How does an ac motor work?
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How does an ac motor work?

Views:0     Author:Site Editor     Publish Time: 2021-09-17      Origin:Site

An Ac motor is a motor driven by alternating current (AC). Ac motors are widely used in industry mainly because of their high efficiency and the ability to generate constant torque up to the rated speed. The two most widely used types of Ac motors are induction motors and synchronous motors. We will briefly describe how they work.


(1) How do Ac motors work?

(2) How is the Ac motor controlled?

(3) Difference between Ac motor and DC motor


(1) How do Ac motors work?

The two basic components of an Ac motor are the stator (fixed outer drum) and the rotor. The rotating interior of the motor, which is connected to (and drives) the motor shaft. Both the stator and the rotor generate a rotating magnetic field. In the windings of the stator, the rotating magnetic field is inherently provided by the sinusoidal characteristics of alternating current. In the rotor, the magnetic field is generated by permanent magnets, reluctance salient poles or other electrical windings.

Because the rotor of a synchronous motor has permanent magnets or electromagnets, they will generate a rotating electromagnetic field, so they operate synchronously with the frequency of the supply current in a locked state.

In an induction motor, the magnetic field in the rotor winding is "induced" by the magnetic field of the stator. In order for this induction to generate torque, the speed of the rotor magnetic field must lag behind the speed of the stator magnetic field. This speed difference is called "slip", which is why induction motors have a "nameplate RPM" rating, which is about 5% smaller than its synchronous speed. For example, Ironhorse model MTRP-001-3DB18 (1 horsepower, three-phase, four-pole, AC induction motor) has a synchronous speed rating of 1800 RPM (assuming a power of 60hz), but the "nameplate RPM" rating is 1760. When using the American standard 60 Hz three-phase power "across the line" power supply, the shaft will rotate at 1760 RPM.


(2) How is the Ac motor controlled?

When simple on/off control is required, contactors or manual motor starters are usually used. Contactors (large three-phase relays) allow PLCs or other controllers to switch power to Ac motors. Reverse motor starters are a dedicated version with two connected contactors, so they also allow the direction of rotation of the motor shaft to be reversed. The manual motor starter includes a manual knob that allows the operator to switch power. All of these types are called "across the wire" control-the motor is directly connected to the input power "line" (via a contactor or motor starter).


(3) Difference between Ac motor and DC motor

Historically, industrial DC motors have always been brushing types. Compared with Ac motors, DC motors with brushes and commutators have many disadvantages: increased maintenance (brush replacement), limited speed range and shorter overall life. AC induction motors have no brushes and have a longer service life.

The speed of the DC motor is controlled by changing the armature current, while the speed control of the Ac motor is achieved by changing the frequency of the alternating current, usually using a variable frequency drive (VFD).

In the past few decades, the advent of brushless DC motors has begun, mainly due to the appearance of semiconductor control circuits required to operate them and the appearance of high-quality permanent magnets. Brushless DC motors do not require brushes or physical commutators, so they have a longer service life. They also overcome the speed limitation of the brushed version.


If you want to specify a motor for a new application, first determine the voltage, speed and horsepower required, and the type of application. If you want to replace a motor of the right size in an existing application, you can find all the necessary information on the motor nameplate of the existing motor. If you have any needs, you can contact us.

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