Crawling and Cogging of Induction Motor

The induction motor works on the principle of electromagnetic induction and it induces current in the rotor resulting in developing torque which is widely used in various industries due to its efficiency and reliability. But sometimes it faces operational challenges like Crawling and Cogging decreasing the efficiency of the induction motor. So, it is necessary to solve these problems to make sure an induction motor operates smoothly. In this article, we will cover the Crawling and Cogging of Induction Motors the reasons for the occurrence of these operational problems, and how to reduce them. After reading this article your doubts will be cleared and you will learn the reasons for the occurrence of these operational problems and how to reduce them.

Crawling and cogging are unwanted results shown by induction motors, impacting their efficiency and stability. Crawling is the slow motion of the motor at low speed and on the other hand cogging is resistance shown by the motor in smoothly starting and operating. In crawling the motor shows odd behaviour affecting the efficiency of an induction motor. the motor’s motion resembles a crawl instead of a smooth motion on the other hand When cogging occurs the motor experiences some jerky movements at the start of operation making it difficult to achieve seamless motion.

Crawling of Induction Motor

Crawling is a phenomenon in an induction motor that occurs at low speed, typically when the motor is not sure about its next step. In this, the motor shows odd behavior affecting the efficiency of an induction motor. the motor’s motion resembles a crawl instead of a smooth motion. This generally happens when there is an asymmetry in the magnetic field leading to the production of uneven torque.

As shown in above diagram, the crawling occurs in an induction motor when harmonics are developed in the motor. The crawling mainly occurs when the motor runs at 1/7th of its synchronous speed. The induction motor experienced crawling because odd harmonics were present in the air gap between the stator and rotor. It means the 7th harmonic effect is present in the three-phase induction motor and due to this crawling effect produced in the induction motor.

Cogging of Induction Motor

Cogging is basically the resistance shown by induction motor in smoothly starting and operating. When cogging occurs the motor experiences some jerky movements at the start of the operation making it difficult to achieve seamless motion.

As shown in the above diagram, when slots of rotor slots are equal to slots at the stator then they align in such a way that slots of the stator get locked up with the rotor slots resulting in clogging. so, to solve this issue the number of slots at the stator and number of slots at the rotor should not be equal.

Conditions that lead to the Crawling and Cogging Phenomenon given below :

Conditions Leading to Crawling Phenomenon

• Low Speed Operation: Crawling mainly occurs at low speeds, often when motor operates below a certain percentage of its speed.
• Saturation of Magnetic Core: High level magnetic core causes uneven torque causing odd motion
• Insufficient voltage: Insufficient voltage may lead to crawling as it does not allow to set up proper magnetic field causing inefficiency.
• Slot Harmonics: Stator and rotor slot harmonics can be the reason for crawling. The interaction between these harmonics results in an uneven torque production, causing the motor to crawl.

Conditions Leading to Cogging Phenomenon

• Permanent Magnet Interaction: Cogging is mainly found in motors using permanent magnets in the rotor causing imperfect alignment in the magnets which leads to resistance during starting.
• Rotor Design: Rotor design play a crucial role in clogging, specially in permanent magnets. when slots of rotor slots are equal to slots at the stator then they align in such a way that slots of the stator get locked up with the rotor slots resulting in clogging.
• Uneven Air Gap: This can also lead to cogging.

Implications of Crawling and Cogging

Crawling

• Efficiency reduction: Crawling leads to efficiency reduction of induction motor.
• Noise: The jerky motion will create odd noise making it difficult to operate.

Cogging

• Difficulty in Starting: Cogging creates challenge in operating an induction motor smoothly.
• Increased power consumption: As to face cogging we need more torque leading to increase in power consumption.

Reducing Crawling Phenomenon

• Providing Appropriate Voltage: Providing the appropriate voltage will reduce the chances of crawling and helps in establishing a balanced magnetic field
• Adjust Winding Design: Modify the motor winding design to minimize magnetic effects.

Reducing Cogging Phenomenon

• Optimize Permanent Magnet Placement: In motors, that uses permanent magnets we can optimize the placement of these magnets to ensure better alignment.
• Modify Rotor Geometry: To achieve uniform magnetic field we can modify the geometry and design of the the rotor, specially in permanent magnet motors.

The Rotor and Stator Slot Harmonic Order

In induction motors, the connection between the rotor and stator slots plays a crucial role in determining the motor’s performance. harmonic orders can influence the magnetic field affecting the motors operations.

Rotor Slot Harmonic Order

Definition: Harmonics from rotor slot arrangement.

Effect: Impacts torque, affecting motor performance.

Stator Slot Harmonic Order

Definition: Harmonics from stator slot arrangement.

Impact: Influences stator-rotor interaction, affecting motor behaviour.

Solved Problems on Cogging and Crawling of Induction Motor are given below :

Solved Problem on Crawling in an Induction Motor

Q. Lets take an example as your ceiling fan moving in a jerking way at the low speed. It is similar to the crawling of induction motor making awkward movements when it should run smoothly. It might be because of poor design of rotor slots.

Think of it like adjusting the fan blades. By changing the angle of the rotor slots, we can make the motor run smoother, avoiding that awkward crawling motion.

Solved Problem on Cogging in an Induction Motor

Q. Imagine you are pedaling your bicycle but it rest your effort. This is the same thing that happen in an induction motor during cogging.

Think of it as adjusting the gear for your bicycle. Adjust the number of stator and rotor slots to prevent cogging.

In conclusion we can say that phenomenon of Crawling and Cogging of Induction creates unwanted problems in smooth operations and understanding these issues and solve them is very important as they can decrease the efficiency of induction motor. So. above are the reasons for the occurrence of these operational problems and how to reduce them. Follow all the steps to reduce the Crawling and Cogging of Induction motor.

Can crawling damage my induction motor over time?

While crawling itself may not directly damage the motor, it can lead to increased heating and reduced efficiency of the motor.

Can I reduce crawling and cogging without hiring a professional?

Yes, certain adjustments can often be made without professional assistance. Optimizing winding designs, adjusting voltage levels are some adjustments which can be done without professional assistance.

How do I know if my motor is experiencing cogging issues?

Cogging is typically identified by jerky movements during the motor’s startup.