# Control the Speed of DC Shunt Motor ABOVE Rated Speed & Draw the Speed Characteristics

Experiment No.: 3(A)

Experiment Name: Control the Speed of DC Shunt Motor Above Rated Speed & Draw the Speed Characteristics.

Objective:

1. To control the speed of a DC Shunt Motor by field control method.
2. To draw the field current vs. speed characteristics.

Theory:

The flux of a DC motor can be changed by changing the field current with the help of a shunt field rheostat.

A resistance is necessary to connect in series of the armature to control initial high starting current because armature has low resistance. When motor armature rotates, the armature conductor also rotate and hence cut the flux. According to the Flemming’s Right Hand rule, is in opposition to the applied voltage, it is referred as Back e.m.f., Eb . The voltage applied across the motor armature has to overcome the back e.m.f. and supply the armature ohmic drop. Hence

V = Eb + Ia Ra

Eb = V – Ia Ra ————- (1)

The equation for back e.m.f. is

Eb=(NΦZP)/60 A [Where N is in r.p.m.]

Or, Eb = k Φ N [where k = Z.P / 60 A]

Putting value of Eb in (1),

k Φ N = V – Ia Ra

therefore,

N = (V – Ia Ra) / k Φ

It is seen from the above that N α 1/Φ. By decreasing flux the speed can be increased and vice versa. Changing field current with the help of a shunt field rheostat can change flux. This method is used when the speed above the normal speed is required.

Circuit Diagram:

Observation Table:

Characteristics Curve:

Result: Speed is increases as the field excitation decreases

Apparatus Used:

Remarks:

To start with, the whole armature control resistance in the circuit. While reducing the field resistance, must control the high speed of the motor.

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