Thread: transformer sizing for 480v VFD

no....

is it a 3phase input VFD?..if so you will need 3phase input

get a 240V single phase input VFD to 240V 3phase output (you'll need delta wound motors) or as someone else has done on here a single ph 240V input to 415V 3phase output ..a special from UK I think it was

Some VFD's like my 5Hp 3 phase in to 3 phase out Allen Bradley, have a DC input set of terminals. So IF you had the right VFD you'd have to build a DC power supply involving a step up transformer, bridge rectifier and filter bank of capacitors. This would cost a fair bit of money to implement correctly. It would probably be cheaper to buy an inverter that steps up the voltage, i.e. 230 single phase in, 415V three phase out. I priced a 5Hp one out of the USA at $700 retail in 2008.

Originally Posted by oreos40

I was given a 480v input VFD. I have 220 power coming into the house. I would like to use the VFD, Here are my questions.

1. Can all transformers be used bidirectionally. IE can I hook up 240 at the secondary and get 480 off the primary?

2. What would the transformer have to be rated at for it to drive 3 HP? 2 HP? (conversion from KVA)

Thanks for any input!

Firstly let me say that I am NOT an electrician, and other than being a lapsed Amateur Radio Operator, or Ham, I have NO electrical qualifications what so ever.

Hi oreos40, as Eskimo has said, the short answer is no, if you are referring to the VFD as a transformer (which it is not), and you most certainly can not connect power to the output terminals of a VFD and expect it to be happy. I am uncertain as to what you meant by transformer here, It sounded like you were thinking the VFD was a transformer, but if you mean a separate transformer feeding the VFD, then the answer would be maybe.
If your VFD is a single phase input, you could use a transformer to provide the correct input voltage to it, in this case 480V but you must have the right current rating, otherwise you will have poor voltage regulation. For instance if the transformer was too small, you might have 480V with no load, but when you put the load on, the voltage would sag to a lower figure, and if the transformer was way too small, it would probably burn out.
For 3HP you would need a 3kva transformer*, although if you had a transformer with 220V input, and 260V output, and you wired it in auto-transformer mode for 480V you could get away with a 2Kva transformer.
A really good and cheap reference book which deals with this subject in an easy to understand manner, is "Three Phase Conversion" by Graham Astbury, one of the Workshop Practice Series books, (No.47) and it covers transformer selection and rewinding to suit your requirements, very well. I got my copy through The Book Depository in the U.K. and they do it for about $12, post free, and you would have it in about a week if my experience is anything to go by.
To answer your first question, transformers can be used bi- directionally, but you need to pay attention to the resistance losses in the windings. It may be that you will not be able to get the current you need, because the wire size in one of the windings was optimised for the original use say as a step down transformer, and you want to use it as a step up transformer.

* I am assuming here that you are able to start the motor under no load conditions, or are able to do a soft start, because if the motor needs to start in a fully loaded condition, with no soft start, you would need to have a larger transformer, perhaps by a factor of 4 or 5 ,but in real world situations you may get away with less than that perhaps as little as 2 or 3.
All the best,
Rob.

Originally Posted by oreos40

I was given a 480v input VFD. I have 220 power coming into the house. I would like to use the VFD, Here are my questions.

1. Can all transformers be used bidirectionally. IE can I hook up 240 at the secondary and get 480 off the primary?

2. What would the transformer have to be rated at for it to drive 3 HP? 2 HP? (conversion from KVA)

Thanks for any input!

The answer to the first question is yes, transformers are reversible. The turns ratio between primary and seconday detemines the voltage ratio.

The second question, I'd just get the biggest transformer I could, but you would want at least 2.5kw 220 to 480 transformer, which could be expensive..

If you were in Australia, I'd suspect you might have a 480V single phase input VFD, but since you are in the US, I'm not so sure, Even if it's 3 phase input you might still be able to run with just single phase input, the input is converted to DC before the 3 phase output is generated.

It's nice to get something for free, but these days VFD's are cheap, and transformers are still expensive... There are cheaper ways of running a 3hp 3 phase motor.

Regards
Ray

PS Model number and specs for the VFD would help to answer the question, but you might be able to run it directly from 220V and use a 3 phase motor with delta connection.

Thanks ray just checking on the transformer stuff your info was helpful. I was not thinking that i could use the VFD as a transformer. VFD's take input power and turn them into dc first then turn it back into variable frequency and voltage to drive the motor. The VFD I was given just needs 480 in it can be either single phase or three phase. the internals need the primary voltage for operation

Hi Oreos...

Like Ray said, dry transformers are reversible. What you propose to do should work fine. Industrial junk yards often will have 240:480 V tranaformers from scrapped machines. They also appear on US eBay frequently, but as you know weight will be a killer for out of state deals.

I think you'd be safe with a three KVA transformer, but you may find larger sizes more easily.

Greg

On edit, "480 v transformer" on us ebay gives over 2100 hits. If nothing there suits try HGR Industrial in Ohio: they've always given me good service and fast shipping.

Hi Oreos

I am not sure of what you are trying to achieve. I presume you are hoping to end up with 220v three phase. If this is the situation have you tried to just connect up your 220v to your input and set your parameters to output 220v.

The reason I say this is that I had a 380-500v VFD that I was hoping to get 415V three phase out. I conected it up to 240v single phase. An error code came up saying that the maximum volts I could get out of it was 380v DC. If I lowered my parameters output to 240v it could provide it at three phase 240v. In the end it I did not go down that path finding that when I connected it up with 480v two phase I could get the desired 415v three phase.

The VFD I am referring to is a vacon 380-500v NX0025. The supply voltage determines the output. Ie the higher the supply voltage the greater the HP motor you can connect to it. What I am getting at that if you are lucky enough to get it to work on 220v you will not be able to run a motor that the VFD is rated at.

I hope this helps.

Emu

1. Can all transformers be used bidirectionally. IE can I hook up 240 at the secondary and get 480 off the primary?

yes

2. What would the transformer have to be rated at for it to drive 3 HP? 2 HP? (conversion from KVA)

a) assuming you have 3x 220V into your house, and you are using a 3 phase step-up transformer, you would choose a 2kVA or greater transformer for 2HP and a 3kVA for 3HP.

b) assuming you only have 220V single phase to your home, you can ily use a single phase step-up transformer too, and you may have to read your VFD manual if a derating is necessary. In theory, a 1.732 times greater current flows into the VFD when hooked up single phase. Usually the VFD is derated by 50%. This is so for two reasons, once the current flows to only one rectifier diode instead of three, and second the ripple into the filter capacitor has a 3 times lower frequency. So, a 3HP VFD can only drive a 1.5HP motor, and the transformer can be choosen accordingly about 1.5kVA. But check with your VFD manual, because some of the better VFD's in the power range below 3HP are designed to be hooked up either 3phase or single phase without need for any derating.

Chris

Oreos most likely has 120v single phase service to his house. This type of service commonly has an additional conductor allowing 240v single phase, although it is composed of two 120v active lines 180 degrees apart. (As distinct from our 240 v active + 0 volt neutral)

For him to make 400v class three phase (415 to 480v) with a VFD he needs to step up his single phase voltage to 400v class, then introduce that to the VFD. So then, a typical 240:480v transformer (single phase) is all he needs assuming that his VFD can be derated yet still power his motor.

Greg

Thank you for all your help!