Speed control methods.

Speed control methods.

• A. AC speed is controlled by changing voltage; DC speed by changing frequency.
• B. DC speed is controlled by changing frequency; AC speed by changing voltage.
• C. AC speed is controlled by changing frequency (via VFD); DC speed is controlled by changing voltage.
• D. Both are controlled by adjusting rotor resistance.

Answer: (C)

Speed control relies on how the motor’s speed relates to its drive variable. For AC motors, the speed of an induction motor is set mainly by the supply frequency, so a variable-frequency drive changes the speed by altering frequency (and typically adjusts voltage to keep torque reasonable). For DC motors, speed is controlled most directly by the voltage applied to the armature (with the field kept constant), so increasing or decreasing armature voltage changes speed. That makes the choice correct: AC speed is controlled by changing frequency (via a VFD), while DC speed is controlled by changing voltage. The other options mix up which quantity governs speed for each type, and rotor resistance control is not a general method for speed control across both motor types.

Speed control relies on how the motor’s speed relates to its drive variable. For AC motors, the speed of an induction motor is set mainly by the supply frequency, so a variable-frequency drive changes the speed by altering frequency (and typically adjusts voltage to keep torque reasonable). For DC motors, speed is controlled most directly by the voltage applied to the armature (with the field kept constant), so increasing or decreasing armature voltage changes speed.

That makes the choice correct: AC speed is controlled by changing frequency (via a VFD), while DC speed is controlled by changing voltage. The other options mix up which quantity governs speed for each type, and rotor resistance control is not a general method for speed control across both motor types.