Phase converter and transformers

Hello !! First post but have been lurking around for awhile. My question is hooking up a three phase step up transformer to a rotary phase converter. 230 1 ph to 600 3 ph. I have assembled many phase converters over the years and have found you cannot perfectly balance the phases. The generated phase always draws less amperage. Also the voltage to ground of this phase is double of the normal phases. With a Y - Delta Xmer you would think this would be a problem. The centre point of the Y primary is bonded to ground. I spoke to a chap from Cedarburg and he spoke of a second transformer to adjust the generated leg but he lost me. Thanks in advance if anyone can enlighten me on this subject. I do know that one solution is to use a single phase Xmer before the converter but once you start buying 600 volt capacitors it gets expensive.

Just to clarify you already have a phase converter and you want to step up the 3phase 240 to a 3 phase 600V?

First, you would NOT ground the wye point of the transformer. The RPC neutral and the transformer wye point are NOT AT THE SAME VOLTAGE. Don't do that.

The Cedarburg guy was talking about a buck-boost transformer, that can be used to even out the phase voltages if needed. It can work, but adds even more series impedance, so it may not be as great as it sounds, unless your load is pretty constant.

The generated leg voltage on the RPC varies with load, so if you have a variable load, it is hard to know just how much compensation you need. With a heavy load, you'd need more, and with a light load you'd need less, so you'd have to be adjusting the transformer taps to change the compensation.*

A three phase transformer should just pass through the imbalance. The windings are fairly independent when decently balanced within a few percent.

* if the wye-delta transformer you have does have its own "taps" for different voltages, you can use it to make at least some of the adjustment. Most likely they are on the delta side which you would be using for output.

With a Wye Delta transformer, the Wild Leg is not passed but gets "spread across" two of the three output legs....


Since a Delta Wye will also shift the phases by 60 degrees electrically.


There is a dedicated section for three phase, inverters, converters, and transformers.
Much More knowledge to be gained there.
Try to re-post in that section, if the thread does not get moved by the moderator.

I have a 240-600V open delta step-up setup on the output of my American Rotary RPC. This powers my 600V mill. It all works very nicely with no issues whatsoever. The 600V side is corner-grounded. The diagram is posted here.

Don Gitzel said:

Hello !! First post but have been lurking around for awhile. My question is hooking up a three phase step up transformer to a rotary phase converter. 230 1 ph to 600 3 ph. I have assembled many phase converters over the years and have found you cannot perfectly balance the phases. The generated phase always draws less amperage. Also the voltage to ground of this phase is double of the normal phases. With a Y - Delta Xmer you would think this would be a problem. The centre point of the Y primary is bonded to ground. I spoke to a chap from Cedarburg and he spoke of a second transformer to adjust the generated leg but he lost me. Thanks in advance if anyone can enlighten me on this subject. I do know that one solution is to use a single phase Xmer before the converter but once you start buying 600 volt capacitors it gets expensive.

Click to expand...

If you are feeding the output of the RPC to the Wye side of the transformer, you very definitely do not want the wye center grounded. An RPC running on the typical 120-0-120 line should read 120 V to each side of the line and the generated leg should read 208 V to ground because the line ground is not at the center of the three phase triangle. The voltage will vary with load and there is only a limited amount you can do about it.

A three leg three phase transformer will tend to balance the voltages because each leg shares its return path with the other legs. A Wye delta balances more because each voltage is a composite of all the applied voltages.

These issues have been flailed about on the Transformers, Phase Converters and VFD forum more than Obama care in the news.

Bill

You can use a Delta/wye with the wye used as secondary winding. I thought at Delta/wye only had a 30° Phase shift. You can also use the auto transformer. They don't have any phase shift, and they don't care if you're stepping up or stepping down the voltage. Autotransformers are usually labelled high and low where isolation transformers are usually labelled primary and secondary.

My main problem with using an isolation transformer is the potential for a leak to ground, and ground sharing through main panel. My system which I only run autotransformers feed through ground fault breaker's, seem a little safer in my eyes.

