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UNIT-IVCONVENTIONAL AND SOLID STATE SPEED CONTROL OF D.C. DRIVES

PART-A

1. What are the ways of speed control in dc motors?Field control - by varying the flux per pole. -for above rated speedArmature control- by varying the terminal voltage -for below rated speed

Voltage control

2. Give the Limitation of field controla. Speed lower than the rated speed cannot be obtained.b. It can cope with constant kW drives only.

c. This control is not suitable to application needing speed reversal.

3. What are the 3 ways of field control in DC series motor? Field diverter control Armature diverter control

Motor diverter control

Field coil taps control

Series-parallel control

4. What are the main applications of Ward-Leonard system? It is used for colliery winders.

Electric excavators

In elevators

Main drives in steel mills and blooming and paper mills.

5. What are the merits and demerits of rheostatic control method? Impossible to keep the speed constant on rapidly changing loads. A large amount of power is wasted in the controller resistance.

Loss of power is directly proportional to the reduction in speed. Henceefficiency is decreased.

Maximum power developed is diminished in the same ratio as speed. It needs expensive arrangements for dissipation of heat produced in the

controller resistance. It gives speed below normal, not above.

6. What are the advantages of field control method? More economical, more efficient and convenient.

It can give speeds above normal speed.

7. Compare the values of speed and torque in case of motors when in parallel andin series.

The speed is one fourth the speed of the motor when in parallel.

The torque is four times that produced by the motor when in parallel.

8. What is the effect of inserting resistance in the field circuit of a dc shunt motoron its speed and torque?

For a constant supply voltage, flux will decrease, speed will increase and torque will

increase.

9. While controlling the speed of a dc shunt motor what should be done to achievea constant torque drive?Applied voltage should be maintained constant so as to maintain field strength

10.What is meant by armature resistance control?A resistance is connected in series with armature. By varying the controller resistance R,

the potential drop across the armature is varied. Therefore, the motor speed also varied.

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11.What is meant by flux control method?The speed of the dc motor can be controlled by varying the field flux. This method of

speed control can be used for increasing the speed of the motor above its rated speed,because the speed of the motor is inversely proportional to the field flux.

12.What is meant by Ward leonard control system?The speed of the DC motor is controlled by armature and field control method. The

armature control can be achieved by varying the field of the DC generator. The flux

control method can be achieved by varying the field of the controlled DC motor.13.What is meant by controlled rectifier?It converts fixed ac voltage in to variable dc output voltage.

14.What is meant by DC drives?A dc motor speed can be controlled by using power electronic converters. It is called as

dc drives.

15.What are the different types of dc drives?a. Single phase dc drives b. Three phase dc drives c. Chopper drives

16.What is chopper? What are the different types of chopper?It convert the fixed dc voltage in to variable dc voltage.a. First quadrant chopper b. Second quadrant chopper

c. Third quadrant chopper d. Four quadrant chopper17.What are the different types of single phase dc drives?(i) Single phase half controlled DC drives(ii) Single phase full controlled DC drives

18.Name the solid state controllers used for the speed control of dc shunt motor andseries motor.

a. Phase controlled rectifier fed DC drivesb.Chopper fed dc drives

19.How is chopper used in speed control of dc motor?DC chopper converts fixed DC voltage in to variable DC output voltage. The variable

output voltage can be obtained by varying the duty cycle of the chopper. This variable

voltage is fed to the motor. Due to this, DC motor speed can be changed.20.What are the electrical parameters affecting the speed of the DC motor?a. Armature voltage b. Field current

21.State control strategies of choppers.a. Time ratio control

b.Current limit control

22.Write the applications of DC chopper.a. Electric traction b.Steel mills c. Paper mills d.Machine tools e. Printing mills

23.Define firing angle.The firing angle is defined as the angle between zero crossing of the input voltage and theinstant the thyristor is fired. The firing angle is denoted as .

