Servo - November 2008

THE UNIVERSAL

MOTOR

by Fred Eady

Electric motors come in a seemingly endless variety of shapes and sizes. If you’re into robots and mechanical devices that move about freely, DC (Direct Current) motors capable of operating on battery power are almost always your most practical motor choice. However, not every robot created by man or alien is a fully mobile Robby running around on forbidden planets. If your robot is a stationary collection of nuts and volts that’s at home working next to a wall outlet, you may be able to use the advantages of an AC (Alternating Current) power source to drive your mechanical animal’s motors.

It is often desirable to be able to control a motor’s speed and direction. I’ll bet that most of you have experience with controlling the speed of a standard brushed DC motor. The first brushed DC motor speed control circuit that comes to my mind is shown in Schematic 1A and consists of a PIC microcontroller squirting a PWM (Pulse Width Modulation) signal into the gate of a MOSFET whose job it is to switch

current to a brushed DC motor. Adding a trio of MOSFETs in Schematic 1B forms an H-bridge configuration that allows us to change both the speed and the direction of the brushed DC motor with a few bits of PIC I/O.

Can you conjure up a similar circuit in your mind’s eye for a simple AC motor speed control? I see a PIC microcontroller punching an optoisolated DIAC (short for the words DIode AC Switch) triggering a TRIAC, which is controlling the flow of AC voltage to an AC motor (Schematic 1C). The problem with the DIAC/TRIAC motor circuit is that the DIAC and a series resistor have all of the “control” and that control is very limited. Another problem with the AC motor control circuit I’ve envisioned lies in the need to provide a separate DC power supply for the PIC microcontroller. What

VDD

DRIVE A+

DRIVE B+

BRUSHED DC MOTOR

Schematic 1A. The duty cycle of the PWM signal that is applied to the gate of the MOSFET determines the speed of the motor. In this case, the duty cycle percentage (0% to 100%) is directly proportional to the speed of the motor shaft.

PWM IN

DRIVE B-

DRIVE A-

SCHEMATIC 1A V+

SCHEMA TIC 1B

Schematic 1B. By selectively energizing diagonally opposing MOSFETs, this H-bridge configuration allows for both speed and direction control of the DC motor. The PWM signal is applied to one of the diagonally opposing MOSFETs while the other associated diagonally opposing MOSFET is held in an energized state.

FROM MICRO

MT2

MT1 _~

AC MOTOR

Schematic 1C. This is about as simple as it gets for AC motor control. Fact is, we don’t have much “control” here as the DIAC break over voltage characteristics and the value of the resistor between MT2 and the DIAC determine the TRIAC’s triggering point.