factor of the induction motor.

The power factor of the induction motor can be improved by reducing the magnetizing current and leakage reactance. The magnetizing current can be reduced by reducing the length of air gap. The leakage reactance can be reduced by the depth of stator & rotor slots, by providing short chorded winding and reducing the overhang in stator winding.

Q: Why the air gap of an induction motor is made as small as possible?

The mmf and the magnetizing current are primarily decided by length of air gap. If air gap is small then mmf and magnetizing current will be low, which in turn increase the value of power factor. Hence by keeping small air gap, high power factor is achieved.

Q: What are the methods adopted to reduce harmonic torques?

The methods used for reduction or elimination of harmonic torques are chording, integral slot winding, skewing and increasing the length of air gap.

Q:In what respect does a 1-phase Induction motor differ from a 3-phase Induction motor?

Construction wise a plain 1-phase Induction motor is more or less similar to a 3-phase squirrel-cage Induction motor except that  its stator is provided with only 1-phase winding..

Q. What would happen if a 3 phase induction motor is switched on with one phase disconnected?

The motor is likely to burn .

Q. What happens if the air gap flux density in an induction motor increases?

The increase in air gap flux increases iron loss and hence efficiency decreases.

Q: What are the effects of increasing rotor resistance on starting current  and starting   torque?

The additional external resistance reduces the rotor current and hence the current drawn from the supply. It improves the starting torque developed by improving the power factor in high proportion to the decrease in rotor current..

Q: why the three phase induction motor draws heavy current at starting?

At the time of starting, the rotor speed is zero hence slip is 1, the emf induced in rotor winding is very high. Hence rotor current is also very high. The starting current is 5 to 7 times more than its full load current. 

DC MACHINE QUESTION AND ANSWER

Q: What are the essential parts of DC machine?

A:

1. Magnetic frame or yoke

2. Poles

3. Armature

4. Commutator

5. pole shoes

6. armature windings

7. interpoles 

8. Brushes

9. bearings

10. shaft

Q. What is the purpose of yoke in d.c machine?

1.It acts as a protecting cover for the whole machine and provides mechanical support for the poles.

2.It carries magnetic flux produced by the poles

Q. What is the purpose of Field pole in d.c machine?

To produce flux with the help of field winding

Q. What is the purpose of Field winding in d.c machine?

To produce flux

Q. What is the purpose of pole shoes in d.c machine?

1. They support field coil

2.Decrease reluctance

3.Increase cross section area of magnetic circuit

Q. What is the purpose of Armature core in d.c machine?

To store armature windings and provide the path for main flux

Q. What is the purpose of Armature winding in d.c machine?

To develop main electrical power

Q. Why the armature core in d.c machines is constructed with laminated steel sheets instead of solid steel sheets?

Lamination highly reduces the eddy current loss and steel sheets provide low reluctance path to magnetic field i.e decreases hysteresis loss

Q.What is the purpose of brushes?

Its function is to collect current from commutator  and supply  to external load circuit

Q: What are the materials used for brushes in dc machines?

1.  Natural graphite

2.  Electro graphite

3.  Hard carbon

4.  Metal graphite

Q.Why the brushes are made of carbon?

1.To some extent carbon brush can act as a self lubricating brush;

2.On moment, polishes the commutator segments;

3.Damage to the commutators is less then when copper brushes are used on occurrence of sparkover.

Q: How to design the number of brushes for a dc machine?

The numbers of brush locations are decided by the type of winding. In lap winding the number of brush locations is equal to number of poles and in wave winding it is always two. In each location there may be more than one brush mounted on a Spindle, whenever the current per brush location is more than 70A. Hence the number of brushes in a spindle is selected such that each brush does not carry more than 70A.

Q:What is the function of bearings?

To reduce friction and thereby help in smooth and easy rotation.

Q What is meant by self excited and separately excited dc generator?

•Self-excited generator are those whose field magnets are energized by the current produced by the generator themselves.

•Separately excited generator are those whose field magnets are energized from an independent external source of dc current.

Q:How can one differentiate between long shunt compound generator and short shunt compound generator?

In a short shunt compound generator, only shunt field winding is in parallel with the armature winding. In a long shunt compound generator shunt field winding is in parallel with both series field and armature winding.

Q:What is cumulative compound dc machine?
A compound wound DC machine 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.
                                   Фtotal=Фseries+Фshunt
Q:What is differentially compound dc machine?
A compound wound DC machine 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.

Фtotal=Фseries--Фshunt

Q: Explain the principle of DC generator.
It is based on Faraday's law of Electromagnetic Induction i.e Whenever a conductor cuts magnetic lines of force , an emf is induced in the conductor. 