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PART-B

1.Explain the Speed control method of shunt motor

We know that the speed of shunt motor is given by:

where, Va is the voltage applied across the armature and is the flux per pole and isproportional to the field current If. As explained earlier, armature currentIa is decided by

the mechanical load present on the shaft. Therefore, by varying Va andIf we can vary n. Forfixed supply voltage and the motor connected as shunt we can vary Vaby controlling anexternal resistance connected in series with the armature. If of course can be varied by

controlling external field resistanceRf connected with the field circuit. Thus for shunt motorwe have essentially two methods for controlling speed, namely by:

1. varying armature resistance.

2. varying field resistance.

Speed control by varying armature resistance

The inherent armature resistance ra being small, speed n versus armature currentIacharacteristic will be a straight line with a small negative slope as shown in figure . In the

discussion to follow we shall not disturb the field current from its rated value. At no load

(i.e., Ia = 0) speed is highest and Note that for shunt motor voltage applied

to the field and armature circuit are same and equal to the supply voltage V. However,as the motor is loaded, Iara drop increases making speed a little less than the no load

speed n0. For a well-designed shunt motor this drop in speed is small and about 3 to 5%

with respect to no load speed. This drop in speed from no load to full load conditionexpressed as a percentage of no load speed is called the inherent speed regulation ofthe motor.

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It is for this reason, a d.c shunt motor is said to be practically a constant speed motor (withno external armature resistance connected) since speed drops by a small amount from noload to full load condition.

Since for constant operation, Tebecomes simply proportional to

Ia. Therefore, speed vs. torque characteristic is also similar to speed vs. armature current

characteristic as shown in figure.The slope of the n vsIa or n vs Te characteristic can be modified by deliberately

connecting external resistance rext in the armature circuit. One can get a family of speedvs. armature curves as shown in figures for various values of rext. From these characteristics

it can be explained how speed control is achieved. Let us assume that the load torque TL isconstant and field current is also kept constant. Therefore, since steady state operation

demands Te = TL, Te = a k I too will remain constant; which means Ia will not change.Suppose rext = 0, then at rated load torque, operating point will be at C and motor speed will

be n. If additional resistance rext1 is introduced in the armature circuit, new steady stateoperating speed will be n1 corresponding to the operating point D. In this way one can get

a speed of n2 corresponding to the operating point E, when rext2 is introduced in thearmature circuit. This same load torque is supplied at various speed. Variation of the speed

is smooth and speed will decrease smoothly if rext is increased.

Obviously, this method is suitable for controlling speed below the base speed and forsupplying constant rated load torque which ensures rated armature current always. Although,this method provides smooth wide range speed control (from base speed down to zero

speed), has a serious draw back since energy loss takes place in the external resistance rextreducing the efficiency of the motor.

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Speed control by varying field current

In this method field circuit resistance is varied to control the speed of a d.c shunt motor. Let

us rewrite .the basic equat ion to understand the method.

If we varyIf, flux will change, hence speed will vary. To changeIf an external resistanceis connected in series with the field windings. The field coil produces rated flux when

no external resistance is connected and rated voltage is applied across field coil. Itshould be understood that we can only decrease flux from its rated value by adding

external resistance. Thus the speed of the motor will rise as we decrease the field currentand speed control above the base speed will be achieved.

Speed versus armature current characteristic is shown in figure for two flux values and

Since no load speed for flux value is than the no load

speed no corresponding to . However, this method will not be suitable for constant loadtorque. To make this point clear, let us assume that the load torque is constant at rated value.

So from the initial steady condition, we have If load torque

remains constant and flux is reduced to new armature current in the steady state is

obtained from

Therefore new armature current is

But the fraction, ; hence new armature current will be greater than the ratedarmature current and the motor will be overloaded. This method therefore, will be suitable

for a load whose torque demand decreases with the rise in speed keeping the output powerconstant as shown in figure. Obviously this method is based onflux weakening of the main

field.Therefore at higher speed main flux may become so weakened, that armature reaction effect

will be more pronounced causing problem in commutation.

