The Permanent Magnet in Motors – Construction and Operating Theory

Permanent magnets are a critical element to virtually any modern step motor. Read on to learn how a magnet, properly used, can convert electrical power to physical power and how such a magnet, whether in a PM motor or a hybrid motor, makes a step motor highly efficient and effective.

Hybrid Motor

Figure 1a depicts a 1.8° hybrid step motor. The rotor contains a permanent magnet similar to those found in permanent magnet step motors. Hybrid rotors are axially magnetized, one end polarized north and the other polarized south. Both the rotor and the stator assemblies of hybrid motors have tooth-like projections. These "teeth" align in various configurations during rotation.

The permanent magnet in a hybrid motor

To understand the rotor’s interaction with the stator, examine the construction of a 1.8° (the most common resolution) hybrid step motor. First, the rotor is composed of two 50-tooth rotor cups enclosing a permanent magnet. The two cups are oriented so that the teeth of the top cup are offset to the teeth of the bottom cup by 3.6°. Second, the stator has a two-phase construction. The winding coils, 90° apart from one another, make up each phase. Each phase is wound so that the poles 180° apart are the same polarity, while the poles 90° apart are the opposite polarity. When the current in a phase is reversed, so is the polarity, meaning that any winding coil can be either a north pole or a south pole.

As shown in fig. 1b below, when phase A is energized, the windings at 12 o’clock and 6 o’clock are north poles and the windings at 3 o’clock and 9 o’clock are south poles. The windings at 12 and 6 would attract the teeth of the magnetically south end of the rotor, and windings at 3 and 9 would attract the teeth of the magnetically north end of the rotor. The desired direction of travel determines the next set of poles to be energized.

Hybrid motor winding

The driver controls this phase sequencing. Because there are 50 teeth on the rotor, the pitch between teeth is 7.2°. As the motor moves, some rotor teeth are in alignment with the stator teeth. The other rotor teeth are out of alignment with the stator teeth by 3/4, 1/2 or 1/4 of a tooth pitch. When the motor takes a step, it will move to the next closest position where the rotor and stator teeth are aligned. The rotor will move 1/4 of 7.2°. The motor will move 1.8° with each step.

The motor will move 1.8° with each step.

Permanent Magnet Motor

Figure 2 depicts a permanent magnet type motor, or “PM” motor. The rotor contains a permanent magnet, giving PM type motors their name. Permanent magnet step motors work on the same principles as hybrids but use a slightly different geometry.

The main distinction between a permanent (PM) magnet motor and a hybrid motor is the presence of the tooth-like projections that the hybrid uses to supplement the magnet action and help with rotation. The predecessor of the permanent magnet motor was the variable resistance motor, which did not use permanent magnets to turn the motor and used solely the teeth to achieve the rotating action of the motor. Variable resistance motors were largely replaced when it was realized that a magnet could do the work of the teeth. The realization that using both a magnet and teeth could yield even more accurate results, leading to the hybrid motor, a hybrid of permanent and variable type motors. All motors with permanent magnets have their advantages. The PM motor can be highly effective and is a good choice for people who like permanent magnets but don’t need the precision accuracy and expense of a hybrid.

Permanent magnet motor figures

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.