Lesson Electric motors · Types and application
Three-phase motors and efficiency
Three-phase motors produce a rotating magnetic field — smoother torque, higher efficiency, common on commercial compressors, chillers, and large fans. Lost phase burns windings fast. Understand rotation, efficiency classes, and nameplate data before you touch a rooftop polyphase unit.
What we are trying to understand
Goals:
- Explain why three-phase needs no start capacitor
- Check rotation direction and phase loss
- Read efficiency, service factor, IP codes on nameplate
- Relate poles to RPM on 60 Hz
Rotating field from three phases
Three AC waveforms 120° apart on windings around stator create a field that rotates — rotor follows.
No auxiliary start winding needed for normal start torque.
Typical line: 208 V, 230 V, 460 V — match motor nameplate and contactor.
Rotation and phase reversal
Reversing any two line leads reverses rotation.
On compressors, wrong rotation → noisy, high amps, no pumping.
Verify rotation on first start or after motor swap — bump test briefly if procedure allows.
Lost phase (single-phasing)
One fuse open, one bad contactor pole, loose lug → motor runs on two phases.
Symptoms:
- Hum, low torque
- High amps on remaining legs
- Overheat quickly
Ammeter on each leg — one leg 0 A while others high = classic single-phase.
Fix power, not windings first.
Synchronous vs actual speed
Sync RPM = 120 × f / poles (f = 60 Hz).
| Poles | Sync RPM | Typical full-load (~slip) |
|---|---|---|
| 2 | 3600 | ~3450 |
| 4 | 1800 | ~1725 |
| 6 | 1200 | ~1075 |
| 8 | 900 | ~825 |
More poles → slower, more torque for size — belt and direct applications differ.
Efficiency and energy standards
Nameplate efficiency (NEMA premium, IE classes on some imports) affects operating cost, not your immediate start diagnosis — but replacement should match application.
Higher efficiency motors may have lower FLA for same hp — overload heater size must change with motor swap.
Service factor (SF)
SF 1.15 means motor can operate at 115% of rated hp/current under defined conditions without insulation damage.
Overload devices must be coordinated — SF is not permission to ignore high amps forever.
Open vs hermetic vs semi-hermetic
Open — shaft out, bearings serviceable, end bells off.
Hermetic — motor inside compressor shell — refrigerant cools motor; no field rewind practical.
Semi-hermetic — bolts off for access — commercial refrigeration.
Three-phase hermetic diagnosis uses terminals, amps, voltages — not end-bell switch.
Wye vs delta connection
Motor may be wye or delta for same voltage class — nine-lead motors reconfigurable.
Wrong connection for supplied voltage → weak or destructive.
Follow nameplate diagram or plate voltage tap links exactly.
Technician mindset
Polyphase call checklist:
- All three legs present and balanced?
- Rotation correct?
- Contactor all poles making?
- FLA per leg vs nameplate?
- Mechanical load free?
Three-phase is efficient until one phase disappears — then it is an emergency.
Field case
Situation. New rooftop 460 V compressor — hum, 15 A on two legs, 0 A on third. Contactors new.
How to think it.
- Open fuse on one leg of disconnect — not compressor.
- Replaced fuse, checked lug torque — normal run, balanced amps.
Takeaway: single-phasing is power path, often not motor.
In the field
Symptom
Hum, high amps, overheating, wrong rotation
Where to look
Fuses, contactor poles, lugs, overload, mechanical load
Likely causes
- Lost phase
- wrong rotation
- overload
- bearing drag
What to measure
- V leg-to-leg
- A each leg
- rotation
- FLA total
What not to do
- Run long on two phases
- swap two leads on running compressor
Checklist
- I explain rotating field vs single-phase start aids
- I test for lost phase with amperage per leg
- I calculate/sync RPM from poles and 60 Hz
- I read service factor and efficiency on nameplate
- I verify rotation after motor or contactor work