DC Compound Motor

July 12, 2020
DC Compound Motor

A 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 connects in series with the armature, and a shunt field winding is connected in parallel to the armature.

We can briefly say that a 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, we use it in industrial applications, including:

  • Drivers
  • Freight elevators
  • Stamping Presses
  • Rolling mills
  • Mixers
  • Reciprocating machines
  • Metal shears

Types of Compound Wound DC Motors

The compound motor divided according to the connection of field winding with respect to the armature winding into 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.

long-shunt connection

Let’s look at the voltage and current equation of the long shunt compound motor:

Let’s start with the total supply current- Il

And as it’s clear:

So

And the total supply voltage will be

Short Shunt Compound Wound DC Motor

In a 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.

short-shunt conection

The voltage and current equation of the short shunt compound motor is as follows:

We start with the total supply current- Il

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

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

And finally, we have this equation

Subdivision of Compound Wound DC Motor

Both long and short shunt compound motors 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.

You must know that the 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

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:

The total produced flux is less than the original flux. Hence, the differential motor is used where constant speed is required irrespective of load, especially in elevators and escalators.

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, we can have any number of poles between the armature and series winding 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 so often.

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

Characteristic of the 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 load increases. As such, we have a high speed with an increase in load, which can be dangerous, especially on full load.

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