Ohio Electric Motors

Brushless DC Motors: Low Maintenance and High ... - Electric Motors

The primary advantages of brushless D.C. motors are:

But brushless D.C. motors are not without their drawbacks. They are more expensive due to their complex control system, the use of rare earth magnets and the need for rotor position sensors. (Higher costs are overcome by a longer life and lower maintenance costs.) Hence, they are much smaller in size than a comparable brushed D.C. motor.

Brushless D.C. motors operate in conjunction with an electronic controller and a rotor position feedback sensor. Based upon the actual rotor position, the controller sequentially energizes or switches “on” the stator’s phase windings so that torque is continuously generated as the permanent magnet (PM) rotor rotates. This switching action is called electronic commutation. To sense the rotor’s angular position, position sensors are used. When the PM rotor passes one set of phase windings, a signal from the position sensor is sent to the controller which then sends a signal to switch “on” the next set of phase windings so that the magnetic fields of the rotor and the stator’s phase windings remain synchronized. The torque/speed characteristic of the motor is determined by the magnitude of the signal and the switching rate of the controller.

For example, in a 2-phase motor, when the phase 1 winding is energized, the PM rotor will rotate to align itself with magnetic field produced by the phase 1 winding. When the phase 1 winding is turned “off”, the phase 2 winding is turned “on” and the rotor will continue to rotate to align itself with the magnetic field of the phase 2 winding. This “on” and “off” switching of the phase windings will maintain torque of the PM rotor.

The number of windings and the type of switching sequence determines the phase winding utilization and the torque response of the motor. Fewer windings and fewer switching pulses give the motor less winding utilization, more inertia to overcome and a poorer torque response. The most common configuration of a brushless D.C. motor is “three phase six pulse.” in machine tools because they can be driven at very high speeds with fast acceleration, deceleration and reversing responses. The larger horsepower sizes have found application in electric vehicles.

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