2-Speed Motor Wiring Diagram

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Technical wiring reference for two-speed AC induction motors covering Dahlander pole-changing connections and separate winding configurations with terminal identification and starter interlocking.

A two-speed AC induction motor provides two distinct operating speeds from a single motor frame. Two principal methods achieve this: the Dahlander (pole-changing) winding, and the separate (two-winding) design.

In a Dahlander motor, a single set of windings can be reconnected between two different pole configurations — typically a 2:1 speed ratio such as 4-pole/8-pole (giving approximately 1 450/725 RPM at 50 Hz, or 1 750/875 RPM at 60 Hz). The terminal arrangement uses six external terminals labelled 1U, 1V, 1W (for the low-speed, high-pole connection) and 2U, 2V, 2W (for the high-speed, low-pole connection). At low speed, terminals 1U/1V/1W receive the three-phase supply and 2U/2V/2W are left open. At high speed, terminals 2U/2V/2W receive the supply, and 1U/1V/1W are shorted together (star point). The exact connection depends on whether the motor is delta/double-star (for constant torque applications) or star/double-star (for constant power applications) — this is motor-specific and must be verified from the motor nameplate or wiring diagram inside the terminal box.

In a two-winding motor, two completely separate and electrically independent windings are wound on the stator, each with their own terminal set. The two windings are individually switched to the supply; only one winding is energised at a time. Two winding motors allow more flexibility — the two speeds can have any ratio and different power ratings — but at the cost of additional copper and a larger motor frame.

Critical to safe operation: both speed configurations require electrical interlocking between the high-speed and low-speed contactors to prevent both from engaging simultaneously. Simultaneous energisation of both connections causes a phase-to-phase short circuit or, in the case of a two-winding motor, winding damage. Mechanical interlocking (fitting a mechanical interlock bar between the two contactors) should supplement electrical interlocking for a higher-integrity solution.

Two-speed motors and multi-speed motors use different winding configurations to achieve variable speed: a Dahlander (consequent-pole) winding achieves two speeds with a 2:1 ratio from a single winding, while a separate-winding design achieves two independent speeds from two distinct windings. Three-phase dual-speed motors are common in HVAC fans, pumps, and compressors. Speed Queen commercial laundry motors are a well-known application requiring careful attention to the multi-lead wiring diagram. You can diagram any multi-speed motor control circuit free at circuitdiagrammaker.com.

How to wire 2 speed motor wiring diagram

Identify the motor type and obtain the terminal wiring diagram Open the motor terminal box and locate the wiring diagram inside the cover. Confirm whether the motor is a Dahlander single-winding (6 terminals: 1U, 1V, 1W, 2U, 2V, 2W) or a two-winding motor (6 or more terminals for two independent windings). Record the pole counts, rated speeds, rated currents at each speed, and the declared connection type (delta/double-star or star/double-star for Dahlander).
Select and specify the two-speed starter A two-speed motor starter requires two three-pole contactors (one for each speed) plus the control circuit with interlocking. Contactor ratings must be selected for the full load current at each speed — on a Dahlander motor, the high-speed and low-speed currents are different; each contactor is rated for its respective speed's current. On a Dahlander motor, a third shorting contactor may be required to create the short-circuit link at 1U/1V/1W terminals for high-speed operation.
Wire the low-speed (high-pole) contactor Connect the three-phase supply (L1, L2, L3) to the main contacts of the low-speed contactor (KM1). Connect the output of KM1's main contacts to the motor's 1U, 1V, and 1W terminals. Terminals 2U, 2V, and 2W are left open (disconnected) during low-speed operation. This gives the delta (or star) connection for the low-pole-count winding.
Wire the high-speed (low-pole) contactor Connect the three-phase supply to the main contacts of the high-speed contactor (KM2). Connect the output of KM2's main contacts to the motor's 2U, 2V, and 2W terminals. For a Dahlander motor in delta/double-star configuration, simultaneously short 1U, 1V, and 1W together via a third contactor (KM3) or a set of auxiliary contacts — this creates the star point required for the double-star high-speed connection.
Wire electrical interlocking between speed contactors Connect a normally-closed auxiliary contact of KM2 in series with the coil supply of KM1 — so that KM2 energised prevents KM1 from energising. Conversely, connect a normally-closed auxiliary contact of KM1 in series with the coil supply of KM2. This mutual interlocking means neither contactor can pull in while the other is already energised. Add mechanical interlocking bar between KM1 and KM2 as a secondary safety measure.
Wire the overload relays Fit thermal overload relays (or electronic overload relays) on both the low-speed and high-speed contactors. Set each overload relay to the motor's rated current at that speed as declared on the nameplate. Both overload relay NC contacts are wired in series with the control circuit stop path so that overload on either speed trips the motor out.
Test and commission With the motor disconnected (leads isolated at the terminal box), energise the control circuit and verify: KM1 (low speed) energises on the LOW SPEED command and drops out on STOP; KM2 (high speed) energises on the HIGH SPEED command and drops out on STOP; neither KM1 nor KM2 can be energised simultaneously (test the interlock). Reconnect the motor, run at low speed, verify rotation direction, measure per-phase current and compare to nameplate rating. Switch to high speed and repeat.

