Ans. Starting compensators may be wound for any voltage or current for which it is practicable to build motors. For very large motors the switching device is generally separate from, the compensator itself and consists of triple and four pole switches for three phase and two phase motors respectively. One double throw switch or two interlocked single throw switches are required for the motor and a single throw switch for energizing the compensator, the running side of the motor circuit being provided with fuses or automatic circuit breakers, or the switches provided with low voltage and overload release attachments.
Star Delta Switches.—These are starting switches, designed for use with small three phase squirrel cage motors having their windings so arranged that they may be connected in star for starting and in delta for running.
Ques. Describe the operation of a star delta switch.
Ans. In starting the motor, the drum lever is thrown in the starting direction which connects the field windings of the motor in star. When the motor has accelerated and has come partially up to speed the starting lever is quickly thrown to the running position in which position the field windings are connected in delta. The effect of connecting the field winding in star at starting is to reduce the voltage applied to each phase winding, while in the running position each phase of the field winding has full line voltage impressed upon it.
Figs. 2,470 to 2,474.—General Electric time limit overload relay for starting compensator. In case of overload, the relay armature is raised and at the end of its travel, opens the small switch at the top which in turn opens the circuit of the low voltage release coil causing the compensator switch to return to the "off" position. The oil dash pot provides a certain time element and can be adjusted to operate immediately upon overload or at any interval up to five minutes. Each relay has five calibrating points, the lowest being the normal full load current of the motor, the highest 300 per cent. full load current. The scale on the calibration tube reads direct and shows various values of current at which the relay may be set to operate. To change overload setting: 1, loosen set screw; 2, turn relay plunger on piston rod until white mark comes opposite required value of current; 3, tighten set screw. Time element adjustment: Removing oil dash pot by turning to the left will expose the cup shaped piston, made of which are two concentric discs (B and C) held together by a milled lock nut, A. There is a hole in each disc through which the oil must pass when the plunger of the relay is raised. The time element may be varied by changing the size of the opening between these discs, that is, to have the relays operate in a shorter period of time, increase the size of the opening and vice-versa. To change the time setting: 1, remove the oil dash pot; 2, raise the discs B and C on the piston rod; 3, loosen the lock nut A; 4, change the opening between B and C, giving a larger opening for shorter time of operation, and a smaller opening for longer time; 5, tighten lock nut A; 6, replace discs in piston D; 7, replace oil dash pot.
Figs. 2,475, and 2,476.—Front and side views (oil tank removed) of Cutler-Hammer star delta switch for starting small three phase squirrel cage motors. In construction, the switch consists of one set of stationary fingers and a rotating wooden cylinder, carrying two sets of contacts. These parts are supported from the switch frame casting and are enclosed in a steel tank which contains an insulating oil. Flexible oil proof cable leads are brought out through insulated bushings in the top of the switch and tagged for convenience in connecting to the lines and motor. To prevent seepage of oil, the leads are sealed into the top of the cover with an oil proof sealing wax. The lever of the star delta switch is arranged with an interlock which prevents its being thrown directly into the running position from the off position. It is necessary to throw the lever first into the starting position and then with an uninterrupted movement to the running position. The circuit of the motor is broken only for an instant in changing from star to delta and no heavy inrush current occurs. No voltage release protection is provided by a latching solenoid which holds the spring centered drum cylinder in the running position. The no voltage release coil is mounted in the lower part of the starting switch, immersed in the oil tank, and is protected against mechanical injury and grounding. The coil is in circuit during the running period only and requires not more than 8 to 15 watts to hold the switch in the running position. The operation of this protective device is such that on failure of voltage the star delta switch will immediately be returned to the off position. Overload release protection consists of two relays on a small slate panel, which is mounted directly on the side of the star delta switch. The switch contacts of the overload release are connected in series with the connections to the no voltage release coil so that when an overload occurs the overload relay operates to open the circuit to the no voltage release coil, thus permitting the switch lever to return to the off position. The overload relays do not afford overload protection during the starting period, and when such protection is desired starting fuses should be installed. These fuses, if used, should have a capacity of 250 to 300 per cent. of the normal full load current of the motor.