Contactor Coil Symbol
Definition: The Contactor Coil symbol represents the electromagnetic operating coil of a contactor or relay, drawn as a rectangle with two terminals (A1 and A2), used in control-circuit wiring diagrams to denote the coil that, when energised by control voltage, produces the magnetic force that closes or opens the associated power and auxiliary contacts, as defined under IEC 60617-07 and ANSI/IEEE 315.
Also known as: contactor coil symbol, KM coil, magnetic coil, relay coil symbol, solenoid coil, A1 A2 coil.
What the Contactor Coil symbol means
The contactor coil symbol marks the control element of an electromechanical contactor or relay. When control voltage is applied across terminals A1 (positive or phase) and A2 (negative or neutral), current flows through the wound coil wire, creating a magnetic field that pulls in the armature. The armature movement mechanically closes the main power contacts and any associated auxiliary contacts simultaneously.
In a wiring diagram the coil symbol is always drawn in the control circuit, separate from the power contacts, because it operates at the control voltage (commonly 24 V DC, 110 V AC, or 230 V AC) rather than the main circuit voltage. The coil symbol carries the same designator as the contactor — for example, KM1 coil energises KM1 main contacts — allowing the reader to trace the relationship between the control circuit and the power circuit across the diagram.
How to identify the Contactor Coil symbol
The contactor coil symbol is a small rectangle (typically elongated horizontally) with one terminal on each short end, labelled A1 (top or left) and A2 (bottom or right) per IEC 60617-07 convention. The rectangle represents the winding of the coil and is distinguished from a relay coil (which may use the same symbol with a different designator) only by its label. Some drawings add diagonal lines inside the rectangle to indicate a coil winding, or a sinusoidal wave symbol inside the box. The coil symbol is connected in series with control elements such as push buttons, auxiliary contacts, and safety relays in the control-circuit rung.
Function in a circuit
The contactor coil converts electrical energy from the control supply into mechanical force. When energised, the coil's magnetic field attracts the contactor's iron armature, overcoming the return spring and closing all associated main and auxiliary contacts. When the coil is de-energised (control circuit opens), the spring returns the armature to its rest position, opening all normally-open contacts and closing all normally-closed contacts. The coil is rated for a specific voltage and frequency; applying the wrong voltage can cause overheating (overvoltage) or failure to operate (undervoltage). Coils are available in AC and DC versions; DC coils use a free-wheeling diode or varistor across A1–A2 to suppress the inductive voltage spike when de-energised.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 (switching devices) depicts the contactor coil as a rectangle with A1 and A2 terminal labels. IEC 60947-4-1 defines coil voltage tolerances: AC coils must operate between 85%–110% of rated voltage; DC coils between 75%–110%. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 represents the coil as a rectangle in the same manner as IEC. NEMA ICS 2 specifies coil operating voltage ranges and temperature ratings consistent with the IEC standard. |
| Key difference | The coil symbol is identical in IEC and ANSI/IEEE: a rectangle with two terminals. The terminal labelling convention differs: IEC uses A1/A2 for the coil terminals; older NEMA drawings may label the coil terminals simply as 1 and 2, or omit labels and rely on the circuit layout. Modern North American practice aligns with the IEC A1/A2 convention. |
Terminals / pins
| Pin | Name |
|---|---|
| a1 | A1 |
| a2 | A2 |
Typical values
Standard coil voltages: 24 V DC, 24 V AC, 48 V AC, 110–120 V AC, 230–240 V AC, 400 V AC (IEC range). Coil power consumption: 5 VA–150 VA (AC hold) depending on contactor size. DC coil impedance: typically 20–500 Ω. AC coil frequency: 50 Hz or 60 Hz (rated separately). Minimum pick-up voltage: 85% of rated voltage (AC coil, IEC 60947-4-1). Maximum coil inrush current: 5–15× steady-state coil current (AC coil at moment of closing).
Where the Contactor Coil symbol is used
- DOL motor-starter control circuits — the contactor coil KM1 is wired in series with a start push button (NO), a stop push button (NC), and an overload relay contact; pressing start energises the coil and holds via a KM1 NO auxiliary contact in parallel with the start button.
- PLC output control — PLC digital outputs (24 V DC sourcing or sinking) drive contactor coils directly or through interposing relays to switch motors, heaters, and pumps from programmable logic controllers.
- Timer-controlled switching — time-delay relays in star-delta starters energise the delta contactor coil after a timed interval following the star contactor coil energisation, automating the transition without operator intervention.
- Interlocking in reversing starters — the NC auxiliary contact of the forward contactor (KM1) is wired in series with the coil of the reverse contactor (KM2), preventing both coils from being energised simultaneously and avoiding a three-phase short circuit.
- Emergency stop circuits — the ESTOP relay coil is de-energised by pressing the emergency stop button, opening the safety relay contacts, which then de-energise all motor contactor coils in the panel.
- Building automation control — HVAC compressor and fan motor contactor coils are driven by BMS outputs (24 V AC or DC) controlled by room thermostats, occupancy sensors, and scheduling software.