Q. Write down the emf equation for d.c.generator?

E= (ФNZ/60)(P/A)V.

p---------no of poles

Z---------Total no of conductor

Ф---------flux per pole

N---------speed in rpm.

Q: Mention the two types of winding used in the dc machines.

1.  Lap winding ; used for low voltage, high current machine

2.  Wave winding ; used for high voltage, low current machine

	LAP winding	WAVE winding
1.	Number of Parallel paths = Number of Poles	Number of parallel paths=2
2.	Number of brush pairs= Number  Of poles	Number of Brush pairs= 2
3.	used for low voltage, high current machine	used for high voltage, low current machine
4.	Ends of armature coil are connected to adjacent commutator segments	Ends of armature coil are connected to commutator segments some distance apart.
5.	It requires equalizing connection	It does not requires equalizing connection
4.	It does not require dummy coil	It requires dummy coil
5.	Resultant pitch= Back pitch – front pitch	Resultant pitch= Back pitch + front pitch

Q: What are dummy coils?

The addition of dummy coils is done in case of small machines for getting the mechanical balance

Q: What is meant by equalizer connections?

In lap winding, due to the difference in the induced emf in various parallel paths, there may be circulating currents in brushes and winding. The connections that are made to equalize the difference in induced emf and to avoid circulating currents through brushes are called equalizer connections.

Q. How are armatures windings are classified based on placement of coil inside the armature slots?

Single and double layer winding

Q: What is meant by commutation?

The process of current reversal in a coil is called commutation.

Q:What is the purpose of mica strip between two adjacent commutator segments?

Mica is placed in between two commutator segments in order to insulate the segments from each other.

Q: What type of copper is used for commutator segments?

A:The commutator segments are made of hard drawn copper or silver copper(0.05% silver)

Q: Why square pole is preferred?

If the cross section of the pole body is square then the length of the mean turn of field winding is minimum. Hence to reduce the copper requirement a square cross section is preferred for the poles of the dc machines.

Q: What is the purpose of slot insulation?

A:The conductors are placed on the slots in the armature. When the armature rotates the insulation of the conductors may damage due to vibrations. This may lead to a short circuit with armature core if the slots are not insulated. 

Q: What is Pole Pitch?

It is the distance between two adjacent poles

Q: What is Coil Pitch?

It is the distance in terms of slots between two sides of coil.

Q:What is full pitched coil?

If the coil pitch is equal to pole pitch then it is termed as Full pitched coil

Q:What is fractional pitched coil?

If the coil pitch is less than pole pitch then it is termed as Fractional pitched coil

Q: What is Back Pitch?

It is the distance measured in terms of armature conductors away from commutator.

Q: What is Front Pitch?

It is the distance measured in terms of armature conductors between coil sides connected to same commutator segment.

Q: What is Resultant Pitch?

It is the distance measured in terms of armature conductors between beginning of 1 coil & beginning of next coil.

Q: What is Commutator Pitch?

It is the distance measured in terms of commutator segments which the two ends of coil are connected.

Q:How to calculate number of conductors?

Number of conductors= number of slots x number of conductors per slot.

Q. Distinguish between shunt and series field coil construction?

Shunt field coils are wound with wires of small section and have more no of turns.

Series field coils are wound with wires of larger cross section and have less no of turns.

Q: How is the voltage built up in shunt generator?

Generator is rotated by prime mover , due to presence of  flux emf is induced by Faraday’s law of Electromagnetic induction

Current will also flow through field winding , hence more flux is produced , so more emf is induced in armature so again more field current and hence more flux. This process continues.

Q.What do you mean by residual flux in DC generator?

The magnetic flux retained in the poles of the machine even without field supplies called the residual flux.

Q.What is critical resistance of a DC shunt generator?

The resistance of field winding at which the generator just excites for a given speed and starts voltage building is called critical resistance.

Q.What is critical speed of a DC shunt generator?

The speed for which the given field winding resistance act as critical resistance is called as critical speed.

Q.What are the conditions to be fulfilled for a shunt generator to build up voltage?

a)There must be some residual magnetism in the field poles.

b)The shunt field resistance should be less than critical resistance.

c)The field coils should be connected with the armature in such a way that current flowing through them should increase the emf induced by the residual magnetism.

Q. Why is the emf not zero when the field current is reduced to zero in dc generator?

Even after the field current is reduced to zero, the machine is left out with some flux as residue so emf is available due to residual flux.

Q. On what occasion dc generator may not have residual flux?

The generator may be put for its operation after its construction, in previous operation, the generator would have been fully demagnetized.

