Speed Control of the DC Shunt Motor

The speed of DC motor is related to following equation
N a Eb / F
N = K ( V – IaRa ) / F

Therefore the speed of the DC motor can be controlled by varying
  • Supply voltage
  • Flux per pole ( Flux control )
  • Armature resistance control ( OR Rheostatic control )

Speed control of DC Shunt Motor
( 1 ) Flux control method
  • The speed variation is obtained by inserting variable resistance is in series with the field circuit. 
  • If the supply voltage is kept constant, back emf Eb is also constant.

        N a 1 / F
  • It means that an increase in field resistance reduces the field current consequently reduction of field current and increase in speed. 
  • As the field current is very small the field copper loss is very small. Therefore this method is very efficient and economical. 
  • The maximum speed can be obtained by minimum value of flux which affects the effect of armature reaction
  • At the same time increase in armature current causes over heating of armature, poor commutation and instability. 
  • Therefore there is some limitation to obtain high speed. 
  • The maximum to minimum speed can be obtained in the ratio of 6: 1 in the inter polar machine whereas it will be ratio of 2 : 1 in the non inter polar machine. 
  • This method of speed control is applicable to achieve speed above normal or rated speed.

( 2 ) Armature resistance control

  • A variable resistor is inserted in series with the armature winding in this method. 
  • As the supply voltage is kept constant, voltage across armature is equal to supply voltage minus voltage drop across rheostat. 
  • As a variable resistor increases, the voltage drop across resistor increases resulting voltage drop across armature decreases. This will result in speed of the motor decreases. 
  • Greater the value of variable resistor, greater fall in speed.

Ia1 = Armature current in first case
Ia2 = Armature current in second case
N1 = Speed in first case
N2 = Speed in second case
V = Supply voltage
Ra = Armature resistance in first case
Ra + R = Armature + variable resistor in second case
( N2 / N1 ) = ( Eb2 / Eb1 )
( N2 / N1 ) = [ V – Ia2 ( Ra + R ) ] / [ V – Ia1 Ra ]
If we consider no load speed N0
( N / N0 ) = [ V – Ia ( Ra + R ) ] / [ V – Ia0 Ra ]
Neglecting Ia0 Ra as compared to supply voltage V
( N / N0 ) = [ V – Ia ( Ra + R ) ] / V
            N = N0 [ V – Ia ( Ra + R ) / V ]
            N = N0 [ 1 – Ia ( Ra + R ) / V ] …………. ( 1 )
  • For a given value of ( Ra + R ) in the armature circuit, the speed is linear function of armature current.

By putting N = 0 in the equation ( 1 )
N0 [ 1 – Ia ( Ra + R ) / V ] = 0
Ia = V / ( Ra + R )
  • This is maximum current and it is known as stalling current. 
  • This method is employed when speed below the normal speed is required for short period duty i.e. printing machine, crane, hoist etc.

  • This method is wasteful because large power loss in the armature circuit.
  • Poor voltage regulation is obtained particularly at lower speed

( 3 ) Voltage control ( Ward – Leonard method )


  • The field winding is permanently connected to fixed supply voltage and voltage across armature varies by means of either variable supply voltage system or motor – generator set. 
  • The speed of motor is approximately proportional to voltage across armature. 
  • The M is main DC shunt motor whose speed control is required as shown in the Figure A. 
  • The field of the motor is connected to exciter
  • The variable voltage across armature is supplied through induction motorDC generator set. 
  • The DC generator is driven by induction motor whose shaft is coupled to an exciter. 
  • The voltage of the generator is varied from maximum to minimum value by means of a field regulator. 
  • The generated voltage is given to the DC shunt motor. 
  • The generator voltage and direction of motor M is reversed by reversing switch RS. 
  • The induction motor – generator set always run in the same direction.


  • Smooth speed control
  • Wide range of speed control from maximum to minimum
  • Good speed regulation is achieved

  • Two extra machines require therefore the initial cost is high
  • Low efficiency of the system particularly at light load
  • More space requires as there are two extra machines require

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