Single-phase, two-phase and three-phase motors

Electric motors let us obtain mechanical energy in the simplest and most efficient manner. Depending on the number of supply phases, we can find single-phase, two-phase and three-phase motors with coiled startup winding and with coiled startup winding with a capacitor. And choosing one or the other will depend on the necessary power.
If you are involved in a project and don’t know what type of motor you should use, in guide we tell you about each motor and their differences. Let’s get to it!

 What is a single-phase motor?

A single-phase motor is an electrically-powered rotary machine that can turn electric energy into mechanical energy.
It works by using a single-phase power supply. They contain two types of wiring: hot and neutral. Their power can reach 3Kw and supply voltages vary in unison.
They only have a single alternating voltage. The circuit works with two wires and the current that runs across them is always the same. For more details see Figure 1 below 

Figure 1 - sine curve on single phase motor

In most cases these are small motors with a limited torque. However, there are single-phase motors with a power of up to 10 hp that can work with connections of up to 440V.
They do not generate a rotating magnetic field; they can only generate an alternate field, which means that they need a capacitor for startup.
They are easy to repair and maintain, as well as affordable.
This type of motor is used mainly in homes, offices, stores and small non-industrial companies. Their most common uses include home appliances, home and business HVAC and other appliances such as drills, air conditioners and garage door opening and closing systems.

What is a two-phase motor?

A two-phase motor is a system that has two voltages 90 degrees apart, which is no longer in use nowadays. The alternator is composed of two windings placed at 90 degrees from each other.
They require 2 live and one ground wire that work in two phases. One increases the current up to 240v for the motion, and the other one maintains the fluidity of the current for the use of the motor.

What is a three-phase motor?

A three-phase motor is an electric machine that transforms electric power into mechanical energy by means of electromagnetic interactions. Some electric motors are reversible – they can transform mechanical energy into electric power acting as generators.
They work by using a three-phase power source. They are driven by three alternating currents of the same frequency, which peak at alternating moments. They can have a power of up to 300KW and speeds ranging between 900 and 3600 RPM.
Three conductor lines are used for transmission, but the final use requires 4-wire cables, which correspond to the 3 phases plus neutral. For more details see Figure 2 below

 Figure 2 - sine curve on three phase motor

Three-phase electric power is the most common method in use in electric grids around the world since it transfers more power, and it sees significant use in the industrial sector.

After a general explanation we go into specifics

What is a 3-Phase Motor?

A 3-phase motor is an electric motor that operates on a three-phase power supply, utilizing three alternating currents to generate a rotating magnetic field, resulting in a smoother and more efficient power delivery compared to single-phase motors. They are a type of AC motor that is a specific example of a polyphase motor. These motors can be either an induction motor (also called an asynchronous motor) or a synchronous motor. The motors consist of three main components – the stator, the rotor, and the enclosure.
The stator consists of a series of alloy steel laminations around which are wound with wire to form induction coils, one coil for each phase of the electrical power source. The stator coils are energized from the three-phase power source.
The rotor also contains induction coils and metal bars connected to form a circuit. The rotor surrounds the motor shaft and is the motor component that rotates to produce the mechanical energy output of the motor.
The enclosure of the motor holds the rotor with its motor shaft on a set of bearings to reduce the friction of the rotating shaft. The enclosure has end caps that hold the bearing mounts and house a fan that is attached to the motor shaft which spins as the motor shaft turns. The spinning fan draws ambient air from outside the enclosure and forces the air across the stator and rotor to cool the motor components and dissipate heat that is generated in the various coils from the coil resistance. The enclosure also typically has raised mechanical fins on the exterior that serve to further conduct heat to the outside air. The end cap will also provide a location to house the electrical connections for the three-phase power to the motor. For more details see Figure 3 below

Figure 3 - Motor internal parts

How Does a 3-Phase Motor Work?

