[Cferrell84]

I'm in the process of wiring up a CNC mill that is 480v 3 phase and have 240v single phase coming in
I have a rotary phase converter already to produce 240 delta 3 phase and just purchased a 63kva transformer to use for step up
After doing a ton of research I'm confused on how the ground should be hooked up if at all on the transformer. The transformer is a delta primary wye secondary which I am reversing and my incoming 3 phase to it is delta and feeding the machine with delta so from what I've gathered I need to disconnect the ground lead from the x0 tap going to the case of the transformer and only connect an earth ground to the case and then carry it to the machine is this correct

 

[psgama]

You still ground the XO of the Secondary side and bond to the case. You'll just be feeding your three phases to your CNC machine. Your method of installing a rotary phase converter ahead of the transformer seems very suspect to me though, as they are designed for a specific load.

Actually, I'm not sure I read your post correctly, what your doing sounds off though.

 

[Big_John]

If you have a wye:delta transformer then you need to be careful: The low-side wye will be rated for 208V. If you feed that with 240V you will get well over 500V out of your high side which your machines will not like very much m

 

[micromind]

If the low side has X0, do not connect it to anything, even the case ground. It MUST be completely isolated. If not, eventually the transformer will burn up.

If the high side is 480, then connect one of the phases (usually H2) to ground. This will give you a grounded B system. Not common these days but still seen in older installations.

Also, as noted, the low side is very likely 120/208. If so, the high side voltage will be really high and the transformer will run really hot regardless of load.

Rob.

P.S. We'll find out the actual knowledge level of posters here; those who state that H2 cannot be connected to ground do not understand transformers.

 

[ddawg16]

For all the money you spent on the transformer, I think you would have been better off going with a VFD.

I trust you have a good sized 240Vac feed to your phase converter?

 

[psgama]

You would have been better off just stepping up the voltage and then using an oversized VFD single phased on the supply side.

You do not want a corner grounded delta system like the previous poster mentioned.

A project like this can be dangerous for a DIY, not to mention that electrical codes state the maximum voltage to ground for a residential installation is 150v, if this project is in your garage.....

If you are set on completing this, I would suggest you hire a competent electrician. For a 480v system, you should have ground fault detection lights, a proper disconnect that will protect your transformer, a proper disconnect for your CNC machine, and conductors that are sized accordingly.

Stay safe.

 

[Know A Little]

Another issue to consider is a backed transformer has a higher inrush and you need to know what lock rotor current is of the rotary converter and if it can handle a 6-12 times (Maybe higher). the transformer rating for inrush.

How many taps does this transformer have?
What is your actual incoming voltage? High and Low measurements taken over a period of time.

You do not want a corner grounded delta system like the previous poster mentioned.

While I agree with you this is a complicated project DIY'er what is your issue with a corner grounded delta?

 

[psgama]

No issues with a corner grounded delta in general, but they can be very dangerous to an unqualified person, and if this transformer is not in an electrical room. Dangerous step and touch voltages, ground currents, and higher dielectric stresses on two phases, combined with the fact that this is being generated by a rotary phase converter just really makes me worry about the safety of this installation.

But as far as a corner grounded delta goes on a real 3 phase supply, we used them all the time in older oilfield installations, haven't saw anyone do a new one in quite some time though.

 

[Cferrell84]

The transformer is 63kva 480v delta primary and 230v wye secondary NOT 208y so I should be close to 480 output if not then it has taps and the CNC transformer has taps for 340v up to 510v with about 15 options in between. I got the step transformer local for a steal and already had the phase converter or I would have went a different route.

