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Saturday, 25 January 2020

Characteristics of DC Compound Motor

Characteristics of D.C. Compound Motor:


Electrical Characteristics :

The torque equation for d.c. compound motor is

T = K1ΦIa = K1[Φsh + Φse] la

With increase in armature current Φse increases and consequently the torque. If the shunt field is more stronger than the series field the torque current characteristics approaches as that of d.c. shunt motor. And in case series field is stronger than the shunt field, the torque current characteristics approaches as that of d.c. series motor. The comparison curves are drawn in Fig.

In the case of differential compound motors since series field opposes the shunt field, the net flux decreases as load is applied to the motor; the result is, there is a decrease in the rate at which the motor torque increase with load. Therefore, such motors are not in common use. Another draw-back is, weakening of flux with increase in load, there is a tendency of speed unstability.

Speed and Armature Current Characteristics :

For the long shunt compound motor

V = Eb + Ia (Ra + Rse)
and Eb = K1ΦN = K1[Φsh + Φse]N

Where Φsh and Φse are flux due to Ish and Ise
N = Eb / K1[Φsh - Φse] =   V - Ia(Ra + Rse) / K1[Φsh + Φse]

With the increase in la, Φse increases and V - Ia(Ra + Rse) decreases, Thus with the increase in Ia, the speed drops at faster rate in cumulative compound motor than in shunt motor. If the shunt field is stronger than series field, the curve tends to shunt motor and if the series field is stronger than shunt field, it tends to series field curve. The comparison characteristic is shown in Fig.

Speed-Torque Characteristics :

The torque in d.c. compound motor is given by

Ta = K1 [Φsh + Φse] la
Or Ia = Ta/ K1[Φsh + Φse]
But N = V - Ia(Ra + Rse) / K1[Φsh + Φse]
N = (V/ K1[Φsh + Φse] ) - ( Ia(Ra + Rse) / K1[Φsh + Φse])
Substituting the value of Ia
N = (V/ K1[Φsh + Φse] ) - ( Ta(Ra + Rse) / K2[Φsh + Φse]2)

It is seen that an increase in torque increases the armature current and so also the flux, decreasing the speed rapidly. The decrease is more predominant than that compared with d.c. shunt motor. Therefore, the speed torque characteristics approaches the shunt motor characteristics if the shunt field is stronger. In case series flux is stronger than the shunt flux, the speed torque characteristics approaches the series motor characteristics. Depending upon the relative strength of shunt and series field the speed torque can occupy any position between the curve 1 and 2 in Fig.


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