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Two wires control brushless servo motor

Circuitry combines brush motor cable advantages with high speed/torque-to-weight and low temperature/EMI of brushless motors

By David J. Bak, Senior Technical Editor -- Design News, March 3, 1997

Ronkonkoma, NY--Applying DC brushless motors to servo applications offers several benefits over brush-type designs: high speed, long life, and larger power-to-size or power-to-weight ratios, for example.

Additionally, because power transistors replace electromechanical commutation brushes, brushless DC motors reduce EMI and operating temperatures. There's only one drawback: complex and bulky cabling between the motor and amplifier.

Solution? True electronic implementation of mechanical commutation. A new system, developed by MFM Technology Inc., features a two-wire connection to an H-drive amplifier. This design, through a patented circuit module that mounts on the motor, achieves full four-quadrant system control.

A permanent magnet rotor, Y-connected stator coils, and three rotor position sensors make up the polyphase dc brushless motor. The circuit module encompasses a signal-responsive stator-coil switch circuit, commutation logic, direction-polarity control circuit, and an auxiliary dc power supply.

Power for the auxiliary dc power supply, which is used to supply commutation/direction control logic, is derived from the output voltage of the PWM H drive amplifier. This voltage is present regardless of the power level or polarity of the amplifier output.

Such a system allows full four-quadrant operation, maintaining complete torque control through zero speed without any dead-band. The design also permits operation of the brushless motor directly from simple dc power supplies or inexpensive SCR or Triac-type controllers.

Size of the two-wire "Brushless Plus" motor is virtually the same as a comparable brush motor, allowing direct replacement of existing brush-based assemblies.

Additional details…Contact MFM Technology, Inc., 200 Thirteenth Ave., Ronkonkoma, NY 11779, (516) 467-5151.

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