What is meant by a control circuit?

Motor control circuit starters are one of the major inventions for motor control applications. For starting a motor to control the electrical power, a starter is used. To reverse, stop, and protect the electric motors, these starters are used. The two major components of a starter are as follows:

1. Contactor: To control the electric current going to the motor, a contactor is used. In the circuit, a contactor can make or break power.
2. Overload relay: The motor can burn out due to overheating and overdrawing too much current and so the motor can become practically useless. From any potential danger, the overload relays can save and prevent this from happening.

Motor-controlled electrical equipment or an electric motor is turned on or off with the help of a starter, which is the assembly of these two components. To the circuit, the necessary overload protection is provided by the starter control circuit.

Types of motor starters

There are many types of motor starters. However, the two basic types of motor starters are-

Manual starters

These are the types of devices that are started or operated manually. Operating these starters is straightforward and also does not require experts intervention. To turn the connected equipment on or off by the user, the starter consists of a button or a rotary knob. The button has mechanical linkages which help the contact to open or close, which makes the motor start or stop. Over other types, the following features of this motor starter make it a preferred choice:

• These starters are good in economical and safe operations.
• For a wide range of applications, they are suitable because of the compact range of the control device.
• To the motor, they provide overload protection to protect it from any potential damage.
• With a vast choice of the enclosure, these devices come.
• The initial cost of these types of starters is very low.

Magnetic motor starters

Another main types of motor starter are the magnetic motor starter. These starters are operated electromagnetically. It means that the load connected to the magnetic motor starter is started and stopped by using a safer and lower voltage than the motor voltage. Just like other types of starters, these starters also have an electrical contactor and overload relay which help to protect the device from huge current and overheating.

Motor starter control circuitry

There are two types of circuits in a motor control starter, which are given below:

Power circuitry

The motor is connected with the help of power circuitry. Through the starter contacts, the overload relay, and then to the motor, the transmission of electricity is provided by it. The motor current is being carried out by the power contacts of the contractor.

Control circuit

To turn it on or off, this is the other motor starter control circuit that operates the contactor. For allowing or interrupting the flow of electric current to the motor, the contactor's main circuit is responsible. The contacts of the control circuit are either opened or closed to do this. An electromagnetic field is created by the control circuit that is energized by the control circuit. To a closed position, the power contacts are pulled by this electromagnetic field. Between the motor and the line, the circuit is completed by this. In this way, by the basic control circuit, remote operations are possible. These control circuits can be wired in the following two ways:

Method 1: For wiring the control circuit, one of the most important and widely used methods is referred to as the "two-wire method". In the two-wire method of wiring, the control circuit, a maintained contact type pilot device like thermostat, presence sensor, or float switch is used.

Method 2: The "three-wire control circuit method" unlike the two-wire method of wiring, uses the momentary contact pilot devices and a holding circuit contact.

The power can be derived by the motor control circuit in either of the following three ways:

1. Common control: When the power source of the control circuit is the same as that of the motor, this type of control is used.
2. Separate control: This control is one of the most popular types of control. In this arrangement, the control circuit derives power from a separate source as the name suggests. As compared to the motor's power source, generally the power derived is lower in voltage.
3. Transformer's control: Here, the power is derived by the control circuit from a control circuit transformer.

Types of magnetic motor starter

There are many types of magnetic motor starters on how they are connected in a circuit. They are-

Direct online starters

Other than a manual starter, the direct online starter is one of the simplest forms of motor starter. A simple push-button is the controller of the starter. The contactor is closed when the start button is pressed and it is connected to the main supply and the motor. The supply current of the motor is provided by this. A stop button is provided to turn the motor off. Normally closed auxiliary contacts are used to wire the control circuit to protect it from the over current. The normally closed auxiliary contacts open when the overload relay trips and the contactor coil is de-energizing and opens the contactor main contacts.

Rotor resistance starter

In these types of starters, the three resistances are connected in such a way that they are connected in series with the rotor windings. The rotor current is considerably reduced due to this as well as the motor torque is increased.

Stator resistance starter

Three resistors are present in the stator resistance starter that is connected in series with each phase of the series windings. A voltage drop is caused at each resistor so it becomes very important to apply a low voltage to each phase. During the motor start stage, these resistances are set at the start or the maximum position. In this type of starter, the starting current is kept minimum. The starting torque also needs to be maintained for the motor.

Auto-transformer starter

A certain percentage of the primary voltage to the secondary voltage of the transformer is supplied by the transformer with an auto-transformer starter. In a star-type configuration, an auto-transformer is connected. In this type of starter, the three secondary coils of the transformer device are connected to the motor's three-phase.

Star-delta starter

The star-delta starter is used on a large scale as compared to other types of starters. In the star-delta starter, as the name suggests the three windings are connected in a star type of connection. The timer or any other controller circuit sets a certain time. The windings are then connected in delta connection after this time is passed. The phase voltage is reduced to 58% and the total current is drawn to 58% of the normal current in the start star connection. A reduced torque is obtained due to this.

Context and Applications

This topic is significant in the professional exam for graduate and postgraduate courses like

• Bachelors of Technology in Electrical Engineering
• Masters of Technology in Electrical Engineering

Practice Problems

1. What is used to start a motor to control the electric power?

1. Starter
2. Circuit breaker
3. Capacitor
4. Solenoid

Answer: Option a

Explanation: For starting a motor to control the electrical power, a starter is used. For reversing, stopping, and protecting electric motors, these starters are used.

2. What is used to control the electric current going to the motor?

1. Overload relay
2. Contactor
3. Armature
4. Switch

Answer: Option b

Explanation: To control the electric current going to the motor, a contactor is used. To the circuit, a contactor can make or break power.

3. How the magnetic motor starters are operated?

1. Mechanically
2. Electrically
3. Electromagnetically
4. None of the above

Answer: Option c

Explanation: The magnetic motor starters are operated electromagnetically. It means that the load connected to the magnetic motor starter is started and stopped by using a safer and lower voltage than the motor voltage.

4. With the help of which circuitry is the line motor connected?

1. Power circuitry
2. Control circuitry
3. Control system
4. None of the above

Answer: Option a

Explanation: With the help of power circuitry, the line motors are connected.

5. In the case of star-delta starters, the three windings are connected first in which way?

1. Star
2. Delta
3. Spiral
4. None of the above

Answer: Option a

Explanation: In the star-delta starters, as the name suggests, the three windings are connected in a star type of connection.

Related Concepts

• Motor control center
• Power distribution
• Transformer
• Switchgear
• APFC