HVACopediaCompressor electrical values
Winding resistance, power, megger readings and replacement rules.
Typical single-phase winding resistance
| Compressor size (hp) | C to R (Ω) | C to S (Ω) | R to S (Ω) |
|---|---|---|---|
| 1/2 | 4.7 | 18.6 | 23.3 |
| 3/4 | 3.3 | 11.9 | 15.2 |
| 1 | 2.5 | 11.0 | 13.5 |
| 1 1/2 | 1.9 | 8.4 | 10.3 |
| 2 | 2.1 | 6.4 | 8.5 |
Approximate values. Always compare with the compressor data sheet. C = common, R = run, S = start.
Hermetic compressor power, 120 V single phase
| Size (hp) | Running watts at 70 °F | Running watts at 110 °F | Starting watts |
|---|---|---|---|
| 1/16 | 66 | 100 | 375 |
| 1/9 | 117 | 160 | 740 |
| 1/8 | 108 | 163 | 743 |
| 1/7 | 160 | 218 | 970 |
| 1/4 | 235 | 320 | 1250 |
On a wattmeter a good start circuit shows a short pause at the combined start and run watts before dropping to the run watts.
Megohmmeter readings (winding to shell)
| Reading | Condition | What to do |
|---|---|---|
| Over 100 MΩ | Excellent | Nothing |
| 60 to 100 MΩ | Some insulation breakdown or moisture | Change the filter-drier; clean the windings if you can reach them |
| 40 to 60 MΩ | Overheated winding, dirty oil or moisture | Change the filter-drier. A burnt smell in the oil means an overheated winding; no smell means dirty oil, so change it |
| 20 to 40 MΩ | Badly contaminated oil, failure likely soon | Change the oil and the driers. If the oil is burnt, check every motor, including the fans, for worn bearings |
Test at 500 to 1000 V DC. Run the compressor at least an hour first so readings are comparable. Never megger a system that is under vacuum. Record the readings and watch the trend.
Relays and capacitors
- Potential relay: the contacts are normally closed (terminals 1 and 2) and the coil (terminals 2 and 5) is high resistance, in parallel with the start winding. A substitute may be rated 90% of the original voltage, never higher.
- Current relay: must be an exact match, since small spring or weight differences can slow the motor. Mount a weight-type relay level and upright. It should close and then open within about 3 seconds.
- PTC relay: cold, 3 to 12 Ω (acts like closed contacts); hot, 10 to 20 kΩ (acts like open). It needs 2 to 5 minutes to cool before a restart.
- Replacement capacitor: same µF, or up to 10% larger. Never smaller. Start and run capacitors are not interchangeable.
- Discharge every capacitor before touching it, with a 20 to 30 kΩ resistor across the terminals for a few seconds. A capacitor can hold up to about 600 V.
- A bulged capacitor top means its internal protector has opened. Discard it.
- Single phasing: if a three-phase motor loses one leg, the other two legs carry about 1.5 times the full-load current. Check the voltage on all three legs and compare the three winding resistances.
- Hard-start kit: choose by voltage and horsepower. Stuck compressors have causes such as long idle time, plating inside, or liquid refrigerant in the compressor.
Add-on start capacitor for a PSC compressor
| Run capacitor (µF) | Add-on start capacitor (µF) |
|---|---|
| 20 | 18 |
| 25 | 18 or 25 |
| 30 | 25 |
| 35 | 25 |
| 40 | 25 or 45 |
| 45 | 45 |
| 50 | 45 |
Use only with a start relay that drops the capacitor out once the compressor is running, and follow the manufacturer when there is a kit for the model.
Take it with you
All of these tables, plus the calculators, are in the HVACopedia app, offline.
Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.