Induction motors are sometimes subjected to high voltage conditions and to protect them from injury, high or excess voltage relays are employed to trip the automatic circuit breaker. These relays are of similar design and wired in the same manner as the low voltage relays.
Fig. 2,343.—Characteristic curves of Condit time limit relay as illustrated in fig. 2,342. Settings: curve A, 5 amperes; curve B, 6 amperes; curve C, 8 amperes; curve D, 12 amperes.
Reverse phase relays have been developed for operating conditions under which a reversal of phase would cause trouble, as for example, in the case of elevator motors.
These are so designed that any phase reversal that would reverse an induction motor, would operate the relay and disconnect the automatic circuit breaker.
The design is based on the principle of the induction motor, and in the case of low voltage motors of limited capacity, the relay may be connected in series in the motor leads. If the voltage or capacity of the motor make this arrangement inexpedient, the relay may be placed in the secondaries of current or pressure transformers connected in the motor leads.
Underload relays are often used to trip the automatic circuit breaker that is placed in the primaries of arc lighting circuits to prevent an abnormal rise of secondary voltage in case of a break in the secondary circuit.
Fig. 2,344.—Diagram showing storage battery and charging dynamo protected by double pole single coil underload circuit breaker. In operation, the circuit breaker disconnects the battery when fully charged, and protects the dynamo from reverse current.
The underload relay is similar in design to the low voltage relay excepting that it acts on a decrease of current.