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Steve Steven
01-05-2009, 07:31 PM
I have posted before about my friend Frank who has bought a Colchester lathe. It has a two speed Canadian motor on it rated to operate at 575V AC three phase. I am hoping to operate it with a VFD.

First, let me say I am not an electrician and never played one on TV!

We have just gotten a transformer that came with it that is a REX three phase unit. Our problem is that the manufacturers label was painted over, it was stenciled not stamped. When I tried to get the paint off so I could read the label, it took off the stenciling also. What I was able to read was "Output Volts 575" and "HP 5.5". (the motor is 3HP) The Input Volts part of the label was ruined but it may have started with a "2".

I tried calling the manufacturer, he was no help at all. (even tho I had a part number [00746] off of the transformer coil)

My question to the group is, "What is the standard voltage in a Canadian industrial three phase system that would start with a '2'?" I am hoping that this is a 240 to 575 volt transformer, or that it will be close enough to work with a 240V input.

The next thing I am going to try is to put 120V to the input side of one of the transformers and read the output side with a voltmeter, this should give me the ratio and I can get a feel of what it will do with 240V input.

However, this three phase transformer set is a bit different than what I am used to, there are only three wires coming out from each of the three transformers in the box, with only two places to connect to on each transformer. There is a "X" terminal which is output, an "H" terminal which is input, on each transformer. There is a wire at the bottom of each transformer which connects all three transformers together. How does this work? I assume that the wire connecting the three transformers is a common for both the low voltage and the high voltage?

Three phase electricity has always confused me, there are only three wires going in and three wires coming out, it must work but I don't understand it!

Please help!

Steve

millwrong
01-05-2009, 07:56 PM
Hi Steve, It seems to be a 240V, three phase- 575V-three phase transformer, which is common industrial voltages here in Canada. Do you have three phase?

J Tiers
01-05-2009, 09:10 PM
I don't meet your specifications, but I do work with power stuff right now.

That sounds like an auto-transformer setup, where the primary and secondary are not separate, but share part of the winding. Evidently wired "Y", since the 3 you say are connected together.

But, if it IS wired "Y" and has a 240V primary, it would be to connect to 415V three phase..... which may not be right for your setup....... because 415V has 240V to neutral.

It sounds like you need to first determine what line voltage you have, and then determine what the motor is wired for.

Then the ratio of the transformer will show what your choices are. I was going to suggest a ratio check, but you seem to have that idea already. That's good, because I really really hate giving advice in this sort of situation when I KNOW I cannot see all the details.... Much better when I have Some idea of the technical savvy on the "other end" .

Watch it, though..... first, is that a single-piece transformer? it sounds like it is three separate transformers in an enclosure, unless you are just describing the three coils on one core. A one-piece unit may not give as accurate a reading with only one winding powered as one of the three separate ones would.

Then also, that transformer may take a fairly heavy in-rush current..... getting it powered up for a test may pop the breaker, if you just plug it in. if you have a variac, use it. otherwise consider checking ratio with a lower voltage. it will affect accuracy, but may be at least do-able

hardtail
01-05-2009, 09:17 PM
208 is another common 3ph voltage, I must be misunderstanding the nameplate data, is this the motor plate as it's hp? The Xformer should be rated in kva.......which you should require about 6 kva minimum if it's 575V, 5.5 hp and you intend on running at full load amps.

Steve Steven
01-06-2009, 07:23 AM
Thanks for the help.

Millwrong, No I don't have three phase available to me, just 220V with two hots. A small business shop. What I am hoping for is that it is in fact a 220 to 575 volt transformer set.

J. Tiers, Yes, I do believe it is wired as a autotransformer. I couldn't think of that phrase when I first posted. I don't understand your next sentenance,
"But, if it IS wired "Y" and has a 240V primary, it would be to connect to 415V three phase..... which may not be right for your setup....... because 415V has 240V to neutral." Do you mean that 415V is the "normal" high voltage? I can actually read the nametag where it says output voltage is 575V, I just can't read the input voltage.

