[Illustration: FIG. 2,427.--Westinghouse voltage regulating ~secondary~ relay; view showing relay removed from oil tank. The secondary relay is practically a motor starting switch of the double pole double throw type, electrically operated through the contacts of the primary relay. It is provided with contact points of one-half inch rod. The relay is suitably connected for starting, stopping and reversing the motor and for properly operating the motor brake. The parts are: A, solenoid; B, laminated field; C, movable contact arm; D, stationary contact arms; E, removable brass contact points; F, terminal block; G, terminals.]
Ans. For the reason that a primary relay, of sufficient accuracy and freedom from errors due to temperature and frequency variations, could not be made sufficiently powerful to carry the relatively large current required for operating the motor.
~Ques. What names are given to the relays?~
~Ques. What difficulties were encountered in the operation of relays? ~
[Illustration: FIG. 2,428.--Diagram of connections of General Electric ~direct current~ (form S) ~voltage regulator,~ for 125, 250, and 550 volts. ~The range of voltage~ is given in the following table:
----------------------------------- |Regulator|Range of voltage | | |------------------------ | |16 |17 |18 |19 |20 | ----------------------------------- |125 |105 |110 |115 |120 |125| |250 |210 |220 |230 |240 |250| |550 |550 | | | | | -----------------------------------]
Ans. Vibration or chattering at the contacts of both relays and tendency of the movable contact arm of the primary relay to hug closer to one of the stationary contact points than to the other, thereby operating too often.
~Ques. What causes vibration or chattering at the contacts?~
Ans. This is due to the voltage frequently approximating the value required for closing a contact, thereby causing the contact points to barely touch and make several poor contacts in succession.
~Ques. What objectionable action is produced by vibration at the contacts?~
Ans. Arcing, burning and pitting of the contacts, even when alloys of the rarer metals are used, such as those of the platinum group, having extreme hardness and high melting points.
[Illustration: FIG. 2,429.--Diagram of connections of automatic induction regulator and auxiliary apparatus on single phase circuit.]