Specifications

Motor type	AC induction motor — Dahlander (pole-changing) or two-winding (separate winding)
Dahlander terminal designations (IEC 60034-8)	Low speed: 1U, 1V, 1W; High speed: 2U, 2V, 2W
Typical Dahlander speed ratios	2:1 (e.g. 4-pole/8-pole: 1 450/725 RPM at 50 Hz; 1 750/875 RPM at 60 Hz)
Dahlander constant torque connection	Low speed: delta (Δ); High speed: double-star (YY); 1U/1V/1W shorted as star point
Dahlander constant power connection	Low speed: star (Y); High speed: double-star (YY); 1U/1V/1W shorted as star point
Supply voltage (typical)	3-phase 380–415 V AC, 50 Hz (or 440–480 V AC, 60 Hz)
Interlocking requirement	Electrical (NC auxiliary contact cross-interlock) and mechanical (contactor interlock bar) — both mandatory
Applicable standards	IEC 60034-1 (motors); IEC 60034-8 (terminal designations); IEC 60204-1 (machinery electrical equipment); BS 7671 / IEC 60364 (installation)

Safety warnings

Both electrical and mechanical interlocking between the high-speed and low-speed contactors is mandatory. If both contactors are energised simultaneously on a Dahlander motor, a direct phase-to-phase short circuit occurs across the supply — this can cause catastrophic damage to the motor, cables, and switchgear, and presents a risk of arc flash and fire.
Never attempt to change speed while the motor is decelerating from the opposite speed without a suitable time delay. On Dahlander motors especially, the transition from high to low speed involves the motor acting as a generator into the supply — consult the motor manufacturer for the recommended speed-change sequence.
All work on motor starters must be carried out with the supply isolated and locked off (lock-out/tag-out procedure per applicable safe work procedure). Three-phase motor supplies are 400–480 V and carry enough fault energy to cause fatal electrical burns even at short contact duration.
Motor overload relays must be set to the correct current for each speed separately. Setting based on the higher-speed current and running at low speed continuously will not protect the motor at low speed — low-speed operation typically has a higher per-phase current and the overload relay must be set accordingly.
All wiring and installation must comply with IEC 60364, BS 7671, NEC/NFPA 70, or the applicable national electrical standard. Motor control panel design for machinery must additionally comply with IEC 60204-1 (Safety of Machinery — Electrical Equipment of Machines). Work must be carried out or supervised by a licensed electrician and/or competent electrical engineer.

Tools needed

Calibrated digital multimeter (AC voltage, resistance, continuity)
Calibrated clamp meter (AC current, for verifying per-phase currents during commissioning)
Insulated screwdrivers (flat-blade 4 mm and 6 mm for contactor and terminal block screws)
Torque screwdriver (for terminal tightening to specified torques)
Calibrated non-contact voltage tester
Ferrule crimping tool (for control cable terminations)
Rotation direction tester (phase rotation meter) or procedure-based rotation check

Common mistakes

Failing to interlock the speed contactors electrically and mechanically — the most dangerous omission in two-speed motor wiring; simultaneous energisation causes a short circuit.
Using the same overload relay setting for both speeds — low-speed operation draws a different current from high-speed; an overload relay set for one speed will either not protect or will nuisance-trip at the other speed.
Reversing the motor terminal connections for one speed — if L1/L2/L3 connect to 1U/1V/1W at low speed and the phase sequence reverses for 2U/2V/2W at high speed, the motor will reverse direction on speed change, causing mechanical damage to driven loads that are sensitive to rotation direction.
Omitting the star-point shorting contactor (KM3) on a Dahlander motor — without shorting 1U/1V/1W for the high-speed double-star connection, the motor will not run correctly at high speed and winding damage can result.
Setting overload relay trip time class incorrectly for high-inertia loads — fans and centrifuges have long run-up times; a Class 10 overload may trip on run-up before the motor reaches speed; Class 20 or 30 may be required.

