Symbol for Electric Motor: IEC, NEMA, and IEEE Schematic Reference

The baseline symbol for an electric motor is a circle enclosing the letter M. However, in professional schematics, a generic circle is never enough. The exact representation changes based on the motor type (AC, DC, stepper, servo) and the governing standard—primarily IEC 60617 (International/Europe) and IEEE 315 / NEMA (North America). Misreading these symbols or their associated terminal designations (like IEC's U/V/W vs. NEMA's T1/T2/T3) is a leading cause of wiring faults on the bench and in the field.

The Complete Electric Motor Symbol Reference Table

Use this table to cross-reference schematic symbols with their physical terminal markings. Always verify which standard your drawing follows before landing wires on the terminal block.

Regional Standards and the "Rows People Get Wrong"

The divide between IEEE 315 / NEMA standards and IEC 60617 goes beyond the drawing itself; it dictates how the physical motor is terminated. In North America, a 9-lead dual-voltage 3-phase motor will have terminals labeled T1 through T9, wired in a specific Wye or Delta configuration for 230V or 460V. In Europe and most of the world, that same motor will have U1, V1, W1, U2, V2, W2, and rely on physical linking bars on the terminal block to achieve Star (Y) or Delta (Δ) connections.

The Rows People Get Wrong

When reading schematics, these specific motor symbols cause the most misinterpretations on the bench:

• Wound Rotor vs. Squirrel Cage: Technicians frequently miss the three small lines extending from the bottom of a wound rotor symbol. If you treat a wound rotor symbol as a standard squirrel cage and wire it directly across the line without the external resistor bank, you will trip the main breaker instantly due to massive inrush current.
• DC Shunt vs. DC Series: In a shunt motor symbol, the field winding is drawn parallel to the armature. In a series motor (like a universal motor), it is drawn in series. Wiring a series motor with no mechanical load will cause it to overspeed and destroy its own bearings (runaway condition).
• Generator vs. Motor: The symbol for a generator is nearly identical to a motor, but the generator symbol often features an arrow pointing out of the circle (indicating power flow out), whereas a motor has an arrow pointing in, or no arrow at all. In IEC, a generator is often labeled 'G' instead of 'M'.

Safe Interpretation When Nameplates and Markings Fade

Physical motor nameplates fade, get painted over, or fall off entirely. When the schematic is missing and the physical markings are gone, you must identify the motor type electrically before wiring it. Here is the definitive multimeter decision path:

1. Test for Brushes (DC / Universal): Set your multimeter to the lowest ohms range. Measure across the main power terminals while slowly rotating the shaft by hand. If you see slight fluctuations in resistance (e.g., jumping between 2.1Ω and 2.4Ω) as the commutator bars pass the brushes, it is a brushed DC or Universal motor.
2. Test for 3-Phase Induction: You should find three distinct pairs of wires with identical, very low resistance (often under 5Ω, depending on motor size). If you measure U1-U2, V1-V2, and W1-W2 and get three matching low readings, and infinite resistance between U and V, it is a 3-phase induction motor.
3. The Back-EMF Spin Test: If you suspect a Permanent Magnet (PM) DC motor or a synchronous motor, chuck the shaft in a cordless drill. Spin it at a known RPM and measure the terminals with your multimeter set to AC and DC volts. A PM motor will generate a measurable DC voltage. An induction motor will generate zero voltage unless it has residual magnetism and capacitors attached (acting as an induction generator).

For deeper symbol identification and standard reference charts, the All About Circuits reference library provides excellent visual cross-sections of legacy and modern schematic standards.

Frequently Asked Questions

What is the standard symbol for a 3-phase AC induction motor?

The standard IEC 60617 symbol is a circle with the letter 'M' inside, accompanied by a three-phase sine wave symbol (~ or 3~) next to it. The terminals are marked U1, V1, W1 for the line connections. In North American IEEE 315/NEMA schematics, it is a circle with 'M' and terminals marked T1, T2, T3 (for 3-lead) or T1 through T9 (for dual-voltage 9-lead configurations).

How do you draw a symbol for an electric motor with a built-in brake?

There is no single universal symbol for a brake motor. Standard practice is to draw the base motor symbol (e.g., a 3-phase induction circle) and add a secondary, smaller rectangle or circle adjacent to the shaft side labeled 'BRK' or 'MB' (Magnetic Brake). The brake coil is usually drawn as a separate solenoid symbol wired in parallel with the motor's main contactor, often with a diode or RC snubber symbol across it to suppress voltage spikes when the brake disengages.

Is there a different symbol for electric motors used in DC vs AC circuits?

Yes. While both use the base circle with an 'M', DC motor symbols explicitly show the internal commutator and brush arrangement (two parallel lines touching the inner circle) and separate field windings (shunt or series). AC induction motor symbols omit the brushes entirely and instead show the stator windings, often accompanied by the AC waveform symbol (~). A universal motor (which runs on both) is drawn as a series DC motor, as its physical construction relies on a commutator and series field.

What does the arrow inside or next to the motor circle indicate?

An arrow pointing into the circle indicates power flow into the device, confirming it is a motor. An arrow pointing out of the circle indicates it is operating as a generator. In some older schematics, an arrow inside the circle pointing to the right or left is used to denote the designated direction of rotation (Clockwise or Counter-Clockwise) when viewed from the drive shaft end.