Induction Motor Torque

"A 1 phase (230V) induction motor's speed is changed by changing the voltage applied at the constant supply frequency (50Hz)."

You are starting off with a flawed premise. This is only true for a very small sub-set of single phase induction motors.

Before moving on, you need to first narrow down exactly what type of the myriad single phase motors you want to discuss.

I mean fancoil motors : induction ones, with a run capacitor, and with an autotransformer for speed control.

The addition : the motor itself being 1-speed one (3 leads - also the fourth wire being the earthing of course).

"I mean fancoil motors : induction ones, with a run capacitor, and with an autotransformer for speed control.

The addition : the motor itself being 1-speed one (3 leads - also the fourth wire being the earthing of course)."

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I'm even more confused than before. The official types of single phase induction motors that I know of are:

• Permanent Split Capacitor (PSC) type (capacitor always there but in series with a motor winding)
• Capacitor Start (capacitor only for starting)
• Capacitor Start - Induction Run (2 capacitors, one for starting, one for running)
• Split Phase Resistance / Inductance start (no capacitor, but resistance difference or an external inductor to create the phase split))
• Shaded Pole (no capacitor)

So if your's has a capacitor, it is one of the first 3. Of those, only the PSC can be speed controlled effectively because the others will have a mechanical Start switch involved in the phase splitting action required to make them start. If you slow them down, those switches re-close and the starting system comes back on, which it is not designed to do.

You can effectively control the Shaded Pole type by reducing voltage, but there is no capacitor. When you do that, reducing the voltage reduces the torque, and the loss of TORQUE effectively reduces the speed. You are not really directly controlling the speed.

I aslo have no idea what 3 leads would be for. Unless this is indeed a Cap Start motor, and how you tie in the 3rd lead affects the direction. In which case, then we are back to not being able to reduce the voltage to change the speed at all.

Hello JRaef

They are split not shaded poles : the fancoil unit's motors (water cooling/heating). The motor has 3 leads and the split capacitor - the usual design. Speed is changed by supplying voltage through the taps of an autotronsformer (6 taps, but used are 3). By time bushes become weared out, the motor has problem starting at the lowest speed (re-oiling does not help). Was thinking about invertor so that frequency was also reduced proportionally. Now get confirmed on an other site the torque would be better then.

"By time bushes become weared out, the motor has problem starting at the lowest speed"

BRUSHES???? If you have brushes, then this is not a normal AC induction motor at all! it is either a "Universal Motor" which is essentially a DC motor designed to run on AC input and there is no capacitor, or it is a very OLD type of motor no longer made, called a Repulsion - Induction motor, and there is no capacitor. So if you have brushes on the motor and you have a capacitor on the motor, I have no idea what the heck you have.

I maybe used the wrong term, "bushes" what serves as bearings (cheaper and less noisy than bearings).

But the qwestion is cleared as I said, thank you for your replies.

It is the mechanical load that determines the torque, not the motor.