During the start of a DC motor using a rheostat, which statement about current is correct?

• A. Field current is high and armature current is low
• B. Field current is low and armature current is high
• C. Both field and armature currents are high
• D. Both field and armature currents are low

Answer: (A)

At start, the motor’s back EMF is zero, so the armature current is governed mainly by the circuit resistance. A rheostat placed in series with the armature limits this current, reducing the starting armature current to a safe, low value. The field winding, on the other hand, is supplied separately and is kept at its normal, relatively high current to establish the magnetic flux needed for torque. So, during start with a rheostat, the field current is high while the armature current is low.