ac power flow

Do you have an oscilloscope and know how to use it?

or

Get a power meter from your electricity provider and watch the wheel spin backwards when you source the source.

Good luck with this.

r e e

That would work but I'm looking for a circut to install for long term monitoring and control on a wind generator. Hooking up my o-scope would not be the answer. Thanks P

You can use Current transformers in line with the motor and service. when both are equal, no current will be registering through the xformer. To detect differential speeds, you will get output and it will generate a phase shift compared to service, depending if you are in slip or regen.

That shift can be used to determine what you're after.

I suppose something like that could be done in spice :)

Jamie

Forget the o-scope, then. Buy the electric meter from your electric provider and watch the cute little wheel spin.

You're welcome.

Two options for you.

1) buy something like a PML 7650 power meter. In one box, it will give you AC power measurement and enough logic capability to control your process. 2) P = V*I*cos (theta). Use a pair of opposite polarity optocouplers connected to the line to two signals giving the polarity of your incoming voltage (or transformer couple your line voltage into a clipper circuit to turn it into a square wave. Run your motor feed through a CT. Convert your CT output to a voltage (one resistor), then multiply that voltage by +1 or

-1, depending on the polarity of your voltage. Low-pass filter the hell out of this signal and you will get something close enough to signed power for your purposes.

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I'll figure it out. Thanks anyway John

"Pat Roche"

** Doing this would be highly illegal in most places.

For safety and other reasons.

... Phil

r e e

Thanks Jamie, I'll look at that. The generator/motor is tied through a branch breaker and there are a lot of circuts on the service. I think I need to identify a direction change by looking at some sort of polairity indicator through a reactance value change. Possibly across an inductor like the transformer you mentioned only maybe a step down not a ct?

Thanks Jamie, I'll look at that. The generator/motor is tied through a branch breaker and there are a lot of circuts on the service. I think I need to identify a direction change by looking at some sort of polairity indicator through a reactance value change. Possibly across an inductor like the transformer you mentioned only maybe a step down not a ct?

If you persist in this exercise, which I personally don't recommend, be careful with any transformer device that may be current driven, such as using the LV winding of a conventional transformer to measure current. Firstly, LV windings may not have insulation suitable for use at mains voltage, but also if the output winding is open circuited you can get lethal voltages on it, way above the 240 on the mains. The same applies to CTs.

Where are you?

In some parts of the world (most parts outside of the Americas), you could be feeding power into one phase of a three phase network.

There would be complications with a capacitor start-run motor, as well.

Not a good idea.

--
"For a successful technology, reality must take precedence 
over public relations, for nature cannot be fooled."
                                       (Richard Feynman)

What are you going to _do_ with the set of dry contacts?

You can't generate power from the induction machine unless it's excited by the power line -- so it's not like you can keep it disconnected from the grid until it's generating power and then let it rip.

Whatever you're doing, if it has to do with wind power then someone has probably done it already, and you should start Googling. Making this sort of thing work in theory is easy, making it work safely is harder, and making it code-compliant is hardest. You can beat your head against the wall for years trying to talk your local electrical inspector into lighting up your home-brew circuit, where a little black box that says "UL" on it will be accepted in a minute.

The way that I know to do this is to measure the instantaneous current and voltage, multiply them, and average the result. _How_ this is done is open -- it can be done electronically with all-analog circuitry, electromagnetically (which is how house power meters work), or digitally with some analog, some ADCs, and some software.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com

There are wattmeters available that use a chip to do the instantaneous V * I multiplication to compute real Watts. Things like the "Kill-a-Watt" use a cheaper circuit, I think, but the same principle. Since you want to control a device, an isolated circuit would be better. Check on Digi-Key for an Analog Devices power meter chip, and read the application notes. They should have a reference circuit for use with a current and voltage transformer. You just need a zero-center meter to display power flow both directions.

I'm not so sure single-phase AC motors are the best choice as induction alternators. You generally need a capacitor bank across the motor to maintain the rotor excitation.

Jon

You may not need the cap bank if you're overrunning a motor already connected to the mains.

I've sort of been wanting to test this out, but I don't have any DC motors to overrun the induction motor I was last using as a generator.

I do recall having to run it way past the nameplate RPM to get 60Hz out of it when I had it attached to an engine. It may have been 2200RPM into a

1450 RPM motor to get it to do anything.

Yow, I think that is an indication of the problem. Let's see, 1450 RPM would indicate 50 Hz mains? You would get the same slip at 1550 RPM, assuming 1500 RPM is the synchronous speed, so that should be enough to get the same stator current. But, a 1450 RPM motor on 60 Hz is not your typical induction motor. It may be a shaded pole motor, which due to the shorting rings on the stator poles will not self-excite well at all. The typical 60 Hz induction motor should have a nameplate speed of about

1725 RPM, and will have a starting switch and start winding.

Were you trying to self-excite this without the grid? Then, putting some big caps across the stator terminals should help build up the magnetic field. Some motors have low-remanance laminations, and just won't self excite.

Jon