T E M A ______
…bringing science to industry
PERMANENT MAGNET MOTOR EFFICIENCIES
Dr. Sc. Branimir Ružojčić, dipl. eng.
The permanent magnet motor as technically advanced and highly efficient transformer of electrical to mechanical energy
New permanent magnet (PM) materials and production technology are making it possible to offer a wide range of motors with horsepower suited to nearly every industrial application. With higher efficiency and flexible rotor design, PM motors are being specified for applications from micro electronics to computers, robotics, industrial automation, heavy industry, military, and marine – nearly everywhere induction motors are currently used.
When we compare synchronous PM and traditional induction motors over a range of shaft speeds, PM motors offer higher efficiency at all speeds. Here, efficiency describes the relationship between electrical energy input and mechanical energy output. Some energy will always be lost to electrical, magnetic, or mechanical forces, but synchronous PM motors typically operate at efficiency levels between 93% and 98%. Some designs even approach 99% efficiency (near their theoretical limit). Since the traditional induction motor typically operates in a range from 80% to 85% efficiency, PM motors offer cooler operating temperatures, smaller standard dimensions, and dramatically lower energy consumption.
Given an input power factor of 1.0, and controlled by an electronic drive, high efficiency is achieved using pure sinusoidal phase voltage. Rotor heat is negligible, and stator heat is easily dissipated.
PM motor efficiency unsurpassed at any shaft speed
Induction motors transfer energy for rotor excitation from the stator winding to the rotor cage, producing a magnetic field and generating torque as the magnetic fields of the rotor and stator merge. The transfer of energy to the rotor generates heat, resulting in degradation of the motor’s efficiency. Both asynchronous and synchronous induction motors are subject to this form of energy loss.
PM motors do not require the transfer of excitation energy from the stator to the rotor. Neither do they require a separate energy source to produce a magnetic field in the rotor like an induction motor. The rotor’s permanent magnetic field generates torque as the magnetic fields of the rotor and stator merge, but without the accompanying electromagnetic losses inherent in an induction motor.
In a PM motor, the motor rated power is dedicated to the maximum motor load, but not only at full power. The PM motor’s sophisticated drive keeps efficiency higher at all RPM’s through phase alignment and other carefully controlled parameters and related motor design characteristics. In fact, at very low RPM’s, where conventional motors are especially inefficient, PM motors demonstrate an even greater efficiency improvement.