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.
What is 3-Phase Power?
To understand three-phase motors, 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 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.
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. 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 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. 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. 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.