Motor Controls
Introduction
Since most heavy machinery is mechanically powered by electric motors, the system designer must be familiar with techniques for controlling electric motors. Motor controls cover a broad range from simple on-off motor starters to sophisticated phase angle controlled dc motor controls and variable frequency ac motor drive systems. In this chapter we will investigate some of the more common and popular ways of controlling motors. Since most heavy machinery requires large horsepower ac motors, and since large single phase motors are not economical, our coverage of ac motor controls will be restricted to 3-phase systems only.
AC Motor Starter
In its simplest form, a motor starter performs two basic functions. First, it allows the machine control circuitry (which is low voltage, either dc or single phase ac) to control a high current, high voltage, multi-phase motor. This isolates the dangerous high voltage portions of the machine circuits from the safer low voltage control circuits. Second, it prevents the motor from automatically starting (or resuming) when power is applied to the machine, even if power is removed for a very short interval. Motor starters are commercially available devices. As a minimum, they include a relay (in this case it is called a contactor) with three heavy-duty N/O main contacts to control the motor, one light duty N/O auxiliary contact that is used in the control circuitry, and one light duty N/C overload contact which opens if a current overload condition occurs.