Question for Electronics Experts

[buffo]

Hey guys...

I've got a question regarding the following variable AC voltage speed controller:

http://www.banggood.com/4000W-AC110V...r-p-87369.html

It's rated for 4KW, but only for resistive loads. I understand that when driving inductive loads with an SCR or a triac the phase offset can prevent the device from switching off, at least at higher currents. But I will be operating at something like 1/50th of the max capacity of the controller. My question then is this:

I plan to use this controller to operate a 1/10th HP AC motor. Do you think I will experience problems with the controller sticking in the on position, given the relatively low power of the motor relative to the rating of the controller? And if so, do you think I could mitigate the problem by adding a small resistive load in parallel with the motor, or would I need to add some capacitance to adjust the phase angle? (Really trying to avoid that route...)

Adam

[logsquared]

Unless you have a brushed motor you will need to change the frequency of the supply to change the speed reliably. I think the controller you are looking at is basically a "dimmer" it just cuts off part of the ac wave effectively reducing the average voltage.

[buffo]

Yeah, that's how it works. At present I'm using a Variac to control the speed of the motor, but it's a bit of a Kludge. That's why I wanted to go with a solid-state solution.

Adam

[logsquared]

My guess is it would work then. If its a dual scr circuit you should have no problems as it would probably turn off at the zero voltage crossing even if the current is out of phase. If its a triac circuit it may get dodgy at the low end. A resistor in parallel would probably be all you need to help that. At 1/10 hp you could use a small variac. I have some around here that IIRC are rated 1 amp and are about the size of an apple.

[CountFunkula]

The SCR or triac-flavored "dimmer" should work ok for decreasing your rotational speed, but your motor slip goes waaay up and you'll be getting a LOT less than 1/10 hp out of it.

I was really hoping you'd found an $8 variable-frequency drive...but naaah

Any particular reason you're using an AC motor as opposed to a DC motor? If you can live with a set of discrete RPM values, what about gearing up for fewer RPM?

[buffo]

The AC motor in question drives a blower for my wood-burning stove insert. I replaced the motor years ago, and the only one I could find that fit was the 1/10 HP unit. It's a bit under-powered compared to the original, which was either 1/4 or 1/6 HP (can't remember).

I've been running it on the variac because I can pump the voltage up to 140 volts, which runs the fan a bit faster, so I get pretty close to the same airflow as before. But this SCR would let me run it up to 150-170 volts, which would be ideal. Then I could still dial it back to 120-140 for lower speed operation. That's the theory anyway.

I don't want to change the motor again, because the mount is rather flimsy and probably would need to be rebuilt if I tore it down again. And of course the company that made the blower has been out of business for nearly 2 decades, so forget about OEM parts.

Worst case, I could buy a transformer with variable taps and wire it with a multi-throw switch, but this controller looked like a cheap solution that would do it all...

Gearing is out of the question - too complicated and no space in the housing.

A VFD is another option, and I think I saw one at Harbor Freight a while back (was designed to power a router), but I'd still need to step the voltage up with a transformer first before using the VFD, since I need to bring the speed up a bit from where it is when running on standard 110V AC.

Adam

[CountFunkula]

Ah! ok! Context always helps so much!

VFD is THE way to go, but not always an option $$$-wise.

The dimmer-type solution may work well enough in this situation but you risk shortening the life of the motor substantially. If those windings get too hot, you could be replacing that motor [again] MUCH sooner than you'd want to.

[logsquared]

A VFD is another option, and I think I saw one at Harbor Freight a while back (was designed to power a router), but I'd still need to step the voltage up with a transformer first before using the VFD, since I need to bring the speed up a bit from where it is when running on standard 110V AC.

Most routers use a motor with brushes. Basically a dc motor. So, it was probably a SCR based circuit. Sounds like the set-up you have is probably the best solution. Is the blower for draft or to push heat into the room? My uncle had a problem with a draft motor on his boiler. We tried several different motor fan/ blower combos. Finally, settled on a over sized motor and a mechanical damper to control the combustion.

[steve-o]

Adam, I'm sure that you already know this but if you do decide to experiment with the little Chinese thyristor box, you might want to build a small blast shield around it. You know .. just because ..

[buffo]

The dimmer-type solution may work well enough in this situation but you risk shortening the life of the motor substantially. If those windings get too hot, you could be replacing that motor [again] MUCH sooner than you'd want to.

Well, I never plan to run it below 110 volts. If anything, I'll be running it above normal voltage. So I don't think the motor windings will get too hot. (I've been running it off the variac at ~ 140 volts for several years without trouble.) Of course, now that I've said that I'm sure the damn thing will melt down tonight!

Is the blower for draft or to push heat into the room?

Just to push heat into the room; it's a completely separate air path from the firebox. The combustion air is drawn in to the firebox from the bottom through adjustable inlet dampers, and the natural draft of the chimney creates more than enough negative pressure to draw in air. (The outlet damper is completely closed on the stove during normal operation, but it's designed to have a "leak by" path to allow the fire to smolder slowly, to allow for maximum heat extraction of a given load of wood.)

Adam, I'm sure that you already know this but if you do decide to experiment with the little Chinese thyristor box, you might want to build a small blast shield around it. You know .. just because ..

HAHA! Good point Steve. Though it will be mounted inside the blower housing, which is made out of sheet steel, so if it does decide to go bang, I should be protected. (Might leave some carbon marks on the flagstone hearth, but that should scrub off easily enough!)

Once I get it hooked up I'll post back here and let everyone know how it worked...

Adam