PlantFlow 3D › Guides › Two-speed Dahlander motors
Two-speed Dahlander motors
How one tapped winding gives two speeds, why a third contactor makes a double star, and why two phases are swapped for high speed.
One winding, two speeds
A Dahlander (pole-changing) motor has one three-phase winding with a centre tap in each phase. By changing how the halves are connected, the number of magnetic poles is halved, so the motor runs at two speeds in a ratio of 2:1, for example about 720 and 1440 rpm.
Two-speed motors are common on fans, conveyors and machine tools where two fixed speeds are enough and a variable frequency drive is not needed.
Low and high speed connections
For LOW speed the supply goes to terminals 1U, 1V and 1W and the winding works in delta, with 2U, 2V and 2W left open. For HIGH speed the supply goes to 2U, 2V and 2W, and a third contactor links 1U, 1V and 1W together. This turns the winding into a double star (YY).
So a typical starter has three contactors: KM1 for low speed, KM2 for high speed and KM3 for the star bridge. KM3 closes first and KM2 is fed through an auxiliary contact of KM3, so the motor never receives high-speed voltage without its star point.
Same direction at both speeds
Inside a Dahlander winding the phase order seen on the high-speed terminals is reversed compared with the low-speed terminals. To make the motor turn the same way at both speeds, two supply phases are swapped on the high-speed contactor. If they are not swapped, the motor reverses when it changes speed, which can damage the machine.
Interlocks and protection
The low-speed contactor must never close together with the high-speed or star-bridge contactors: that would short-circuit the supply through the winding. Each coil circuit therefore passes through normally-closed auxiliary contacts of the other contactors.
The current is different at each speed, so there are two overload relays, each set to the nameplate current for its own speed, with both 95-96 contacts in series in the control circuit.
Try it yourself
Choose “Two-speed motor” in the simulator, run at low speed, stop, then run at high speed and watch KM3 close before KM2. The Check tab tests that both speeds turn forward.
Open the PlantFlow 3D simulator →Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.