E O Potthoff J R Hendershot

In past years, the electric motor industry has been dominated by the use of essentially single and constant speed motors when line fed. There were many mechanical methods used to achieve adjustable speed using a single-speed prime mover. There was also widespread use of dc motors for adjustable speed over the last century because only the voltage must be adjusted for speed change. Although operation at speeds well below synchronous can be accomplished through adjustment of the secondary resistance in wound-rotor induction motors, the ubiquitous squirrel-cage ac induction motor requires the adjustment of the frequency to change speed. This was not so easily accomplished until the recent utilization of pulse width modulation (PWM) inverters with insulated gate bipolar transistors (IGBTs) for power transistors plus microprocessors.

Electric drives are available featuring a single rotating element and some associated control for performing the adjustable-speed driving function. The use of a single rotating element differentiates this type of drive from the eddy-current coupling motor, hydraulic coupling motor combinations, and adjustable-speed belt drives, all of which are tandem drives.

The speed of the single-unit adjustable-speed drive is controlled through the interaction of the control and the motor. For this reason, one must consider these two elements as a drive and not consider the motor alone.

Subsection 6.1.1 described motors that will be further discussed in this subsection. Previous discussions of control were limited to that required for starting and protection; this subsection will also describe specific controls required for speed control. The discussion will cover the following types of drives:

• Ac adjustable-voltage drives

• Wound-rotor induction motors with several different types of secondary controls

• Adjustable-frequency drives

• Modified Kraemer drives

• Dc motors with silicon-controlled rectifier (SCR) power supplies

All have physical features, operating characteristics, or prices that make them particularly valuable in some specific segment of the pump driver spectrum.

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