SOLUTION

Relay

Relays are switches that open and close circuits electromechanically or electronically. 

Or

An electric device that is designed to interpret input conditions in a prescribed manner and after specified conditions are met to respond to cause contact operation or similar abrupt change in an associated electric control circuit. Inputs are usually electric but may be mechanical, thermal, or other quantities. A relay may consist of several units, when responsive to specified inputs, the combination providing the desired performance characteristic. The relays can be electrical, thermal, pneumatic, hydraulic, and other.

Some of the automatic relays used to control and protect electrical equipment are described in the following paragraphs.

Thermal overload relays are provided on starters to prevent equipment from burning out if abnormal operating conditions increase the load beyond the pump’s design capacity. When current is excessive, the relays (sometimes called heaters) placed on each phase of the power supply open the control circuit and stop the motor. These relays are normally set to stop the motor when the current exceeds the design load by 25 percent.

Fuses or circuit breakers are placed in the main power wiring to each motor to protect against short circuits. They are normally located in the safety switch just ahead of the starter.

Overcurrent relays are often referred to as overload relays. Their purpose is to sense current surges in the power supply and to disconnect the motor if a surge occurs. They can be fitted with time-delay or preset thermal-overload mechanisms. The overload relays may reset automatically or they may require manual resetting after they are tripped.

Voltage relays are frequently used to detect a loss of power and to initiate a switchover to an alternate power source. Undervoltage relays are also used to shut down motors if the voltage drops too low. Voltage relays generally have a timing mechanism that allows minor functions (ones that won’t damage the motor) to continue before power is actually disconnected from the motor.

Frequency relays respond to changes in the frequency (in cycles per second) of an AC power supply. They are most often used where local power generation is involved. Frequency relays are also used on synchronous motor starters to sense when the motor has reached synchronizing speed.

Phase-reversal relays are installed to detect whether any two of the three lines of a three-phase power system are interchanged. If the phase sequence should be reversed, all motors will run backward. A phase-reversal relay senses this change and opens the control circuit to disconnect the motors. A phase reversal can be particularly serious for deep-well pump applications. The pump shafting can become unscrewed, allowing the pump to fail.

Loss-of-phase relays are installed in most wiring systems to detect the loss of one of the three phases. Loss of one phase is not unusual, and if the power is not cut off, the motor may burn out within a few minutes while operating on only two Phases.

Differential relays are frequently used on large equipment or switchgear. These units check whether all of the current entering a system comes back out of the system. If it does not, the relay closes a contact that shuts down the equipment. An equipment shutdown by a differential relay indicates major trouble, so an electrician should always be called.

Reverse-current relays sense a change in the normal direction of current or power flow and activate an alarm for the operator. They can also open circuits to isolate the faulty portion of the system.

Time-delay relays are used when some condition needs to last for a specified length of time before some other action is begun. For example, if a pump motor must be given sufficient time to reach full speed before the discharge valve opens, a time-delay relay can be energized when the motor is started. The relay can be set to close its contacts several seconds later to activate the valve-opening control circuits.