A rotating magnetic field is a magnetic field that has moving polarities, where opposing poles rotate about a central point or axis. It is produced by a system of coils symmetrically placed and supplied with polyphase currents. A rotating magnetic field is the key principle in the operation of induction machines, such as the induction motor, which consists of a stator and rotor. In the stator, a group of fixed windings are arranged so that a two-phase current, for example, produces a magnetic field that rotates at an angular velocity determined by the frequency of the alternating current. A permanent magnet in such a field will rotate so as to maintain its alignment with the external field. This effect was utilized in early alternating-current electric motors.

The rotating magnetic field is the result of two orthogonal coils with a 90-degree phase difference in their alternating currents. However, in practice, such a system would be supplied through a three-wire arrangement with unequal currents. The rotating magnetic field results in a rotating radially-inward force being applied to the core across a diameter, causing an ovalising distortion moving.

The concept of a rotating magnetic field is essential to the operation of AC motors and generators. Tesla and another inventor named Ferraris realized that AC current could cause a rotating magnetic field without a moving commutator. The magnetic poles of a motor winding appear opposite of the poles in a permanent magnet.