checking voltage on 3 phase

[dlenox]

Guys,

First of all - thanks for all the suggestions and great reading on this board!

I have a 48v 17' wind turbine that I have the 3 wires (from the star connected 3 phase) coming into my garage.  I did not run the 4th common wire from the turbine.

For giggles, I would like to monitor the voltage coming out of each of the 3 phases individually.

Since there is no neutral is the only way to put a voltmeter across 2 of the phases? If so I would assume that I would be reading the combined RMS voltage as a combination of both phases?

Dan Lenox

[Flux]

You and a lot of others here seem to have a strange idea about 3 phase voltages.

In a star 3 phase system the phase voltage is that between any line ( output terminal) and the star point.

In a 3 phase 3 wire system you measure the line voltage between any pair of lines, it so happens that with delta the phase voltage is the same as the line voltage. With a star 3 wire system you can measure any of the 3 line voltages and divide by 1.73 to get the actual phase voltage if the waveform is a sine wave.

With a balanced system the line voltages all should be equal so there is no real point in measuring more than one.

When feeding a rectifier on load then these voltages become virtually meaningless, the waveform will be so far removed from a sine wave that the result will tell you nothing.

Below cut in all three voltages should be equal so why measure more than one.

I do agree that this is a bit confusing as a commercial 3 phase system is based on the line voltage. I do often see such schemes described in terms of phase voltage when they really mean line voltage.

I am glad that you only want to do this for giggles, as it is effectively useless even if you measure it.

Flux

[dlenox]

Flux,

Thanks for the info.

I am software engineer and develop command and control software for the DOD (US Navy and other branches), and eventially would like to put computer behind turbine and develop software to monitor and control it, hence monitoring the output of each phase as well as rpm and total rectified output.

Since I did not bring out the center star (common) connection from the stator, and hence can only perform this measurement between 2 phases.

My initial thoughts were that in an idea world all phases should be equal, however if a problem started occurring in the stator that possibly one or more phases would show a different voltage than the other phases.

I am not so much concerned with what the actual meter reading of the voltage would be but use it as a reference between all phases, to indicate that a problem has/is occurring.

This may be a moot point as once this condition exists that irrepairable damage probably has all ready been done within the stator, however if monitored by computer it could quickly respond and shut down the system to contain the problem.

While I have your attention perhaps another somewhat related question: I have a 17' 48v turbine very similar to the one that the OtherPower.com guys built, only difference is that I wound my 80turn coils with 1 in hand of #12 wire.

I have some local tower erectors/electricians helping me and they are looking for a suitable inverter/grid tie system for my application and are asking what the max voltage and amperage that the turbine will be outputting.  I have no response for them, obviously the minimum cut in voltage is 48v, but unsure what else to tell them for helping.

Always appreciate the help.

Dan Lenox

[Ungrounded Lightning Rod]

Even in a star you'd see a difference in the voltage between pairs of wires if one of your phase windings was in trouble.  It's just that with a star you'd see a voltage drop on TWO of the three pairs for ONE winding in trouble.

But that assumes a resistive load, or something else that lets the voltages vary.

I presume you have either a genny/rectifier/battery or a genny/rectifier/max-power-point-controller/battery setup.

In both cases the voltage at the output terminals will be clamped:  In one case by the diode drop and battery voltage, in the other by the diode drop and state-of-charge of the input capacitor on the controller.  So you won't see any significant difference in measured voltage on the various lines.

If you're looking for a difference in the coils' generation voltage, or other imbalances between the coils (like shorted turns), you want to look at the CURRENT in each phase wire.  That is proportional to the difference between the generated voltage and the clamping voltage divided by the winding resistance.

With a star connection a small change in one of your coils will produce a big change in the observed current on its leg, which works out very well for what you want to see.

Of course current will be going up and down all the time as the wind changes.  So you'll need to measure the average current in the three legs simultaneously and ratio them to see if you have an issue.

[Flux]

In these circumstances checking input volts will be meaningless as ULR has pointed out ( and I implied in the first reply). Current would be more useful but I suspect dc current and speed would pick up any departure from normal ( you may need to factor battery volts in this.

I strongly suspect you are right about it being no use, the only hope may be that you could shut it down before burnt coils fouled the magnets. You would be well in trouble before you detected problems and a burnt coil may fail mechanically before it gave any indication of shorted turns.

Regarding your other question I am not sure what you are doing. A battery operated grid tie inverter suitable for 48v would be fine as long as it could handle upwards of 3kW. The batteries dictate its requirements not the alternator.

For direct grid tie your alternator volts will be too low so that is not an option.

Flux

[CaptainElectrode]

As to the last post, I also have a 48 volt generator that I am now in the process of building.  I have the 3x1.5x.75 magnets and looking at winding the coils now.

The previous post said that this would be too small for a direct grid tie in.  In other  words, I want to go straight from the generator to the grid without batteries.  Is this possible?  If so what do I need in order to do this?

If not possible can I go from batteries to grid? if so what do I need in order to do this?

Any and all advice is greatly appreciated.

David