Speed Control of a DC Shunt Motor
OBJECTIVES :
1. To plot the speed versus field-current characteristic curve for a dc shunt motor.
2. To plot the speed versus armature voltage characteristic curve for a dc shunt motor.
APPARATUS : A dc shunt motor (with armature and field terminals); 220-V, dc supply; One 1000-Ω, 2-A, rheostat; One 25-Ω, 20-A, rheostat; One dc voltmeter (0 - 300 V); One dc ammeter (0 - 2 A); One dc ammeter (0 - 20 A); One tachometer.
CIRCUIT DIAGRAM : The circuit arrangement is shown in Fig. 16.22.
Fig. 16.22 Circuit diagram for studying the speed control of dc shunt motor.
BRIEF THEORY : The equation governing the speed of a dc shunt motor is given as
a a
V I R
N −
∝ Φ
whereI Ra ais the voltage drop across the armature, which is usually not more than 5 % of the terminal voltage V. Hence, we can say that
N ∝V Φ
The speed is, therefore, inversely proportional to the flux Φ (or field current If) and almost directly proportional to the terminal voltage V.
By putting suitable rheostats in the field circuit and armature circuit, we can vary both the field current and the armature terminal voltage, and plot the speed control characteristics.
PROCEDURE :
1. Make the connections as shown in Fig. 16.22.
2. Keep the armature-control rheostat Rac to its maximum and the field control rheostat Rfc to its minimum value.
3. Switch on the dc supply. The motor starts running at slow speed.
4. Bring the armature-control rheostat Rac to its minimum value so that the armature terminal voltage is at its rated value.
5. Gradually increase the rheostat Rfc to decrease the field current If in steps up to a level where speed does not become exorbitantly high. Measure the corresponding speed using the tachometer. Note down the values.
6. Draw the speed versus field-current characteristic curve.
7. Bring the field-control rheostat Rfc to its minimum value so that the field current If is at its rated value.
8. Now, gradually increase the armature-control rheostat Rac to decrease the terminal voltage V in steps. Measure the corresponding speed using the tachometer. Note down the values.
9. Draw the speed versus terminal-voltage characteristic curve.
10. Switch off the dc supply.
(a) Speed versus field-current characteristic. (b) Speed versus voltage characteristic.
Fig. 16.23 Speed control of dc shunt motor.
OBSERVATIONS :
If
(in A)
N (in rpm)
V (in V)
N (in rpm) 1
2 3 4 5 No.
Flux Control Method Armature Control Method
RESULTS :
1. The speed versus field-current characteristic curve and the speed versus armature-voltage characteristic curve are plotted in Fig. 16.23.
2. As the field current If is decreased (and hence the flux Φ is decreased), the speed N increases (Fig. 16.23a).
3. As the armature voltage V is decreased, the speed N also decreases almost proportionately (Fig. 16.23b).
PRECAUTIONS :
1. Before switching on the supplies, the zero readings of the ammeters and voltmeter should be checked.
2. The terminals of the rheostat should be connected properly.
3. The dc ammeter and voltmeters should be connected with correct polarity.
4. Before starting the motor, the field-control rheostat should be at its minimum and the armature-control rheostat should be at its maximum.
VIVA-VOCE :
1. Q. : What is the most important precaution in any experiment with a dc shunt motor ?
Ans. : Before switching on the dc supply, sufficient resistance should be put in series with the armature of the dc shunt motor.
2. Q. : What will go wrong, if the above precaution is not observed ?
Ans. : At the start (N = 0), the back emf is zero. The armature resistance is very low (less than 1 ohm). If the dc supply is directly switched on, the armature current may become damagingly high.
3. Q. : If the rated speed of a dc shunt motor is 1500 rpm, which of the two methods do you suggest for reducing the speed to 1000 rpm ?
Ans. : The armature-control method is suggested, as the armature voltage can be easily reduced by increasing the rheostat Rac.
4. Q. : On the other hand, if you wish to increase the speed to 1600 rpm, which method should be used and why ?
Ans. : Flux-control method should be used because the flux is to be reduced which can easily be done by increasing the resistance of the rheostat in the field circuit. On the other hand, the armature control method requires increasing the armature voltage beyond the rated value, which may damage the armature.
5. Q. : Is there any limitation of the flux-control method of speed control of a dc motor ? Ans. : Yes, speeds below the rated value cannot be achieved by this method.
6. Q. : What would you do to reverse the direction of rotation of the dc shunt motor ? Ans. : The torque is given as τ = Φk Ia. To reverse the direction of rotation, the direction of torque is to be reversed. For this, we reverse the connections to either the field or the armature.
7. Q. : What will happen if the field winding of a dc shunt motor running on no load is suddenly opened ?
Ans. : The magnetic flux will reduce to almost zero (in fact, to the residual magnetism value), and hence the speed will become dangerously high. The parts of the motor may even fly apart.
8. Q. : What will happen if the shunt field winding of a loaded dc shunt motor accidentally breaks ?
Ans. : The flux will reduce to a very low value, which results in a very low torque (τ = Φk Ia). The motor sudden comes to a stop. As a result, the induced emf E reduces to zero. Whole of supply voltage acts to force a very heavy current in armature winding, because of which it may even burn.