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Cogging & crawling in induction motors

Slot-harmonic parasitic phenomena. Cogging = locks at start when S=R. Crawling = stuck at n_s/7 from 7th space-harmonic torque. Universal cure: skew rotor bars one stator slot pitch.

Junior ~10 min

Step 1 — Two parasitic phenomena in cage induction motors

Animation speed 0.55×

n — torque — issue —

Reference notes

Squirrel-cage induction motors can suffer two parasitic phenomena that come from the discrete slot/bar distribution rather than from the AC supply: cogging (failure to start) and crawling (locked at ~1/7 of synchronous speed). Use Next → to walk through the slot-harmonic origin of each, the design rules that prevent them, and the universal cure — skewing the rotor bars by one stator slot pitch.

Cogging — magnetic locking

When the number of stator slots S equals the number of rotor bars R (or integer ratios apply), every rotor tooth aligns with a stator tooth at the same time. The slot-permeance harmonic creates a strong alignment torque that pulls the rotor into specific angular positions with zero net acceleration torque. Symptoms: motor hums but doesn't rotate, draws large current (~600 % FLA), eventually trips on thermal overload. Bump-start by hand and it accelerates fine — the lock exists only at rest.

Cogging avoidance rules

S ≠ R always.
Avoid R being an integer multiple of S, and vice versa.
Popular choice: R = S ± 2P (P = pole pairs). Systematically detunes alignment forces.
Example: 4-pole 36-slot stator (P = 2) → choose R = 28, 34, 40, 44; never 36.
Skew the rotor bars by one stator slot pitch (see below).

Crawling — stuck at n_s/7

The actual air-gap MMF is not a pure sinusoid — it contains space harmonics from the discrete slot/bar distribution. The dominant unwanted components are:

5th space harmonic — rotates BACKWARD at n_s/5 relative to fundamental.
7th space harmonic — rotates FORWARD at n_s/7 relative to fundamental.

Each harmonic produces its own torque-slip curve, scaled by its amplitude. The 7th-harmonic torque peaks at n = n_s/7. If the load torque is between the fundamental torque (low at this speed since it's near s = 1) and the 7th-harmonic torque, the motor settles at n_s/7 — it crawls. For a 4-pole 50 Hz motor (n_s = 1500 RPM), the crawl point is ~215 RPM.

The 5th harmonic, rotating backward, produces only braking torque during forward acceleration — no stable crawl point. Higher harmonics (11th, 13th) could in principle crawl at n_s/11 and n_s/13 but their amplitudes are small and rarely problematic.

The universal cure — rotor-bar skewing

Skew the rotor bars by approximately one stator slot pitch across the rotor axial length. Why this addresses both phenomena:

Cogging: at no rotor position do all bars align with stator teeth simultaneously. Any alignment torque is averaged to zero along the bar length.
Crawling: the 7th space-harmonic flux has angular period = 1/7 of a slot pitch. Over one full slot pitch of skew, each bar spans exactly 7 harmonic periods → integrated flux linkage = 0. Similarly weakens 5th, 11th, 13th.

Manufacturing: the rotor lamination stack is twisted slightly during assembly so the bar slots form a helix. End-to-end twist = 1 stator slot pitch. Cost: essentially zero. Universal in modern industrial cage motors.

Other practical mitigations

Fractional-slot designs — choose stator and rotor slot counts that aren't simple integer ratios.
Slot-opening optimization — narrower slot openings reduce the amplitude of slot-permeance harmonics.
Closed-slot rotors — rotor slots with a thin iron bridge over the slot opening reduce slot-harmonic effects further (at the cost of slightly more leakage reactance).

Field diagnostics

Motor hums, won't rotate from rest → cogging suspected, or rotor lock-up from another cause (mechanical jam, bearings).
Motor reaches ~14 % of nameplate sync speed and stays there, current 3–5× FLA, thermal overload trips → crawling suspected. Verify against expected n_s/7 number from nameplate (for 1500 RPM sync, expect ~215 RPM; for 1800 RPM sync, ~257 RPM).
Broken rotor bars can mimic crawl (sets up effective S = R imbalances and weakens fundamental torque). Diagnostic: motor current signature analysis (MCSA) finds sideband frequencies around the fundamental at ±2·s·f.

Why it matters less today

Modern industrial induction motors (NEMA Design A/B/C/D, IEC 60034) are designed with proper R/S ratios and bar skewing — cogging and crawling are designed out. Where these problems still appear: cheap mass-market motors (ceiling fans, low-end HVAC, washing-machine direct drives) where design margins are cut, or motors with significant damage. The phenomena remain a classic exam/interview topic because they cleanly illustrate how the discrete geometry of slots and bars affects machine performance — and how a simple manufacturing step (bar skew) addresses two independent-looking problems at once.

Take-away. Cogging = motor won't start because S = R puts the rotor in a slot-alignment lock. Crawling = motor stuck at n_s/7 because the 7th forward-rotating space harmonic produces a parasitic torque peak there. The 5th harmonic rotates backward and only brakes. The universal cure for both: skew the rotor bars by one stator slot pitch — averages out cogging AND nulls the 7th-harmonic flux linkage. Choose R = S ± 2P for additional cogging safety. Modern industrial motors essentially eliminate both problems by design; cheap mass-market motors can still suffer them.

Keyboard shortcuts

Cross-section panel shows S and R; torque-speed panel shows the 7th-harmonic crawl peak when the rotor is unskewed and the fundamental + load-line intersection when skewed.