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Device guide / Motor supply and protection
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Induction motor

The three-phase squirrel-cage induction motor: rating plate markings, star and delta connection in the terminal box, and changing the direction of rotation.

Motorrating plate400/690 V Δ/YStar (Y)W2U2V2U1L1V1L2W1L3Delta (Δ)W2U2V2U1L1V1L2W1L3690 V mains400 V mains
The arrow shows the direction of rotation seen from the D-end (the shaft end). Links in the terminal box connect the windings in star or delta.

Direction of rotation

MainsMotorL1L2L3U1V1W1D-end (drive end, shaft side)

Seen from the D-end (the shaft end), the shaft turns clockwise. Swapping any two phases reverses the direction.

Two phases swapped: the shaft turns anticlockwise. Swapping any other pair of phases does the same.

The three-phase squirrel-cage induction motor is the most common electric motor in industry. It is simple and rugged: three windings in the stator, and in the rotor a short-circuited cage winding with no slip rings or brushes.

Terminal box

The terminal box has six terminals: the winding starts U1, V1 and W1 and the winding ends U2, V2 and W2. Links between the terminals connect the windings either in star or in delta.

  • Star (Y). The winding ends are joined together. Each winding gets a lower voltage.
  • Delta (Δ). The end of each winding is joined to the start of the next winding.

Rating plate markings

The marking 400/690 V Δ/Y means that the motor is connected in delta on a 400 V supply and in star on a 690 V supply. The rating plate also gives the rated current, which the motor protection is set to.

Direction of rotation

The direction of rotation changes when two phases are swapped. In a reversing circuit, this is done with two contactors that are interlocked against each other.

Starting

  • In direct-on-line starting, the motor draws a starting current that is many times its rated current.
  • Star-delta starting reduces the starting current. It requires the delta voltage of the motor to be the same as the supply voltage, and all six winding ends to be brought out to the terminal box.
  • If one phase is missing, the motor does not start but hums and heats up. The motor protection or a phase failure relay stops it.

A two-speed Dahlander motor has a single winding, and changing how it is connected gives two speeds.

In your region

Showing notes for

Australia & New Zealand

In Australia the nominal voltage used to be 240 V before 230 V, so older equipment may be marked 240 V.

Source: Electrical Connection: When voltage varies (2012) · Standards Australia: AS IEC 60038:2022 Standard voltages

Other IEC countries

Check the supply before connecting: IEC 60038 gives 230/400 V, but 220/380 V and 240/415 V systems still exist, and some countries use 60 Hz.

Source: IEC 60038:2009 Standard voltages (public copy) · IEC: World Plugs, voltage and frequency by country

United States, Canada

NEMA motors mark their leads T1 to T12 instead of U1 to W4. On a six-lead motor, T1, T2 and T3 are U1, V1 and W1, and T4, T5 and T6 are U2, V2 and W2. On diagrams the motor itself is marked MTR.

Dual-voltage motors, such as 230/460 V, may have nine leads, which are connected according to the motor's connection table. Nine-lead motors cannot be started wye-delta.

Nameplate voltages are a little below the system voltage: a 460 V motor is used on a 480 V system, a 575 V motor on 600 V and a 200 V motor on 208 V. The mains frequency is 60 Hz.

Source: WEG: motor installation manual, NEMA and IEC lead markings · AEMC Instruments: testing three-phase dual-voltage motors (connection tables) · Baldor: motor and system voltages (Power Transmission Engineering, 2017) · PacifiCorp: power quality standards, ANSI C84.1 system voltages · NFPA 79 First Draft report, 2024 edition (13.2.3, Annex E Table E.1 device designations)

UK & Ireland, Europe

No regional notes on this page yet. If something is different in your country, please send feedback.

Frequently asked questions

Should a motor be connected in star or delta?

The rating plate tells you. On a 400 V supply, a 230/400 V Δ/Y motor is connected in star and a 400/690 V Δ/Y motor in delta.

What is the difference between star and delta connection?

In star, the winding ends U2, V2 and W2 are joined, and each winding gets a lower voltage. In delta, each winding end is joined to the start of the next. Links in the terminal box set the connection.

Why does a three-phase motor hum but not start?

Usually a missing phase, such as a blown fuse or a loose wire. The motor hums and heats up, so switch off at once. Then check the fuses and measure all three phases at the motor. A jammed load or bearing can also cause it.

How do you reverse a three-phase motor?

Swap two phases, for example L1 and L3. Shifting all three by one place does not change the direction. A reversing circuit swaps them with two interlocked contactors.

Can a 230/400 V motor be started star-delta?

Not on a 400 V supply. Star-delta needs the delta voltage of the motor to equal the supply voltage, so the plate must read 400/690 V. All six winding ends must also be in the terminal box.

Sources

  1. ABB: requirements for star-delta starting
  2. Electrical Connection: When voltage varies (2012)
  3. Standards Australia: AS IEC 60038:2022 Standard voltages
  4. IEC 60038:2009 Standard voltages (public copy)
  5. IEC: World Plugs, voltage and frequency by country
  6. WEG: motor installation manual, NEMA and IEC lead markings
  7. AEMC Instruments: testing three-phase dual-voltage motors (connection tables)
  8. Baldor: motor and system voltages (Power Transmission Engineering, 2017)
  9. PacifiCorp: power quality standards, ANSI C84.1 system voltages
  10. NFPA 79 First Draft report, 2024 edition (13.2.3, Annex E Table E.1 device designations)