The most commonly used method of speed control of a D.C motor is by varying the

The most commonly used method of speed control of a D.C motor is by varying the

Right Answer is:

Field Strength


Field control is the most common method and forms one of the outstanding advantages of shunt motors. The method is, also applicable to compound motors. Adjustment of field current and hence the flux and speed by adjustment of the shunt field circuit resistance or with a solid-state control when the field is separately excited is accomplished simply, inexpensively, and without much change in motor losses.

The speed of DC Motor is inversely proportional to the field current and flux.

N ∝ 1 ⁄ I
or N ∝ 1/φ

By decreasing the flux the speed can be increased and vice versa. Hence this method is also called as a flux control method.

  • The lowest speed obtainable is that corresponding  to the maximum field current
  • The highest asleep is limited electrically by the effects of armature reaction under weak-field conditions in causing motor instability and poor commutation.
  • Since the voltage across the motor remains constant, it continues to deliver constant output. This characteristic makes this method suitable for fixed output loads.


The merits of this method are:

  1. Good working efficiency.
  2. Compact controlling equipment
  3. The capability of minute speed control.
  4. The speed is not affected by load, and speed control can be performed effectively even at light load
  5. Relatively inexpensive and simple to accomplish both manually and automatically.
  6. Within limits, field control does not affect ape regulation in the cases of the shunt, compound, and series motors.
  7. Provides relatively smooth and stepless control of the speed.


The demerits of field control as a method of speed control are:

  1. Inability to obtain speeds below the basic speed.
  2. Instability at high speeds because of armature reaction.
  3. Commutation difficulties and Possible commutator damage at high speeds.

Shunt Motors:

  • The flux of a D.C. shunt motor can be changed by changing shunt field current (Ish) with the help of a shunt field rheostat as shown in Fig. Since the field current is very small, the power wasted in the controlling resistance is very small.

  • In the non-interpolar machine, the speed can be increased by this method in the ratio 2:1.
  • In machines fitted with interpoles the ratio of maximum to minimum speeds of 6: 1 is fairly common. 

Series Motor

In a series motor, the variation of flux can be brought about in any one of the following ways

  1. Field Diverter
  2. Armature Diverter
  3. Tapped field control
  4. Paralleling Field control
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