What is a 3-Phase Motor and How Does it Work?

Three-phase motors (also annotated numerically as 3-phase motors) are widely used in industry and have become the workhorse of many mechanical and electromechanical systems because of their relative simplicity, proven reliability, and long service life. Three-phase motors are one example of a type of induction motor, also known as an asynchronous motor, that operates using the principals of electromagnetic induction. While there are also single-phase induction motors available, those types of induction motors are used less frequently in industrial applications but are widely used in domestic applications such as in vacuum cleaners, refrigerator compressors, and air conditioners, owing to the use of single-phase AC power in homes and offices. In this article, we will discuss what a three-phase motor is and describe how it operates. To access other resources about motors, consult one of our other motor guides covering AC motors, DC motors, Induction motors, or the more general article on the types of motors. A full list of related motor articles is found in the section on related articles.

To understand single phase motor, it is useful to first understand three-phase power.

In electrical power generation, alternating current (AC) that is created by a generator has the characteristic that its amplitude and direction changes with time. If shown graphically with the amplitude on the y-axis and time on the x-axis, the relationship between the voltage or current vs. time would resemble a sine wave as shown below:

Electrical power carried to homes is single-phase, meaning that there is one current-carrying conductor plus a neutral connection and a ground connection. In three-phase power, which is used in industrial and commercial settings to run larger machinery that has greater power needs, there are three conductors of electrical current, each of which is operating at a phase difference of 120o of 2π/3 radians apart. If viewed graphically, each phase would appear as a separate sine wave, which then combines as shown in the image below:

Three-phase motors are powered from the electrical voltage and current that is generated as three-phase input power and is then used to produce mechanical energy in the form of a rotating motor shaft.

What is a 3-Phase Motor?

Three-phase motors are a type of AC motor that is a specific example of a reducer 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.

How does a 3-Phase Motor Work?

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. 

These principles form the basis for understanding how a three phase induction motor works.

Figure 3 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.

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. 

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. 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 3-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.

Summary

This article presented a brief discussion of what 3-phase motors are and how they operate. To learn more about motors, explore our related articles listed below. For information on other products, consult our additional guides or visit the Thomas Supplier Discovery Platform to locate potential sources of supply or view details on specific products.

The first concept we need to understand about a 3 phase induction motor is the first part of its name – 3 phase power.  A single phase power supply uses two wires to provide a sinusoidal voltage.  In a three phase system, three wires are used to provide the same sinusoidal voltage, but each phase is shifted by 120°.  At any point in time if you were to add up the voltage of each phase, the sum would be constant.  Single phase power is fine for residential or other low power applications, but three phase [JS2] power is typically required for industrial or higher power applications.  This is because it can transmit three times as much power while only using 1.5 times as much wire.  This makes for a more efficient and economical power supply. 

Induction motors offer many advantages, including reduced upfront and maintenance costs. Because of their basic, economical design, induction machines usually cost less than synchronous and dc motors. This makes them an ideal choice for industrial, fixed speed applications like wind power and wind turbine generators. 

The sheer simplicity of induction motors also makes maintenance easier and less frequent, decreasing operating costs over time. This cost efficiency gives induction machines a significant edge over synchronous and dc motors, which feature many additional components, like slip rings, commutators, and brushes. 

Durability is another strength of induction motors. These rugged machines can run for several years with little attention and maintenance, even in demanding environments. The absence of brushes (and sparks) allows induction motors to operate safely in explosive or other environmental conditions, creating a flexible solution for Oil & Gas, material handling, and more. 

3 phase induction motors bear unique advantages as well, including self-starting torque. This feature eliminates the need for starting capacitors, which are typically required for a single phase motors. 3 phase machines also deliver exceptional speed regulation and overload capacity, making them viable for a wide range of applications.