240v to 480v downgrade

claya · Jan 8, 2020

I bought a 3ph 480v I-R compressor that uses a PAIR of 1ph isolation transformers as a power conditioner. So the PC is actually setup 480v in/ 480v out. I only have 240v, 3ph power. The isolation transformers can be connected as 240v primary. With what I have, is it possible to setup the PC as 240v in / 480v out? Or am I just better off buying a 240v / 480v 3ph transformer.

Jraef · Jan 9, 2020

Power conditioner on a compressor? Odd. Might it be a buck-boost setup to compensate for a voltage drop? That makes more sense on a compressor. It would help if you could post photos of the transformer nameplates.

claya · Jan 9, 2020

240V to 480V

They added the power conditioner because the air dryer blew out without it. According the the previous owner, it was sold without the PC, then added when the dryer blew out.

The isolation transformers are wired together.
Line in: X1-X3 & H4(tied together on both)
Line out: X2-X4 & H1, H4(tied together on both)

Mark Rand · Jan 9, 2020

There should be no problem at all with reconnecting the existing transformers for a 240V primary voltage. That was good luck on the purchase!

claya · Jan 9, 2020

240V to 480V

Mark Rand said:
There should be no problem at all with reconnecting the existing transformers for a 240V primary voltage. That was good luck on the purchase!

Based on the diagram, How do I change the input for 240V? It is currently wired for 480V.

They are NOT wired like the diagram, but rather ( I believe) as a power conditioner. Two of the three line-in phases are wired to the secondary (X1-3). Load is connected to the Primary (H1) & (H4) AND secondary (X2-X4)I will try and upload the current wiring diagram.

My licensed commercial electrician is not familiar with this setup. Hence the query for expertise.

SAF · Jan 9, 2020

Those transformers are a Buck-Boost pair. They can add or subtract 24 or 48 Volts to your line voltage.
They cannot be used to go from 240V to 480V. You will need a 3Φ transformer or a rewiring of the compressor controls and motor.

SAF Ω

Mark Rand · Jan 9, 2020

Damn, you're right. I read the 48 and 24 as 480 and 240. :dopeslap:

claya · Jan 9, 2020

480v to 240

SAF said:
Those transformers are a Buck-Boost pair. They can add or subtract 24 or 48 Volts to your line voltage.
They cannot be used to go from 240V to 480V. You will need a 3Φ transformer or a rewiring of the compressor controls and motor.

SAF Ω

See the attached. This is how they are wired. (480V in / 480V out) Pretty sure this is not buck/boost. But rather for power conditioning. Educate me, but any voltage variation on the one of the three lines is balanced by the other two, correct??

I CAN get a 240V to 480V transformer and connect it conventionally to the pair, but if there is a simple way to re-wire the existing setup for 240V in/480V out, that is preferred.

SAF · Jan 9, 2020

From your diagram, looks like they were setup for the boosting configuration.
460 to 483V. Conventional connection diagram.

The transformer you need is a 240Delta Input with a 480 Wye output, so that you can properly ground the Wye output and your compressor frame.

SAF Ω

claya · Jan 9, 2020

Curious why boost is setup with the line inputs connected to the secondary (Xx..) I would expect the opposite, as the primary is the higher amperage?

The compressor nameplate is 380v-480v. Is it possible they had this configured as Buck? With the primary on the load side?

All I know is at the original location, it did not have the transformer pair like it was supposed to have, and it blew out the dryer as a result. So the transformer pair got added at the second location. (I am 3rd owner) And they are calling it a Power Conditioner.

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SAF · Jan 9, 2020

They can be connected either way, buck or boost. You had the info, that it was needing to be bucked down closer to the machine nameplate. But it appears that you like to tease that information out of us.

Either way, the pair will not do what you need. Boosting 240 volts another 48V, won't get you anywhere near 480V.

SAF Ω

claya · Jan 9, 2020

They have been calling it "power conditioning" all along. And is what the original dealer described it as in the sales invoice. There has been no mention of buck/boost or a need for it, by anyone involved. This came out of a shop with 10 machining centers, and three other new compressors. I am not aware of buck/boost on any of them, except for the I-R compressor I have. I was told it was running on 480v.

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claya · Jan 10, 2020

240V to 480V

Can I use THREE 1Ph transformers like this to get from 240V Delta to 480V WYE? I only barely understand AC electrical, obviously.

The smallest 240V to 480V 3Ph transformers I have found thus far are 7.5KVA. With the 480v to 240v conversion, will I need 7.5KVa? At 240V, it looks like the compressor wants about 20A FLA.

