Compressor Symbol
Definition: The Compressor symbol represents the hermetic motor-compressor of a refrigeration, air-conditioning, or heat-pump system — a single-phase induction motor sealed with its pump in one welded shell — shown in wiring diagrams as a circle marked COMP with three motor terminals labeled C (Common), R (Run), and S (Start).
Also known as: hermetic compressor, AC compressor, refrigeration compressor, condenser compressor, sealed unit, compressor motor, hermetic motor-compressor.
What the Compressor symbol means
The Compressor symbol denotes the electrical side of the system's heart: the motor that drives the refrigerant pump. In single-phase hermetic compressors the motor has two windings — a low-resistance RUN (main) winding and a higher-resistance START (auxiliary) winding — joined internally at one point, which is brought out as the COMMON terminal. The three shell terminals are therefore C (Common, fed from one leg of the supply, usually through the internal overload), R (the run winding's other end, fed from the second supply leg), and S (the start winding's other end, fed through the run and/or start capacitor).
How S is fed defines the motor type. A PSC (permanent split capacitor) compressor keeps a run capacitor permanently connected between R and S, giving the start winding a phase-shifted current for both starting and running. A CSR / CSCR (capacitor-start capacitor-run) compressor adds a large electrolytic start capacitor and a potential relay that switches it out once the motor is up to speed, for hard-starting applications. In residential condensing units the compressor's C and R terminals are powered through the contactor from the 240 V line, with the dual run capacitor's HERM terminal feeding S.
How to identify the Compressor symbol
In HVAC wiring diagrams the compressor appears as a circle (sometimes containing 'COMP', a piston symbol, or an M) with three terminal marks arranged as the C/R/S cluster — the same triangular Fusite terminal layout found under the compressor's terminal cover. Adjacent symbols usually include the internal overload (a small switch/heater mark inside or beside the circle), the run capacitor connected between R and S (or to the HERM terminal of a dual capacitor), and the contactor contacts feeding C and R.
IEC-style schematics draw the compressor motor as the standard motor circle with M 1∼ (single-phase) and show the auxiliary winding and capacitor explicitly per IEC 60617 winding symbols, while ANSI/NEMA ladder-style HVAC diagrams typically use the circle-with-terminals shorthand with C, R, S letters. Either way, the C/R/S labels are the giveaway: no other HVAC component uses that exact terminal triplet.
Function in a circuit
At start-up, both windings are energized: the run winding directly across the line, the start winding through capacitance that shifts its current roughly 90° ahead, producing a rotating field that accelerates the rotor. In a PSC design the modest run capacitor (typically 25–70 µF) stays in circuit permanently, so the start winding continuously contributes torque and efficiency. In a CSR design the potential relay drops out the large start capacitor (typically 88–270 µF electrolytic) once back-EMF across the start winding signals that the motor is near speed, leaving the run capacitor in circuit.
The internal overload protector (an OLP disc in series with Common, sensitive to both current and shell temperature) opens the circuit on locked-rotor or overheat and re-closes after cooling. Winding resistances follow a fixed relationship used in field diagnosis: R(C-R) is the smallest (run winding), R(C-S) is larger (start winding), and R(S-R) equals their sum — any deviation, or continuity to the shell, condemns the compressor's motor.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 represents the compressor motor with the single-phase machine symbol (circle, M 1∼) plus explicitly drawn main and auxiliary windings and capacitor per the winding and capacitor symbols; IEC 60335-2-34 governs the safety of hermetic motor-compressors. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 motor symbols plus NEMA ladder-diagram practice yield the familiar HVAC unit diagram: compressor circle with C/R/S terminals, contactor contacts, dual run capacitor (C/FAN/HERM), and overload marks. UL 60335-2-34 (formerly UL 984) is the North American compressor safety standard; AHRI standards cover performance ratings. |
| Key difference | IEC drawings render the compressor as an explicit two-winding single-phase motor circuit, while North American unit-wiring diagrams compress it into the circle-with-C/R/S shorthand that mirrors the physical terminal layout. The electrical content is identical; the C/R/S lettering convention itself is universal across manufacturers for single-phase hermetics. |
Terminals / pins
| Pin | Name |
|---|---|
| c | C (Common) |
| r | R (Run) |
| s | S (Start) |
Typical values
Residential and light-commercial single-phase hermetics: 208/230 V AC 60 Hz (or 220–240 V 50 Hz), sizes from about 1/3 HP (small refrigeration) to 5 tons of cooling. Nameplate electrical data: RLA (Rated Load Amps) commonly 8–30 A and LRA (Locked Rotor Amps) typically 4–6× RLA (e.g. a 3-ton unit: RLA ≈ 15 A, LRA ≈ 80 A). Run capacitors 25–70 µF at 370/440 V AC; start capacitors 88–270 µF electrolytic. Winding resistance examples: C-R 0.5–2 Ω, C-S 2–8 Ω on residential sizes. Refrigerants R-410A, R-32, R-454B in current equipment.
