AC Motor VS DC Motor

Although both AC and DC electric motor converts electric energy into mechanical energy, however their construction and way of functioning is different. AC refers alternating current which means the direction of current is reversing ( changing ) after every half cycle. DC simply means the current is flowing in one direction. DC motors are powered from direct current (DC) source such as batteries and converts converts dc electrical energy into mechanical energy. AC motors on the other hand functions on AC supply and converts AC electrical energy into mechanical energy. Now, lets talk about some common differences between AC motor vs DC motor.

Constructional Difference :

DC motors are constructed with brushes and commutator ↗. Armature and static field (or permanent magnet) are the main parts of dc motor. Coil ends of armature winding is connected to the commutator segments.

AC motor has stator and rotor as its main parts. Stator has coils that are supplied with an alternating current in order to produce rotating magnetic field. Rotor of AC motor is inside the motor coils ( stator ) and it is attach to the output shaft.

Operational Difference :

Power in dc motor is supplied to both armature winding and field winding. In case of dc motor, armature rotates and it is placed between the constant magnetic field which are physically stationary. Torque is produced due to the interaction of armature field and stator field flux due to which armature starts to rotate.

For detail working of dc motor, kindly read the following article

How DC Motor Works ? ↗

AC motors on the other hand continuously produce rotating magnetic field while armature doesn’t rotate in AC motors. Moreover, it is not always necessary to supply the power to both rotor and stator in case of AC motors. The example is of induction motors ↗.

Phase Difference : 

AC motors can be single phase or three phase depending on the requirement while dc motors on the other hand are all single phase.

Size difference :

AC motors are usually bigger in size as compared to DC motors.

Speed Control :

One more basic difference between AC and DC motor is the speed control. In case of dc motor, it is easy to control the speed over a wide range. The speed of dc motor is controlled by varying the armature winding current.

On the other hand, although the use of variable speed drives makes it possible to get the speed control of AC motors, however it makes the motor more noisy. AC motors may also gets heated with variable speed drives. Moreover, it is difficult in AC motors to get control over a wide range of speeds.

Commutator and Brushes :

Alternating current produce in the armature of dc motor needs to be converted. For that purpose, dc motors are constructed with brushes and commutator. The presence of commutator and brush assembly makes the current to rotate in one direction. However, this arrangement makes dc motor bulky and heavy. It also needs regular maintenance which makes them expensive. Life time of dc motors also becomes less due to commutation .

AC motors on the other hand doesn’t requires inversion and conversion due to which commutation is not required in these motors. This factor makes them less expensive. They also requires less maintenance. Life time period of AC motors are more than that of dc motors.

AC VS DC Motor Torque :

DC motor produces more torque as compared to AC motor. Starting torque of dc motor is almost three times then that of AC motors.

DC VS AC Motor Efficiency :

DC motors have more efficiency as compared to AC motors with respect to the power losses. In case of dc motor, only resistive losses takes place. On the other hand, AC motor has active as well as reactive component of current due to which impedance losses takes place in AC motor. It is to be noted that efficiency is not only the criteria to choose between ac vs dc motor. There are many reasons and criteria’s when choosing between ac and dc motor.

Conclusion :

Both AC and DC motor have different advantages which makes them suitable in different applications. If you require high torque or need to control the speed of motor, it is best to go for DC motors. AC motors are mostly used where constant speed is required and it is used for large (high voltages and current ) powered equipment’s.


That’s all. Hope this will helps you.

Related Posts :

Ideal Transformer and its characteristics ↗

Armature Reaction in Alternator ↗

Difference between synchronous Motor and Induction Motor ↗

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