Q.A DC generator fails to self excite. List the cause for the failure 

a)Residual magnetism may not be there in the poles

b)Direction of rotation may be wrong

c)The field resistance may be more than critical resistance

d)There may be disconnection in the field winding

e)Brush contact may be poor

f)The field coils may be connected with the armature to oppose the emf due to residual magnetism

Q: Which losses have highest proportion in DC machine?

Armature copper loss has highest proportion of 30 to 40% of losses.

 Q: What is the condition of maximum efficiency in Dc Machine?

Variable losses= constant loss

Q: What is core loss? What is its significance in electric machines?

When  a  magnetic  material  undergoes  cyclic  magnetization,  two  kinds  of  power losses occur  on  it.  Hysteresis  and  eddy  current  losses  are  called  as  core  loss.  It  is important  in determining heating, temperature rise, rating & efficiency of transformers, machines & other A.C run magnetic devices.

Q what do you mean by hysteresis loss? 

 The magneto motive force or mmf applied in the core is alternating. For every cycle due to this domain reversal, there will be extra work done. For this reason, there will be a consumption of electrical energy which is known as Hysteresis loss.

Q. What is eddy current loss?

When a magnetic core carries a time varying flux, voltages are induced in all possible path  enclosing  flux.  Resulting  is  the  production  of  circulating  flux  in  core.  These circulating current do no useful work are known as eddy current and have power loss known as eddy current loss.

Q:How hysteresis and eddy current losses are minimized?

Hysteresis loss can be minimized by selecting materials for core such as silicon steel & steel alloys with low hysteresis co-efficient and electrical resistivity. Eddy current losses are minimized by laminating the core. 

Q:What are factors on which hysteresis loss?

It depends on magnetic flux density, frequency & volume of the material.

Q: What is the overall efficiency of DC generator?

95%

Q:Differentiate between geometric neutral axis (GNA) and magnetic neutral axis (MNA).

GNA is the axis which is situated geometrically or physically in the mid way between adjacent main poles. MNA is the axis which passes through the zero crossing of the resultant magnetic field waveform in the air gap.

Q. Define armature reaction in dc machines?

The  interaction  between  the  main  flux  and  armature  flux  cause  disturbance  called as armature reaction.

Q: What are the effects of armature reaction?

A:The various effect of armature reaction are :

1 Due to demagnetizing effect, generated emf is reduced

2Magnetic flux density increases over one of pole and decrease over other half , hence total flux decreases and so terminal voltage decreases.

3Due to high magnetic flux density , maximum voltage between adjacent commutator segments occur , resulting in sparking

4. Flux wave is distorted & there is shift in position of MNA in direction of rotation for generator and against direction of rotation for motors.

Q: What are the methods to eliminate armature reaction effect?

1. Provide Extra De- magnetizing AT on field windings

2. Using compensating winding

Q:What are compensating winding?

Auxilliary windings placed in slot of pole shoe connected in series with armature winding in such a way that flux produced by them is in opposition to armature flux.

Q:What is commutation?

The process by which current in short circuit coil is reversed while it crosses MNA.

Q.Name the two methods of improving commutation.

•Emf commutation( Brush shift and use of interpoles)

•Resistance commutation

Q.What is reactance emf in dc machine?

The self induced emf in the coil undergoing commutation which opposes the reversal of current is known as reactance emf.

Q: What are Interpoles?

These are small poles fixed to yoke in between main poles & its winding is connected in series with armature winding.

Q:To what polarity are the interpoles excited in dc generators?

The polarity of the interpoles must be that of the next main pole along the direction of rotation in the case of generator. 

Q: How can the voltage in a DC generator be increased?

Increasing the main field flux and the speed of the armature can increase the voltage in a DC generator

Q: Give application of all types of DC generators.

Separately excited : Electro-platting, Electro –refining

DC shunt- Battery charging and lighting purpose

DC series- Booster  & braking of DC locomotives

Dc cumulative-Transmission of power and domestic lighting purpose

Dc differential- Electric arc welding

Q. How does d.c. motor differ from d.c. generator in construction?

Generators are normally placed in closed room and accessed by skilled operators only. Therefore on ventilation point of view they may be constructed with large opening in the frame. Motors  have  to  be  installed  right  in  the  place  of  use  which  may have  dust,  dampness, inflammable gases, chemical etc. to protect the motors against these elements the motor frames are used partially closed or totally closed or flame proof

Q: List all the important information on name plate of a DC motor.

kW, nominal speed, armature current, insulation class.

Q: What is working principle of DC motor?

