Understanding Phase Difference in Three-Phase Induction Motors

A three-phase induction motor is a type of AC electric motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding. The stator of a three-phase induction motor typically has three separate windings. These windings are physically placed around the stator core.

Phase Windings and Their Arrangement

In a three-phase system, the AC voltage supplied has three distinct phases. For a three-phase induction motor to operate correctly and produce a rotating magnetic field, the three stator windings are designed to interact with these three phases of the power supply. The windings are typically distributed spatially around the stator bore.

Angular Phase Difference Explained

The term "angular phase difference" refers to the difference in the phase angle of the voltage or current waveforms between the different phases in an AC system. In a balanced three-phase system, the three phase voltages or currents are displaced from each other by a specific angle.

For a standard three-phase system, the phase voltages (or currents) are displaced by an angular difference of 120 degrees electrically. This means that if phase A is at its peak, phase B will reach its peak 120 electrical degrees later, and phase C will reach its peak 120 electrical degrees after phase B (or 240 electrical degrees after phase A).

To effectively utilize this three-phase power and create a smoothly rotating magnetic field, the three windings in the stator of a three-phase induction motor are typically placed and connected in such a way that they are also electrically spaced 120 degrees apart. When the three-phase voltages are applied to these windings, they produce magnetic fields that are also displaced by 120 electrical degrees in time and space, resulting in a rotating magnetic field.

Analyzing the Options

Let's look at the given options for the angular phase difference:

• 360°: This would mean the phases are in sync, which is not a characteristic of a multi-phase system like three-phase.
• 120°: This is the standard electrical phase difference between phases in a balanced three-phase system. The windings are designed to match this.
• 180°: This represents an anti-phase relationship, typical in split-phase systems or between opposite ends of a single winding, but not the difference between distinct phases in a standard three-phase supply or motor winding arrangement.
• 90°: This is the phase difference found in two-phase systems (like some types of single-phase motors using auxiliary windings), not in a three-phase system.

The angular phase difference between each phase winding of a three-phase induction motor is designed to align with the electrical phase difference of the supply, which is 120 degrees.

Summary Table

Therefore, the angular phase difference between each phase winding of a three-phase induction motor is 120°.

Revision Table: Three-Phase System Basics

Additional Information: Electrical vs. Mechanical Degrees

It's important to distinguish between electrical degrees and mechanical degrees. Electrical degrees relate to the phase difference in time of the waveforms, while mechanical degrees relate to physical angle in space (like shaft rotation). In a motor, these are related by the number of poles:

Electrical degrees = Mechanical degrees × (Number of poles / 2)

The 120° phase difference discussed for the windings is typically referred to in electrical degrees, matching the electrical phase difference of the power supply.