Types of DC Motor

Types of DC Motor Points : Types of DC Motor, Permanent magnet , Separately excited , Series wound , Shunt wound, Compound wound dc motors Similar to dc generators, the dc motors can also be classified as
1. Permanent Magnet
2. Separately Excited
3. Series Wound
4. Shunt Wound
5. Compound Wound DC Motors.
6. Cumulative Compound Wound Motor
7. Differential Compound Wound Motor 1. Permanent Magnet Motor It consists of an armature and one or several permanent magnets encircling the armature. Field coils are usually not required. However, some of these motors do have coils wound on the poles. If they exist, these coils are intended only for recharging the magnets in the event that they loose their strength. Schematically, this motor appears as only an armature.

2. Separately Excited DC Motors These motors have field coils similar to those of a shunt wound machine, but the armature and field coils are fed from different supply sources, as illustrated and may have different voltage ratings.
In a separately excited dc motor,
Armature current, I_a = Line current, I_L = I (say)
Back emf developed, E_B = V- I R_a
Power drawn from supply mains, P = V I
Where V is supply voltage
Mechanical power developed, P_m = Power input to armature – power lost in armature = V I - I² R_a = I (V- I R_a = E_b I 3. Series Wound DC Motors As the name implies, the field coils, consisting of few turns of thick wire, are connected in series with the armature, as illustrated. The cross sectional area of the wire used from field coils has to be fairly large to carry the armature current, but owing to the higher current, the number of turns of wire in them need to be large. In a dc series motor.

Armature current, I_a = Series field current, I_se
= Line current, I_L = I
Back emf developed. E_b = V – I (R_a + R_se)
Power drawn from supply mains = V I
Mechanical power developed
P_m = Power input – losses in armature and field
= V I - I² (R_a + R_se)
I [V – I (R_a + R_se)] + E_b I

4. Shunt Wound DC Motors The word “shunt” means “parallel”. These motors are so named because they basically operate with the field coils connected in parallel with the armature. The field winding consists of a large number of comparatively fine wire so as to provide large resistance. The field current is much less than the armature current, sometimes as low as 5%.
The current supplied to the motor is divided into two paths, one through the field winding and second through the armature i.e.,
Input line current, I_L = I_a + I_sh
where I_a is the armature current and I_sh the shunt field current and is given by the expression
I_sh = V/R_sh where V is the supply voltage and R_sh is the shunt field resisiarice.
Back emf developed, E_b = V - I_a R_a
Power drawn from supply mains, P = V I_L
Mechanical power developed, P_m Power input
losses in armature and shunt field

= VI_L - V I_sh - I_{a2 Ra
= V (IL - Ish) - Ia² Ra
= VIa - Ia² Ra = Ia (V - Ia Ra) = Eb Ia}

5. Compound Wound DC Motors A compound wound dc motor has both shunt and series field coils. The shunt field is normally the stronger of the two (i.e., has more ampere-turns). Compound wound motors are of two types namely cumulative compound wound and differential compound wound motors.

6. Cumulative Compound Wound Motor Is one in which the field windings are connected in such a way that the direction of flow of current is same in both of the field winding. In the motor of this type the flux due to series field winding stren8thens the field due to the shunt field winding. 7. Differential Compound Wound Motor Like compound wound dc generators, may be either long shunt connected or short-shunt connected. In long shunt connected (or long shunt) compound wound motors, series field and armature are connected in series with each other and in parallel with thee shunt field. In short shunt connected (or short shunt) compound wound motors, the armature and shunt field are in parallel with each other and the pair is in series with the series field. Long shunt connection sometimes results in simpler wiring. Changing from long to short shunt, or vice-versa, has little effect on motor performance.