Where the Compressor symbol is used
- Residential central air-conditioning condensing units, fed through a 2-pole contactor with a dual run capacitor
- Heat-pump outdoor units, working alongside the reversing valve and defrost control board
- Window and portable room air conditioners with PSC compressors and combined fan circuits
- Domestic refrigerators and freezers, using a PTC or current relay start device on smaller RSIR/RSCR hermetics
- Commercial refrigeration — walk-in coolers, display cases, ice machines — with CSR compressors and potential relays
- Mini-split and packaged HVAC systems, where inverter-driven variants replace the C/R/S single-phase arrangement with a three-lead BLDC drive
Example
In a 3-ton condensing-unit diagram, L1 from the contactor feeds the compressor's C (Common) pin through the internal overload, L2 feeds the R (Run) pin directly, and the S (Start) pin connects to the HERM terminal of a 45/5 µF dual run capacitor whose common terminal returns to L1 — making the compressor a PSC motor; a technician verifying it measures roughly 1 Ω from C to R, 3 Ω from C to S, and 4 Ω from S to R, confirming healthy windings before condemning a weak capacitor instead.
Key facts
- C, R, S = Common, Run, Start: Common is the internal junction of the two windings; Run is the low-resistance main winding; Start is the higher-resistance auxiliary winding.
- Winding-resistance check: R(S-R) = R(C-R) + R(C-S), with C-R the smallest reading; deviation from this pattern or any continuity to the shell indicates a failed compressor motor.
- PSC compressors run with a permanent run capacitor between R and S; CSR/CSCR compressors add a start capacitor switched out by a potential relay for hard starting.
- In a dual run capacitor, the HERM terminal feeds the compressor's S terminal, FAN feeds the condenser fan, and C is the shared common.
- LRA (Locked Rotor Amps) is typically 4–6 times RLA (Rated Load Amps); both appear on the nameplate and size the breaker, contactor, and wire.
- An internal overload protector in series with Common opens on overcurrent or shell overheat — a compressor reading open C-to-anything when hot may just be on internal overload, not dead.
- Hermetic means motor and pump share one welded, refrigerant-filled shell: the motor is cooled by suction gas and cannot be serviced separately.
- Short-cycle protection (typically a 5-minute delay) protects the compressor from restarting against head pressure, which would trip it on LRA.
Diagrams that use this symbol
Frequently asked questions
What do C, R, and S mean on a compressor?
Common, Run, and Start — the three terminals of the single-phase motor inside the hermetic shell. The run (main) winding lies between C and R, and the start (auxiliary) winding between C and S; C is where the two windings join internally. Power feeds C and R, while S receives its phase-shifted current through the run capacitor (and start capacitor, if fitted).
How do I identify C, R, and S with a multimeter if the labels are missing?
Measure resistance across all three terminal pairs. The pair with the HIGHEST reading excludes Common — the remaining terminal is C. From C, the LOWER resistance goes to R (run winding) and the higher goes to S (start winding). The arithmetic check is R(S-R) = R(C-R) + R(C-S). Also verify no terminal shows continuity to the shell, which would mean a grounded winding.
What is the difference between a PSC and a CSR compressor circuit?
A PSC (permanent split capacitor) circuit uses only a run capacitor, permanently connected between R and S — simple, efficient, adequate starting torque for systems whose pressures equalize off-cycle. A CSR/CSCR (capacitor-start capacitor-run) circuit adds a large electrolytic start capacitor in parallel via a potential relay that disconnects it once the motor approaches speed — needed for hard-starting duty such as refrigeration with expansion valves that hold pressure differential. Hard-start kits retrofit PSC compressors toward CSR behavior.
What do RLA and LRA mean on the compressor nameplate?
RLA (Rated Load Amps) is the current rating used for sizing conductors, overloads, contactor, and breaker under normal running. LRA (Locked Rotor Amps) is the inrush the motor draws at zero speed — every start — typically 4 to 6 times RLA. A compressor that repeatedly draws LRA without accelerating (clicking off on internal overload) points to a failed capacitor or start device, low voltage, or a mechanically seized pump.
Why does the compressor hum and trip instead of starting?
The motor is drawing locked-rotor current without developing enough torque to accelerate. The most common cause is a failed or weak run capacitor (or start capacitor/potential relay in CSR systems); other causes are restarting against unequalized head pressure, low supply voltage, or a mechanically stuck compressor. Check the capacitor's microfarads against its rating first — it is the cheap, frequent culprit — and always allow the pressures a few minutes to equalize before a restart.
Can I test whether the compressor windings are bad without running it?
Yes — three ohmmeter checks with power off and the wires removed: (1) winding pattern C-R < C-S, with S-R equal to their sum, all in the expected low-ohm range (fractions of an ohm to a few ohms for residential sizes); (2) no reading of 0 Ω (shorted winding) or infinite on a COOL compressor (open winding — note a hot compressor may be legitimately open on internal overload until it cools); (3) no continuity from any terminal to the copper suction line or shell (grounded winding, megohm-test for certainty). Failing any of these condemns the compressor.
Related symbols
- 3-Pole Contactor symbol
- Contactor Coil symbol
- Single-Phase Motor symbol
- Pressure Switch symbol
- Thermal Overload Relay symbol
- Thermostat symbol
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