- Induction Motor Phenomena: Crawling and cogging are important characteristics to understand in the operation of squirrel cage induction motors.
- Crawling Definition: This is when an induction motor runs at much lower speeds than its designed speed, primarily caused by harmonics like the 5th and 7th producing additional torque.
- Cogging in Induction Motor: Occurs when the motor fails to start because the slots of the stator lock up with the rotor slots, often due to matching slot numbers or harmonic interference.
- Preventing Cogging: Adjusting the number of rotor slots to be different from those in the stator and skewing the rotor slots can effectively prevent cogging.
- Understanding Harmonics: Recognizing how harmonic frequencies interact with the motor’s slot frequencies is crucial for diagnosing and solving motor issues like cogging and crawling.
Squirrel cage induction motors can exhibit two different parasitic torque effects: crawling and cogging. Crawling is stable running at an unintended low speed. Cogging is reluctance torque from stator and rotor slotting that can retard or prevent starting. Neither effect is a normal operating characteristic.
Crawling of Induction Motor
Crawling occurs when an induction motor accelerates from rest but settles at a low subsynchronous speed instead of reaching its intended operating point. A well-known case is stable operation just below 1/7th of the fundamental synchronous speed. It arises from air-gap space harmonics produced by the winding and slot distribution. In a balanced three-phase machine, the relevant non-triplen orders include the 5th and 7th harmonics; a 3rd-order component does not produce the same balanced rotating field.
The harmonic fields have different directions and synchronous speeds. The triplen component represented by Ns/3 does not form the same useful balanced rotating field, the fifth-order field corresponds to Ns/5 in the reverse direction, and the seventh-order field corresponds to Ns/7 in the forward direction. Focusing on the 5th and 7th orders, the fifth-harmonic torque opposes forward rotation, while the 7th-harmonic field can add a local forward-torque feature near Ns/7. Each component has its own slip and torque-speed curve. Crawling occurs only if the combined fundamental and 7th-harmonic torque intersects the load torque at a stable point near one-seventh speed. Higher harmonics can create other local torque features; a reported 13th-harmonic case lies near Ns/13, but its importance depends on the winding, rotor and load.
Cogging of Induction Motor
Cogging in an induction motor is magnetic tooth-locking or reluctance torque caused by stator and rotor slotting. Severe cogging can prevent acceleration from rest. Low supply voltage reduces useful starting torque and can make the symptom worse, but low voltage is not the underlying cogging mechanism.
Both the stator and cage rotor have teeth separated by slots. Air-gap reluctance changes as their teeth pass one another, producing a position-dependent torque. Equal or otherwise unfavourable slot and pole combinations can align many teeth at once and create a large locking torque.
Cogging can cause noise and vibration even when it does not stop the motor. Harmonic interactions between slot permeance, winding magnetomotive force and rotor currents can add synchronous or asynchronous parasitic torques, so designers evaluate the complete stator-slot, rotor-slot and pole-number combination rather than applying one equality rule alone.
Methods to overcome Cogging
Cogging is reduced during electromagnetic design rather than corrected after installation. Two common measures are:
- Select stator and rotor slot counts that differ and avoid other slot-pole combinations known to produce strong parasitic torques.
- Skew the rotor slots along the shaft so teeth cannot align over the full core length at once. Skew reduces cogging but also slightly reduces coupling and increases leakage reactance, so its angle is a design trade-off.
