12/10/2017

Speed Control of the DC Series Motor

• The speed of the DC Motor is related to following equations
N α Eb / Ф
N = K ( V – IaRa ) / Ф
• Therefore the speed of the DC Motor is controlled by varying
( 1 ) Supply voltage V
( 2 ) Flux per pole ( Flux control )
( 3 ) Armature Resistance Ra ( Rheostatic control )
• The speed of the DC series motor can be controlled by following methods
Armature resistance control
Flux control
Series – parallel control
Armature resistance control

Armature resistance control

( 1 ) Variable resistance in series with the motor
• The speed of the DC series motor depends upon voltage drop across variable resistance.
• Higher the voltage drop across the armature, lower the speed. However there is considerable power loss in the variable resistance.
• This method is economical for constant torque drives.
• This method is employed for driving cranes, hoists, trains etc.
( 2 ) Armature – diverter
• A variable resistor is connected across the armature in this method.
• If the load torque is kept constant, the armature current reduces due to armature diverter.
Ta α ΦIa
• As the torque Ta is constant, the flux must be increase due to decrease in armature current.
• Therefore the motor will take more current from the supply result in increase in the flux and decrease in the speed ( N α 1 / Φ ).
• The speed variation is obtained by varying the diverter resistance.

Flux control

( 1 ) Field diverter
• The field winding is shunted by variable resistance R. Due to divert the field current through the variable resistance path, the flux Φ reduces.
• This will result in increase the speed of the motor. Finally we conclude that the speed of the DC series motor increases above its normal speed due to field diverter.

Tapping of the field winding
• The number of tapping is taken out from the field winding in this method.
• The speed of the DC series motor increases by cutting the field winding step by step from position 1, 2, 3 and 4. This method is useful in the traction system.
Paralleling field coils
• Let us consider a four pole DC motor. All the field coils are connected in series as shown in the Figure E. The DC motor runs at N1 RPM.
• The series combination of the two field windings is connected in parallel as shown in the Figure E. The speed of the motor increases due to decrease in flux. Obviously N2 > N1

What is limitation of field control method?
• The speed above normal speed is obtained by field current or flux control. The commutation becomes unsatisfactory particular at weaker field due to effect of armature reaction.
Series – parallel control
• This method is particularly used in the electrical traction in which two or more motors are mechanically control.
• When two DC series motors are connected in series, necessary high starting torque and low speed is obtained at starting.
• Similarly when two DC series motors are connected in parallel, high speed and low starting torque is obtained during running condition.
( 1 ) Series connection
• When two DC series motors are connected in series, voltage across each motor is half that of the supply voltage.

Speed N1 α ( Eb / Φ )
α ( V / 2 ) / I
α ( V / 2I )………..( 1)
Torque Tα ( ΦI )
α ( I )( I )   ( As Φ = I )
α ( I )2…….……..( 2)

( 2 ) Parallel connection
• When two DC series motors are connected in parallel, current through each path reduces to ( I / 2 ) from I.

Speed N2 α ( Eb / Φ )
α ( V ) / ( I / 2 )
α ( 2V ) / ( I ) ………( 3)
Torque Tα ( ΦI )
α ( I / 2 )( I / 2 )   ( As Φ = I )
α ( I / 2 )2 …………( 4 )

From equation ( 1 ) and ( 3 )
N1 / N2 = ( V / 2I ) / ( 2V / I ) = 1 / 4
• It means that when two DC series motors are connected in series, its speed reduces 1/4th that of parallel connection.

From equation (  2 ) and ( 4 )
T1 / T2 = ( I2 ) / ( I2 / 4 )
= 4
• It means that when two DC series motors are connected in series, the starting torque increases up to 4 times that of parallel connection.
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