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Relay Questions and Answers

Electrical Relay Questions

These questions are related to Solid State Relay Timers, ON and OFF Delay Timers, Magnetic Motor Contactors and Starters which are covered in detail here:

ON Delay Timer and OFF Delay Timer

Solid State Timers

Motor Contactors and Starters

  1. Electromagnetic relays operate in which way?

An electromechanical relay consists of a coil and contacts. It turns a load circuit ON or OFF by energizing an electromagnet that opens or closes contacts in the circuit.

  1. Relays are associated with two types of circuits, how do they interact with each other?

A relay is made up of two circuits: the coil input or control circuit and the contact output or load circuit. Closing the switch in the control circuit energizes the electromagnet, which in turn closes the relay contacts in the load circuit to switch the load on.

  1. What is the difference between a normally closed (NC) and a normally open (NO) relay contact?

Normally open contacts are those contacts that are open when the coil is DE energized and closed when the coil is energized. Normally closed contacts are those contacts that are closed when the coil is de energized and open when the coil is energized.

  1. There are three common relay control applications, describe them.

(1) To control a high voltage load with a low voltage control circuit. (2) To control a high current load with a low current control circuit. (3) To control multiple switching operations by a single, separate current.

  1. For trouble shooting purposes, what are two commonly used relay options?

Two common relay options used for troubleshooting are an on/off indicator to indicate the state of the relay coil and a manual override button to move the contacts into their energized position for testing.

  1. Relays are specified in six different ways, list them.

(1) Type of operating current (AC or DC). (2) Normal operating voltage or current. (3) Permissible coil voltage variation (4) Coil resistance (5) Power consumption. (6) Contact rating (AC or DC) maximum current rating at specific voltage

  1. In a relay contact switching arrangements, the terms poles, throw, and break are defined as:

Pole is the number of switch contact sets. Throw is the number conducting positions, single or double. Break designates the number of points in a set of contacts where the current will be interrupted during opening the contacts.

  1. Solid state relays have what main advantage over electromechanical relays?

More reliable

  1. How is the electrical isolation of the input and output of a solid state relay is accomplished?

Electrical isolation of the input and output sections of a solid state relay is accomplished by using an LED in the control circuit and a photodetector in the load circuit.

  1. Explain how output contacts in conventional relays are switched differently than that of time delay relays.

In a conventional control relay, the contacts immediately change when the control circuit is energized. With a time-delay relay, the contacts do not change state until a predetermined time after the input is either energize or de energized.

  1. What is the difference between an on-delay timer and an off-delay timer?

The contacts of an on-delay timer change state a fixed time after the control circuit is energized. The contacts of an off-delay timer change state after a fixed time after the control circuit is de energized.

  1. A two coil latching relay state is changed by what means?

In a two coil latching relay, energizing one coil will latch the contacts closed and they will stay in that position. When the second coil is energized the contacts will change state and stay in that position even if power is removed.

  1. A single coil latching relay state is changed by what means?

In a single-coil latching relay, the direction of current through the coil determines if the contacts will be latched or unlatched state.

  1. A relay and a contactor differ in which way?

They both operate on the same principle, but the contactor is capable of handling heavier loads currents.

  1. You must combine two components to form a magnetic motor starter, what are they?

A magnetic motor starter is a contactor with an overload protective device attached.

About Ahmad Faizan

Mr. Ahmed Faizan Sheikh, M.Sc. (USA), Research Fellow (USA), a member of IEEE & CIGRE, is a Fulbright Alumnus and earned his Master’s Degree in Electrical and Power Engineering from Kansas State University, USA.

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