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DC Compound Motor

July 12, 2020

DC compound motor or Compound wound DC motor is also a self-excited motor, but it is a compound of both series and shunt field coils connected to the armature winding. The series field winding is connected in series with the armature and a shunt field winding is connected in parallel to the armature.

Briefly, we can say that DC compound motor is a combination of both a shunt-wound DC motor and a series-wound DC motor. This means that the compound motor has the advantage of high starting torque and efficient speed regulation. Thus, it can be used in industrial applications include:

  • Drivers.
  • Freight elevators.
  • Stamping Presses.
  • Rolling mills.
  • Mixers.
  • Reciprocating machines.
  • And metal shears.

Types of compound wound DC motor

Compound motor divided according to the connection of field winding with respect to the armature winding to two major types:

Long shunt compound DC motor

In the long shunt motor, we connect the shunt field winding parallel across the series combination of both the series field winding and the armature.

Let’s look at the voltage and current equation of the long shunt compound motor.
Let’s start with the total supply current- Il

Il=Ia+Ish.

And as it’s clear: Ia=Ise.

So:

Il=Ise+Ish.

And the total supply voltage will be:

V= Eb+ Ia.(Ra+Rse).

Short shunt compound wound DC motor

In short shunt motor, we connect the shunt field winding parallel across only the armature winding and we connect the series field winding to the supply current.

The voltage and current equation of short shunt compound motor are as follows:
We start with the total supply current- Il

Il=Ise.

As it’s clear, the supply current pass through the series field winding:

Il=Ia+Ish.

And we will get the voltage by applying KVL to the circuit and it’ll be:

V=Eb+Ia.Ra +Ise.Rse.

And when we make up we will have:

V=Eb+Ia.Ra+Il.Rse.

Subdivision of Compound Wound DC Motor

Both long and short shunt compound motor can be subdivided according to the excitation or nature of compounding. They are:

Cumulative compound DC motor

We can say that the compound motor is cumulative when the shunt field flux produced by the shunt winding aids the effect of the main field flux, produced by the series winding.

Φtotal=Φseries+Φshunt.

You must know that cumulative motor is the most common because it provides high starting torque and good speed regulation at high speeds. This makes it the best in:

  • electric shovels.
  • stamping machine.
  • reciprocating pumps.
  • hoist.
  • compressors, and so on.

Differential compound DC motor

We can say that the compound motor is differentiated when the flux produced by the shunt field winding decreases the effect of main series winding flux; so:

Φtotal= Φseries- Φshunt.

The total produced flux is lesser than the original flux. Hence, the differential motor is used where constant speed is required irrespective of load, especially when it is used in the elevator and escalator.

Compound Inter-Pole motor

The inter-pole is slightly different from the cumulative and differential compound motor as it has inter poles connected in series with the armature. Specifically between the armature and series winding and we can have any number of poles to strengthen the field.

The polarity of this additional inter-pole must match with the polarity of the series windings.

The great advantage of the compound motor is that:

It prevents the armature and brushes from arcing so the motor will last longer and wouldn’t need to cut down the armature as often.

Also, it allows the armature to carry large shaft load and draw heavier currents.

Characteristic of compound wound DC motor

The most important part of any machine is its characteristics.

The shunt field winding in cumulative motor produces the definite flux and the flux from series winding added to it produces a large amount of flux. This flux causes a large amount of torque at low speed, so cumulative motor run at a reasonable speed on light or no-load conditions.

In contrast, in the differential motor, both series and shut fluxes oppose each other. Thus, a flux decreases with increases in load. As such, we have a high speed with an increase in load and this can be dangerous especially on full load.