Dual Speed or two speed electric motors


Dual Speed Motor

 Dual speed motors, also known as two-speed motors, are electric motors designed to operate at two distinct speeds. Most of these motors typically achieve their dual-speed capability through Pole-changing motor they also known as Dahlander motors because they were created by  Robert Dahlander (1870–1935).l

Principle of operation.

A regular induction motor. works on the principle of electromagnetic induction where the alternating current passing through the stator winding induces a rotating magnetic field. This magnetic field then induces a current in the rotor winding, causing it to rotate and generate torque.

The rotor of the induction motor rotates because it tries to catch the rotating magnetic field (Ns) generated in the stator. It can never catch this magnetic field because of the slip and it turns chases this field until power is removed from the stator coils and the rotating magnetic field disappears.

The speed of the rotor depends on the speed of the stator's rotating magnetic field (Ns) simply put the Motor RPM depends on the Synchronous speed (Ns)

Mathematically, Synchronous Speed (Ns) is given by 120f / P


Ns = 120f / P


Where:

Ns = Synchronous speed 

f = Frequency of the power supply (Hz)

P = Number of poles


Looking at the formular above we can only change the Synchronous Speed (Ns) by either changing the Number of poles(P) or changing the supply frequency (Hz).

Before the invention of Variable Frequency drives the frequency supplied to the motor was fixed and couldn't be changed so the only option was to change the number of poles. if the number of poles can be doubled, effectively, synchronous speed (Ns) would be halved and if Ns is halved the rotor speed would also be halved.

How The Poles are changed in dual speed motors.

The trick is in the very design of the motor. The motor's stator winding is designed with multiple taps, which can be reconnected to form different numbers of poles. The actual switching between pole configurations is typically done through contactors. Take a look at the diagram below.


Fig 1.0 Internal configuration of a dual speed motor.





Fig 1.1 Power Cricut of dual speed motor



 The control circuit is wired so that only K1 (the Slow Speed Contactor) closes to feed power to 1U, 1V, and 1W. Based on the winding design, in this configuration, the motor runs in a DELTA configuration with four poles. More pole pairs result in a slower speed.

To run this same motor at high speed, K1 opens, then K2 and K3 close. When this occurs, power is fed to 2U, 2V, and 2W through K3 (the High-Speed Contactor), and 1U, 1V, and 1W are shorted by K2 (another High-Speed Contactor). According to the winding design, this configuration forms a DOUBLE ST winding with two poles. Fewer pole pairs result in a higher speed.

Fig 1.2 Internal configuration for High Speed.








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