CNC is 15hp w/ 480v transformer

The phase converter is a North American phase converter 25hp not homemade

Feeding the phase converter panel is #2 with a 100amp breaker

From the phase converter to the transformer is #4 wire with a 100amp non fused disconnect (I would've thought a fused disc. would've been better here but the tech I spoke with at the phase converter company advised to use a non fused and don't energize the trans. till the phase converter is running since it acts as a load

Transformer to CNC is #10 wire with a 30amp fused disconnect with 20amp fuses which came with the machine

The ground is the biggest thing I was worried about and want to go the safest route obviously so would that be to tie my ground coming in the transformer to the case only and leave x0 empty? I've heard of corner grounding before but never dealt with it and unsure how it works

 

[Cferrell84]

Transformer is an Acme DTGA-063-4s and says on the inside panel that it is rated for reverse feeding or step up but if it is "T" connected that x0 must not be used

 

[AllanJ]

With the delta secondary (as used here) connect the three hot lines to the respective hot output terminals and connect the ground to the transformer case only. If there is no neutral output terminal (center tap between two of the hot terminals then don't run a neutral alongside the output lines.

Whether it is a plain (no neutral; no corner grounding) delta supply coming in from the utility pole or a plain delta supply from your transformer secondary, hot to ground voltage is technically undefined and that is okay.

 

[Cferrell84]

Thank you. Should the disconnect between the phase converter and the transformer be fused or non fused? The phase converter has a 100amp
Breaker feeding it.

 

[Cferrell84]

Phase converter question

Just finished wiring in everything for a Fadal vmc 15 15hp CNC. The setup is as follows-
240v single phase in to 25hp North American phase converter
From phase converter to a 63kva step transformer wired for step up to 480v
Then to the machine

Fired up the RPC and transformer and these are the readings in getting
Output of the phase converter-
L1-L2. 244v
L1-L3. 265v
L2-L3. 244v

480 voltage checked at the disconnect at the mill-
L1-L2. 470v
L1-L3. 500v
L2-L3. 507v

Transformer has 5 taps and is a 480d-230y
The mill has a transformer in its cabinet with taps for voltages 340-510 are these readings good to go or do I need to get them closer or can I set the taps on the 470v line different then the 500v lines?

 

[ddawg16]

Re: Phase converter question

I'm personally not real thrilled about the voltage differences between legs on the 480v side. Open ckt, they will be a little high....but the 30v spread?

I'm betting part of the problem is your L1-L3 voltage....that one is too high. I bet if you swap a leg, the voltage will follow.

What kind of current are you supplying your phase converter? If your phase converter is in fact 25 HP, then you would need to be supplying it with something like 175A @ 230Vac.

I have a feeling your going to run into problems.

 

[Cferrell84]

I just pulled one of the caps on the RPC and got the high reading out of the RPC down but it also dropped one of my other readings.
Now it reads coming out of RPC
L1-l2 247v
L1-L3. 258v
L2-L3. 235v

Is it acceptable to adjust the taps on the step transformer independently like having one coil tapped on 2 one on 4 one on 5 or do they have to be all on the same setting

 

[ddawg16]

Re: Phase converter question

They should be closer....but, it's not going to hurt the motor.....assuming it works at all.

You didn't say how much current you are feeding it.

 

[Cferrell84]

Haven't checked the current. I went to grab my meter and realized my dad borrowed it so headed to get it now.

Should I adjust the taps on the transformer individually to get them closer at the machine

 

[Cferrell84]

The RPC is a North American 25hp phase converter the tech guy advised to run it on a 100amp breaker with #2 wire. Said they were rated from 60-125 amp

 

[ddawg16]

Re: Phase converter question

You are NEVER going to get 25HP from that thing if you are only feeding it with 125A.

25A @240Vac is over 175 amps.

Assuming your CNC motor is 15hp, it's going to draw a little over 17Kw. So your RPC is going to need at least 71.4A @240Vac. This is not taking into account PF and losses in the RPC

I see that 100A breaker being right at the ragged edge. Lets hope is has a long OC curve.

 

[Cferrell84]

Ok this thing has me stumped. I've got the incoming voltage into the transformer near perfectly balanced 244, 245, 247v but I still have a 30 volt drop in one of the legs coming out of the transformer its reading 483,485, 452 makes no sense to me. I tried moving the tap to a different lug on the low leg but it wouldn't run, it tripped the main breaker soon as I tried to energize it so had to move it back to the same tap number as the other 2 legs