Separately Excited DC Motor : Definition,Characteristics , Application and Torque Equation

DC Compound Motor 

DC Compound motor are special types of motor in which two types of field winding is available. Its one field winding is connected in series and the other is connected across the armature. Due to both types of winding in DC Compound motor, its characteristics are intermediate to DC series motor and DC Shunt Motor.DC Compound Motor are classified into two categories 

• Cumulative Compound Motor
• Differential Compound Motor

Cumulative Compound Motor

if the magnetic flux produced by the series winding assists the magnetic flux produced by the shunt then is called a Cumulative compound motor. The cumulative compound motor also classified into two categories

• Long shunt type
• Short Shunt type

Long Shunt Compound DC Motor

When the shunt field winding is connected parallel to both armature and series winding then this motor is called a long shunt compound motor. The circuit diagram of the long shunt compound dc motor is given below.

Short Shunt Compound DC Motor

When the shunt field winding is connected parallel to armature winding only then this motor is called a short shunt compound dc motor. The circuit diagram of the short shunt compound dc motor is given below.

Characteristics of Cumulative Compound DC Motor

Characteristics of Cumulative dc motor are given by curve. This curve is drawn between two different parameters of the compound dc motor.

Torque Load Characteristic

Torque load characteristics of cumulative compound dc motor are given below.

From this figure, it is very clear that the torque developed in the cumulative compound dc motor is less than the shunt motor at full load but is greater than the series motor. the starting torque of the cumulative dc motor is greater than the starting torque of the shunt motor. from the figure, it is also clear that as the load current increases, the torque also increases.

Speed torque characteristics 

Speed torque characteristics of cumulative compound dc motor is given below by a curve between speed and torque.

From the given curve it is very clear that at low torque the speed of the motor is very high but as the torque increases the speed of the motor decreases slowly. It means that this motor is capable to bear a high load but not able to bear a sudden overload.

Speed torque characteristics

Speed torque characteristics of cumulative Compound DC motor are given below by a curve between speed and torque.

From give figure it is shown that the speed of the motor reduces as the load current increases. But the speed variation of this motor is intermediate of series and shunt dc motor.

Differential Compound DC Motor

A compound dc motor in which the series winding is so connected that it will oppose the field flux of shunt field winding this type of dc motor is called a differential compound dc motor. Differential compound dc motors are also classified into two types i.e (a) Long Shunt type (b)Short Shunt Type
During the starting of the differential compound dc motor, the series winding is short circuit due to this this motor starts as a shunt motor with a constant speed.

Long Shunt Type

In this type of DC motor, the shunt field winding is connected across the armature and series field winding. The series field winding is connected in series with armature winding so that it will produce a magnetic field that opposes the field flux of the shunt. the circuit diagram of the long shunt differential motor is given below.

Short Shunt Type

In this type of DC motor, the shunt field winding is connected across the armature winding. The series field winding it will produce a magnetic field that opposes the field flux of the shunt. the circuit diagram of the long shunt differential motor is given below.

Characteristics of Differential Compound DC Motor

Different characteristics of differential compound dc motor is given below by curve between two different parameters of the motor.

Torque load Characteristics

The torque load characteristics of the differential compound dc motor are given below between the torque and load curve. From the given curve it is clear that when the load current increases, the torque also increases for a given value of load current, beyond the value of this load current torque is almost constant irrespective of the load current.

Speed Torque Characteristics

Speed torque characteristics of differential compound dc motor is given below by a cure between speed and torque. From the curve, it is clear that the speed of the motor increase along with the increase in torque. But it increases slowly due to overloading and chances of reverse rotation increase.

Speed Load Characteristics

Speed load characteristics of the differential compound motor are given below by a curve between speed and load. form curve it is looking that when the load current increases the speed of the motor also increases with the load current. The reason for this increase in speed of the motor is the reduction in resultant magnetic flux.  

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