What is a relay?

Relay is defined as the switch which is operated electrically or electromechanically. It is used for the opening and the closing of the circuit. Relay is used to open or close the contact of one circuit to another circuit. Relay has its input and output terminals by which it gets connected. Relays connect or disconnect the small value current in a circuit. There is some exception while controlling those devices which consume power, i.e., relay can’t control power consuming devices, exceptionally solenoid, and small motors.

What is a starter?

A starter is an electrical device that is used to start heavy motors. Starter also controls the supply power to be applied to the equipment. The starter is composed of two parts, i.e., contactors and the overload.

A contactor limits or manages the flow of electric current in the motor, and it continuously disturbs the circuit. It works on the principle of electromechanics. Overload is used to safeguard the motor from the overload current or high current and overheating. Overload is composed of a current sensing unit and has trip capability.

The starter is also used as protecting device. When the circuit experience any abnormal condition or heavy current, the starter rapidly trips the circuit and safeguards the elements of the circuit.

Uses of relay

Relay is used in switching device and protecting the circuit elements from damage. On the other hand, a relay is used as a switch in the automation process. The circuit element (LED) is directly connected to the relay's input-output pin, which gives the ON or OFF signal only when the required conditions are met.

The relay works as a switching device and an isolating device that isolates a circuit with a low voltage to the circuit of having a high voltage circuit, changes over, protects elements of a circuit and detects an abnormal condition in the circuit, etc. If in consideration of microprocessor, heavy loads are managed by relays in test light.

Classifications of relay

Relays play an important role as a switching device or as a protective device, due to which they are classified in many ways.

Types of the relay-

• Electrothermal relay
• Electromechanical relay
• Solid state relay
• Hybrid relay

Types of the relay on the basis of polarity-

• Polarized relay
• Non- polarized relay

Explanation of different types of relay

Electrothermal relay

An electrothermal relay is also known as a thermal relay. These relays are made of two different metallic strips (bimetal) whose thermal expansion coefficients are different. When these bimetallic strips are connected in a circuit, they work as a relay contact. As the temperature rises, the bimetallic strips expands non-linearly because their thermal coefficients are different, resulting in an open contact or closed contact.

Electromechanical relay

These types of relay work mechanically with the principle of the electromagnet. These relays consist of electromagnets that produce magnetic fields. This is responsible for the moving of contacts when electrical supply is available.

Solid-state relay

A solid-state relay is similar to an electromechanical relay, but the key difference is that they don’t have a moving part. That’s why the operation of opening and closure of contact takes a long time.

Hybrid relay

As the name suggests, these relays are hybrid, meaning they are made up of electromechanical and semiconductors. This kind of relays has minimum disadvantages because they acquire the advantages of both.

The solenoid is also described as a relay or a part of a relay used to switch high currents. A solenoid is defined as a coil wounded on a metal rod. This solenoid is used to produce the required field. The field obtained by the solenoid gives the required movement of the contacts of the relay. Due to this, the solenoid itself works as a relay in some conditions.

Types of relay

Relays are of many types-

• Reed relay
• EM relay
• Latching relay
• Electronic relay
• Non-latching relay
• High- voltage relay
• Small signal relay
• Time delay relay
• Multi-dimensional relay
• Thermal relay
• Differential relay
• Distance relay
• Automotive relay
• Frequency relay
• Polarized relay
• Rotary relay
• Sequence relay
• Buchholz relay
• Ground fault relay
• Supervision relay
• Safety relay
• Moving coil relay

Types of starters

Types of starters:

Manual starters

Manual starters operate manually by manpower. This type of starter is less reliable but economical. Nowadays, this type of starter is used as a secondary starter when an automatic starter fails in some cases. These starters need proper attention of manpower at the time of motor starting to safeguard from damage.

Magnetic motor starters

These starters are automatic types of starters that work themselves when required. These are more reliable than manual starters.

Types of magnetic motor starters-

• Direct-on-line starter
• Rotor resistance starter
• Stator resistance starter
• Autotransformer starter
• Star delta starter

Uses of the starter

There are various application of starters like-

• In lathe factories
• In spinning mills
• It is automated like it turns off when supply is off
• Protects the circuit from overload and short circuit
• Safeguard motors from high starting current

Common Mistakes

There is always confusion between the classification and the types of the relay. Remember, classification and types are two different things.

Context and Applications

In each of the expert exams for undergraduates and graduates, this topic is huge and is mainly asked in

• Bachelor of Technology in the Electrical and Electronics Department
• Masters of Technology in the Electrical and Electronics Department
• Bachelor of Science in Physics
• Master of Science in Physics

Related Concepts

• Circuit breaker

Practice Problems

Q1. Which one is not the type of starter?

1. DOL

2. Autotransformer

3. Star delta

4. Reed

Correct option- (d)

Explanation- Reed is the type of relay.

Q2. Which starter is more reliable?

1. Manual starter
2. Magnetic starter
3. Both are the same
4. None of the above

Correct option- (b)

Explanation- The magnetic starter is more reliable than a manual starter because they work automatically.

Q3. Relay works as ______.

1. switching device
2. resisting device
3. isolating device
4. switching and isolating devices

Correct option- (d)

Explanation- Relays work both as switching and isolating devices because they are torn on or off and isolate the circuit.

Q4. Starter safeguards the motor from ________.

1. resistance
2. inductance
3. high stability
4. high current

Correct option- (d)

Explanation- Starter protects the motor from the high current by tripping the circuit.

Q5. Solenoid refers to a  _____.

1. circuit breaker
2. tripping device
3. relay
4. starter

Correct option- (c)

Explanation- Relay and solenoid somehow do similar work by generating magnetic force.