Question

A DC motor operates with 220 V mains. Initially, it draws a current of 20 A. the resistance of the starter is 6 \[\Omega \]. When the motor runs at its full speed it consumes 500 W power. Find the current drawn by it.
(A) 5 A
(B) 10 A
(C) 15 A
(D) 20 A

Answer 1

Hint:It is mentioned that the motor operates at 220 V. the initial current is given and the power is given when running at full speed, so we have to consider here emf and potential difference separately.

Complete step by step answer:
Using ohm’s law V=Ir
\[220=20\times r\]
r=11 \[\Omega \]
so, the resistance comes out to be 11 \[\Omega \], also given the resistance of the starter is 6\[\Omega \]. So, the total resistance in the circuit comes is 11\[\Omega \] but the resistance of the starter is (11-6) =5\[\Omega \]
Now we need to find the current drawn by the motor when it runs at full speed consuming 500 W of power.
Using the formula, \[P={{i}^{2}}R\]
\[\begin{align}
& 500={{i}^{2}}\times 5 \\
& i=10A \\
\end{align}\]
Thus, when the motor runs at its full speed it consumes 500 W power and the current drawn by it is 10 A
So, the correct option is (B).

Note: Power and current has direct relationship. Also, the total resistance in the circuit is also determined using the formula. Ohm’s law can be applied to any conductor which is Ohmic and follows ohm’s law at all conditions of temperature and pressure.DC motor converts direct current electrical energy into mechanical energy. It is just the opposite of an electric generator where mechanical energy is converted into electrical energy.