Every sector of industry strives for improving efficiency and reliability in their equipment, so that they can better serve their customers. One technology set to redefine expectations is the permanent magnet AC motor, which is an increasingly attractive alternative to the currently ubiquitous induction or squirrel cage motor. Rick Munz assesses its advantages.
Permanent magnet motors are not completely new; high-performance servo motors are based on the same concept. But servo motors are expensive and what is new is the emerging class of PM or brushless AC motors.
To date induction motors have fulfilled the role of driving equipment such as pumps, fans, compressors, conveyors and production machinery; servo motors are used where the performance requirements justify their extra cost. PM motors bridge the gap between the two ends of the drive spectrum.
PM motors often use ferrite magnets glued onto the rotor and typically outperform standard induction motors by about 25% in terms of both energy efficiency and power density (power-to-volume ratio). If powerful neodymium super magnets are used, performance is 50%–100% better than an induction motor.
Permanent magnet synchronous motors (PMSMs) are synchronous with mains frequency, at any torque up to the motor’s operating limit. Three-phase PMSMs are permanently excited so are highly responsive to changing load demands. They also have a high overload capability, so are robust and need little maintenance.
PMSMs are fitted with an electronic controller, which uses current-switching to both activate the motor and to control its output torque. While the controller is an extra expense, it provides the servo-like control that induction motors lack.
The advantages of a PMSM over an induction motor can be summed up as: better efficiency, more precise speed control, higher power density and reduced operating temperature (resulting in longer bearing and insulation life).
PMSMs are ‘synchronous machines’, ie the rotor spins at exactly the same speed as the magnetic field produced by the stator windings, whereas an induction motor has typically 1-3% slip. This synchronicity aids dynamic performance and speed control.
Another advantage of PMSMs is that they typically have a wider speed range than induction motors. As a general rule, PMSMs are rated for 20:1 speed range without feedback or 2000:1 closed loop. However, the drive selection also plays a part in speed range, so each situation has to be considered individually.
PMSM motor losses are around 15–20% lower than induction motors. Depending upon size, electricity price and duty cycle, users can therefore expect to recover the extra cost of a PMSM motor in 3–15 months.
PMSMs are suitable for almost any application currently serviced by an induction motor, and many where a large (very expensive) servomotor is used. In situations where the primary concern is motor efficiency, centrifugally loaded variable speed applications like fans and pumps become very attractive indeed.
In many cases replacing an induction motor with a PMSM can make the need for extra power transmission equipment, such as belts, chains or gearboxes, redundant – driving up overall system efficiency and reducing initial purchase cost, commissioning time and maintenance commitment.
PMSMs are generally designed to be direct replacements for EC and NEMA induction motors, allowing users to easily switch to modern, high efficiency, low maintenance and reliable motors. With energy prices increasing, many plant operators are switching to PM Motors.
Rick Munz is with REGAL C&I.