Speed Control of D.C. Motors

The relationship given below gives the speed of a D.C. motor

The above equation shows that the speed depends upon the supply voltage V, the armature circuit resistance R_a, and the field flux Ф, which is produced by the field current. In practice, the variation of these three factors is used for speed control. Thus, there are three general methods of speed control of D.C. Motors.

1. Resistance variation in the armature circuit: This method is called armature resistance control or Rheostat control.
2. Variation of field flux Ф
   This method is called field flux control.
3. Variation of the applied voltage.
   This method is also called armature voltage control.

1. Armature resistance control (Rheostat Control):

Figure: (a) Speed control of a d.c. Shunt motor by armature resistance control.
(b) Speed control of a D.C. Series motor by armature resistance control.

In this method, a variable series resistor R_e is put in the armature circuit. The figure (a) above shows the process of connection for a shunt motor. In this case, the field is directly connected across the supply and therefore the flux Ф is not affected by variation of R_e.

Figure (b) shows the method of connection of external resistance R_e in the armature circuit of a D.C. series motor. In this case, the current and hence the flux is affected by the variation of the armature circuit resistance.

The voltage drop in R_e reduces the voltage applied to the armature, and therefore the speed is reduced.

This method has the following drawbacks:

1. In the external resistance R_e a large amount of power is wasted.
2. Control is limited to give speed below normal and increase of speed cannot is obtained by this method.
3. For a given value of R_e, the speed reduction is not constant but varies with the motor load.

This method is only used for small motors.

2. Variation of field flux Ф (Field flux control):

Since the field current produces the flux, and if we control the field current then the speed can be controlled. In the shunt motor, speed can be controlled by connecting a variable resistor R_c in series with the shunt field winding. In the diagram below resistor, R_c is called the shunt field regulator.

Figure: (a) Speed control of a D.C. shunt motor by variation of field flux.
(b) The diverter in parallel with the series of D.C. Motor.

gives the shunt field current

Any of the one methods can vary the field current of the series motor:

• A variable resistance R_d is connected in parallel with the series field winding. The resistor connected in parallel is called the diverter. A portion of the main current is diverted through R_d.
• The second method uses a tapped field control.

Here the ampere-turns are varied by varying the number of field turns. This arrangement is used in electric traction.

Figure: Tapped series field on D.C. motor

The advantages of field control are as follows:

• This is an easy and convenient method.
• The power loss in the shunt field is small because shunt field current I_sh is very small.

3. Armature Voltage control:

We can control the speed of the D.C. motors by varying the applied voltage to the armature. Ward-Leonard system of speed control works on this principle of armature voltage control. In this system, M is the main dc motor whose speed is to be controlled, and G is a separately excited dc generator. The generator G is driven by a 3- phase driving motor which may be an induction motor or asynchronous motor. The combination of ac driving motor and the dc generator is called the motor-generator (M-G) set.

Figure: Ward-Leonard drive

Advantages of Ward-Leonard Drives:

1. This drive has a smooth speed control of dc motors over a wide range in both directions.
2. It has inherent regenerative braking capacity.
3. By using an overexcited synchronous motor as the drive for the dc generator, the lagging reactive volt-amperes of the plant are compensated. Therefore the overall power factor of the plant improves.

Drawbacks of classical Ward-Leonard system:

1. Its initial cost is high because of the use of two additional machines (M-G set) of the same rating as the main dc motor.
2. It has a large size and weight.
3. It requires more floor area and costly foundation.
4. Very frequent maintenance is required.
5. The losses are higher because of lower efficiency.
6. Its drive produces more noise.