Switches put a new spin on motors

Sept. 5, 2002
Recent advances in motor technology can be found in areas such as automation, power/driver integration, and rare-earth magnets.

By Doug Toman
Vice President
DynaMotors Inc.
Cleveland, Ohio

Edited by Miles Budimir

The DynaMotor can run up to 20,000 rpm making it suitable for high-speed fans, furnace blowers, aircraft accessories, and electric-vehicle propulsion.

In recent years, improvements in motor and drive designs have generally been incremental. That has changed with recent development by DynaMotors Inc., Cleveland, of a totally integrated motor/drive that provides adjustable speed and high torque over a wide speed range even from a single-phase source. It is brushless, inexpensive, and produces no high-frequency electromagnetic interference.

The primary proprietary feature of the DynaMotor is a solid-state switch inserted into each coil of the motor's wound rotor. When the switch is open, there is no current or torque. Closing the switch lets current flow, which produces flux and torque. The flux from the salient-pole ac-stator field induces a voltage in the armature conductors. Each switch is turned on by an associated photodetector which is illuminated as it rotates past stationary LEDs mounted on the motor end-bell. Selectively turning one or more of the LEDs on or off controls the current, torque, speed, and direction of the motor.

Changing the timing and duration of switch actuation controls the magnitude and polarity of the torque. Because there are multiple coils on the armature, and the coils can be controlled individually and energized simultaneously, it is possible to maintain continuous positive or negative torque. In practice, the switches turn off when the current nears zero to eliminate transient switching losses.

An internal speed sensor provides feedback so the motor becomes a selfcontained, closed-loop drive system. A potentiometer or keypad manually controls the motor, or a transducer or serial link can provide automatic control.

The motor combines features of ac and dc motors but also has unique characteristics. Its salient-pole stator is similar to a dc machine and does not create a rotating field, as used in other induction motors. However, as in an ac motor, the stator field induces a rotor current. But because it doesn't follow a rotating field, speed is not related to line frequency. As a result, the motor's speed-torque curve is similar to that of a dc motor: high torque at low speed and maximum speeds higher than 3,600 rpm.

It can be configured to operate up to 20,000 rpm, and has good speed regulation without encoder feedback. A unique feature is that rotation of any horsepower motor can be reversed without contactors or additional power semiconductors merely by changing the sector in which the switches are closed. Also, having only a single stage of power conversion (both ac and dc drives need two) and no power capacitors reduces power components and losses.

The motor uses standard parts with normal tolerances and off-the-shelf electronic components. Having fewer electronic parts typically increases reliability and reduces maintenance. The motor runs directly from a single or three-phase ac line, which eliminates the external drive-control cabinet as well as the interconnecting power and feedbacksignal wiring. The switches operate at low frequency and all semiconductor switches are on the armature so the motor stator acts as an effective filter cutting power-line noise and EMI, eliminating the need for external filters and chokes. The motor's line current is sinusoidal, which eliminates the problems associated with discontinuous line current generated by other types of drives.

The DynaMotor can run efficiently from 400-Hz aircraft power, where the motor's high speed gives it a favorable power-to-weight ratio. It can also operate directly from a varying-speed alternator in hybrid vehicles, eliminating the need for variable-ratio transmissions. For battery-only electric vehicles, two or more motors can operate from a single square-wave inverter with each motor running at different speed/torque points.

While development efforts have thus far concentrated on single-phase motors, DMI plans to build larger three-phase units. An additional goal is a regenerative motor/drive without additional power devices.

Stationary infrared LEDs mount on the motor end-bell. Rotating photodetectors and solidstate switches mount on the shaft outside the main motor housing to isolate them from heat and dirt and for easy maintenance. Armature coil conductors connect to the switches via slots in the shaft.

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