Name the three basic methods that are used to vary the speed of a DC motor?

Answer 1

speed of a dc motor can be varied by mainly two methods:-

1)field control method

2)armature control method

further ,

field control method can be divided in 3 parts:-

1)field rhestat control method---speed variation is acomplished by means of a variable resistance inserted in series with shunt field

2)reluctance control method---by changing the reluctance of magnetic circuit by choosing right material..

3) field voltage control method-requires variable field voltage supply.

and armature control method can be divided in 3 parts :-

1)armature resistance control---variable resisitance connected in series with armature

2)shunted armature control.

3)armature voltage control.

------------mrityunjay pandey

(kiit university ,btech 2nd year)

Answer 2

Bassicaly the speed of DC motor is vary or controlled mainly by Armature controlled and field cotrolled. and in both the methods we can controlled the speed by controlling flux, by adding resistance and by changing current.

Answer 3

(i) Flux/pole, Φ (Flux Control)

(ii) Resistance Ra of armature circuit (Rheostatic Control) and

(iii) Applied voltage V (Voltage Control).

Speed Control of Shunt motor:

(i) Variation of Flux or Flux Control Method: By decreasing the flux, the speed can be increased and vice versa. The flux of a dc motor can be changed by changing Ish with help of a shunt field rheostat. Since Ish is relatively small, shunt field rheostat has to carry only a small current,

which means I2 shR loss is small, so that rheostat is small in size.

(ii) Armature or Rheostatic Control Method: This method is used when speeds below the no-load speed are required. As the supply voltage is normally constant, the voltage across the armature is varied by inserting a variable rheostat in series with the armature circuit. As controller resistance is increased, voltage across the armature is decreased, thereby decreasing the armature speed. For a load constant torque, speed is approximately proportional to the voltage across the armature. From the 65 speed/armature current characteristic, it is seen that greater the resistance in the armature circuit, greater is the fall in the speed.

(iii) Voltage Control Method:

(a) Multiple Voltage Control: In this method, the shunt field of the motor is connected permanently to a fixed exciting voltage, but the armature is supplied with different voltages by connecting it across one of the several different voltages by means of suitable switchgear. The armature speed will be approximately proportional to these different voltages. The intermediate speeds can be obtained by adjusting the shunt field regulator.

(b) Ward-Leonard System: This system is used where an unusually wide and very sensitive speed control is required as for colliery winders, electric excavators, elevators and the main drives in steel mills and blooming and paper mills. M1 is the main motor whose speed control is required. The field of this motor is permanently connected across the dcsupply lines. By applying a variable voltage across its armature, any desired speed can be obtained. This variable voltage is supplied by a motor-generator set which consists of either a dc or an ac motor M2 directly coupled to generator G. The motor M2 runs at an approximately constant speed. The output voltage of G is directly fed to the main motor M1. The voltage of the generator can be varied from zero up to its maximum value by means of its field regulator. By reversing the direction 66 of the field current of G by means of the reversing switch RS, generated voltage can be reversed and hence the direction of rotation of M1. It should be remembered that motor generator set always runs in the same direction.

Answer 4

We can control the speed of the Induction Motor by

1)By changing Applied Voltage,

2) By changing the Applied Frequency,

3) By changing Number Of Stator Poles.

Answer 5

frequency controls the speed

single pole motor at 60Hz is 3600RPM