Single vs Three-Phase Motor (Explained for Beginners)

SIngle-Phase MotorThree-Phase Motor
1Not self starting (need a starting capacitor)Self starting (No starting capacitor)
2Lower Power FactorHigher Power Factor
3Need 2 cores cableNeed 3 cores cable
4Consumes more electrical energyMore efficient (Consumes less electrical energy)
5Hard to reverse its rotating directionEasy to reverse its rotating direction
6Need higher current protection devices, and larger cable size due to the high current consumption Draws lower current than the same power rated single phase motor
Single vs Three-Phase Motor

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 everywhere and in almost all industries.

In this article, we will discuss what are the differences between single and three-phase motors.

In order to know the main differences between single-phase and three-phase motors 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.

The notable distinctions between single-phase and three-phase induction motors to be aware of are:

  • Three phases motor is self-starting, while a 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 starting 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 wires.
  • Because a single winding carries all of the currents 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 motors:

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.

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Maintenance Cost of Single-Phase and Three-Phase Motor

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

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

voltage levels of Single Phase and three-phase motors

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 that of a 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 requires 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 do 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 wires.

An induction motor with only one phase has a low power factor when compared to a three-phase induction motor.

It is mainly used in industrial and commercial applications. The power output remains constant. During work, its power never drops to zero.

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 currents.

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

So, 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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