Rotating Magnetic Fields & AC Motor Operation

To understand AC motor operation, it’s important to look into the development of rotating magnetic fields. These magnetic fields follow the fundamentals of electromagnetism to rotate the shaft of an AC motor.

Let’s take a closer look at an electric motor stator. Remember that the construction of an AC motor stator is a hollow cylinder filled with coils of insulated wire.

STATOR COIL ARRANGEMENT

Use the diagram below to view the interaction between the stator coils. In this example, there are 6 coils (2 coils per 3 phases). Known as “motor windings,” these coils operate in pairs and are wrapped around the iron core material that makes up the stator.

The motor windings each become a separate electromagnet. The coil pairs feature opposite polarities (one north pole, one south pole) due to how they’re wound. In the diagram, suppose that coil A1 is a north pole and its coil pair A2 is a south pole. When the electric current changes direction the polarity of the poles will switch.

POWER SUPPLY

In the following diagram, the motor stator is attached to a 3-phase AC power supply. Motor windings A1 and A2 are connected to Phase A of the power supply. Imagine also that windings B and C are respectively connected to power supply phases B and C.

Motor windings are usually separated by 120º. The number of times a motor winding appears determines the number of poles. This example shows a second set of 3-phase windings. Each winding appears 2 times, which makes this a 2-pole stator. However, if each winding appeared 4 times it would be a 4-pole stator.

Electric current flows through the windings when AC voltage is applied to the stator. The direction of the current flow that runs through a motor winding determines how the magnetic field develops. Use the chart below as a reference for the next few diagrams. They will show how a rotating magnetic field is developed. According to the chart, suppose that a positive electric current flow in the A1, B1, and C1 motor windings create a north pole.

START CURRENT FLOW

To make visualizing a magnetic field easier, the diagram below shows a start time when no current is flowing through one of the windings. Observe the starting line:

• Phase A has no current flow
• Phase B has a negative direction (-) current flow
• Phase C has a positive direction (+) current flow

According to the chart above, B2 and C1 are north poles while B1 and C2 are south poles. Magnetic lines of flux depart from the B2 north pole and arrive at C2, the nearest south pole. Lines of flux also depart from the C1 north pole and arrive at B1, its nearest south pole. As a result, a magnetic field is created (as shown by the arrow).

TIME 1

From the starting point, let’s monitor the magnetic field in 60º segments. When the field rotates 60º at Time 1:

• Phase C has no current flow
• Phase A has a positive direction (+) current flow
• Phase B has a negative direction (-) current flow

Now windings A1 and B2 are north poles and windings A2 and B1 are south poles.

TIME 2

At Time 2, the magnetic field rotates another 60º:

• Phase B now has no current flow
• Phase A maintains a positive direction (+) current flow (although it’s decreasing)
• Phase C now has a negative direction (-) current flow

Because current flow has changed directions in Phase C windings (started in a positive direction, but switched to negative direction by Time 2), the magnetic poles have reversed polarity (C1 north pole and C2 south pole became C1 south pole and C2 north pole).

360º ROTATION

After six 60º time segments, the magnetic field will have rotated one full revolution of 360º. Using a 60 Hz power supply, this process will repeat 60 times per second.

SYNCHRONOUS SPEED

Speed is important to the rotating magnetic field of an AC motor. It is known as “synchronous speed.” This speed is calculated by dividing 120 times the frequency (F) by the number of poles (P). As an example, the synchronous speed for a 2-pole motor operated at 60 Hz is 3,600 RPM.

As the number of poles increase, synchronous speed decreases. The chart below illustrates how an increasing number of poles equates to a decreasing amount of synchronous speed at 60 Hz.