Dahlander motor switch replacement

© J.N. Simonnet – March 2014

Problem statement

This design is intended as a replacement for a 3-position switch doing 0/3000rpm/6000rpm/0 on a Lurem wood machine[1]. This switch is natively equipped with a mechanical device preventing going backwards, which would damage the equipment if spinning brutally down to 3000 rpm from 6000rpm. The main motor is a dual-speed asynchronous Dahlander motor.

Remember that the kinetic energy varies with the square of the rotating speed, so the machine is designed to spin down freely to zero from 6000 rpm.

The mechanical part of the switch is broken and the switch has to be replaced.

The Lurem machine is no longer supported as the Lurem company has disappeared, and the broken original Dahlander switch cannot be sourced again.

Some vendors still have spare parts, but none still have stock for the original. Some of these vendors have switches doing 0/3000rpm/6000rpm/3000rpm/0 without the mechanical device preventing from going from 6000 to 3000 rpm. As already mentioned, this is not adequate. Additionally, these sell for around 200 €, and it is cheaper to re-implement with an Arduino[2] and 3x 3-phase relays.

The electrical motor is a Leroy-Somer LS100L2[3], 3.7kW/380V.

This photo shows the broken switch. You may notice the left hand side which no longer has the original mechanical device preventing backwards rotation.

Dahlander motors

Dahlander motors [4][5]are asynchronous 3-phase motors, wired to allow a ratio of two between the low speed and the high speed. There are two different models: quadratic couple (symbol YY/Y) and constant couple (symbol YY/D):

Constant couple model (YY/D)

This is the most common model. It corresponds to a load couple which remains constant with the rotation speed.

Low speed: each winding is fed with U/2 or 190V. Windings are under-supplied, which reduces slightly the available power. High speed: each winding is fed with 220V, the nominal value. Hence the Leroy-Somer LS100L2 motor is rated for 3 kW for the low speed and 3.7 kW for the high speed.

Quadratic couple model (YY/Y)

Quadratic couple means that the load couple varies as a function of the square of the rotation speed, like a fan. Model type is YY/Y. 

Each winding receives 220V in high speed mode, and 110V in low speed mode.

Connections to mains

Both models are connected as follows:

  • Low speed: U1/V1/W1 to mains 3-phases, U2/V2/W2 unconnected
  • High speed: U2/V2/W2 to mains 3-phases, U1/V1/W1 shorted

In the Lurem machine, we have a mechanical speed increase by a factor of 2, as the highest rotation speed one can achieve at 50 Hz is 2850 rpm.

It is important to ensure that the switch from low to high speed does not generate a short-circuit . This is something to watch when implementing the switching with relays, and it implies putting some delays between relay activations/de-activations.

Reference wiring with contactors

Here is the wiring for the two speeds. The implementation makes use of 3 relays. 

Though the drawing shows a constant couple model, the wiring is actually the same regardless of the actual type:

Overall design

As there is enough room in the existing control box, I have chosen a DIN rail mounting, with a maximum length of 230 mm. The 3 relays will then be DIN rail mounted. I have also selected a 24V DIN rail mount power supply, which will be connected between 1 phase and the neutral to obtain the 220V supply required.

DIN rail mounting

As for the electronics, I have selected a custom DIN rail box. It contains the custom interface board to steer the relas, as well as the Arduino Pro Mini.
The custom box can be seen to the right of the first picture, with a red LED on (Arduino).

Relay selection

I have chosen Moeller relays which switch 3 phases L1/L2/L3 and also have a 4th contact, which we are going to use for a light indicator. The relay coil is operated at 24V. The light indicators are 24V lights.

These relays consume 2.6W as per their datasheet, which translates to 110...

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