DC Motors vs. AC Motors: The Differences Explained

I will talk about the working principle and difference between DC motor and AC motor in the simplest and most understandable way.

DC motor physical model

The picture above is the simplest physical model of a DC motor.

Working principle

DC Motor

  • The DC power supply current flows along the positive pole of the power supply to the left side of the brush.

The brush and the commutator rub against each other.

The current flows through the left commutator (which has two left and right commutator segments) that flow into the coil and flow out from the right side of the coil.

After the right commutator and the right brush flow back to the negative pole of the power supply, a closed loop is formed.

  • Since the coil is in the magnetic field of the main magnetic pole (N and S in the figure), the coil is subjected to the electromagnetic force.

The two sides of the coil are different in direction of current (current on the left flows inward and outward on the right), so the two coil sides are subjected to electromagnetic forces of opposite magnitudes.

These two electromagnetic forces just form the electromagnetic torque.

With the pulling of the electromagnetic torque, the coil starts to rotate.

In the DC motor, the coil is embedded in the rotor slot and the motor starts to rotate.

  • The left and right reversing pieces rotate with the rotating shaft, and the brush is fixed.

After one turn, the coil on the right goes to the left and the coil on the left goes to the right.

However, due to the existence of the commutator, the current in the coil on the left side flows in the same direction as the current in the coil on the left side, so the direction of the electromagnetic force received is the same, and the right side is also the same.

Therefore, from the perspective of space, the direction of the electromagnetic force received at the coil side of the same position is constant, which ensures the cyclic rotation of the motor.

  • But for a coil, because the magnetic field is different when the coil is turned to different positions, the electromagnetic force of the coil is also changing.

Therefore, the coil turns unstable and suddenly slows down.

Therefore, it is possible to ensure uniform and stable coil force by installing a plurality of coils.

Then the following one is what we get:

DC motor physical model

Even such a motor model.

DC motor physical model

Besides, the two outer magnetic poles are actually the electromagnets generated by the excitation coils.

There are permanent magnets in small motors, and electromagnets are used in slightly larger ones.

Magnetic pole and magnetic circuit of DC motor

The model is just a model, but the real motor rotor is like this.

real motor rotor

AC Motor

AC motors are divided into synchronous motors and asynchronous motors.

Synchronous motor is mainly used as the generator, and the asynchronous motor is mainly an electric motor.

I mainly talk about asynchronous motors.

Because the asynchronous motor has a simple structure, low price, convenient maintenance and reliable operation, it has been widely used.

Although the AC motor has a simple structure, the working principle is actually more complicated than that of the DC motor.

If you want to understand clearly, it will be more difficult.

AC motor principle

A three-phase symmetrical alternating current is applied to the stator of the AC motor, as shown in the figure above.

The stator does not move, and only a change in current can produce a rotating synthetic magnetic field.

This magnetic field acts like a magnet that rotates around the stator.

With this rotating magnet, everything is easy to handle.

A closed coil is placed inside the stator, and an electromotive force and current are induced in the closed coil, and an electromagnetic force is generated.

The closed coil will turn.

It can also be understood in this way.

There is a rotating magnet on the stator. The rotor closing coil is actually an electromagnet due to induction charging.

When the outer electromagnet is turning, it will turn with the electromagnet inside, and the rotor of the AC motor will turn.

The rotational speed of the stator magnetic field is called the synchronous speed.

The inner rotor is actually driven by the stator magnetic field, so its speed will be slower than the stator magnetic field, so it is called asynchronous speed.

So there is the name of the asynchronous motor.

rotor of an AC motor

The rotor of an AC motor is such a simple closed coil, or a closed conductor.

Like a rat cage, it is also called a rat-cage asynchronous motor.

In addition, since the electromotive force and current inside the rotor are induced by the stator magnetic field, the asynchronous motor is called an induction motor.

Therefore, the three-phase asynchronous motor has many different names: AC motor, asynchronous motor, induction motor, are all saying it.

It is the name given to it from a different angle.

If you want to know more, you can ask questions in the comments, I will try my best to answer them in detail.

4 thoughts on “DC Motors vs. AC Motors: The Differences Explained”

    1. Arnie Kwiatkowski

      I believe that you can reverse an AC 3 phase motor by just changing where the input wires land in order to reverse the direction of the rotating magnetic field. I do not think you can change the direction of a DC motor.

      1. Yes, you can reverse the direction of a d.c. motor. If it has permanent magnets for the field, simply reverse the d.c. connections and the motor will reverse. If it is a shunt or series wound machine, you will need to reverse the connections to either the armature or the field windings, not both. On some d.c. motors the connections are not brought out separately, so it will be more difficult to do this.

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