Permanent Magnet Motor Construction



PM Motors Figure 2 and 3

Figure 2 depicts a permanent magnet motor, or “PM” motor. The rotor contains a permanent magnet, giving PM motors their name. Permanent magnet step motors work on the same principles as hybrids but use a slightly different geometry. PM rotors are radially magnetized, north and south poles alternating along the circumference of the rotor. A pole pitch is the angle between two poles of the same polarity, north to north or south to south. Both the rotor and the stator assemblies of PM motors are smooth.

The stator sections of the A phase and the B phase are mechanically offset by one quarter pole pitch. Each phase’s stator section has projections offset by one half of a pole pitch. As current is passed through the windings a rotating magnetic field is established. The rotor of the permanent magnet motor will move in synchronism with this rotating field. See Fig. 3.

What Are Permanent Magnets?

To understand why PM motors can be beneficial for a variety of motor needs, it may be helpful to know what permanent magnets are. They are magnets that maintain their own persistent magnetic fields. Rare earth magnets, powerful magnets made from rare earth metals, are the type typically used for this purpose. Rare earth magnets are not particularly rare; they just happen to come from the class of metals known as rare earth metals. There are other metals that only become magnetic when magnetized by an electric field and only stay magnetized as long as that electric field is in place. This concept is at the heart of how PM motors work. In PM motors, a wire winding serves as an electromagnet when electricity passes through it. The electromagnetic coil is attracted to the permanent magnet, and this attraction is what causes the motor to rotate. When the source of electrical power is removed, the wire loses its magnetic qualities and the motor stops. In this way, the rotation and motion of PM motors can be managed by a motor driver that controls when and for how long electricity and, by extension, the electromagnet, allows for rotation of the motor.