Ques. Are starting compensators necessary for small motors? Why?
Ans. No, because the full voltage starting current taken, although equal to several times the load current, is nevertheless so small, compared with the capacity of the station alternators or feeders, that it does not materially affect the regulation of the circuit.
Fig. 2,464.—Diagram of connections of General Electric three phase starting compensator with low voltage release and fuses.
Fig. 2,465.—Diagram of connections of General Electric two phase starting compensator with no voltage release and overload relays for 1,040 to 2,500 volt circuits.
Motors larger than about 7 horse power cause an objectionably heavy rush of current if thrown directly on the line. Starting compensators obviate such sudden variations of line load and are accordingly recommended for motors above 7 horse power except in cases where voltage variations and excessive starting currents are not objectionable.
Figs. 2,466 and 2,467.—General Electric three phase hand operated starting compensator. Fig. 2,466, compensator in case; fig. 2,467, compensator with case removed. The compensator consists of a core and windings, a cable clamp, and a switch, assembled in a substantial metal case with external operating handle and release lever. The windings consist of coils wound on separate legs of a laminated core, and tapped at several points, the connections terminating at the switch contacts. The shaft of the switch extends through the sides of the compensator case, and is operated by a lever at the right, being held in the running position by a lever at the left. It is provided with wiping contacts. The switch is immersed in oil, and is intended to be used as a line switch as well as for starting the motor. The lever has three positions: "off," "starting," and "running." In the off position, both compensator and motor windings are disconnected from the line. In the starting position, the switch connects the line to the ends and the motor to the taps of the compensator winding without overload relays or fuses in circuit. In the running position, the compensation winding is cut out and the motor is connected to the line through suitable fuses or overload relays mounted directly above the compensator. To prevent the attendant throwing the motor directly on the line, and thereby causing a rush of current which it is the object of the compensator to avoid, an automatic latch is provided and so arranged that the lever at off position can be thrown only into the starting position (backward); and can be thrown thence into the running position (forward) only by a quick throw of the lever, whereby any appreciable drop in speed and consequent increase in current in passing from the starting into the running position is avoided.