Cogging and Crawling in Electrical
Machines
Cogging and Crawling are two undesirable phenomena that can affect the performance of
induction motors, particularly squirrel cage motors. Understanding these issues is
important to prevent inefficiencies and operational failures in motor-driven systems.
Cogging (Magnetic Locking)
Cogging is a phenomenon where the induction motor fails to start or experiences jerky
movement at low speeds. It is also known as magnetic locking. This issue arises due to the
interaction between the rotor teeth and the stator magnetic field, especially when the rotor
teeth align perfectly with the stator teeth.
Causes:
• Cogging occurs when the number of stator slots and rotor slots are equal or have a
common factor, which causes reluctance torque (locking torque).
• Uneven magnetic pull between rotor and stator leads to 'locking' at certain rotor positions.
Effects:
• The motor may not start at all or start with significant difficulty.
• This problem is more evident in motors running at low speeds and with heavy loads.
Prevention:
• Skewing rotor slots: By slightly skewing the rotor slots with respect to the stator slots,
cogging can be reduced. Skewing breaks the alignment between the rotor and stator slots.
• Proper slot combination: Choosing an appropriate ratio of rotor and stator slots can
minimize the chance of cogging.
Crawling
Crawling is a phenomenon in which an induction motor runs at a speed much lower than its
synchronous speed, typically around one-seventh of the synchronous speed. This results in
the motor “crawling” instead of reaching its normal operating speed.
Causes:
• Crawling is mainly caused by the presence of harmonics in the motor. The 7th harmonic of
the magnetic field in the stator induces a torque that opposes the fundamental torque,
preventing the motor from reaching its normal speed.
• Crawling is more common when the motor is lightly loaded, as the harmonic effects are
more pronounced.
Effects:
• The motor operates at a significantly reduced speed (around 1/7th of the synchronous
speed).
• This can lead to inefficiencies, overheating, and poor performance, particularly in systems
requiring precise speed control.
Prevention:
• Improved design: Manufacturers often design motors with skewed rotor bars and
optimized stator slot configurations to minimize the impact of harmonics and prevent
crawling.
• Harmonic filtering: Reducing the effect of harmonics through electrical design and filtering
can help prevent the crawling issue.
Conclusion
Both cogging and crawling reduce the efficiency and effectiveness of induction motors.
These issues can be mitigated through proper design techniques such as skewing rotor
slots, optimizing stator-rotor slot combinations, and minimizing the effects of harmonics.
Understanding and addressing these phenomena is crucial for maintaining reliable motor
performance in industrial applications.
