What are the stator and rotor magnetic fields?

The stator and rotor magnetic field is the resulting magnetic field generated by a system of coils that are symmetrically aligned. They are supplied with polyphase currents. The magnetic field is generated by poly-phase (two or more phases) current or single-phase current. In the latter case, two field stator windings are supplied and two resulting electromagnetic fields are generated beyond the stage.

These electromagnetic fields are often used for electromechanical applications. For instance induction motors, electric generators, synchronous generators, and induction regulators.

Rotating magnetic field

In the operation of induction machines, the rotating magnetic field plays a vital role. The induction motor consists of a rotor and a stator. A two-phase current, for instance, is a group of fixed windings in a stator arranged to produce a rotating magnetic field at an angular velocity determined by the frequency of the alternating current. The coils are wound in slots in the rotor or armature, which are short-circuited and induce a variable flux current generated by the field pole. The armature current generates the flux and responds to the four poles of the field and the armature is set to rotate in a certain direction.

A symmetrical rotating magnetic field can be generated with two polarity wound coils operating at a right angle facing. However, three sets of coils are almost always used because they are compatible with the symmetric three-phase AC sign current system. The three coils are operated in phase 120 degrees from one set to another. For this example, the magnetic field is considered to be the linear function of the coil current.

mode and plan of operating electric motors by progressive shifting
CC BY-SA 3.0 | Image Credits: https://commons.wikimedia.org | Oxyman

In such a field the permanent magnet rotates to maintain its alignment with the external field. This effect was used in early alternating-current electric motors. A rotating magnetic field can be generated using two orthogonal coils with a phase difference of 90-degree in their alternating currents. In practice, however, such a system is supplied by a three-wire arrangement with unequal currents. This inequality by the permanent magnet can cause serious problems in the validation of conductor sizes. To overcome this, three-phase systems are used in which the three currents are equal in size and have a 120-degree phase difference. Three identical coils with geometric angles of 120° to each other form a rotating magnetic field in this case. The ability of a three-phase system to create the rotating field used in electric motors is one of the main reasons why three-phase systems dominate the world’s power supply systems.

The rotating magnetic field is also used in induction motors. The magnets wear out over time in induction motors. Induction motors use short-circuit rotors that follow the rotating magnetic field of the multicolored stator. In these motors, short-circuit turns of the rotor develop eddy currents in the rotating region of the stator. It helps in moving the rotor through the Lorenz force. These types of motors are usually not synchronized, but a 'slip' degree must be included to generate electricity due to the relative motion of the field and rotor.


Depending on the configuration of the spinning electromotive device or induction motor the stator field can act in various roles. It can behave like a magnet, interact with the armature to create motion or draw its energy from the field coils moving on the rotor. These effects can be achieved. The first DC generators (called dynamos) and shunt motors placed the field coils on the stator. The power generation or motor reaction coils on the rotor are also placed by DC motors. This is necessary because it requires a continuously moving power switch called a commutator to properly align the field in the spinning rotor. The commutator must become larger and stronger as the current increases.

The stator of these devices can be a permanent magnet or an electromagnet. Where the stator is an electromagnet, the coil that powers it is called the stator field coil or stator field winding.

The coil can be iron core or aluminum. To reduce loading losses in motors, manufacturers use copper as a conductive material in windings. Aluminum has low electrical conductivity. It is an alternative material in fractional horsepower motors. Especially when the induction motors are used for a very short duration of time.

The AC alternator can generate power on multiple high-current power output coils connected in parallel, which eliminates the need for a commutator. For transferring high-voltage and low-current power to the rotating field coil, it can be done by placing field coils on the rotor allows an inexpensive slip ring mechanism.


In a three-phase induction machine, the alternating current supplied to the stator winding creates magnetic flux and torque that rotates it. A magnetic field is generated in the air gap between the stator and the rotor with the help of flux. It also produces a voltage that induces a current through the rotor bar. The rotor circuit is short and current flows through the rotor conductors. The action of the rotating flux and current produces the force that produces the torque to start the motor.

The alternator rotor is made up of a wire coil wrapped around an iron core. The magnetic part of the rotor is made of a piece of steel to assist the conductor slots in specific shapes and sizes. As currents flow through the wire coil, a magnetic field is created around the core, called the field current. The current strength of the rotor field controls the strength level of the magnetic field. Direct current (DC) drives the field current in one direction and transmits it to the wire coil through a set of brushes and slip rings. Like any magnet, the generated magnetic field has north and south poles. The normal clockwise direction of the motor by the rotor can be changed using the magnets and magnetic fields installed in the rotor design. So that the motor gets magnetized and runs in a reverse or counterclockwise direction.

Torque of rotor

Torque is generated by the force generated by the interaction of a magnetic field and a current. The force and torque of the rotor can be calculated as follows:


Where F is force, T is torque, r is radius of rotor rings, B is magnetic field, I is current and L is length of rotor bar.

Context and Applications

This topic is important for professional exams in both undergraduate and postgraduate courses like:

  • Bachelors in Technology in Electrical Engineering
  • Masters in Technology in Electrical Engineering

Practice Problems

1. What is the speed at which the rotating magnetic field revolves?

  1. Synchronous speed
  2. Rotating speed
  3. Induction speed
  4. Torque of stator

Answer: Option a

Explanation: Synchronous speed is the speed at which the rotating magnetic field revolves.

Q2. Which of the following statements are correct for each phase of the Magnetomotiveforce (MMF) in rotating machines?

  1. It is a rotating MMF wave.
  2. It is not a rotating MMF wave but its amplitude hardly pulsates.
  3. Torque generating sinusoidal MMF waves.
  4. None of the above.

Answer: Option b

Explanation: Each MMF phase is not a rotating MMF wave, its amplitude pulsates vertically and perpendicular or alternate with the axis of its phase.

Q3. If the excitement of a single-phase winding takes place by alternating current, which of the following statements is correct?

  1. It produces many stationary MMF wave inductances.
  2. Produced MMF pulsates along with its magnetic axis.
  3. Both a and b.
  4. None of these.

Answer: Option b

Explanation: It is a single-phase MMF decay that transforms into two orbiting MMF waves, but is not produced and produces a single solid MMF wave that propels in its generated magnetic axis.

Q4. Which of the following is true, at any time the magnitude of rotating flux and when it achieves torque at the start?

  1. Changes
  2. Pulsates
  3. Remains constants
  4. None of the above

Answer: Option c

Explanation: Fr = 3 / (2Fm) (In a 3 phase machine). The continuous rotating amplitude of the MMF or rotating field is produced in the air gap of a three-phase machine.

Q5. On which of the following the synchronous speed of linear induction motor does not depend?

  1. Width of the pole pitch
  2. Number of two-poles
  3. Supply frequency sinusoidally
  4. All of the above

Answer: Option a

Explanation: The synchronous speed of the linear induction motor does not depend on the width of the pole pitch.

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