## 14/11/2017

### Speed Control of the DC Shunt Motor

Introduction
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.

Let
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
As
( 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