Compressor Symbol

Compressor schematic symbol
The Compressor schematic symbol. Standard-specific details are listed below where available.

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.

Live editor identity: Live-library identity: `compressor`, 60 × 60 canvas units, 3 logical terminals. Match this exact SVG and terminal list when identifying the placeable editor symbol; do not identify a physical package from the schematic outline alone.

Function and usage boundary

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.

Usage boundary: Use the Compressor abstraction only when the intended circuit can be expressed through these logical ports: `c` (C (Common)), `r` (R (Run)), `s` (S (Start)). Select a versioned physical profile and exact manufacturer datasheet before assigning package pins, ratings, or fabrication dimensions.

Standards: IEC vs ANSI

IEC 60617IEC 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 315ANSI 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 differenceIEC 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.

The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.

Variant availability

Logical terminals

The live editor exposes `c` (C (Common)), `r` (R (Run)), `s` (S (Start)). These are stable logical terminal IDs for diagram connectivity, not a promise that they equal physical package pin numbers.

Logical IDEditor terminal name
cC (Common)
rR (Run)
sS (Start)

Polarity and direction

This editor symbol does not encode a universal positive/negative orientation. Do not infer physical polarity or package pin numbering from left/right placement alone.

Reference designator

Project-defined: No single reference letter is asserted for this broad symbol. Apply the project standard and equipment-classification rules consistently.

Where the Compressor symbol is used

Key facts

Common interpretation mistakes

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.

Sources and verification

Geometry fingerprint: 4616b527ca952ac5. A changed SVG, canvas size, or terminal definition invalidates the recorded review.

Related symbols

Physical breadboard, PCB, and panel mapping

No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.

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