Correct Answer - Option 4 : remains constant
Concept:
Speed of DC shunt motor is given by,
\(N \propto \frac{{{E_b}}}{\phi } \propto \frac{{V - {I_a}{R_a}}}{\phi }\)
But generally, Ra term is small, hence this term is neglected.
\(\Rightarrow E \propto V\)
Now the speed is,
\(\Rightarrow N \propto \frac{{{E_b}}}{\phi } \propto \frac{V}{\phi }\)
From data, as V is half, ϕ becomes half, such that speed becomes constant. Also given that,
\(T \propto {N^2},\;T \propto \phi {I_a}\)
Since speed is not changed even V is half, the torque also remains unchanged.
Hence to get constant torque with field flux fallen to half, armature current has to be doubled.
