12.1 Electric Motors, Compressors & Motor Protection
Key Takeaways
- On a hermetic compressor the three terminals are identified by resistance: common to run is the lowest reading, common to start is higher, and start to run equals the sum of the other two.
- Locked rotor amperage typically runs five to eight times rated load amperage, which is why a compressor that will not start pulls LRA until the overload opens.
- A potential relay senses back electromotive force across the start winding through its coil and opens its normally closed contacts to drop the start capacitor out of the circuit.
- A permanent split capacitor motor uses a run capacitor continuously and has no start winding switch, while a capacitor-start capacitor-run motor adds a start capacitor and a relay for high starting torque.
- A winding-to-ground reading below roughly one megohm indicates insulation breakdown, and a compressor that reads shorted to ground must be replaced, not restarted.
12.1 Electric Motors, Compressors & Motor Protection
[!NOTE] Electrical Applications for HAC Systems is a ten-question block on the Alabama blueprint - the largest single technical block outside Systems and Sizing. This section covers motors and compressors; the two sections that follow cover control circuits and branch-circuit wiring.
Single-Phase Motor Types
| Type | Starting torque | Start components | Typical HVAC use |
|---|---|---|---|
| Shaded pole | Very low | None (shading coil) | Small condenser fans, draft inducers |
| Split phase | Moderate | Start winding + centrifugal switch | Legacy open-drive applications |
| Permanent split capacitor (PSC) | Low | Run capacitor only, energized continuously | Blowers, condenser fans, small compressors |
| Capacitor start, induction run (CSIR) | High | Start capacitor + relay, dropped after start | Small hermetic compressors |
| Capacitor start, capacitor run (CSCR) | Highest | Start and run capacitors + relay | Larger single-phase compressors, hard-start retrofits |
| ECM / variable speed | Programmed | Electronic module, DC brushless | High-efficiency air handlers and inducers |
A PSC motor has no centrifugal switch and no start relay. Its run capacitor stays in the circuit at all times, shifting the phase of the current in the start winding to create a rotating field. A failed run capacitor produces the classic symptom: the motor hums, draws locked rotor amperage, and does not turn.
An ECM is a DC brushless motor with an onboard electronic control. Constant-torque and constant-airflow ECMs will increase their own speed to maintain airflow as static pressure rises, which masks a duct restriction until the motor overheats. Always measure static pressure on ECM equipment - the airflow symptom will not appear.
Hermetic Compressor Terminals
A single-phase hermetic compressor has three terminals in a triangle: C (common), S (start), and R (run).
Identifying Them by Resistance
With power off and capacitors discharged, measure between all three pairs:
- The highest reading is always S to R.
- The terminal not involved in that highest reading is C.
- From C, the lower reading goes to R; the higher reading goes to S.
- Verify: $R_{C-S} + R_{C-R} = R_{S-R}$.
Worked example: readings are 2 ohms, 6 ohms, and 8 ohms. The 8-ohm pair is S to R. The remaining terminal is C. From C, 2 ohms goes to R (the run winding is heavier wire and lower resistance) and 6 ohms goes to S. Check: $2 + 6 = 8$. Consistent.
Reading the Faults
| Reading | Diagnosis |
|---|---|
| Infinite resistance across a winding | Open winding - compressor is failed |
| Near-zero resistance where a value is expected | Shorted winding - compressor is failed |
| Any winding to the shell below about 1 megohm | Grounded winding - compressor is failed |
| Correct resistances but will not start | Check capacitor, start components, supply voltage, and mechanical seizure |
Insulation resistance testing uses a megohmmeter, not an ordinary ohmmeter, applying 500 volts direct current between a winding and the compressor shell. A reading above roughly 1 megohm is generally acceptable; below that, insulation is breaking down. Never megger a system containing electronic controls without disconnecting them.
Starting Current and Overloads
- Rated load amperage (RLA) is a derived value printed on the nameplate for use in circuit sizing.
- Full load amperage (FLA) is the running current of a fan or blower motor at rated load.
- Locked rotor amperage (LRA) is the current drawn at the instant of start, or any time the rotor cannot turn. It typically runs five to eight times RLA.
A compressor that will not turn draws LRA continuously until a protective device opens. Two families of device do that job:
- Internal (inherent) overload - a bimetallic device embedded in the windings that opens on temperature and current and resets automatically after cooling. A compressor "off on internal overload" is hot; it may take 30 to 60 minutes to reset, and restarting it repeatedly destroys it.
- External overload - a line-break device clamped to the shell, used on smaller compressors.
- Motor protection modules on larger equipment monitor discharge temperature, phase loss, phase reversal, and current, and lock out until manually reset.
Hard-Start Components
When a compressor struggles to start against unequalized pressures or low voltage, a start assist adds torque.
- A start capacitor is a large-value electrolytic device (for example 88 to 108 microfarads) rated for intermittent duty only. Leaving one in the circuit destroys it and the start winding within minutes.
- The potential relay removes it. The relay coil is connected across the start winding, where it senses the back electromotive force the motor generates as it comes up to speed. Its contacts are normally closed in the start capacitor circuit. As the motor accelerates, back EMF rises until it reaches the relay pick-up voltage, the coil energizes, and the normally closed contacts open, dropping the start capacitor out. On a standard potential relay, the coil lands on terminals 2 and 5 and the contacts are terminals 1 and 2.
- A current relay performs the same job on small compressors by sensing run-winding current instead, using a normally open contact that closes on high inrush.
- A start capacitor must carry a bleed resistor across its terminals - commonly 15,000 to 20,000 ohms at 2 watts - to drain the residual charge between cycles so the relay contacts do not arc and weld.
Three-Phase Considerations
- Single-phasing is the loss of one of three supply legs. The motor continues to turn but the remaining two legs carry far more current, and the windings overheat quickly. Suspect it when a three-phase motor hums, runs hot, and reads normal voltage on only two legs.
- Phase rotation matters on scroll compressors and on three-phase blowers. A scroll compressor running backward makes an abnormal noise, does not build pressure differential, and will be damaged if it is left running. Reverse any two of the three line leads to correct rotation.
- Voltage unbalance above 1 percent should be investigated, and current unbalance runs several times the voltage unbalance percentage.
- Three-phase motors have no start windings, no start capacitors, and no start relays - the rotating field is inherent in the three-phase supply.
A technician measures the following resistances across a single-phase hermetic compressor: terminal 1 to 2 is 3 ohms, terminal 2 to 3 is 9 ohms, and terminal 1 to 3 is 12 ohms. Which terminal is common and which is start?
What does a potential relay sense, and what does it do when the compressor reaches operating speed?
A three-phase scroll compressor was reconnected after a service disconnect replacement. It now runs with an abnormal noise, draws low amperage, and builds almost no pressure differential. What is the most likely cause?