Pay attention to what you're high/low taps are labeled and/or primary/secondary. If it is designed for step up, it will be labelled accordingly.

575-600V RPC Methods

Have a look at these links for some food for thought on 600V setups.

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/240-v-575-v-3-phase-converter-project-314554/?highlight=600V+RPC

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/rotary-phase-converter-designs-plans-101231-post2899411/#post2899411

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/575-3-230-single-146328/?highlight=600V+RPC

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/230-1ph-600-3ph-102027/?highlight=600V+RPC

http://www.practicalmachinist.com/vb/transformers-phase-converters-and-vfd/wired-rpc-600-transformer-issues-310872/?highlight=600V+RPC

SAF Ω

Thank you all for your responses. Your knowledge amazes me. I sent e-mails to three large converter manufactures and was supersized to have engineers phone me back. They lost me pretty quick with a lot of theory. I have cataloged all of your responses and will study the information. I will also experiment when I have the opportunity.

Don Gitzel said:

I will also experiment when I have the opportunity.

Click to expand...

Be careful with 600 V! I always hook up meters while circuits are dead and double-check grounds, then energize from a remote disconnect. If no arc flash, I can approach. (Not that I've ever had a 600 V arc flash.)

Gorgoth209 said:

Just to clarify you already have a phase converter and you want to step up the 3phase 240 to a 3 phase 600V?

Click to expand...

I have been assembling rotary phase converters for many years as well as servicing and taking in on trade many different commercially made models. I have found that the commercially manufactured models only place the run capacitors across only one phase. I divide the capacitance and place it across two of the 230 volt lines. I found this balances the amperage and also brings the voltage closer together between the wild phase and the two other phases. Here is a real example. My converter using a 7.5 hp 3600 rpm idler and using an old Lincoln 10 hp motor as a load. L1 to L2 244 volts, L2 to L3 238 volts, L1 to L3 242 volts, L1- 10 amps, L2- 10 amps, L3 -9.6 amps. Now here are the values using the same load and a 7.5 WYN converter made in USA. L1 to L2 244 volts, L2 to L3 235 volts, L1 to L3 214 volts. L1 - 12 amps, L2 - 17 amps, L3 - 4.5 amps. You can see clearly how much better balanced my converters are. Now having said this, motor loads do not mind this imbalance however my concern is feeding a transformer.

Don Gitzel said:

I have been assembling rotary phase converters for many years as well as servicing and taking in on trade many different commercially made models. I have found that the commercially manufactured models only place the run capacitors across only one phase. I divide the capacitance and place it across two of the 230 volt lines. I found this balances the amperage and also brings the voltage closer together between the wild phase and the two other phases. Here is a real example. My converter using a 7.5 hp 3600 rpm idler and using an old Lincoln 10 hp motor as a load. L1 to L2 244 volts, L2 to L3 238 volts, L1 to L3 242 volts, L1- 10 amps, L2- 10 amps, L3 -9.6 amps. Now here are the values using the same load and a 7.5 WYN converter made in USA. L1 to L2 244 volts, L2 to L3 235 volts, L1 to L3 214 volts. L1 - 12 amps, L2 - 17 amps, L3 - 4.5 amps. You can see clearly how much better balanced my converters are. Now having said this, motor loads do not mind this imbalance however my concern is feeding a transformer.

Click to expand...

Just plug a transformer in and then repeat your testing with load and no load.

I generally start my RPCs with my transformers online. I have not had a problem yet, and I'm pretty protected up stream. Sure it adds to a little bit of additional hard start. But just go for it, and let us know how it works.

I used to be worried about the hard start with the transformer online. Well I forgot to...two or three times, and I blew some fuses on my 240 V disconnect which feeds my 600 V transformer then fused disconnect. I watch my amps pretty close, as they are in my face. On my 240 v disconnect now has 30 A fusing(From 20). this is my no load with transformer on line. Just shutting down for the night. This picture clearly displays what the transformer is drawing at Idle.

I do have a larger 20 hp RPC with 15 kVA transformer set up outside.