SAF · Jan 10, 2020

Before looking for anything, it would be best if you measured your building voltage, phase to phase voltage, and phase to ground. Then provide the nameplate from the compressor you wish to run. Guessing on what you need can prove to be expensive.

Finding a qualified electrician would also be a major plus, as your transformer will need some specific grounding connections on its output, to make it safe.

SAF Ω

claya · Jan 10, 2020

Agreed. Will be taking measurements tommorrow. My electrician has a lot of commercial experience. Just limited with buck/boost, apparently.

The compressor nameplate is 480v-380v/3/60. Nothing about Amps, or VA. Odd. The closest manual that I can find, that matches the compressor suggests 460v @ 18.8A. It lists compressor as "star delta", which I think means, the motors are started as Star (wye) wound, then run as Delta, correct? Control voltage is 120v.

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claya · Jan 13, 2020

SAF said:
Before looking for anything, it would be best if you measured your building voltage, phase to phase voltage, and phase to ground. Then provide the nameplate from the compressor you wish to run. Guessing on what you need can prove to be expensive.

Finding a qualified electrician would also be a major plus, as your transformer will need some specific grounding connections on its output, to make it safe.

SAF Ω

Measured voltages:
1-2: 241.8v
2-3: 245.8v
1-3: 246.2v
1-g: 120.2V

The above is the 3PH output of the Rotary phase converter. The nameplate on the compressor says 480-380/3/60.

hitandmiss · Jan 13, 2020

When running on a RPC, you want only a Delta/Delta connection on any transformers for voltage conversion.
The "wild leg" is passed through on only 1 phase.
When using a Wye connection on a transformer, the "wild Leg" instability is now transferred to all 3 phases.

I figured this out when a new customer was running a $250,000 foreign made stone saw with an RPC powering a Delta /Wye transformer to step up from 240 to 380 VAC.
They were burning up servo and spindle motors when the control power to the VFD's that powered the motors caused the control software to crash allowing DC on the motor windings until the software rebooted.
The only solution was to power it with a generator of the correct voltage and frequency, end of problem, this was several years ago.

Bill

SAF · Jan 13, 2020

Now that we know that your using a phase converter with a 240V delta output, what is the HP rating of the motor on your phase converter and the HP rating of the compressor motor?

Are you aware that a rotary converter should be sized at twice the size of a hard starting load, such as a compressor? Is the 1Φ circuit feeding the phase converter, big enough to feed this load? What size is the 1Φ circuit feeding the converter?

Tags of the two motors, would be much better than guessing. A photo of the compressor control panel, would likely show if you have an across the line starter or a wye/delta starter.

An across the line starter would have one motor starter. A wye/delta setup would have one starter and two additional contactors and a timer.

SAF Ω

claya · Jan 14, 2020

SAF said:
Now that we know that your using a phase converter with a 240V delta output, what is the HP rating of the motor on your phase converter and the HP rating of the compressor motor?

Are you aware that a rotary converter should be sized at twice the size of a hard starting load, such as a compressor? Is the 1Φ circuit feeding the phase converter, big enough to feed this load? What size is the 1Φ circuit feeding the converter?

Tags of the two motors, would be much better than guessing. A photo of the compressor control panel, would likely show if you have an across the line starter or a wye/delta starter.

An across the line starter would have one motor starter. A wye/delta setup would have one starter and two additional contactors and a timer.

SAF Ω

40hp RPC. 15hp (VSD) compressor motor. Yes, I am aware of the 2x RPC HP requirement. Will get pictures of the motor control, etc..

1ph: 125a.

I thought one of the side benefits of a buck/boost/isolation transformer for regular utility supplied power was to condition the 3ph voltages to be more balanced? In the case of the RPC, could we put a buck/boost on the wild leg to balance the 3ph input? Is the wild leg voltage differential on our RPC enough to be concerned about?

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claya · Jan 14, 2020

Control panel, and 15hp motor nameplate attached. That is a contactor and some solid state relays for a fan. This is a VSD compressor, so the 15hp motor is connected directly to the drive. I fear this is much more complex than a motor contactor for the 15hp motor. Looks like the motor nameplate shows it wired as WYE, correct?

There is a separate 120v plug on the compressor also. It powers an automatic drain and the dryer motor.

Schematic attached. It shows a 200-230v or input, but not sure I trust that, as the decal/nameplate is 380v-480v.

240v to 480v downgrade

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