VFD for 480 Volt single phase

[Kentower]

Whilst I have had a little experience using 2 and 3 hp VFD's on 240 volt. I am pretty happy with how to wire them, how to dig out the star point, separate the wires, solder tails on, wire them in delta etc. My nephew wants to run a 7.5 kw 3 ph 415 volt (currently wired in star but with the connections already there to change to 240 v Delta). one can go out and spend over a $1000 on a 480volt VFD. Has anyone bought the correct VFD and are they happy with the operation of it. Are there other options like getting a cheaper 415 Volt VFD? Do they run on 2 live 240 volt wires or 3? Would it handle 480 Volt? (The 480 volt I am talking about is commonly known as single phase and was used widely in rural areas. It consists of 2 live 240 volt wires)
Can anyone suggest a way that is of reasonable cost to run this 7.5 kw 415 volt 3 ph motor on 480 volt single phase? (a VFD is highly desirable to assist with speed control)
Another simple question whilst everyone's brains are so attuned. If the voltage is right, are VFD's just as happy to run motors wound in star configuration?
Any help greatly appreciated. Thanks Ken

[jhovel]

Motore NEED to be run at the correct voltage. That is a motor wired for 415V has to be run at 415V or less. Certainly NOT set to 240V star and then fed
415V! And in star configuration, it needs to be made for 480V operation OR the VFD needs be be able to limit the output to 415 (or 440V as the case may be).
In essence, the VFD doesn't 'know' where it's feeding the power. Star or delta or whatever.

Cheers,
Joe

[Ropetangler]

G'day Ken, depending on what you are running with the 7.5Kw motor, and whether or not you can afford some power loss, you might get away with building a static converter to run your motor. It is nothing much more than some capacitors, a contactor or two and not much else.
There are available from the U.K, VFDs which take 240V single phase, and output 415V 3phase, but I'm not sure that they go to 7.5 Kw (or 10 H.P.) and if they do, I would expect that one to be pretty expensive. I would think that the 415V 3 phase motor would be ok on 480V, and some 415 Volt VFDs would possibly be ok on the 480V input, you would have to check the specs, but just how much headroom you would have left could be an issue. The 480V nominal voltage would fluctuate up and down, just as the 240V supply does, and would almost certainly top 430V at times, so not sure if a 415V rated box could keep all the smoke in at 430Volts.
One solution may be a small transformer, set up as an auto-transformer, in buck mode rather than boost mode. If you had a 480V transformer with a 60V secondary winding, you can connect the 2 windings in series such that the voltages either add together, or they subtract. If they add, it is boost mode and if they subtract one from the other, they are said to be in buck mode. Graham Astbury in his book "Three Phase Conversion" (No.47 in The Workshop Practice Series explains it much better than I will ever be able to, and I recommend his book to you for more info pertaining to your situation. The Book Depository will have it for about $11.00 Au and no postage charge, and you should have it within a week.
The answer to the last question you had is yes. VFDs don't care, star or delta, if the voltage is right, it's all good.
Another way would be to build a Rotary Phase Converter, which is basically a static converter with another 3 phase motor, not driving anything, but just generating the extra phase. In fact, a 480V supply would be ideal I would think for you to generate 3 phase to just hook up any existing 3 phase equipment without further modification.
I'm sure that there will be other replies from far better qualified bods than myself, to add to your knowledge, good luck,
Rob.

Hi Ken, I just noticed that Joe has beaten me with his reply, and he said that it would not be OK to run a 415V motor on 480V. Joe may well be correct here, but I have been under the impression that many motors will work over that range, - I may have been misinformed on this, or equally I could have the bull by the t**s. Better to be sure rather than sorry on anything electrical at these power levels.:oo:

[GuzziJohn]

Hi Ken,

It certainly is a viable option to use a 3 phase 415 volt VFD on a "single phase" 480 volt supply.
You would need a 3 phase VFD that will function with only 2 lines present (check the manual for the de-rating required)
Check the input supply voltage ratings.
Voltage out to the motor is set to 415 v.

