AC Controllers

Nov. 15, 2002
One operation of all motors, whether powered by ac or dc, calls for a controller.

Controller manufacturers are improving features such as motor protection and brushless dc commutation. One operation of all motors, whether powered by ac or dc, calls for a controller.

The simplest of controllers, called starters, merely connect and disconnect a motor to a power source. But controllers also protect the motor from overload and prevent excessive branch-circuit current. In some cases they protect the equipment they power as well as the operators. More sophisticated controllers, sometimes called drives, regulate motor torque, speed, or horsepower in response to remote commands.
Adjustable-speed ac drives offer advantages over dc drives because of the simplicity, high-speed capability, and low-maintenance requirements of squirrel-cage motors. Squirrel-cage motors can also adapt to adverse conditions, such as dirty air, explosive atmospheres, and inaccessible locations.

Ac motor drives: The ac induction motor is sometimes considered a constant-speed motor, and it is when connected to a 60-Hz power source. However, its speed can be adjusted if power is supplied by an adjustable-frequency drive. Speed is also adjustable by means of eddy-current drives. The primary elements of an adjustable-frequency drive are a rectifier and an inverter that convert 60-Hz power to adjustable-frequency ac. Two ways to provide this conversion are the six-step and PWM methods.

Two conversion methods are hard to compare because there are many variations. However, the attributes include:

The six-step method:

• Can operate at higher frequencies.
• Produces less motor noise.
• Produces less stress on motor insulation.

The pulse-width modulaiton (PWM) method:

• Produces less motor losses in some versions.
• Provides low-speed torque without cogging.
• Can provide stall torque.

Eddy-current Drives: The primary elements of an eddy-current drive are an ac motor, an eddy-current clutch, a tachometer, and a solid-state regulator. The eddy-current clutch consists of a drum driven at constant speed by the ac motor and, concentric with the drum, a rotor for driving a load.

Torque is transmitted from the drum to the rotor through an adjustable magnetic field that is established in an air gap between the two members. The magnetic field is produced by a coil on the rotor. Power for the coil, which is equivalent to about 2% of the drive rating, is provided by the regulator. The regulator adjusts coil excitation to a level where output speed as indicated by a tachometer is equal to that set on a speed-reference potentiometer.

Eddy-current drives typically have a 30-to-1 speed range at constant torque, produce intermittent torque up to 200% of rated, provide 0.5% speed regulation from no load to full load, and when stalled, deliver up to full-load torque without pulsations.

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