TYPES OF DC MOTORS

TYPES OF DC MOTORS

•          DC motors are classified according to the way in which their fields excited

1. Separately excited dc motor

2. DC series motor

3. DC shunt motor

4. DC compound motor

SEPARATELY EXCITED DC MOTOR

•          These motors have field coils similar to the field coils  of a shunt wound motor.

•          The field coils are excited by the separate dc source.

•          Armature current I_a = I = line current

•          Back emf equation E_b = V – I_aR_a

DC SERIES MOTOR

•          The field coils consisting of few turns of thick wire are connected in series with the armature. The field coils have low resistance  The cross sectional area of the wire used for field coils has to be fairly large to carry the armature current

armature current I_a = series field current I_se = I

DC SHUNT MOTOR

•          The word ‘shunt’ means ‘parallel’ ,The field coils are connected in parallel with armature, The field coils consists of large number of turns ,The field coiles have high resistance

•          I = I_a + I_sh

•          Shunt field current I_sh =  V/Rsh

              where V = supply voltage

                     Rsh = shunt field resistance

•               Back emf equation E_b = V - I_aR_a

DC COMPOUND MOTOR

•          DC compound motor has both shunt and series field coils

•          The shunt field is normally the stronger of the two i.e has more ampere turns

•          DC compound motor has two types

  (a) long shunt dc compound motor

•          The shunt field winding is in parallel with both the armature and series field winding as shown in fig

•          I_sh = V/R_sh

•          I_a = I_se = I –I_sh

•          E_b = V – I_a(R_a+R_se)

  (b) short shunt dc compound motor

•          The shunt field winding is in parallel with the armature alone and the combination is in series with the series field

•          I_a = I –I_sh

•          I_sh = (V – I_seR_se)/R_sh

•          E_b = V – I_aR_a - I_seR_se

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