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Motors & controllers

Nov 30, 1995 12:00 PM

Included in this category are all types of motors and starters; auxiliaries such as brakes and clutches; monitors for high temperature and over-load and grouped motor controllers such as MCCs and starter racks.

Motors have evolved into highly efficient units capable of providing a variety of functions, depending on the application and its requirement. Their corresponding controllers have also evolved into sophisticated devices, some with solid-state construction, some in a reduced physical size but still capable of handling a large motor. As components of HVAC, HACR, and process equipment, their importance cannot be minimized.

Electric motors

Available in a wide variety of types, designs, constructions, and configurations, motors can be chosen to meet any specific application, requirement, and environment. For example, squirrel-cage induction motors are widely used because of their simple design and relatively low cost. Modern, high-efficiency designs have become more common while offering increased reliability as well. Wound-rotor motors are often used for high starting torque and low starting current applications, such as in cranes, hoists, and reciprocating machines. External variable-resistance banks permit some range of control. Synchronous motors are applicable to most constant-speed loads, such as compressors, and have the ability to improve the power factor of a system when connected. Ease of speed control using DC motors, along with the simplicity of the associated drives, make them useful for both constant- and variable-torque loads. Other specialized motors and their controllers include steppers, linear motors, and pancake types.

NEMA designations for induction motors

The National Electrical Manufacturers Association (NEMA) has designated "design types" that determine the electrical characteristics of induction motors. Design B motors have a high starting torque for accelerating high-inertia loads and can handle short-duration overloads. Design A motors have the same starting torque as Design B motors, but develop a higher breakdown torque. The Design C motors has the low starting current and high starting torque suitable for loads requiring rapid acceleration. Design D motors are best suited for high-inertia loads such as punch presses, cranes, and elevators.

Motor speeds and applications

Constant speed motors operate at a practically uniform speed during normal operation. Multispeed motors are available for loads that can be operated at two or more speeds. These motors can be of the single-winding type with a 2:1 speed ratio, the 2-winding type with two independent speeds, or other variations. PWM motors allow the speed ratios to be tailored to the load requirements. Multispeed motors are available as variable torque types for fans and centrifugal pumps, constant torque for conveyors and positive-displacement pumps, or constant horsepower for winches and machine tools.

Starters and starting methods

Single-phase motors, usually in fractional horsepower sizes, are typically operated with manual starters, which provide an ON/OFF means, as well as overload protection. Where overload protection is provided as integral with the motor, only a starting switch is needed. Short-circuit protection is provided by the branch circuit protective device.

Full-voltage starting of induction motors is used unless there are limitations on inrush currents, or if the load is of the high-breakaway type.

Reduced-voltage starting provides a cushioned start that lowers the starting current and stabilizes line voltage. There are several types of reduced-voltage starters: part-winding, reactor or resistor, autotransformer, wye-delta, and more recently, solid-state types.

Solid-state starters are now widely accepted because of their performance and reliability. Not only do they keep starting current to a minimum under close control (called ramping), but they also offer excellent control of motor torque. This means that the motorcar be started without "jolting" (termed "soft-start"),or it can also be stopped smoothly. Other features include reversing, phase-loss protection, and energy-savings options.

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