A 3-phase motor works by harnessing three alternating currents to produce a rotating magnetic field, which, in turn, drives the motor’s rotor and induces rotation. This design enhances efficiency and performance, making 3-phase motors suitable for a wide range of industrial applications.
Three-phase motors operate by the principle of electromagnetic induction which was discovered by the English physicist Michael Faraday back in 1830. Faraday noticed that when a conductor such as a coil or loop of wire, is placed in a changing magnetic field, there is an induced electromotive force or EMF that is generated in the conductor. He also observed that current flowing in a conductor such as wire will generate a magnetic field and that the magnetic field will vary as the current in the wire changes in either magnitude or direction. This is expressed in mathematical form by relating the curl of the electric field to the rate of change in time of the magnetic flux:

These principles form the basis for understanding how a three-phase motor works.
Figure 4 below is an illustration of Faraday’s law of induction. Note that the presence of an EMF depends on the motion of the magnet which results in a changing magnetic field to exist.

Figure 4 - Principle of Electromagnetic Induction

For induction motors, when the stator is powered from a three-phase electrical energy source, each coil generates a magnetic field whose poles (north or south) change position as the AC current oscillates through a complete cycle. Since each of the three phases of the AC current are phase-shifted by 120o, the magnetic polarity of the three coils are not all identical at the same instant of time. This condition results in the stator producing what is known as an RMF or Rotating Magnetic Field. As the rotor sits in the center of the stator coils, the changing magnetic field from the stator induces a current in the rotor coils, which in turn results in an opposing magnetic field being generated by the rotor. The rotor field seeks to align its polarity against that of the stator field, the result being a net torque is applied to the motor shaft and it begins to rotate as it seeks to bring its field into alignment. Note that in the 3-phase induction motor, there is no direct electrical connection to the rotor; magnetic induction causes the motor rotation.

With three-phase induction motors, the rotor seeks to maintain alignment with the RMF of the stator, but never achieves it, which is why induction motors are also called asynchronous motors. The phenomenon which causes the rotor speed to lag the speed of the RMF is known as slip, as is expressed as:

where Nr is the speed of the rotor, and Ns is the synchronous speed of the rotating field (RMF) of the stator.

Synchronous motors operate in a similar fashion to induction motors except that in the case of a synchronous motor, the stator and rotor fields are locked into alignment so that the stator RMF will cause the rotor to turn at the exact same rate of rotation (in synch – therefore the slip is equal to 0). For more information on how this is accomplished, refer to these articles on reluctance motors and brushless DC motors (BLDC motors). Note that synchronous motors, unlike induction motors, need not be powered by AC power.

Motor Controllers for 3-Phase Motors

The speed that is generated by a three-phase AC motor is a function of the AC supply frequency since it is the source of the RMF in the stator coils. Therefore, some AC motor controllers operate by using the AC current input to generate a modulated or controlled frequency input to the motor, thereby controlling the speed of the motor. Another approach that can be used to control motor speed is by altering the slip (described earlier). If the slip increases, the motor speed (i.e. the speed of the rotor) decreases.

To learn more about the approaches for motor control, review our article on AC Motor Controllers.

3-Phase Motor Types

There are several types of 3-phase motors, each designed for specific applications:

• Induction Motors: Also known as, asynchronous motors, are widely used for general-purpose applications due to their simplicity and reliability. Two common types of induction motors are squirrel cage and wound rotor.
• Synchronous Motors: Maintain synchrony with the supply frequency, making them suitable for precision applications such as clock motors.
• Brushless DC Motors: Utilize electronic commutation for improved efficiency and longevity, often found in applications like robotics and electric vehicles.

When Would a Three-Phase Motor Be Used?

A three-phase motor would be used in various applications, including:

• Industrial machinery and manufacturing equipment
• HVAC systems for large commercial buildings
• Water pumping stations and wastewater treatment plants
• Oil and gas processing facilities
• Electric vehicles and transportation systems
• Large-scale air compressors and pumps