Difference Between AC Motor and DC Motor

Difference Between AC Motor and DC Motor:

  • AC motors is defined as Alternating Current Motors which is energized from alternating currents typically the main source of ac motors are AC. In case of DC Motor is energized by Direct current and the main source for DC motors are DC. However, both motors are used to deliver mechanical output from various sources.
  • The main part of the AC motors is of starter (the stationary Part), rotor (the rotating Part) and AC source. These three element are enough to rotate a motor in AC source. But In DC motor’s main parts are armature (rotating parts), field (stationary part), Commutator, carbon brushes and DC Source. Absence of any of these elements, the DC motor do not work.
  • AC Motor does not have carbon brushes (expect slip ring induction Motor). In DC motors carbon brushes are mandatory.
  • Three phase AC Motors are self-starting Motor but single phase motors are not self-starting motor and it needs some external starting devices. DC motors are self-starting motors and it does not require external starting devices.
  • The direction of the motor rotation is depending on the direction of Rotating magnetic field and In DC motor the direction of motor rotation can be found from the Fleming left hand rule.
  • AC Motor consist of three terminals RYB and DC motor consist of four terminals which are armature positive, negative terminals and field positive and negative terminals.
  • In AC Motor the direction can be reversed by changing the phase sequence of the motor i.e RYB is clockwise direction means, the same can be reversed by RBY – anti clockwise direction. In DC motor we can reverse the motor by changing the polarity of field winds or armature winding.


Note: You should not change the polarity of both armature and field, you should change at one place.

  • In Ac Motor Harmonics affects the motor performance. But In DC motor there is no harmonics.
  • AC motor stator core needs core laminations to avoid eddy current losses and hysteresis losses but, DC does not require lamination, Since DC does not have frequency losses.
  • Ac Motor needs costly speed control devices (VFD) but in DC motor does not require such costly device.
  • AC motor Speed control can be done by changing the input frequency or input starter voltage or pole changing method or changing the rotor resistance (slip ring induction motor). In DC motor The speed control can be done but changing the armature voltage and field flux control (field weakening method).
  • AC Motor takes high starting current typically four to five times of its full load current. But DC motor takes very less starting current typically two to three times for few Milli seconds.
  • AC motors are wound such as full pitch winding or short pitch winding. In case DC motors are wound lap winding or wave winding.
  • AC motors needs less maintenance cost, but DC motor needs high maintenance cost.
  • AC motor needs lot of protection such as locked rotor, short circuit protection, over current, Earth Faults, Over voltage etc… But DC motor does not require such protection, only it needs short circuit protection, field zero current detection (very important), over current protection, Earth fault circuit etc.
  • AC motor has low starting torque; DC motor has high starting torque.
  • AC motor performance is affected by power factor, But IN DC Motor, there is no power factor.
  • Bar winding affects the motor performance in AC motor, since presence of skin effect. But In DC motor, Skin effect does not affect the motor performance (DC do not have skin effect)
  • AC motor cannot be used as AC generator (except synchronous motor) but DC motor can be used as generator as well as motor.

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