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to: Can a Variable Speed Drive control Single Phase Motor?
in Category: FAQ
    
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In a nutshell, all AC motors, of which the induction motor is one, create a rotating magnetic field in the stator (the non-rotating winding in the motor housing). The rotor follows this rotating field more or less accurately. If the rotor is independently magnetized say with permanent magnets, it follows the rotating field exactly. This would then be a "BLDC" or "permanent magnet synchronous" motor.

Your regular induction motor however does not contain a magnet, but what is called a squirrel cage winding in the rotor. The rotating magnetic stator field induces currents in this cage, pulling the rotor along, causing it to rotate. The torque is generated by magnetic induction, hence the name of the motor. The induction machine rotor never follows the stator field exactly, there is always a certain amount of slip. The slip is the difference in rotation speed of the stator field and rotor, and it is necessary for induction to happen.

When you first connect the motor to an AC supply, the stator field starts rotating at the supply frequency while the rotor is stationary. There is maximum slip and due to effects i am not going to go into, the generated torque is relatively low. When the rotor starts turning, the slip will decrease and torque increases until it reaches maximum with a small slip. The torque and thus actual rotation speed is a complex function of the load, supply voltage and frequency.

Proper speed control means that you need to vary both the motor voltage and frequency to control slip and keep it in a sweet zone in relation to the actual rotation speed. This is what Variable Speed Drives do. A fly in the ointment however is that for single phase induction motors this idea does not really work. You noted that i wrote earlier that the stator windings create a rotating magnetic field. Only you won't get rotation with just one phase, you get a pulsing voltage only. For this reason single phase motors artificially create a second phase to create the rotation. This second phase can be created in many ways using auxiliary windings and phase shift capacitors or coils. Combining a variable speed drive and a single phase motor is at best a horrible kludge and usually does not even begin to work. All proper variable speed drive controlled induction motors are 3 phase, as are the outputs of the variable speed drives. 3 phase shifted voltages is the minimum required to create a rotating field.

Back to single phase motors. In certain kinds of application, you can skip variable speed drives and simply control the motor by controlling the effective stator voltage using a triac or thyristor or whatever. This does nothing to the supply frequency, which is the standard 50/60 Hz all the time. So what is controlled is the amount of slip, by crude indirect torque control by stator voltage. If the load curve is monotonously increasing and sufficiently steep, the rotation speed will follow the stator voltage. Not linearly and not with world class dynamics, but it will follow. Now it happens that a fan is typically this kind of load and for that reason this simple scheme works quite OK for most fans.

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