AC Motor Working Principle

Electric motors are essential devices that convert electrical energy into shaft rotation. These motors come in several types and classifications, but the AC motor remains one of the most commonly used types across several industries today.

Alternating current (AC) motors operate by converting alternating current into mechanical power based on the principles of magnetism. As a result, they offer several advantages over Direct current (DC) motors, such as increased power output and higher efficiency. But before delving into the basic construction and working principles of AC motors, it is essential to understand how alternating current differs from direct current.

Figure 1: AC motors operate by converting alternating current into mechanical power based on the principles of magnetism

Source: [teptong/ Adobe Stock]

Figure 2 shows a typical AC voltage profile.  Alternating current describes the flow of charge that changes direction periodically, causing the voltage level to reverse along with the current. For instance, the voltage typically starts from zero, grows to a maximum, decreases to zero and then to a maximum (in the opposite direction) before it returns to 0. AC motors require this kind of voltage to operate.

Figure 2: AC voltage profile

Source: Temitayo Oketola

In contrast, the current flow in DC is unidirectional, as shown in Figure 3. DC voltage is far more consistent in voltage delivery than AC since the magnitude of voltage remains constant as time progresses. DC motors require DC voltage to operate and are commonly used in applications where high starting torque is important.

Figure 3: DC voltage

Source:FatmirJashari/ CC[SA][3.0]

There are several versions of the AC motor, but most AC motors comprise the following essential parts:

• Stator 
• Rotor 

Figure 4: The basic construction of an AC motor

Source: www.researchgate.com

The stator is the stationary part of the motor, and it consists of coils covering its entire periphery. In contrast, the rotor is the rotating part of the motor made of a permanent magnet or laminated core and attached to the motor shaft.

When AC electricity is supplied to the stator windings, it energizes these electromagnets to generate an alternating flux (or rotating magnetic field). According to Faraday's law of induction, this rotating magnetic field generates an induced current that opposes the magnetic field in the rotor, causing it to rotate.

Consider a single-pole AC motor illustration shown in Figure 5. In this scenario, the rotor is a permanent magnet that moves freely between two electromagnetic stator poles (Poles A and B). These stator poles connect directly to an AC source through the stator windings to operate the AC motor.

Figure 5: Single-pole AC motor illustration

Source: Temitayo Oketola

Now, consider the positive half cycle of the AC waveform (the interval in which the magnitude of the AC voltage increases from 0, attains its peak and returns to 0). In this case, the stator poles form magnetic poles, with pole A forming the north pole (N) and B forming the south pole (S). Due to the laws of magnetism, pole A (North pole) attracts the rotor’s south pole while pole B (South pole) attracts the rotor’s north pole, causing the rotor to revolve by 180 degrees (see Figure 6).

Figure 6: Single-pole AC motor with the stator forming north and south poles

Source: Temitayo Oketola

During the AC supply’s negative half cycle, the polarity of the stator poles becomes reversed. As a result, pole A now attains the south polarity, while pole B attains the north polarity. Therefore, the like poles of the stator and rotor face each other (see Figure 7). 

Figure 7: Single-pole AC motor with reversed north and south poles

Source: Temitayo Oketola

This causes the repulsive magnetic force to deflect the rotor from its position, causing the rotor to align to a new position, as shown in Figure 8.

Figure 8: AC motor’s rotor aligning to a new position

Source: Temitayo Oketola

The stator's magnetic field changes continuously, and the rotor revolves as the AC supply waveform completes its cycle. This phenomenon is called the rotating magnetic in AC motors.

AC motors are broadly classified as:

1. Synchronous motors 
2. Induction motors

Synchronous motors convert AC electrical power into mechanical power while operating at synchronous speeds. This means that they rotate at the same rate as the frequency of the supply current. Synchronous motors have fixed speeds, making them ideal in applications requiring constant and precise speed, like robot actuators and watches.

In contrast, induction motors (also called asynchronous motors) always run slower than synchronous speeds. They have a simple construction and offer high starting torque and efficiency. Engineers commonly used them to run pumps, compressors and drilling machines.

Learn more about the types of AC motors on GlobalSpec.

AC motors can meet the requirements of several applications as long as they are correctly specified. Therefore, engineers are advised to reach out to AC motor suppliers to discuss their application needs.