Motor Theory Fundamentals Reference

Why this matters

Half of field-tech failures involve a motor: blower, condenser fan, compressor, pump, garage door, dishwasher pump, washing machine drive. Knowing how each type works - and what each type's failure modes look like - turns "the motor isn't running" from a guessing game into a measurement-driven diagnosis.

AC induction motor - the workhorse

Most residential and commercial motors are AC induction motors. The fundamentals:

Stator: stationary windings around the outside. When energized with AC, they create a rotating magnetic field.

Rotor: the spinning shaft assembly inside the stator. For an induction motor, the rotor is a "squirrel cage" - conductive bars connecting two end rings, no electrical connection to the outside (no brushes, no slip rings).

How it works:

1. AC in the stator creates a rotating magnetic field
2. The rotating field induces currents in the squirrel cage rotor bars
3. The induced currents create their own magnetic field in the rotor
4. The two fields interact: rotor follows the rotating field
5. Rotor always lags slightly behind the stator field - this lag is called "slip"

Typical slip: 3-5% on residential motors. A 60 Hz / 4-pole motor's synchronous speed is 1,800 RPM (3,600 RPM at 2-pole). At 3% slip, actual speed is ~1,750 RPM (or ~3,475 at 2-pole). This is why nameplate motors read 1,725 or 3,450 RPM, not exactly 1,800 or 3,600.

Single-phase vs three-phase

Three-phase motors (commercial / large industrial):

• Three windings 120° offset
• Rotating field generated automatically by phase shifts in the supply
• No starting mechanism needed (self-starting)
• More efficient per HP
• Smoother torque
• Common in commercial buildings, industrial pumps, large compressors

Single-phase motors (residential / small commercial):

• One winding from a single AC supply
• Single AC source alone produces a pulsating field, not a rotating field - NOT self-starting
• Need an auxiliary mechanism to create the rotating field at startup
• Less efficient than three-phase
• Cheaper, smaller, common in residential

Single-phase motors come in several variants based on the starting mechanism.

Single-phase motor variants

Capacitor-start (CS):

• Run winding (continuously energized) + start winding (energized only at startup)
• Start capacitor (high microfarad value, electrolytic) gives a 90° phase shift to the start winding current
• Centrifugal switch on the rotor disconnects start winding once motor reaches ~70-80% of running speed
• Used for: pool pumps, well pumps, larger appliances
• Failure mode: start capacitor bulged / leaked = motor won't start (hums, trips overload)

Capacitor-start, capacitor-run (CSCR):

• Two capacitors: large start cap + smaller run cap
• Run cap remains in circuit during operation, improves running efficiency
• Used for: high-efficiency motors, A/C and heat pump compressors
• Failure mode: either cap can fail; test both separately

Permanent Split Capacitor (PSC):

• One capacitor permanently in series with the start (auxiliary) winding
• No centrifugal switch (simpler, more reliable)
• Less starting torque (won't start under heavy load)
• Used for: HVAC blower motors, condenser fan motors
• Failure mode: cap weakens over time → motor runs slow or won't start at temperature extremes

Shaded pole:

• Simplest, cheapest single-phase motor
• "Shading coil" loop around part of each stator pole creates a weak rotating field
• Very low starting torque
• Inefficient
• Used for: bathroom exhaust fans, small refrigerator fans, small clock motors
• Failure mode: bearings wear; replace whole motor (rarely worth servicing)

Split phase:

• Start winding + run winding with different impedance characteristics
• No capacitor
• Centrifugal switch
• Cheaper than capacitor-start
• Used for: older / smaller motors, some appliance applications

Brushed DC and Universal motors

Universal motors: run on AC or DC; brushed; high speed; high power-to-size ratio; noisy; brushes wear (every few hundred to few thousand hours of use). Used in: portable power tools (drills, vacuum cleaners), some old kitchen appliances, hair dryers.

Brushless DC (BLDC) motors: electronically commutated; permanent magnet rotor; stator switching electronics. More efficient, longer-lived than brushed. Used in: ECMs (HVAC blower motors), modern pump motors, computer fans. See HVAC Blower Motor Diagnosis Reference.

Key electrical concepts

Locked Rotor Amps (LRA): current draw at the instant of starting, when the rotor isn't spinning yet. Typically 5-7× full-load amps (FLA). LRA is what the breaker, contactor, and conductors must handle for the first ~0.1 seconds.

Full-Load Amps (FLA): current draw at rated load. Nameplate value.

No-Load Amps: current draw with no mechanical load (motor spinning freely). Typically 30-40% of FLA for induction motors.

Power factor (PF): ratio of real power (watts) to apparent power (volt-amps). For induction motors, PF is less than 1 - typically 0.7-0.9. Motors with poor PF need bigger conductors / breakers despite their watts.

Torque vs speed: induction motor torque is highest at startup, drops as speed approaches synchronous. Capacitor-start has 3-4× starting torque vs running; PSC has only ~1.5× starting torque (which is why PSC won't start under heavy load).

Motor protection

Internal thermal overload: small thermal switch inside the motor that opens if winding temperature exceeds rating (typically 250 °F / 121 °C). Self-resetting after cooldown.

External overload relay: at the motor starter; calibrated for the motor's FLA; opens the contactor if current exceeds setpoint.

Class 10 / Class 20 / Class 30 overload: time-current trip class. Class 10 trips fastest, Class 30 slowest. Match to motor application - Class 30 for compressors with long start times.

Built-in thermistor (modern motors): PTC or NTC thermistor in the windings; signals back to controller for thermal protection.

Motor failure modes

Winding burned out (open or shorted):

• Caused by: prolonged stall (locked rotor), overheating from insufficient cooling, voltage surge, ground fault
• Diagnosis: resistance measurement between leads - should be balanced if 3-phase; should be at expected value per nameplate
• Result: replace motor

Bearings worn:

• Symptoms: noise (whining, grinding), vibration, shaft wobble
• Test: spin shaft by hand (power off) - should rotate freely without scraping
• Some motors have replaceable bearings; many residential motors are full-replacement

Capacitor failed (single-phase motors):

• Symptoms: motor won't start (hums), trips overload, runs slow
• Test: capacitance meter; ±6-10% of nameplate
• Replace cap, motor often returns to service

References

• NEMA MG 1 (Motors and Generators standard)
• NEC Article 430 (motors and motor circuits)
• AHRI 540 (electric-motor-driven products)
• ASHRAE Handbook - HVAC Systems and Equipment (fan and pump motors)
• Manufacturer specifications (Genteq, Emerson/Nidec, Regal, Baldor, Marathon, U.S. Electric Motors)