Understanding Rotating Magnetic Fields in Three-Phase Induction Motors and Transformers

When a balanced three-phase supply is provided to a three-phase induction motor, it generates a rotating magnetic field. This field is what enables the motor to operate. However, when the same balanced three-phase supply is given to a transformer, it does not produce a rotating magnetic field. This article will explore how a three-phase induction motor works and why it generates a rotating magnetic field, while explaining why a transformer does not.

How a Three-Phase Induction Motor Works

A three-phase induction motor relies on a rotating magnetic field to function. This field is created by the interaction between the stator and the rotor. The key factors contributing to the development of the rotating magnetic field in an induction motor are:

The Role of Balanced Three-Phase Supply

For a rotating magnetic field to be created in an induction motor, a balanced three-phase supply is essential. The currents in the three-phase windings must be 120° phase apart. This phased relationship is crucial because it creates a cyclic magnetic field that rotates within the motor.

Proper Coil Arrangement

The spatial arrangement of the three-phase coils is also vital. Each coil must be positioned such that they are magnetically 120° apart. This ensures that the magnetic flux peaks at different times, leading to the cyclical rotation of the magnetic field. For instance, the Y-phase coil reaches its maximum flux value 6.66 milliseconds after the R-phase coil, while the B-phase coil reaches its peak 13.33 milliseconds after the Y-phase coil. This phased relationship continues, creating a rotating magnetic field.

Induction and Secondary Flux

The rotating magnetic field in an induction motor also requires the presence of a squirrel cage rotor. This rotor contains a series of conductive bars that induce currents due to the changing magnetic field. The interaction between the primary field from the stator and the induced current in the rotor creates a force, leading to rotational motion.

Why Transformers Do Not Have a Rotating Magnetic Field

In contrast, a transformer does not produce a rotating magnetic field despite receiving a balanced three-phase supply. The reason for this difference lies in the operating principle and intended function of transformers versus motors.

Transformer Operation

Transformers are designed to transfer electrical power between two circuits through electromagnetic induction. To achieve this, only condition 'i' must be satisfied: the three-phase coils must have currents 120° apart. However, the other conditions necessary for a rotating magnetic field are not met in a transformer:

Condition 'ii': The spatial magnetic arrangement required for a rotating field is absent in transformers. Condition 'iii': Transformers do not have a secondary coil or squirrel cage that induces currents, crucial for creating the tangential motive force and rotational motion.

Conclusion

A three-phase induction motor generates a rotating magnetic field due to the balanced three-phase supply, proper coil arrangement, and the presence of a squirrel cage rotor that induces current. Transformers, on the other hand, do not produce a rotating magnetic field because they are designed for a different purpose and lack the necessary components to achieve the rotating field.

Keywords

three-phase induction motor, rotating magnetic field, transformer, induction