Why Squirrel-cage Induction Motors Are The Only Type Compatible With Pole-changing Speed Control

In motor speed regulation theory, there is a concept that puzzles many novice electricians: pole-changing speed control is the simplest and most stable motor speed regulation method. However, it can only be used for squirrel-cage induction motors and is completely unsuitable for wound-rotor induction motors.

Both belong to induction motors, and their speed regulation principle relies on the stator rotating magnetic field, yet they differ greatly in compatibility. Fundamentally, the core difference lies not in the stator structure, but in the structural characteristics of the rotor and whether the rotor pole pairs can automatically change with the stator pole number.

1. Squirrel-cage rotor features "self-adaptation", perfectly suited for pole-changing speed control

The squirrel-cage motor has a special rotor structure. It consists of rotor bars and short-circuit rings at both ends forming a closed circuit. As a short-circuit conductor structure with no fixed windings or predefined magnetic poles, it has no preset coil turns or magnetic pole specifications.

During operation, the rotor itself does not actively generate a magnetic field. It relies entirely on induction from the stator rotating magnetic field to generate current and torque. When we rewire the stator windings to switch the pole pairs of the magnetic field for high/low speed switching, the squirrel-cage rotor will automatically induce pole pairs fully matching the stator magnetic field.

Simply put: if the stator produces a 2-pole magnetic field, the rotor adapts to 2 poles; if the stator switches to a 4-pole magnetic field, the rotor automatically adapts to 4 poles. No hardware modification is required throughout the process, delivering excellent self-adaptability. This is why industrial two-speed and multi-speed motors all adopt squirrel-cage construction.

2. Wound rotor winding structure cannot accommodate pole-changing speed control

The rotor of a wound-rotor induction motor is a prefabricated independent three-phase winding with precisely wound coils. Its coil layout, number of turns and pole pairs are fixed parameters and cannot be altered after manufacture.

If we forcibly rewire the stator for pole changing, the pole pairs of the stator magnetic field will change, while the rotor pole pairs remain fixed. In other words, the rotor parameters fail to match the new pole number, resulting in severe mismatch between stator and rotor pole pairs. Under such operating conditions, the internal magnetic field of the motor will be completely disrupted, and output torque will drop sharply. This will not only cause severe vibration, abnormal noise and failure to start, but also directly burn out the windings and damage the equipment.

Summary

Squirrel-cage rotor of squirrel-cage induction motors: pole pairs are self-adaptive and change flexibly following the stator, supporting pole-changing speed control. Wound rotor: fixed pole pairs with no adjustability, cannot adapt to magnetic field switching. Pole-changing speed control is prohibited.