by Joe Evans, Ph.D

In this particular case rotation would be clockwise. Reversing any two of the phase connections will change the phase peak relationships and cause the motor to rotate in the opposite direction. “Rolling” those connections (for example, moving 1 to 2, 2 to 3 and 3 to 1) will not change the phase relationships, and therefore, the direction of rotation will remain the same.

The Rotating Magnetic Field

We have seen how the voltage can peak in a three-phase circuit and how the stator poles are aligned to match voltage peaks, but why does the rotational magnetic field occur automatically? Figure 4 puts the linear flow of voltage peaks shown in Figure 2 and the pole locations shown in Figure 3 into a rotational perspective.

AC motor pole placement
Figure 3. AC motor pole placement

The stator images show the three sets of poles and their polarity from Points 1 through 7. The graph image shows the phase voltage peaks for the same points.

At Point 1, Phase 1 is at its positive peak and a maximum magnetic field is generated in Poles 1 and 1A. At Point 2, Phase 3 is at its negative peak and the maximum magnetic field is generated in Poles 3 and 3A. At Point 3, the maximum field has moved to Poles 2 and 2A.

If you study the other points you will see that this trend continues in a clockwise direction. As a result, the three phases create an automatic rotating field in the stator. If any two of the incoming phase leads are switched, the magnetic field will rotate in a counterclockwise direction.

The rotating magnetic field

Figure 4. The rotating magnetic field

As mentioned earlier, motor speed depends on both frequency and the number of poles. Motor speed will change in direct proportion to a change in frequency. For example, at 30 Hertz an 1,800 rpm motor will rotate at 900 rpm.

If an additional set of poles is added to each phase of the stator shown in Figure 3, its speed will also be decreased by 50 percent. The time required for one 360-degree rotation of the stator field is proportional to both frequency and the number of poles.

Three-phase motors can be designed to operate at two different speeds, and the speed relationship depends on the winding method employed.

Two-speed, single-winding motors use a stator that is wound for a single speed, but when the winding is connected in a different manner, the number of poles connected is also changed.

For example, in one connection,four poles are connected, but with the alternate connection, eight are connected.

With this winding method, a two to one speed relationship (1,800 rpm/900 rpm) will always exist. Usually, the brake horsepower (BHP) at the low speed will be one quarter that of full speed. However, constant torque designs will maintain one half BHP at the lower speed.

Two-speed, two-winding motors are actually two motors wound on a single stator.

Although these motors are typically larger and more expensive, they are not limited to the two to one speed relationship of single winding motors.

For example, a four- and six-pole, two-winding motor would produce speeds of 1,800 rpm and 1,200 rpm. In this example, the BHP at the low speed will be two thirds that of full speed. Next month, this column will investigate the operation of single phase motors.

Pumps & Systems, April 2011

Click the links below for the rest of the AC Motors series:

AC Motors: Magnetism and the DC Motor

AC Motors Part 3 - Single Phase Operation

AC Motors Part 4: Frame Size, Enclosures & Nameplate Data

AC Motors Part 5: AC Motor Life

AC Motor Torque