Single vs Three Phase Motor (Explained for Beginners)

The most common motors I have ever seen in all industrial fields I worked for, are induction motors. Single and three phase motors are almost every where and at almost all industries.

In this article, we will discuss what is the differences between single and three phase motors. In order to know the main differences between single phase and three phase motor we will go over some of the basics of single and three phase systems.

Key Difference between single and three phase motors

The key difference between a single and a three-phase motor is the power supply single or three phase system.

There notable distinctions between single phase and three phase induction motor to be aware of are

  • Three phase motor is self starting, while single phase one needs starting capacitor.
  • Mechanical noise and vibration are produced by single-phase motors. A three-phase motor, on the other hand, runs quietly and smoothly.
  • A single-phase motor’s rotating direction control is a little more complicated, and it’s done by changing the polarity of the starter winding, but a three-phase motor’s direction control is straightforward, and it’s done by swapping any two input phases.
  • When compared to a three-phase induction motor, a single-phase induction motor has a low power factor.
  • The single-phase motor has two terminals and can be powered with only two wires, but the three-phase motor has three terminals and may be powered with three or four wires (including neutral).
  • Because a single winding carries all of the current in a single-phase motor, copper losses are substantial. Copper losses are reduced in three-phase motors because the current is shared amongst the windings.

Common Features of Single Phase vs Three Phase Motor

A three phase motor does not require: a capacitor or a starter winding.

With a VFD, a three phase motor can operate over a wide range of speeds, not just a single speed like a single phase motor.

Any two of the three phases of an electrical power source, or any one of the three phases and the neutral, can be used to connect a single-phase electric motor, as long as it can handle the different voltages unlike the original three phases, the phase shifting of the different connections is irrelevant.

There will be no rotating effect from a three-phase electrical power supply, and only pulsating effect, from the single-phase power supply is applied to motors with these connections. Two equal rotating shafts in opposite directions make up a pulsating single-phase supply, which has no self-starting capability How to start a single motor with an intermittent supply.

Maintenance of single phase and three phase motor:

Repairing and maintaining a single-phase motor is very simple. It is because it does not use any type of complex or expensive equipment. On the other hand repairing and maintaining a three-phase motor is challenging because of its complexity.

Read also:

Maintenance Cost of Single Phase and Three Phase Motor

As three phase motors are common for larger industrial applications, so that the cost of repairing a three-phase motor is much higher than that of a single-phase one.

So, its essential to make a powerful maintenance program for 3 phase induction motors, to make sure they are working properly.

Output voltage levels of Single Phase and three phase motor:

The single-phase motor requires a voltage of nearly 120/230V to run.

On the other hand, the voltage requirement of three phase motor can be much higher than the single phase motor. A voltage of about 415V is delivered to the three-phase motor.

However, I worked with 3 phase motors working on medium voltage of 3.3KV, This one has a very complicated starting, protection and speed control system.

Starting mechanism of single phase and three phase motor

The single-phase motor does not start on its own and requires the use of auxiliary equipment such as motor starters. When the single phase current passes through the motor, it still does not start and require an extra circuit to work.

This is because a rotating magnetic field cannot be generated by a single-phase supply.

Why Is A three phase motor self starting

The three-phase motors are self-starting and does not require any additional equipment like starting capacitor to start running.

In brief it is self starting because:

  • The stator three phase windings produce a rotating field.
  • According to the transformer action, An EMF is produced in the rotor.
  • A short circuit current is produced in the rotor short circuited winding.
  • Rotor magnetic flux is produced due to the rotor current.
  • Now, the motor has two magnetic fields, one in the rotor and the other in the stator.
  • According to Lenz’s law, the rotor field will try to oppose the causing field, i.e the stator field.
  • The rotor will run in the direction of the stator field.

This is why three phase induction motors are self starting.

Power Supply of Single Phase and Three Phase Motor

An electric motor with only one phase has only two terminals and can be powered by only two wires; a three-phase motor has three terminals and can be powered by three or four (including neutral) wires.

An induction motor with only one phase has a low power factor when compared to a three-phase induction motor. The voltage supply rises and falls that is why its power output is not constant.

As mentioned earlier that a single phase supply is used by the single-phase induction motor to run. While three-phase induction motors require three-phase power supply to run normally. It is mainly used in the industrial and commercial applications. The power output remains constant. During work its power never drops to zero.

The three current carrying wires are connected at the distant of 120-degree from each other. The Phase difference of each wire as compare to other is 1200 which means each single AC power is 1200 out of phase.

Efficiency and consumption

When comparing single phase vs three phase motor power consumption and efficiency, single-phase motors have a low power factor and efficiency since only one winding must carry all of the current.

Because three windings are available to convey the current, the three-phase motor has a high efficiency. Single-phase motor consume more electrical power than three-phase motor.

So that, for large industrial applications, three phase motors are the most common. While single phase motors are just for smaller motors and applications.

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