What is a three-phase motor and single phase motor and how are they different?

What is a three-phase motor?

A three-phase motor works on a three-phase AC supply and this motor’s electromagnets will be connected to one of the three phases of the supply. The three-phase motor can produce more power than a single phase. A three-phase motor operates with the help of three single-phase coils or windings and these windings or the coils will be 120 degrees apart and there would be three hot wire to the motor’s load. A three-phase motor is smaller in size when compared to a single-phase motor, but these motors would need a three-phase supply to operate. As we discussed above the three conductors are placed 120 degrees apart and it would generate electrical power. We can also describe a three-phase motor as a device that is capable to convert three-phase electrical power to mechanical energy according to the principle of electromagnetism.

What are the features of a three-phase induction motor?

  • Three-phase supply will make the magnetic field
  • It could be a squirrel cage or wound rotor
  • This motor is capable of self-start
  • This type of motor has high power
  • It is used in seventy percent of the industries
  • It has certain applications like pumps. Conveyor belts etc

Three-phase AC induction motor

The motor that operates by the electromagnetic induction principle is the induction motor. This motor is not similar to the DC motor in this motor the power supply is connected to the stator winding and there is no electrical connection between the rotor and the power supply. These motors are used for many industrial applications. The induction motor speed is almost constant and its starting current is higher than the full load current.

How does a three-phase induction motor work

The stator winding of the three-phase motor will be connected to a three-phase supply and because of this there a rotating magnetic field is created. The three-phase current will flow through the windings and thus the stator is energized. So when the rotor winding cuts the stator’s rotating magnetic field an emf will be generated and because of this emf current will flow through the rotor.

How to control the speed of a three-phase induction motor?

The induction motor speed is depended on the supply frequency, pole, and relative speed. So we can control the speed of the three-phase induction motor by controlling the stator voltage, pole change, and supply frequency.

What are the types of a three-phase induction motor?

Squirrel cage type

In this type of three-phase motor, the rotor windings have copper bars and it can be seen in rotor slots. This type of motor has low starting torque but it has the good running performance.

Slip ring type

This type of motor has the same number of rotor and stator winding and the rotor winding is star connected.

What are the losses that can be seen in a three-phase induction motor?

  • Stator loss – this loss is because of the copper and iron loss in the stator winding and the core
  • Rotor loss – this loss is because of the copper and iron loss in the rotor winding and the core
  • Mechanical loss is because of the friction and windage losses

What is the difference between the squirrel cage and slip ring induction motor?

  • The squirrel cage motor has low starting torque but the slip ring type has a higher starting torque
  • The squirrel cage has higher efficiency than the slip ring type
  • We cannot control the speed of the squirrel cage induction motor but we can control the speed in a slip ring induction motor
  • The slip ring motor has a low starting current, while the squirrel cage has a high starting current
  • The squirrel cage motor needs a starter while the slip ring motor doesn’t need a starter
  • High maintenance is required for the slip ring motor, while the squirrel cage type needs less maintenance
  • The construction of the squirrel cage is rugged while the slip ring doesn’t have rugged construction
  • The slip ring motor costs really higher than the squirrel cage motor

What is the synchronous and asynchronous motor?

In a three-phase motor, the rotating magnetic field is created in the stator and this rotating speed will be synchronous speed. The synchronous speed is related to the supply frequency and the poles in the device. So in a synchronous motor, the rotor rotation will be the same speed as the stators rotating magnetic field, but in an asynchronous motor, the rotation of the rotor will be less than the synchronous speed.

How to test a three-phase motor?

We should do the power supply test in a motor, we can check this by using a multi-meter. We must check if it receives the required voltage and also we need to check the terminal for the power supply and make sure it is in good condition.

We must do the continuity test of the motor and this can be done by using a multi-meter, we need to check the continuity of the winding from phase to phase. The winding resistance check must be done and it can be done by using the multimeter, so check if each of the windings has the same resistance or not. We also need to check the resistance of the insulation and it can be and we can use a megger to do this. The full load amps of the motor must be checked and it can be done by a clamp-on meter. We must also check the earth’s continuity so we need to determine the resistance between the motor body and earth.

What are the advantages of a three-phase induction motor?

  • It can self start
  • Less maintenance
  • Its construction is robust
  • Value for money
  • The armature reaction is very less because there is no commutators and brushes

What is a single-phase induction motor and how does it work?

A single-phase induction motor has one stator winding and so that it works in a single-phase power and it creates less power than a three-phase motor. The rotor will be a squirrel cage type in a single-phase induction motor. This motor doesn’t have the ability to self-start. The motor will be connected to the single-phase power the alternating current will flow through the winding of a single-phase motor, and this current will create a magnetic field. The rotor of the signal phase motor will be energized by the induction. But the required torque for the starting of the motor is not available so the rotor will only vibrate instead of rotating. So because of this the single-phase induction motor is not a self-starter and needs a starting mechanism. So we use an extra stator winding as the starting mechanism of a single-phase induction motor.

What is the difference between a single-phase and a three-phase motor?

The single-phase motor has a single-phase supply and the three-phase motor has a three-phase supply. A Single-phase motor is not self-starting so it would need a capacitor, the created magnetic field in a single-phase motor will be equal and opposite, and the same criteria for the torque so the net torque will be very low. In a single-phase motor, there will be a high current. The current in a three-phase motor will be low compared to a three-phase motor and because of this the conductor used to transmit power will be of low size. A three-phase motor uses less power than a single-phase and because of the three-phase is economical.


post-graduate in Electronics & communication.

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