When a current carrying conductor is placed in magnetic field it experiences force.

Q. How will you change the direction of rotation of d.c.motor?

Either the field direction or direction of current through armature conductor is reversed.

Q. What is back emf in d.c. motor?

When dc motor is given supply it starts to rotate, due to this the armature conductor cut the magnetic flux hence the emf is induced in the armature conductor which oppose the main supply. This induced emf is called Back emf.

Q: What are the factors affecting torque?

T=PIZ Ф / 2πA

Number of poles (p)

Number of parallel paths(A)

flux (Ф)

Number of conductors (Z)

Armature current(Ia)

Q:Define : Speed regulation.

It is defined as ratio of speed from no load to full load and full load speed.

Q: Why dc shunt motor is called constant flux or constant speed motor?
In DC shunt motor flux produced is proportional to field current. The input voltage is constant so the field current is also constant hence flux is also constant.

Q: Why dc series motor should not be started at no load.

Because its starting speed is very high.

Q:Which type of motor is used for traction ? why?

Dc series motor is used for traction purpose because its starting torque is high.

Q. Enumerate the factors on which speed of a d.c.motor depends?

N= (V-IaRa)/Ф

so speed depends on voltage applied to armature, flux per pole, resistance of armature.

Q: What are the various methods of controlling speed?

1.Field control method

2. Armature control method

3. Ward Leonard method

Q.Why field control is considered superior than armature control method of DC shunt motor?

a.The regulating resistance which has to carry only a small current is easily available.

b.Power wasted in regulating the resistance is very small and hence this method is more economical

Q.Under what condition the mechanical power developed in a dc motor will be maximum?.

The ratio of back emf to applied voltage is 0.5

Q.Why is the starting current high in a DC motor?

The absence of back emf at the time at starting causes the armature current to shoot up to about 20 times the normal current, if no limiting resistance is included.

Q.What is the need for starter in a DC motor?

Starters are used in DC motors to limit the high starting current within about 2 to 3 times the rated current by adding resistance in series with the armature circuit. Apart from starting resistances, starters are invariably fitted with protective devices such as no voltage protection and over-load protection.

Q.What is the function of over-load release coil provided in a DC motor starter?

Due to any overload in the motor, if the line current increases above a preset value, the excess magnetic force causes the lifting of an iron piece. As the iron piece makes an upward movement, a contactor fitted along with it causes the two terminals of NVR coil to get short circuited. Hence the electromagnet fitted with NVR coil loses its magnetic force and releases the starter handle from the ON position towards OFF position, thus protecting the motor against over-load.

Q.What is the function of a no-voltage release coil provided in a DC motor starter?

As long as the supply voltage is on healthy condition the current through the NVR coil produce enough magnetic force of attraction and retain the starter handle in the ON position against spring force. When the supply voltage fails or becomes lower than a prescribed value the electromagnet may not have enough force and the handle will come back to OFF position due to spring force automatically. Thus a no-voltage or under voltage protections are given to the motor.

Q.How does 4 point starter differ from 3 point starter?

In 3 point starter, NVR coil is connected in series with the shunt field coil. The exciting current through the NVR coil in 3 point starter is same as the shunt field current of the motor. In 4 point starter, NVR coil along with a high resistance connected across the supply voltage. Thus the exciting current through NVR coil of a 4 point starter is purely proportional to the supply voltage and independent of shunt field current 

Q: What are the different types of tests done on DC motor?

1.Brake test

2.Swinburne’s test

3. Field test

Q.Give some applications of DC motor.

Shunt : driving constant speed, lathes, centrifugal pumps, machine tools, blowers and fans, reciprocating pumps

Series : electric locomotives, rapid transit systems, trolley cars, cranes and hoists,conveyors

Compound : elevators, air compressors, rolling mills, heavy planners.

Q:State one advantage and disadvantage in the application of each of the three basic types of DC motors

Shunt Motor:Advantage: Substantially constant speed i-e low speed regulation

Disadvantage: Cannot be used for constant speed application

Series Motor:Advantage: High torque low speed (at start) and low torque at high speed. This is typical requirement for traction type of load

Disadvantage: Accidental no load can cause the motor to run at dangerously high speed.

Compound Motor:Advantage: Negligible speed regulation for cumulatively compound motor

Disadvantage: Higher cost. 

Q:Why a differentially compound motor is not used in practice?

In differential compound motor, as two fluxes oppose each other, the resultant flux decreases as load increases, thus the machine runs at a higher speed with increase in the load. This property is dangerous as on full load, the motor may try to run with dangerously high speed. So differential compound motor is generally not used in practice.