The speed equation of a DC motor shows that,
N α Eb / φ
or N α (V – IaRa)/ φ
But the resistance of armature winding is small. Therefore the voltage drops IaRa will be negligible as compared to the external supply voltage V. Therefore, the expression for the speed can be approximated as follows:
N α V/φ (because V>> IaRa)
From this expression we can obtain the dc shunt motor speed control methodsas follows:
- The speed is inversely proportional to flux φ.
- It is directly proportional to armature voltage drop (IaRa).
- It is directly proportional to applied Voltage V.
DC Shunt Motor Speed Control Methods
So by varying one of these parameters, it is possible to change the speed of DC shunt motor. Depending on the parameter being controlled, methods of speed control of shunt DC Motor are classified as follows:
DC Shunt Motor Speed Control Methods by Flux Control
The set up for speed control of DC shunt motor using flux control technique is shown in the figure. In order to change the speed, we have to change flux. This can be achieved by changing the field current. The field current can be changed by changing the rheostat R connected in series with the field.
At the time of starting the motor, we need to run the motor slowly, therefore, the flux should be maximum, because,
N α 1 / φ
To obtain maximum flux at the start, the field current should be maximum at the time of starting. To obtain this, the value of rheostat (R) should be minimum.
The speed of DC shunt motor can be varied by varying the field current. As we increase the resistance R of the rheostat, the field current Ish decreases. So the flux φ decreases. This results in increasing the speed of the motor. As the R is increased, the speed increases.
We can use this technique to control motor speed above its rated value. It is the most commercial method. There is a limit to the maximum obtainable speed by this method due to poor commutation at week fluxes. Most common maximum to minimum speed ratio is 6:1.
DC Shunt Motor Speed Control Methods by Armature Control
The set up for speed control of DC shunt motor by armature voltage control method is shown in the figure. A rheostat is connected in series with armature winding. By varying the value of R we can vary the voltage across the armature.
Because speed N is directly proportional to armature voltage, it is possible to change the speed by changing the value of rheostat R.
We can use this technique to control motor speed below its rated value. But it is neither efficient nor economical method because, in this method, speed is reduced at the cost of power loss in rheostat (Ia2R).
DC Shunt Motor Speed Control Methods by Applied Voltage
In this method, the field winding of the motor is connected to a constant DC voltage. But armature is supplied with different voltages with the help of suitable switch gear as shown in the figure.
- The biggest advantage of this method is that the direction can be reversed.
- Smooth increase and decrease in speed are possible.
- The speed control can be obtained over the wide speed range.
The arrangement for changing the applied voltage is expensive.
DC Motors — 4 | Objective Type Question Answers
#1 In a D.C. generator, the iron losses mainly take place in
#2 During rheostat braking of D.C. series motors the motor is run
as a generator
#3 For which type of D.C. motor, dynamic braking is generally used?
All of the above
#4 During rheostatic braking the braking torque is proportional to
#5 Which method of braking is generally used in elevators ?
#6 In variable speed motor
a weaker commutating field is needed at low speed than at high speed
#7 If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens
this will make armature to take heavy current, possibly burning it
#8 D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the
#9 Which D.C. motor has got maximum self relieving property ?
#10 A 230 V D.C. shunt motor takes 32 A at full load. The back e.m.f. on full load, if the resistance of motor armature and shunt field windings are 0.2 ohms and 115 ohms respectively, will be
#11 One D.C. molar drives another D.C. motor. The second D.C. motor when excited and driven
runs as a generator
#12 Which of the following D.C. motors has the least drop in speed between no-load and nominal load ?
Shunt motor with commutating pole
#13 In a D.C. motor if the back e.m.f. is absent,
motor will burn
#14 What will happen if supply terminals of D.C. shunt motor are interchanged?
Motor will run at its normal speed in the same direction as it was running.
#15 When the electric train is moving down a hill the D.C. motor act as
D.C. series generator
#16 Which of the following methods is most economical for finding the no-load losses of a large D.C. shunt motor ?
none of the above
#17 Which of the following statement is incorrect ? If a starter is not used with large D.C. motor, it will draw a starting current which
will produce very low starting torque
#18 The rated speed of a given D.C. shunt motor is 900 r.p.m. To run this machine at 1000 r.p.m., which of the following speed control scheme will be used?
Field resistance control
#19 In a motor, energy conversion would not have been possible but for the
production of opposing back e.m.f. Eb in the armature
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- Construction of a dc motor
- Working principle of dc shunt motor
- Types of dc motors
- Speed control of dc shunt motor
- Series motor speed control
- DC motor characteristics and applications