In my case I used a Danfoss VLT 6000

See my thread. http://www.woodworkforums.com/showthread.php?t=169363&p=1636465#post16364

John

[Kentower]

Thanks for the replies. Sorry, I should have been more specific, we really want to go down the VFD path as this provides speed control. I am looking at other options that may be cheaper than buying a straight out 480 V VFD. I like the idea of using the step down transformer you suggested Rob. (step the voltage down to 415 from 480).My question is:- if i buy a 415 Volt (input) VFD, can I run it off 2 input wires (415 volt between the two) instead of the usual 3?
Ken

[Ropetangler]

Thanks for the replies. Sorry, I should have been more specific, we really want to go down the VFD path as this provides speed control. I am looking at other options that may be cheaper than buying a straight out 480 V VFD. I like the idea of using the step down transformer you suggested Rob. (step the voltage down to 415 from 480).My question is:- if i buy a 415 Volt (input) VFD, can I run it off 2 input wires (415 volt between the two) instead of the usual 3?
Ken

The answer to that Ken is With some VFDs you can and some you can't. As John mentioned, even with the ones that do allow a single phase supply, you often have to derate them, because all the input power is going in on one phase instead of being shared between 3 phases. You will need to check this with your supplier if buying new, or check the specs on-line if buying second hand. You could always come back to this forum if you are not too confident that you have a full enough understanding to be sure whats what. You may find that you don't have to worry about the buck transformer, if you check with your supplier, because I'm reasonably certain some VFDs can cope with this (the 480V input) OK, but I have no idea of prices.
Rob

[Vernonv]

... and he said that it would not be OK to run a 415V motor on 480V. Joe may well be correct here, but I have been under the impression that many motors will work over that rangeYou are correct Rob, most motors seem to handle over range ok ... it is only 15%. I run a number of 3 phase motors (and a welder) on a 480V static phase converter and have done for many years, without issue.

I run a 3HP dust extractor and 5HP panel saw (at the same time ... I used to run a 7HP saw until I "upgraded" to a newer saw), plus a 3HP metal lathe. I also run an old transformer based TIG welder off single phase 480V (it was designed for single phase 415V).

As mentioned the SPC only requires a few bits and peices (contactor, caps, etc), but as mentioned won't give you variable speed as a VFD would.

[jhovel]

This turned up on eBay: http://www.ebay.com.au/itm/281234448276 so they are obviously available just for that situation.
A quick look at his 'store' showed this: http://www.ebay.com.au/itm/271356627186

[Theberylbloke]

Hi Kentower,

You need to be aware that 480 volts is 2 x 240 volts which are 180 degrees out of phase. The two phases are not 120 degrees apart as per three phase systems. This arrangement is often done in rural areas to provide "dual" phase supplies. The kicker here is that such rural supplies are often run from small transformers, typically capable of only 10KVA output or 40 amps in total. They are likely to be fused appropriately for the transformer configuration. These transformers have two windings each rated at 5 KVA or 20 amps. The windings are connected either in parallel or in series, tail to tail (if that makes sense). While pole mounted transformers can usually survive 30% overload, you will need to exercise some caution on how you load the two phases. Even more so if you are not the only customer connected to the substation! I am assuming you also have a house as well as the shed?! You should speak to your local distribution authority about your transformer, it's current loading and your future loading plans. This could prevent unforseen issues with them and unexpected bills following the previously alluded to unforseen issues!

I suspect that an attempt to run a 7.5 hp motor from one phase of a 480 volt supply will be unsatisfactory, VFD or no VFD.

If you are going to run this from a 480 volt supply then you also need make sure the electrical installation on your property will handle the extra load. Due to the limited nature of such supplies older switchboards and houses may have been set up with two power circuits and one light circuit with the stove split across the two phases. The mains circuit being quite small as not much load was going to be placed on them! Thus check the wiring sizes between the pole transformer, the main switchboard and final circuits. You will probably want a dedicated circuit for this lathe as well. All this will likely require a sparky who is conversant with AS 3008. That standard gives current ratings for cables and can be quite interesting!

A VFD works by rectifying the AC input to DC then inverting the DC back to 3 phases of AC. A three phase in and out inverter will expect the input to be fairly equal across all three phases. If you are only using two of the three phase inputs the VFD will have to be derated as has been pointed out above. Running a three phase VFD on one phase will result in very significant derating. Usually the inverter uses the magnitude of the input voltage as it's reference for the output voltage. i.e. 240 volt input will give 240 volt output. They can often be programmed to put out less volts than the input. A few VFDs have a voltage doubler circuit on the input side thus providing a higher voltage input reference.

Running a machine of this size on household supply is not simply a matter of finding a VFD, static or rotary converter suitable for the task. There will be more involved. Sorry if that tends to spoils the party, I don't mean to, just ensuring you go in with your eyes open.

Cheers

The Beryl Bloke

[Gerbilsquasher]

Hi Kentower,

You need to be aware that 480 volts is 2 x 240 volts which are 180 degrees out of phase. The two phases are not 120 degrees apart as per three phase systems. This arrangement is often done in rural areas to provide "dual" phase supplies. The kicker here is that such rural supplies are often run from small transformers, typically capable of only 10KVA output or 40 amps in total. They are likely to be fused appropriately for the transformer configuration. These transformers have two windings each rated at 5 KVA or 20 amps. The windings are connected either in parallel or in series, tail to tail (if that makes sense). While pole mounted transformers can usually survive 30% overload, you will need to exercise some caution on how you load the two phases. Even more so if you are not the only customer connected to the substation! I am assuming you also have a house as well as the shed?! You should speak to your local distribution authority about your transformer, it's current loading and your future loading plans. This could prevent unforseen issues with them and unexpected bills following the previously alluded to unforseen issues!



