Electric Motors
Electric Motors

DC Compound Motor (Definition, Types, Applications, equations)


DC Compound Motor

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

DC Compound Motor

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

Before taking about types of wound motor you can refresh and revise previous articles as ( working principle of dc motorConstruction of Dc motor, types of dc motorDC shunt motorSeries wound Dc motor ).

Types of compound wound Dc motor:

We said before that 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.

long shunt compound Dc motor

Voltage and current equation of 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.

short shunt compound Dc motor

Voltage and current equation of short shunt compound motor:

We will also start with the total supply current Il:

Il=Ise.

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

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.

Types of DC Compound Motor:

Yes, I repeat the title because and as we said before, both long and short shunt compound motor can be subdivided according to the excitation or nature of compounding to:

Cumulative compound Dc motor:

We can say that the compound motor is cumulative when the shunt field flux produced by the shunt winding aid the effect of main field flux produced by the series winding; briefly:

Φtotal=Φseries+Φshunt.

cumulative compound excited Dc motor

You must know that cumulative motor is the most common because it provides high starting torque and good speed regulation at high speeds which make 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, so the differential motor is used where constant speed is required with irrespective of load especially it’s used in elevator and escalator.

 

Compound inter-pole motor:

The compound motor 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 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 relation between torque and current.

speed and current and speed and torque.

We will start with cumulative compound motor, when the shunt field winding in cumulative motor produces the definite flux and the flux from series winding added to it we have a large amount of flux this flux causes a large amount of torque at low speed so cumulative motor run at reasonable speed on light or no load conditions.

characteristic of compound wound Dc motor

In contrast, in differential motor we have two fluxes oppose each other so we have a decreases resultant flux with increases in load, so we have a high speed with an increase in load and it’s so dangerous especially on full load so and as I mentioned before it isn’t particularly used.