Small single-phase VFD for 120V? (2023 Update)

Do you think frequency control will work on those fan motors? They may be some modern electronic things.

Sounds like you're not dealing with a plain AC fan motor anymore. Papst the cooling fan maker? just change to a DC fan with speed control. VFDs on small motors is completly pointless anyways.

These are traditional AC fan motors sans electronics. They work acceptably though not ideal with a variable transformer but the UPS up front does not like the variable transformer for some reason (overload shutdown).

Yes, that company. They are plain AC motors, IIRC the shaded-pole kind.

This is a bit more challenging. The fans are in an environment that's too hot for electronics, which is also why they are completely metal.

Basically something like this but much smaller and for 120V, not 220V:

Do you need full range control? You could get two speeds with a series R or C, or even putting fans in series.

I'm doing a fan speed controller right now, but with 24 VDC fans, driven by a DAC and an opamp. Planning to adjust fan speed based on box internal temperature.

We'll have a DAC value integer with low and high limits. Once every second, if the temp is below setpoint, tick down, and vice versa. That is a low-drama control loop.

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** Have you tried a Variac?

Work just fine with most kinds of induction motor fans.

.... Phil

In theory yes and I thought about it. However, I need at least four speeds and that really gets old when you have to cram a bunch of big caps into a box. All of which have to be 120V AC rated. Resistors would becomes very toasty and for whatever reason the UPS does not like inductors.

I was looking for something that can dial in a speed via a potmeter. What I find strange is that such small VF drives are available for 230V but not for 120V.

With a meaningful hysteresis that should indeed be low-drama.

Yes, sorry, that was in an answer to John and not in the original post. There is a UPS up front and that doesn't even like the smallest variac I have. The inverter in the UPS starts to buzz and then issues an overload shut-off even without any fans connected to its tap-off. The UPS is a small one with a modified sine inverter, no space for anything bigger there.

This was a puzzler to me because that same variac runs fine on a smaller

200W modified sine camping inverter, with and without fans connected.

Joerg wrote: =============

** FFS - so the real story has nothing to do with fan speed control ????? ** Betcha the "modified sine" has a significant DC offset. Variacs are just like toroidal transformers and insanely sensitive to offsets on the AC supply. A pair of reverse parallel electros in series with the Variac will eliminate such offsets.

See page from my colleague Rod Elliott:

...... Phil

It does. The variac was never that great, fans become a bit louder at lower speeds. A VF drive would solve two problems, better fan control (more quiet) and it should run fine with any UPS. I was always on the lookout for a VFD even before buying this UPS.

Good point, could be a design flaw in the UPS. I can check that.

Try this:

.

It's best to get a small 3-phase motor for this. VFDs really do not like their motor circuit opening and closing, as with a centrifugal switch. I suppose one could use one phase and ignore the others; the manufacturer will know if this is supported.

Joe Gwinn

Joe Gwinn wrote: ================

** Giant HUH ???

** You feeling OK ? Or just got no idea what a Papst AC fan is?

...... Phil

No hysteresis. It's a 1-LSB bang-bang loop.

It's not so strange; many items that benefit from variable frequency drive are tools for metalwork where cutting depends critically on speed, and half-horsepower is tiny (but well over the 100W class), and

240V power is common.

Either use an inductive-rated dimmer and universal (with brushes) motor (drill motor, or other handheld power tool), or grab a DC supply suitable for a stepper motor with its controller. Regulated DC not required, but you'll want isolation, so regulation is kinda... free.

Heck, a UPS is involved? So, you have battery power you could tap?

** The OP needs an all metal, AC fan cos of the hot environment.

** BLDC fans are out - read ALL the posts in a thread sometime.

A Variac is all he needs for speed control. Using a "modified sine wave" AC supply is his one issue.

..... Phil

The 'BL' in BLDC is for brushless, which isn't the suggestion.

That's not speed control for induction motors, though, only universal or rectifier-to-DC types, with brushes... and a triac (like the speed control on a Dremel tool) is cheaper.

I'm OK. The issue is that while VFDs intended for machine tools may be able to drive a single-phase induction motor as well, but it's best to short or bypass the centrifugal switch if using the start winding of a single-phase motor.

They seem to all be single-phase, with a start winding.

Joe Gwinn

Then the hysteresis is one LBS :-)