Question

A dc shunt motor operating in a linear region of magnetizing characteristics runs at 1500 rpm with a voltage rated over the entire portion of the armature in a linear region of magnetizing characteristics. If the applied voltage is reduced by 50%, what will be the speed of the motor?
1. 750 rpm
2. 2500 rpm
3. 3000 rpm
4. 1500 rpm

Answer 1

Correct Answer - Option 4 : 1500 rpm

Concept

In DC shunt motor, the speed (N) is directly proportional to back emf (Eb) and inversely proportional to flux (ϕ).

\(N \propto {E_{b} \over \phi}\)

\(N \propto {V-I{_a}R{_a} \over \phi}\)

By neglecting armature reaction,

\(N \propto {V\over \phi}\)

Flux is directly proportional to the applied voltage. As applied voltage (V) is varying flux is also varied.

⇒ N is constant.

Calculations

Let applied voltage =  V

Reduced applied voltage =  V / 2

No-load speed = 1500 rpm

As the speed is constant irrespective of voltage variation, the speed will be 1500 rpm at reduced voltage.