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Device guide / Circuits
05

Dahlander motor

A Dahlander motor is a two-speed squirrel-cage induction motor with a single winding. At low speed the windings are in delta, at high speed in double star. This is how it is wired with three contactors.

L1L2L3K1K2K3star pointM 3~1U1V1W2U2V2WK1 low speed: L1→1U, L2→1V, L3→1WK2 high speed: L1→2U, L2→2V, L3→2WK3 joins 1U, 1V and 1WLNS01112S22122S11314K11314K32122K22122K1A1A2S21314K31314K12122K3A1A2K33334K2A1A2
Control: S1 low speed (K1), S2 high speed (K3 and K2), common stop S0. The circuit of K1 has the NC contacts of S2, K3 and K2. K2 energises through an NO contact of K3. Power circuit: K1 supplies terminals 1U, 1V and 1W, K2 supplies 2U, 2V and 2W, and K3 joins 1U, 1V and 1W together.

How it works

LNS0S2S1K1K3K2K1S2K3K1K3K3K21Low speed

S1 energises K1, which latches. The motor runs at low speed in delta. The NC contact of K1 keeps the high-speed circuit open.

LNS0S2S1K1K3K2K1S2K3K1K3K3K22S2 pressed

The NC contact of S2 drops out K1, and its NO contact energises K3. K3 makes the star point, and its NO contact energises K2. The motor runs at high speed.

LNS0S2S1K1K3K2K1S2K3K1K3K3K23S1 at high speed

The NC contacts of K3 and K2 are open in the circuit of K1, so S1 does nothing. The only way back to low speed is through stop.

LNS0S2S1K1K3K2K1S2K3K1K3K3K24Stop

S0 breaks both circuits. K1, K2 and K3 de-energise and the motor stops.

A Dahlander motor is a squirrel-cage induction motor with two speeds but only one winding. The speed changes when the winding connection is changed so that the number of poles changes. The pole numbers are in the ratio 1:2, so the speeds are in the ratio 2:1. For example, a 4/2-pole motor has synchronous speeds of 1500 and 3000 rpm on 50 Hz mains.

If the speeds need any other ratio than 2:1, the motor has two separate windings. The Dahlander winding is the most common way to make a two-speed motor.

Terminals

The terminal box has six terminals, marked according to IEC 60034-8: 1U, 1V and 1W for low speed and 2U, 2V and 2W for high speed.

  • Low speed. The supply goes to 1U, 1V and 1W. Terminals 2U, 2V and 2W are left free. The windings are in delta.
  • High speed. Terminals 1U, 1V and 1W are joined together, and the supply goes to 2U, 2V and 2W. The windings are in double star (YY).

Three contactors

Low-speed contactor K1 supplies 1U, 1V and 1W. High-speed contactor K2 supplies 2U, 2V and 2W, and star-point contactor K3 joins 1U, 1V and 1W together. In the control circuit K2 can only energise through an NO contact of K3, and K1 is interlocked against K2 and K3 with NC contacts.

Constant torque or fan duty

Delta and double star (Δ/YY) is a constant-torque winding: the torque is about the same at both speeds. For fans and pumps there are also star and double-star windings (Y/YY). In those, the power at low speed is about a quarter of the power at high speed. The rating plate tells which one you have.

Starting and changing speed

According to VEM, a pole-changing motor is designed to be started direct-on-line at low speed, and high speed is normally reached through low speed. In some circuits, such as ABB's example with a soft starter, the motor is always stopped before the speed is changed.

The example on this page and the Johdota level are simpler: S2 also starts high speed straight from standstill. You may switch from low straight to high, but from high to low only through stop. If the motor must always start at low speed, the high-speed circuit needs an extra condition.

Remember

  • K1 must never be energised at the same time as K2 or K3. If K1 and K3 were both energised, the phases would be short-circuited through the star point.
  • Always wire according to the motor's own connection diagram. The rating plate gives the power and current for both speeds.

Sources

  1. VEM: pole-changing motors and Dahlander terminal connection plan KP 0003 (catalogue, chapter 16, p. 16/6)
  2. WEG: Specification of Electric Motors, section 5.1.2 Dahlander
  3. ABB: PSE soft starter with a Dahlander motor, application diagram 1SFB536005G5023
  4. Sprecher + Schuh: auxiliary contact terminal markings 13–14 and 21–22