Squirrel-cage induction motors

Bars in the rotor iron join to end rings. Without laminations it resembles a pet exercise wheel—hence the name. No slip rings for the main rotor current on a standard cage motor.

The stator field induces voltage in the bars. Rotor current creates torque that pulls the rotor along. If rotor reached exact synchronous speed, induced voltage would fall to zero—so induction motors always run with slip: % slip = (n_s − n)/n_s × 100.

More shaft load → more slip → more induced rotor current → more torque, until you overload. Nameplate rpm already includes design slip at rated load.

At start, slip is 100% and induced rotor currents are large; line current often 5–7× FLA (order of magnitude). Soft starters and VFDs exist because of that reality.

Lose one phase while running and many motors continue with severe heating. Current imbalance and temperature tell the story before the rewind shop does.

Starting torque, pull-up torque, and breakdown torque are different points. A motor that starts a fan may not start a loaded compressor. Match design letter/code and application—not only HP.

A few percent voltage unbalance produces much larger current unbalance and heating. Measure all three line voltages at the motor under load when investigating chronic heat.

Bad bearings raise mechanical load → higher slip and amps. Infrared and vibration complement clamp meters. Not every high-amp motor needs a rewind.

Translate the theory into a two-minute job briefing: what you will measure first, what reading would change your mind, and what you will leave documented for the next shift. If you cannot brief it, you do not own it yet.

Before you speak, write down:

1. What topology or machine you have in front of you.
2. Voltages and currents with the measurement point.
3. Frequency or rpm if they apply.
4. What the nameplate or diagram says.
5. What changes if you isolate one part of the circuit.

A diagnosis without those data is conversation, not the trade.

1. Explain the central block out loud to an imaginary helper.
2. Rewrite the field case with numbers from a real piece of equipment.
3. Complete the checklist without looking.
4. Mark which rows in the mistakes table have already happened to you.

If you cannot say the core idea in one minute, return to the first third.

Energized measurement needs PPE, a meter of the right category, and a plan if the reading does not make sense. Capacitors, inductive fields, rotating shafts, and power neutrals do not forgive haste. If the procedure says de-energize and verify absence of voltage, that rules.

Situation. Motor nameplate 1780 rpm. Clamp shows 2× FLA and tach reads 1680 rpm on a hot afternoon.

How to think. Slip is high—overload or low voltage. Measure voltage at the motor under load and check the mechanical load before condemning the motor.

Conclusion: rpm and current together diagnose induction motors.