Electric Motors

Publisher Summary This chapter focuses on several factors that need to be considered for the practical selection of the proper driver. Electric motors are the most frequently used drivers in pumping stations, primarily because of their versatility, compactness, and low maintenance. The most common machine is the polyphase (three-phase) squirrel-cage induction motor. Induction motors up to about 600 kW (800 hp) are usually used for adjustable-speed drives, but larger drives tend to be more economical with a wound-rotor or a synchronous motor. Electric motors might be mounted in many ways. The motor could be close-coupled to the pump in either a horizontal or a vertical arrangement in which the pump impeller is fastened directly to the shaft of the motor. Wherever possible, however, avoid configurations that lead to high maintenance or difficult replacement problems. Close-coupled pumps are more difficult to maintain because most units must be disassembled to service the seals. The squirrel-cage induction motor operates on alternating current but has no physical electrical connections to the rotor. The rotor consists of a stack of steel laminations with evenly spaced conductor bars around the circumference. The wound-rotor motor is different from the squirrel-cage motor, basically in the design of the rotor and the addition of brushes and slip rings. The rotor windings on a squirrel-cage motor are short-circuited by an end ring.