Control the Speed of DC Shunt Motor BELOW Rated Speed & Draw the Speed Characteristics

Experiment No.: 3(B)

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


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


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 law of electromagnetic induction, e.m.f. is induced in them whose direction accordic to the Flamming’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 = (N Φ Z P) / 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


N = (V – Ia Ra) / k Φ

Above equation shows that if the voltage across the armature is varied by inserting a variable rheostat in series with the armature circuit, speed can be varied.

Circuit Diagram:

Observation Table:

Sl. No.Armature current (Ia) (A)Speed (N) (r.p.m.)
1. 0.41 1911
2. 0.40 1769
3. 0.39 1577
4. 0.38 1482
5. 0.36 1457
6. 0.34 1303
7. 0.31 1289

Characteristics Curve:

To be plotted on graph paper

Result: Speed is very nearly proportional to the applied voltage in case of armature control method.

Apparatus Used:

Sl. No.Name of the apparatusSpecificationQuantityMaker’s Name
1DC Motor 1 HP, 230 V DC, 4.1 A, 1500 rpmTriple-D
2Ammeter Digital 1CABS Electra 
3Voltmeter Digital 1 CABS Electra
4Tachometer Non contact type, Digital 1 METRIX+
5Field Rheostat Wire wound 1 
6Control Panel 230 V AC 1 Microline


To start with, we have to put the whole armature control resistance in the circuit.

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