I have a 3 phase converter installe

[David_M]

I have a 3 phase converter installed in our shop - it runs on a 50 amp breaker and should put out about 30 amps 3 phase.

Today I was working on a cooking appliance that has 6 45 ohm resistance elements used to heat/boil water in a tank.

I noticed a couple of things that seemed a bit strange. First it took a bit longer than I expected to get the unit to temperature. Second, I was expecting the same amp draw on all three legs. - I would have expected around 26 on each leg. Instead I was getting 16, 13 and 12

My question is whether or not these anomalies are a function of how the converter works and combines to make the third phase particularly when connected to heating elements. I have used this converter on other 3 phase equipment and not seen any issues.

To make sure there wasn't an element issue, isolated each element and they all range from 43.5 to 45 ohms. I also traced the wiring to verify the load was split evenly among the phases.

Finally the elements are designed for 208 volts on each phase not 240 +/- so would this be a big enough difference to lower my amp draw?

Thanks

 

[itsmoked]

Depends on the type converter you have. Most that are for motors actually reqire a motor (its inductance) as part of the conversion. If that's the case, you're going to get some wierd results just using resistive loads.

Try including an idling motor to see if things improve. Otherwise just use single phase power to test the individual elements and call-it-a-day.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Voltage?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

If the resistances range over some set of values, the currents will as well. You can only expect identical currents if you have identical impedances.

 

[David_M]

The unit I have is a Power Wave XR7. I think it is basically a big motor and a set of capacitors.

Here are the voltage readings with no load.

Each Phase to Ground

119.5 V
122.0 V
256.0 V

Phase to Phase

1-2 224V
1-3 209V
2-3 222V

I will be moving this unit to a true 3 phase 208V system later this week. It will be interesting to see what I get. This is the first time I have used this with resistance loads and I was surprised at the variation I was getting.

Thanks

 

[David_M]

Another update.

I just looked at this unit - it is 50 Amps in and only 20 Amps out! I think this limitation along with the resistive loads is likely what is causing these anomalies. I will report what I get when I put the unit on true 208, 3 phase power

 

[itsmoked]

What are you getting that's the new one?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

You are looking at a wild leg delta voltage at 256 volts to ground. Rotary phase convertors depend on the driven motor to help even out the phase to phase and phase to neutral voltages.
The action of the capacitors and as a result the voltages and phase angles depends somewhat on the load.
The nominal voltages to ground of a four wire delta system are 120V, 120V and 208V. Your reading of 256V is either in error or a result of the interaction between the capacitors and the induction of the motor. I suspect an error in the reading. 256V does not correlate well with the phase to phase voltages.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[David_M]

Just an update on this post.

First, I was initially mistaken the normal current draw for this appliance is around 16 amps not 26 as I had originally thought.

I contacted the manufacturer of the Powerwave XR7 and they said the 256V reading was not out of line for their machine. They explained why but that is lost on me now. They also said it was not uncommon to get amp draw variation leg to leg.

At 16 amp resistive load I was pretty close to the max capacity of the converter.

I since have put the unit back in service on a true 208V/3 phase system. I am getting almost identical 16 amp reading on each leg.

Also the response/heating time is much quicker and what I would have expected. I am not sure why, if I had a 240V system and put in 208V elements I would have expected different performance but not the other way around.

Thanks for all the suggestions.