Troubleshooting

Motor runs in one direction at low speed but reverses at high speed: Cause: Phase sequence at the high-speed terminals (2U/2V/2W) is reversed relative to the low-speed terminals — the phase rotation presented to the high-speed winding is opposite Fix: With the motor isolated, swap any two of the three supply conductors at the high-speed contactor (KM2) output terminals. Re-energise, run at high speed, and verify correct rotation direction with a tachometer or by observing the driven load. Do not swap at the motor terminals — swap at the starter output.
Overload relay trips on low-speed run but not at high speed: Cause: Overload relay is set to the high-speed current; low-speed current is higher, tripping the relay on low-speed operation — or the motor has a genuine overload (mechanical blockage, excessive load) at low speed Fix: Check motor nameplate for low-speed and high-speed rated currents. Set OL1 (low-speed overload) to the low-speed rated current. Measure actual current at low speed and compare to nameplate. If within nameplate limits, the overload relay was simply set incorrectly.
High-speed contactor will not energise even though the control command is given: Cause: Interlock auxiliary contact from the low-speed contactor (KM1) has not fully opened — KM1 is still energised or its auxiliary contact is slow to open; or the mechanical interlock is preventing engagement Fix: Verify KM1 is fully de-energised (audible drop-out, LED extinguished if fitted) before applying the high-speed command. Introduce a time delay between STOP and HIGH SPEED START if speed is being changed without a full stop. Inspect the mechanical interlock bar for damage or misalignment.

Frequently asked questions

What is a Dahlander winding in a two-speed motor?

A Dahlander winding is a single set of motor windings that can be reconnected to produce two different numbers of poles — typically in a 2:1 ratio. By changing the pole count, the synchronous speed changes (since synchronous speed = 120 × frequency ÷ pole count). The reconnection is done externally by the speed-change starter, using six motor terminals.

Can both speed contactors of a two-speed motor be energised simultaneously?

No — and this must never be permitted. Simultaneously energising both speed connections on a Dahlander motor creates a phase-to-phase short circuit across the supply. On a two-winding motor it causes winding damage and possible fire. Both electrical and mechanical interlocking between the two speed contactors is mandatory.

How do I identify the high-speed and low-speed terminals on a Dahlander motor?

Open the motor terminal box. The terminals are labelled 1U, 1V, 1W (for low speed) and 2U, 2V, 2W (for high speed) per IEC 60034-8. The motor wiring diagram inside the terminal box cover shows the connection diagram for each speed. The nameplate identifies the pole counts and corresponding synchronous speeds.

What is the difference between constant torque and constant power Dahlander connections?

Constant torque (delta/double-star): low-speed winding is connected in delta, high-speed in double-star. Torque is approximately constant across both speeds; power approximately doubles at high speed. Constant power (star/double-star): low speed connected in star, high speed in double-star. Power is approximately constant; torque approximately halves at high speed. The application (fan, compressor, lathe) determines which is appropriate.

Can a two-speed motor change speed under load without stopping?

Speed changing under load is possible but requires careful control. On a Dahlander motor, switching directly from low to high speed under full load causes a high inrush current (up to 6–8× rated) similar to direct-on-line starting. Some applications require a brief stop and restart sequence or a timed transition. Always consult the motor manufacturer's recommendations for the specific load type.

How do I wire a Speed Queen motor using a wiring diagram?

Speed Queen commercial washer motors are typically multi-speed capacitor-run single-phase motors with multiple winding taps for wash and spin speeds. The motor wiring diagram (printed on the motor label or in the service manual) identifies terminals by number; the machine's wiring harness connects specific terminals for each speed through a speed selector switch or relay. Always source the specific Speed Queen service manual for the model being serviced, as terminal assignments differ between washer generations.

What is the difference between a dual-speed and multi-speed motor wiring diagram?

A dual-speed motor wiring diagram shows two speed selections, typically achieved via a Dahlander winding (single-winding, 2:1 speed ratio) or two separate windings. A multi-speed motor diagram extends this to three or four speed taps, each corresponding to a different winding configuration. In both cases, the control circuit uses contactors or a drum switch to connect the appropriate winding terminals while disconnecting the others; interlocking prevents simultaneous connection of two speed configurations.

What does a multi-speed motor wiring diagram show?

A multi-speed motor wiring diagram shows the motor's numbered or lettered terminals, the winding connections for each speed (which terminals are energised, joined together, or left open), and the switching contactors or speed controller. For a Dahlander motor, low speed connects windings in series-delta and high speed reconnects them in parallel-star. The diagram also shows the overload relay, which must be set for the higher of the two speed current ratings.

How do you wire a two-speed 3-phase motor?

A 3-phase two-speed motor (Dahlander type) typically has six or more terminals. For low speed, three terminals are connected to the supply and the other three are left open or shorted together (depending on the winding type). For high speed, the connection reverses: the previously open terminals connect to the supply in delta, and the previously connected terminals are joined together. A two-speed starter uses two interlocked contactors — one for each speed — to prevent simultaneous energisation of both speed configurations.

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