Electrical Drives
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.
AC Controllers
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:
Six-step method:
- Can operate at higher frequencies.
- Produces less motor noise.
- Produces less stress on motor insulation.
PWM method:
- Produces less motor losses in some versions.
- Provides low-speed torque without cogging.
- Can provide stall torque.
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