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ACTUATORS - AC MOTORS



note: I have never actually used an AC motor, so feel free to correct and verify this information

note: this page is a place holder until a better tutorial is written

Unlike DC motors which work using a single constant current, AC motors run under 3 phase current. To have 3 phase power on a robot, you either need a big bulky/expensive DC->AC converter, or you must tether it to a wall socket. You probably won't use AC motors unless your robot is stationary, such as a robot arm or robot pancake maker. Unless you want the pancake maker to also walk your dog or something . . . But here they are anyway:

Voltage

• Polarized (current cannot be reversed)
• Typically from 120-240V AC, usually to match mains power
• Higher voltages generally mean more torque, but also require more power
• Rarely used on mobile robots due to power requirements
• note: A universal motor has brushes like a DC motor, but will operate on AC or DC

Current

• When buying a motor, consider stall and operating current (max and minimum)
• Stall Current - The current a motor requires when powered but held so that it does not rotate
• Operating Current - The current draw when a motor experiences zero resistance torque
• It is best to determine current curves relating voltage, current, and required torque for optimization
• When a motor experiences a change in torque (such as motor reversal) expect short lived current spikes
• Current spikes can be up to 2x the stall current, and can fry control circuitry if unprotected
• Use diodes to prevent reverse current to your circuitry
• Check power ratings of your circuitry and use heat sinks if needed

Power (Root-Mean Squared Voltage x Current)

• Running motors close to stall current often, or reversing current often under high torque, can cause motors to melt
• Heat sink motors if not avoidable

Torque

• When buying a motor, consider stall and operating torque (max and minimum)
• Stall Torque - The torque a motor requires when powered but held so that it does not rotate
• Operating Torque - The torque a motor can apply when experiencing zero resistance torque

Velocity

• Motors run most efficient at the highest possible speeds
• Gearing a motor allows the motor to run fast, yet have a slower output speed with much higher torque
• Remember that torque determines acceleration, so a fast robot with poor acceleration is really a slow robot
• If uncertain, favor torque over velocity

Efficiency

• More efficient than DC motors
• Typically most efficient at rated voltage and frequency
• Use gearing (opt to buy motors with built-in gearing or gear heads)

Control Methods

• Modifying the AC frequency can alter speed and torque
• Encoder - device which counts rotations of wheel or motorshaft to determine velocity for a control feedback loop
• Tachometer - device which measures current draw of motor to control output torque

This circuit will allow you to control the speed of an AC motor.
The bridge rectifier produces DC voltage from the 120VAC line.
A portion on this current passes through the 10K ohm pot.
The circuit comprised of the 10k pot rated at 3W+, the two 100 ohm resistors and the 50uf capacitors delivers gate drive of the SCR.
The diode D1 protects the circuit from reverse voltage spikes.
The ratings of the bridge rectifier and the SCR should be 25 amps and PIV 600 volts.
The diode D1 should be rated for 2 amps with PIV of 600 volts.
The circuit can handle a load up to 10 amps. The SCR should be very well heat sinked.