14 November 2017

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
       ( a ) Supply voltage
       ( b ) Flux per pole ( Flux control )
       ( c ) 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.
field-control-of-dc-shunt-motor.png


( 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.
armature-resistance-control-of-dc-shunt-motor.png


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.

Disadvantage

  • 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 )


ward-leonard-speed-control-of-dc-shunt-motor.png
  • 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 C. 
  • The field of the motor is connected to exciter. 
  • The variable voltage across armature is supplied through induction motor – DC 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.
variation-of-speed-with-respect-to-voltage-in-the-ward-leonard-speed-control.png
Advantages
  • Smooth speed control.
  • Wide range of speed control from maximum to minimum.
  • Good speed regulation is achieved.

Disadvantages

  • 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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