Which type of motors would be expected to have the lowest demand for a starting current?

• A. DC shunt motor
• B. Synchronous motor
• C. Induction motor with autotransformer starting
• D. Wound-rotor induction motor

Answer: (D)

Starting current is minimized when there’s a way to limit the current drawn by the motor at the moment it starts. A wound-rotor induction motor can have external resistance inserted into the rotor circuit during starting. That added rotor resistance reduces the rotor current—and because the rotor and stator are electromagnetically linked, the overall current drawn from the supply can be kept much lower. You can set that rotor resistance to a fairly high value, giving a much smaller starting inrush than with direct-on-line starts or other starting methods. Other methods either apply full supply directly or still allow significant inrush, so they don’t achieve as low a starting current as the wound-rotor approach with starting resistors.