Lesson Electric motors · Types and application
Single-phase motor types
Residential HVAC runs on single-phase power for most fans and many compressors. Shaded-pole, PSC, split-phase, CSIR, and CSCR each start differently and fail differently. Read the nameplate and diagram before you condemn a "burned motor" when the cap or relay is the real fault.
What we are trying to understand
Single-phase field classification:
- Shaded-pole — low torque, no external cap
- PSC — permanent run capacitor
- Split-phase / CSIR / CSCR — start cap and/or centrifugal switch
Each has distinct FLA, starting method, and typical failures.
Single-phase induction basics
Single-phase alone does not create a pure rotating field — needs auxiliary phase (capacitor or shaded pole) to start.
Rotor follows the field with slip — actual RPM below synchronous (e.g., 1725 vs 1800 on 4-pole, 60 Hz).
Nameplate: hp, RPM, voltage, FLA.
Shaded-pole motors
Tiny condenser fan, evaporator fan, some small aux fans.
- Low starting torque, modest efficiency
- Single speed, no external capacitor
- Copper band (shading coil) on pole creates phase shift
Failures: bearings, obstruction, open winding (burn smell).
Do not expect shaded-pole torque on belt-drive loads.
Split-phase (resistance start)
Start winding heavier wire, higher R — phase shift by resistance.
Centrifugal switch removes start winding ~75% rated speed.
Humming without spin → switch stuck open (no start) or closed (high amps, hot start winding).
Some designs add start capacitor → CSIR.
CSIR (capacitor-start, induction-run)
Start capacitor in series with start winding — more starting torque.
Switch removes start cap and winding at speed — runs on run winding only.
Common on belt-drive applications needing higher start torque than split-phase alone.
Listen for switch click at start and stop.
CSCR (capacitor-start, capacitor-run)
Both start and run capacitors — start cap out after start; run cap stays with start winding for efficiency.
Open run cap → motor may start but ~10% high amps, hot at full load.
Among the most efficient single-phase designs for fans and compressors with belt load.
PSC (permanent split-capacitor)
Run capacitor permanently in series with start winding — both windings energized whenever motor runs.
- No start switch — simple, reliable
- Lower starting torque — direct-drive fans, blowers
- Multispeed — tap different winding resistances (2–4 speeds)
Wrong cap µF → hum, low RPM, high line amps.
Multispeed PSC and furnace/cool fan logic
Extra leads (HI/MED/LO) select speed by tapping windings.
Heat often low speed; cool high speed — relay and heat fan switch interlock prevents two speeds at once.
Follow the OEM speed diagram: know which tap is active in each mode.
ECM note (boundary)
ECM (electronically commutated) is not PSC — module on motor, different diagnosis.
Do not apply cap rules to ECM; follow OEM module procedure.
Technician mindset
On site, answer:
- Which type from diagram and cap presence?
- Direct drive or belt?
- Centrifugal switch or potential relay?
- µF on nameplate?
Type drives test order — cap and switch before ordering motor.
Field case
Situation. Condenser fan hum, no rotation. PSC motor; cap bulged.
How to think it.
- Open cap → no phase shift → locked rotor hum → thermal opens.
- Replaced run cap (match µF and V rating) — normal spin, FLA matched nameplate.
Takeaway: PSC failures are often cap, not windings.
In the field
Symptom
Hum, no start, high amps, slow RPM
Where to look
Capacitor, centrifugal switch, relay, bearings, obstruction
Likely causes
- Bad run/start cap
- stuck switch
- open start winding
What to measure
- Voltage at terminals
- cap µF
- amp vs FLA
- spin shaft by hand
What not to do
- Swap cap µF "close enough"
- ignore switch click
Checklist
- I identify shaded-pole, PSC, CSIR, CSCR on equipment
- I know PSC has continuous run cap, no start switch
- I listen for centrifugal switch on split/CSIR
- I read hp, RPM, FLA, µF on nameplate
- I separate ECM from PSC diagnosis