Best way to regulate single phase induction motors (like fans and pumps)

[Miyuki]

Hi folks,

for my little HVAC project I am searching what will be best solution to control speed of fans and pumps
They all are classic single phase induction motors of tens of watts
And I think about three possibilities:
1) classic triac switch part of wave - solution proven by ages but can give lot of switching noise
2) low speed pwm of full waves at like 2-4Hz - to eliminate switching noise and have more linear relation from duty cycle to power
3) high speed pwm at kHz range - not sure if it have real benefits to use
All will be controlled by MCU, so no big implementation difference by all three modes

What do you thing and use ?

[rstofer]

You really can't control the speed of an AC induction motor by controlling voltage, you need to control frequency.
Google for 'microchip ac induction motor control' as Microchip does a lot of this stuff.

For such small motors, a control should be pretty easy to build but what you are building is a Variable Frequency Drive.  I have never seen one for 10s of Watts.

Shaded pole motors can be controlled with voltage as they are intended to slip.
https://electronics.stackexchange.com/questions/358562/shaded-pole-motor-speed-control

So now it gets down to induction motor versus shaded pole motor.

[Benta]

10s of watts? Are you certain they are not shaded pole motors? I don't think I've ever seen induction motors that small.

[Miyuki]

They are capacitor run motor (PSC)

[Siwastaja]

Some fans are specifically designed to run by adjusting voltage (or, often, using a TRIAC control - but using a transformer / an autotransformer works better and reduces audible noise!) While far from optimal, this has been the most typical way of controlling fans, even fairly large ones (hundreds of watts)!

These are induction motor designs that have larger acceptable slip range and don't encounter breakdown torque so easily. The somewhat quadratic nature of the power required (as a function of RPM) by the rotating fan blades also helps here, torque requirement drops when you slow it down, so the unoptimal voltage control works well enough.

Generally speaking, the best and most elegant solution is to actually create a two-phase VFD inverter, since a capacitor run motor is nothing more than a two-phase motor, capacitor generates the second phase (which in theory would be 90 degrees out of phase). But tuning this to be optimal may be a lot of work. I have done this once and it was an interesting project, but makes little sense economically. The end result can be better than the original motor, however, since now you can exactly control the phase shift and the amplitude of the second phase. The capacitor is always a compromise.

[schmitt trigger]

Siwastaja is correct, a two phase VFD would provide the best motor performance, the tradoff being complexity and cost.

The reason being that a capacitor will never provide a full quadrature field. The small motors which I have measured are around 70 to 80 degrees shift.

[Tomorokoshi]

Does this have a low-mass rotor running a gearbox, or a high-mass rotor connected directly to the fan impeller? Can you post a link to a representative motor?

[Simon]

Controlling induction motors by any of the methods used for brushed motors is a really bad idea, they simply do not work like that.

[NiHaoMike]

Generally speaking, the best and most elegant solution is to actually create a two-phase VFD inverter, since a capacitor run motor is nothing more than a two-phase motor, capacitor generates the second phase (which in theory would be 90 degrees out of phase). But tuning this to be optimal may be a lot of work. I have done this once and it was an interesting project, but makes little sense economically. The end result can be better than the original motor, however, since now you can exactly control the phase shift and the amplitude of the second phase. The capacitor is always a compromise.

It can make sense for small (less than 2 tons) HVAC compressors that are typically not available in 3 phase versions.

[Simon]

what is the scope of the project in reality and what could you use that has speed controllable brushless motors? Fans with speed control you can certainly get even for 110/220VAC

[Siwastaja]

what is the scope of the project in reality and what could you use that has speed controllable brushless motors? Fans with speed control you can certainly get even for 110/220VAC

And these fans with speed controls are very widely AC induction motors with a triac (or even autotransformer) speed controller.

[Simon]

what is the scope of the project in reality and what could you use that has speed controllable brushless motors? Fans with speed control you can certainly get even for 110/220VAC

And these fans with speed controls are very widely AC induction motors with a triac (or even autotransformer) speed controller.

whatever they are they are made to speed control. As the ones I see that run on mains come from supplies of DC 24 and 12 volt fans I would not assume triac control which is a really bad idea for induction motors.

[Miyuki]

For fans they say in datasheet:
Speed control
Smooth or stepped speed control can be performed
by external thymistor or transformer.
Several fans can be connected to one speed controller
if the total power and operating current of fans do
not exceed the rated controller values.
230V 80W 0,34A

[Simon]

Well if the fans allow it then just use PWM with a thyristor, it's the simplest solution.

[Siwastaja]

The datasheet is clear. This is the typical kind of fan ACIM that is designed to be "dimmed" using voltage or phase angle modulation. It's not optimal at any operating point, but it's cheap and traditional. So, a good old thyristor dimmer will do, but I hate the mains frequency buzz with such fans; a transformer makes it quiet.

[Simon]

what datasheet?

[Siwastaja]

what datasheet?

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