Induction motors are widely used in industries because they are simple, strong, and require very little maintenance. However, during operation—especially at startup—they can face two common problems: crawling and cogging. These issues mainly occur in squirrel-cage induction motors and affect smooth performance.

What is Crawling in an Induction Motor?

Crawling is a condition where the motor runs at a very low speed (about 1/7th of its normal speed) instead of reaching full speed.

Why Does Crawling Happen?

Crawling occurs due to disturbances in the motor’s magnetic field, known as harmonics.

• The main magnetic field tries to rotate the motor at full speed.
• A specific harmonic (called the 7th harmonic) creates a backward force.
• This backward force can slow the motor down and make it run at low speed.

Key Points About Crawling

• Caused by unwanted magnetic effects (harmonics).
• More common when the motor is running with a light load.
• Reduces motor efficiency and causes unstable operation.

How to Prevent Crawling

• Use proper winding design to reduce harmonics.
• Design stator slots carefully.
• Ensure the motor operates under proper load conditions.

What is Cogging in an Induction Motor?

Cogging, also known as magnetic locking, is a condition where the motor fails to start even when power is supplied.

Why Does Cogging Happen?

Cogging occurs when the stator and rotor teeth align perfectly, creating a strong magnetic attraction.

• This alignment locks the rotor in place.
• It is more likely when the number of stator slots matches or is a multiple of rotor slots.

Key Points About Cogging

• The motor does not start at all.
• Caused by strong magnetic locking between stator and rotor.
• Usually happens due to poor motor design.

How to Prevent Cogging

• Use skewed rotor slots (slightly twisted design).
• Avoid matching stator and rotor slot numbers.
• Improve design accuracy during manufacturing.

Difference Between Crawling and Cogging