CEMP-E
EI 11C201
1 March 1997
where pump speed adjustments may be required, but variable speed operation or electronic
signal for use in on/off and variable speed control of the pumps. On/off controls are usually
provided by pressure or electronic switches. Variable speed control devices consist of (1)
available for many years as controllers for variable speed pumps. These older methods are
inefficient in that the slip losses which developed are lost as heat. The recent development of
solid state electronics has led to the introduction of newer methods of variable speed control
suitable for both squirrel-cage and wound-rotor induction motors. The variable voltage and
variable frequency controls are suitable for use with squirrel-cage rotors. Variable frequency
drives are possible in efficiencies up to 95 percent, and are available in sizes up to 250 hp.
However, variable voltage units are inefficient and are not recommended. Wound-rotor motor
controls come in five categories, (l) fixed step resistors, (2) liquid rheostats, (3) reactance/
resistance controllers, (4) electronic rheostats, and (5) regenerative secondary controls. Of
these, the regenerative secondary control offers the best efficiency, while the other units are
considerably less efficient and require more maintenance. In general, variable speed control
devices are more expensive, less efficient, and require a higher degree of maintenance than
constant speed controls.
6-7
Previous Page
