supplying accurate output voltage < input voltage

Does the voltage have to be regulated -- that is, automatically adjust as the line voltage changes?

If not -- just stick a cheap DVM on the turbine!

I'm not sure. The turbine is very sensitive to changes in line voltage. A change of 1 volt in line voltage causes a noticeable change in airflow. However, there's a rheostat between the turbine and the power source, could be the rheostat is very sensitive to changes in line voltage. I could check that with my voltmeter I guess.

Digital voltmeter?

you didn't say how much power the turbine takes. and if it cycles once in a while, or a lot.

Two ways come to mind:

1. you can use a passive 'regulator' [by Solar?] It's a resonating transformer, so you have to specificy 50 or 60Hz. Very large for the amount of power that goes through them, but they regulate to 1%, which is around 1 Volt, then you could put a Variac after the regulator and adjust the voltage pretty exactly. Not much wasted power here.

1. Another, but complicated way, is to use a Saturable Core Reactor in series with your load. No current through control winding on core, very small voltage to load. Maximum DC current through control winding on core, and the reactor 'disappears', gone, so you get a lot of voltage to the load, but not all. Combine with simple AC to DC bridge circuit, resistors, and feed that current back into the control winding [right phase] and voila! pretty EXACT voltage, and automatic. If the technology is too daunting, post your request on craigslist,
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   for like you can pick up a technician or engineer who'll put it all together for you.

I checked the output voltage of the rheostat on the AC setting of the voltmeter, and it's about 104 volts. I turn it up sometimes briefly. It uses about 1-2 amps. Laura

I have a regulator box that has inside a motorized variac. Moves the variac to compensate for input voltage changes. But its reaction time isn't very fast.

The cheapest Sola constant voltage transformer still costs several hundred dollars.

You could use a Uninterruptible Power Supply (UPS). You'd need the kind that is always regulating the output voltage (an online UPS), rather than just turning on when power is lost or sagging badly (a standby UPS).

See this article for further explanation:

Times have changed. While Sola products are rather expensive, there are plenty of cheap constant voltage transformers on the market. For example:

Like all good resonant power transformers, they buzz and run warm.

Yes, for *very* small values of 'several hundred'. :)

--Winston

In that case: even the cheapest Mercedes still costs $300, because that's how much someone wants for one in the Penny Saver.

Yup.

There aren't too many $300 Mercedes that the seller will accept back if the buyer is not satisfied however. :)

BTDT, forgot to pick up the teeshirt.

--Winston

Ah, now comes the clue.... You don't have problems with your voltage..

It's a simple matter of mass air flow and your motor design..

You need a closed loop PID controller to keep that steady..

Jamie

Would a UPS that outputs voltage exact to within 1% or so be very expensive? Also, can one get a UPS with adjustable output voltage? Laura

Would a UPS that outputs voltage exact to within 1% or so be very expensive? Also, can one get a UPS with adjustable output voltage? Laura

No, it's definitely very voltage-sensitive. The guy who makes and sells the turbine told me that the motor speed is linear with the voltage, in the range of 100-120 V. The motor speed varies greatly over that range, from close to zero to max. Laura

Yes, I hear you and still, it is a mass air flow and motor design problem..

You most likely got a split phase shaded pole/capacitor motor that is improperly being controlled.

Just your basic window fan motor that shifts greatly in speed under just slightly altered conditions, including mass flow.

You need to closed loop controller...

Jamie

I haven't noticed that the motor speed is sensitive to anything else besides voltage, and the guy who makes the motor says that's the only thing that causes changes in speed. You could send your suggestions to Turbine Products, see what he says. In any case I need to deal with this particular motor, not some future version. Laura

Regulating the voltage is only going to fix part of your problem, the issues are that your motor design is very sensitive to RPM changes with load changes.

If you were to sit down and attempt to calculate the difference in voltage needed per RPM, you'll find that it's all over the place..

Motor speed in your case is like an inverse log, once you get it out of the hole, it'll speed up greater than expected. It's all about slip in an AC motor under load changes. AC motors do not have much torque at the lower RPM, unless of course, you're doing vector mode, and I know for a fact you're not doing that.

Your problem will not go away that easy how ever, a thyristor controller may work a little better over having a rheostat, but then again, you may have a slight drop on the high end of those.

what you have there is a low end design.. What you should have is a brushless DC motor which requires a speed controller.. THey do make integrated Brushless DC motors. We use them at work in vacuum loaders and they are good at maintaining speed. They are integrated motors of high RPM with a 0..10 volt input speed control reference. They are a closed loop and thus can monitor its own RPM.

Jamie

Have you considered that Turbine Products, whatever that may be, have sold you a piece of unusable garbage? Why are they not modifying their equipment to make it usable? Or at least providing you the applications support that you are seeking here? They're responsible for the halfassed design of this POS.