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Speed control by armature voltage variation

In this method of speed control, armature is supplied from a separate variable d.c voltage

source, while the field is separately excited with fixed rated voltage as shown in figure. Herethe armature resistance and field current are not varied. Since the no load speed

the speed versus Ia characteristic will shift parallely as shown in figure fordifferent values of Va.

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hence Va can be varied. The potential divider connection uses two rheostats in parallel tofacilitate reversal of generator field current. First the induction motor is started with generator

field current zero (by adjusting the jockey positions of the rheostats). Field supply of the motoris switched on with motor field rheostat set to zero. The applied voltage to the motor Va, can

now be gradually increased to the rated value by slowly increasing the generator field current. In

this scheme, no starter is required for the d.c motor as the applied voltage to the armature isgradually increased. To control the speed of the d.c motor below base speed by armaturevoltage, excitation of the d.c generator is varied, while to control the speed above base speed

field current of the d.c motor is varied maintaining constant Va. Reversal of direction ofrotation of the motor can be obtained by adjusting jockeys of the generator field rheostats.

Although, wide range smooth speed control is achieved, the cost involved is rather high as werequire one additional d.c generator and a 3-phase induction motor of simialr rating as that of

the d.c motor whose speed is intended to be controlled.

In present day, variable d.c supply can easily be obtained from a.c supply by usingcontrolled rectifiers thus avoiding the use of additional induction motor and generator set to

implement Ward leonard method.

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3. Explain the Speed control method of Series motor

Speed control below base speed

For constant load torque, steady armature current remains constant, hence flux also remains

constant. Since the machine resistance a s r +re is quite small, the back emfEb isapproximately equal to the armature terminal voltage Va. Therefore, speed is proportionalto Va. If Va is reduced, speed too will be reduced. This Va can be controlled either by

connecting external resistance in series or by changing the supply voltage.

Series-parallel connection of motors

If for a drive two or more (even number) of identical motors are used (as in traction), the motorsmay be suitably connected to have different applied voltages across the motors for controlling

speed. In series connection of the motors shown in figure , the applied voltage across eachmotor is V/2 while in parallel connection shown in figure, the applied voltage across each motor

is V. The back emf in the former case will be approximately half than that in the latter case.For same armature current in both the cases (which means flux per pole is same), speed

will be half in series connection compared to parallel connection.

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Speed control above base speed

Flux or field current control is adopted to control speed above the base speed. In a series motor,independent control of field current is not so obvious as armature and field coils are in

series.However, this can be achieved by the following methods:

1. Using a diverter resistance connected across the field coil.In this method, a portion of the armature current is diverted through the diverter resistance. So

field current is now not equal to the armature current; in fact it is less than the armature current.Flux weakening thus caused, raises the speed of the motor.

2. Changing number of turns of field coil provided with tapings.

In this case shown figure 39.20, armature and field currents are same. However provision is keptto change the number of turns of the field coil. When number of turns changes, field mmf se f N

I changes, changing the flux hence speed of the motor.

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3.Connecting field coils wound over each pole in series or in. parallel.

Generally the field terminals of a d.c machine are brought out after connecting the field coils(wound over each pole) in series. Consider a 4-pole series motor where there will be 4

individual coils placed over the poles. If the terminals of the individual coils are brought out,

then there exist several options for connecting them. The four coils could be connected inseries as in figure 39.21; the 4 coils could be connected in parallel or parallel combinationof 2 in series and other 2 in series as shown in figure 39.22. n figure For series connection of

the coils (figure 39.21) flux produced is proportional to Ia and for series-parallel connection

(figure 39.22) flux produced is proportional to Therefore, for same armature current

Ia, flux will be doubled in the second case and naturally speed will be approximately doubled as

back emf in both the cases is close to supply voltage V. Thus control of speed in the ratio of 1:2is possible for series parallel connection.

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