Example
In a DOL motor-starter control-circuit rung, the 230 V AC control supply phase is connected through a STOP button (NC contact), then through a START button (NO contact) in parallel with a KM1 NO auxiliary contact (the hold-in or seal-in contact), then through the overload relay trip contact (NC), and finally through the KM1 coil rectangle symbol to the neutral. When START is pressed, KM1 coil energises, closes KM1 main contacts (connecting the motor to 400 V three-phase), and closes the auxiliary seal-in contact — keeping the coil energised after the START button is released.
Key facts
- The contactor coil symbol is a rectangle with terminals A1 (top/line) and A2 (bottom/neutral or negative), drawn in the control-circuit section of the wiring diagram; it carries the same designator (e.g. KM1) as all contacts operated by that coil.
- Contactor coils are voltage-rated devices, not current-rated; applying the wrong control voltage (over or under) causes coil failure — IEC 60947-4-1 requires AC coils to operate correctly between 85%–110% of rated voltage.
- DC contactor coils produce an inductive voltage spike when de-energised; a free-wheeling diode (for DC coils) or a varistor/RC snubber (for AC coils) must be connected across A1–A2 to protect the driving transistor or PLC output.
- AC coil inrush current is 5–15 times the steady-state holding current at the moment the armature closes; this inrush appears as a current surge on the control supply and must be considered when sizing the control transformer.
- The A1 terminal is conventionally the positive (DC) or line-voltage (AC) connection, and A2 is the negative (DC) or neutral (AC) connection; reversing polarity affects only DC coils (which have a polarity-sensitive internal diode).
- Pins on this symbol: A1 (x=20 y=0) — control supply connection; A2 (x=20 y=40) — control return connection.
- The coil symbol is always drawn in the control circuit, never in the power circuit; the separation of coil symbol and contact symbols across different circuit sections is the defining layout convention of IEC-style wiring diagrams.
- When a contactor coil fails (open-circuit), the main contacts remain open and the connected motor does not start; when it fails shorted, the contactor locks in and cannot be stopped from the normal control circuit.
Diagrams that use this symbol
- dol starter diagram
- delta star connection diagram
- star delta circuit diagram
- star delta starter diagram
- star delta wiring diagram
- star delta control circuit diagram
- star delta control diagram
- star delta control wiring diagram
Frequently asked questions
What does the contactor coil symbol look like in a wiring diagram?
The contactor coil symbol is a small rectangle drawn in the control-circuit section of the wiring diagram, with a terminal on each short end labelled A1 (line/positive supply) and A2 (neutral/negative return). It shares the same alphanumeric designator (e.g. KM1) as the power contacts and auxiliary contacts it operates.
What does the contactor coil symbol represent in an electrical schematic?
The contactor coil symbol represents the electromagnetic operating coil that actuates the contactor when energised. When control voltage is applied across A1–A2, the coil creates a magnetic field that pulls the armature and simultaneously closes all associated main power contacts and normally-open auxiliary contacts (or opens normally-closed auxiliary contacts).
What is the difference between A1 and A2 on a contactor coil?
A1 and A2 are the two terminals of the contactor coil. Per IEC 60617 convention, A1 is the line or positive terminal (the supply connection) and A2 is the neutral or negative return terminal. For AC coils either terminal can be the line or neutral; for DC coils with an internal suppression diode polarity is critical — A1 must be positive and A2 negative.
What voltage is applied to a contactor coil?
Contactor coils are available in a range of rated voltages: 24 V DC and 24 V AC are common for PLC-controlled systems; 110–120 V AC is standard in North America; 230–240 V AC and 400 V AC are common in European panels. The coil voltage must match the control supply voltage. IEC 60947-4-1 requires the coil to operate correctly between 85% and 110% of its rated voltage.
Why does a DC contactor coil need a suppression diode?
When a DC coil is de-energised, its inductance generates a voltage spike that can be many times the supply voltage, sufficient to damage the PLC output transistor or relay contact that is controlling it. A free-wheeling diode connected in reverse-bias across A1–A2 provides a safe path for the coil's stored energy, clamping the spike to approximately 0.7 V. AC coils use a varistor or RC snubber for the same purpose.
What happens if the wrong voltage is applied to a contactor coil?
Applying overvoltage to a contactor coil causes excessive current, coil overheating, insulation breakdown, and eventual burnout — the coil burns open and the contactor cannot operate. Applying undervoltage below the minimum pick-up voltage (85% of rated) means the coil does not generate enough magnetic force to close the armature, so the contacts chatter or fail to close, causing arcing and premature contact wear.
How do I identify the contactor coil in a wiring diagram?
The contactor coil is the rectangle symbol labelled KM (IEC) or M (NEMA) in the control-circuit rungs of the wiring diagram, distinct from the main contact symbols which appear in the power-circuit section. The coil symbol's designator (e.g. KM1) matches the designator on all associated contact symbols (KM1 NO, KM1 NC) throughout the diagram, allowing cross-referencing between the control and power sections.
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