Introduction and Overview
From Induction to Permanent Magnets
For decades, alternating current induction motors (ACIM) have provided the driving force for commercial and industrial applications. These asynchronous motors feature simple and robust designs, require little maintenance, function reliably in a range of uses, require very simple electric starters/contactors to operate, and are inexpensive to manufacture. Asynchronous rotors are cast; the magnetic field necessary to oppose the electromagnetic field generated in the stator winding is induced in the rotor. No permanent magnets are used. But ACIM have limitations: Single phase ACIM have low starting torque, are relatively inefficient under lighter loads, and are acceleration-limited due to inherently high rotor inertia.
As demand increased for higher motor acceleration, power density, and overall efficiency, motor manufacturers have turned to synchronous motor designs. Permanent magnet synchronous motors (PMSM) incorporate permanent magnets to supply the necessary opposing magnetic field. This significantly increases torque density and efficiency, particularly during acceleration, by eliminating the extra current required to induce the magnetic field.
Initially, PMSMs were configured with permanent magnets adhered to the inside of the motor housing and electromagnetic windings fixed to the rotor. This configuration, commonly referred to as a brushed DC motor, uses carbon brushes and a copper commutator to feed electrical current to the rotor windings. Brushed DC motors share the ACIM benefits of simple and robust designs, simple electric starters/contactors for operation, and low manufacturing costs. They also share the drawback of limited acceleration due to high rotor inertia and have higher maintenance requirements due to constant wear on the carbon brushes during operation.
Continuing customer demand for higher acceleration, as well as for point-to-point positioning and more accurate position control, spawned the development of brushless DC (BLDC) motors. An inversion of the brushed DC configuration, BLDC motors feature permanent magnets mounted to the rotor and electromagnetic windings fixed to the inside of the motor housing. Locating the permanent magnets on the rotor significantly reduces rotor inertia, enabling higher acceleration. The downside to this approach is the need for a rotor position feedback device and complex external electronics to track rotor position and manipulate current in the stator windings to produce the desired motion output. Higher performance came with higher overall system cost and complexity, and concern over magnet retention—especially at high rotational speeds.
- Properties of Magnetic Materials
- Surface and Interior Mounting
- Evolution of Design
- Magnet Retention Slots
- Magnet Retention Wedges
- Magnet Retention Sleeves