I would have thought an RPC with stepdown transformer from 480 to 415 would have been ideal in this situation. So what you are saying is that even though I have 16mm SDI from the pole to the house and 10mm circ to the shed, I can't pull 40 amps per leg on a two phase supply because Spaznet are cheapskates? I think I need a discount on my power bill.....

[Vernonv]

I would have thought an RPC with stepdown transformer from 480 to 415 would have been ideal in this situation.I keep looking for a suitable transformer (old welder perhaps) to allow my system to run "proper 415V", however you don't see them around a lot unfortunately ... I'll keep running my stuff off 480V until I can locate one.


So what you are saying is that even though I have 16mm SDI from the pole to the house and 10mm circ to the shed, I can't pull 40 amps per leg on a two phase supply because Spaznet are cheapskates? I think I need a discount on my power bill.....What was described is one of many different setups and especially when you consider they vary state to state. For example. we are supplied by a stonking great big transformer that actually supplies a few properties in the area (I believe our service fuses are around 80amps per phase). You really have to look at each individual situation, rather than accept what is generally held to be true.

[Theberylbloke]

I would have thought an RPC with stepdown transformer from 480 to 415 would have been ideal in this situation. So what you are saying is that even though I have 16mm SDI from the pole to the house and 10mm circ to the shed, I can't pull 40 amps per leg on a two phase supply because Spaznet are cheapskates? I think I need a discount on my power bill.....

Essentially the answer is that your distribution company sized the transformer to what the first customer requested or may have upgraded it if a later customer paid for the work. Pretty much as I also said if you are the only house on that transformer it's likely to be smaller rather than larger. Distribution companies are not usually overly generous when it comes to the size of the transformer they install. In this area, the use of 10kVA transformers to feed 3 or more houses is not unknown.

The only real way to know is to ask your distributor.

With regard to your quoted cable sizes, their actual current carrying capacity depends on a lot more than their cross sectional area. e.g. 6mm can be rated anywhere from 58 amps down to about 8 amps depending entirely on how it is run. This completely ignores the cables length and thus any voltage drop considerations.

Also consider the losses involved. Voltage drop in your mains and sub mains, the step down transformer should be fairly efficient, assuming it is correctly sized to your load, the RPC less so. All these things come into play as to how much current you can draw.

Cheers

The Beryl Bloke

[Kentower]

Thanks for replies. It is probably best just to go for the $1200 480 v 11 kw line to line unit off ebay and used off both 240 v lines. Even though the motor is 7.5 kw, it will only be used for light work and intermittently. The supply we are using was originally a dairy farm with refrig vat, ancillary pumps and house off the transformer situated right at the shed. The wiring to and in this shed is all heavy with a relatively new input to/and a new switchboard.
As a side issue I am looking at getting a VFD that will supply about 4kw and running off 415 v, off 2 input wires. I intend to get this 415 v off my old 275 amp ABEL welder. It has provision for either 240 volt or 415 volt input. My theory is to feed 240 v to the 240 v input on this welder (via the 240 v 30 amp outlet I have) and take off 415 v off (the two 415 v inputs of the welder) (I have put an AC voltmeter across these terminals and got 415 v or therabouts) then feed this to the vfd.
Any thoughts on this theory and also any brands of VFD that will run off 2 wire 415 v and supply about 4 kw would be appreciated.
Thanks Ken

[Ropetangler]

That should work well Ken, dual voltage welders are often used for donor transformers to build RPCs, and I see no problem with using it as a voltage step up for your VFD. There should be plenty of capacity there, and the only consideration you may need to take into account would be the electrical efficiency of the setup. There would be higher losses I imagine using the welder transformer to step up the voltage, than there would be if you could just use a VFD capable of accepting a 480V nominal input voltage and outputting 415V 3 phase. On the other hand, if it is used to power equipment which is used on an intermittent basis, for short periods of time, the efficiency becomes much less of an issue.
I can't offer any advice as to which VFD to use, but if you keep an eye out on eBay, Gumtree and the local classifieds, and just Google for specs on anything you feel is promising, you will soon find something, and hopefully a range of choices, and then you can get back to this forum and ask if anyone has thoughts on those specific models. Various members have suggested different brands, which they considered quality brands, so perhaps you can research these brands, and it may give you an idea of the features to look out for. Best of luck,
Rob.