Types Of Electric Motors (2)

* Separately excited DC motor:

If the field current is supplied from a separate source it is a separately excited DC motor.

* Self excited DC motor: shunt motor:

In a shunt motor, the field winding (shunt field) is connected in parallel with the armature
winding (A) . The total line current is therefore the sum of field current and armature current.

Characteristics of a DC Shunt Motor

The following can be said about the speed of shunt motors:

􀂃 The speed is practically constant independent of the load (up to a certain torque after which speed decreases)

and therefore suitable for commercial applications with 

a low starting load, such as machine tools
􀂃 Speed can be controlled by either inserting a resistance in series with the armature (decreasing speed) or by inserting resistance in the field current (increasing speed).

* Self excited DC motor: series motor:

In a series motor, the field winding (shunt field) is connected in series with the armature winding (A).The field current is therefore equal to the armature current.

The following can be said about the speed of a series motor:

􀂃 Speed is restricted to 5000 RPM
􀂃 It must be avoided to run a series motor with no load because the motor will accelerate uncontrollably
Series motors are suited for applications requiring a high starting torque, such as cranes and hoists . Characteristics of a DC Series Motor
* DC Compound Motor:

A DC compound motor is a combination of shunt and series motor. In a compound motor, the field winding (shunt field) is connected in parallel and in series with the armature winding (A) . For this reason this motor has a good starting torque and a stable speed. The higher the percentage of compounding (i.e. percentage of field winding connected in series), the higher the starting torque this motor can handle. For example, compounding of 40-50% makes the motor suitable for hoists and cranes, but standard compound motors (12%) are not .

Characteristics of a DC Compound Motor