I know my line voltage, it is 240V single phase. I spent last night researching, and it looks like I will have to use a RPC instead of a VFD to get three phase from the 240V I have, since the lathe has internal switching and a two-speed motor I want to retain. The motor nameplate says 575V three phase. Yes the transformer bank has three separate coils in it.

Hardtail, the transformer bank nameplate is not readable, but I did find one part that said "HP 5.5" most of the rest of the plate was not readable. I know that transformers are rated in KVA, but I cannot find and read that on this transformer. The motor is a 3HP 575V three phase unit, I can read its nameplate. I have only 240V single single phase available for power.

Thanks to all who replied,

Steve

J Tiers
01-06-2009, 08:53 AM
J. Tiers, Yes, I do believe it is wired as a autotransformer. I couldn't think of that phrase when I first posted. I don't understand your next sentenance,
"But, if it IS wired "Y" and has a 240V primary, it would be to connect to 415V three phase..... which may not be right for your setup....... because 415V has 240V to neutral." Do you mean that 415V is the "normal" high voltage? I can actually read the nametag where it says output voltage is 575V, I just can't read the input voltage.



Transformers or 3 phase loads can be wired more than one way. They may be wired with no neutral, in "delta", which means each load, (or transformer coil) is wired from one of the three wires to the next.

They may be wired "Y", which means that each load, etc, is wired from one of the 3 wires to the neutral. A variant of that is when there is no actual neutral, but the loads are simply "tied together" . That forms a "false neutral", and is often done in motors.

There is a difference in voltage between the different wirings.

For instance, the common (in the US) voltage of "208V 3 phase". it is 208V AC from wire-to-wire (line-to-line). But it is only 120V from line to neutral. There is a factor of 1.73 difference. The "Y" voltage is the "delta" voltage divided by 1.73.

Note that the "delta", or "line" voltage is what the supply is described with.... Like "208 V 3 phase". The 120V part is not used to describe it

Since your set of transformers (which I take it are "three separate lumps of iron", is that right?) is apparently wired in "Y", with a false or "floating" neutral, they will take the lower "Y" voltage on their primary. But we don't yet know what that is (or might be)

IF they are truly set up for 240V "Y", then because they are wired "Y", they would ACCEPT a "line" voltage of 415V (240 x 1.73), and put OUT the 575. That is a less common voltage, and seems less likely. It would not be good for you. If that is the case, your ratio test will show it up, because you will have a ratio of about 1.38 : 1.

A better situation would be that it is wired to ACCEPT 240V 3 phase and put OUT 575V 3 phase. Then you would be in great shape. The ratio for this would be about 2.4 : 1. That would be perfect, because that would mean the input TAKES 240V and puts OUT 575. Exactly what you need.

But, we do NOT know this yet... All we have is speculation based on a partially readable tag.

Hopefully when you measure you will find out that you have a good situation.

Evan
01-06-2009, 09:46 AM
Power specifications in Canada are the same as in the US. The US and Canada are tied together on the North American power grid.

The short answer is that it seems to be a 3 phase transformer. If you don't have three phase power then it is of no use.

hardtail
01-06-2009, 10:00 AM
Generally if your voltage is within 10% it's acceptable so if you have residential service it should 230V, so 208-240 input should work, the gap may grow on the high side but it should be close.

VFD should be derated for single phase input so you'd likely need a 5hp 575V which could start to get costly but check some manufacturers claim no derating for 3 hp and less.

Your other 2 options are a RPC, here you can either produce 3ph on the low voltage side or 3ph and the high side of the xformer. Again if your load is 3hp go with a 5hp idler for your RPC. Basically you already likely have an acceptable xformer you just need an idler.

Edit: Upon further thinking I think you have a 3ph xformer there and I don't think it will work well for a RPC, you would have to have your idler on the low side of the xformer and your 3ph leg isn't going to be very stable to transform.

I would probably look for a single phase transformer and VFD or idler, whichever route you want to go.

J Tiers
01-06-2009, 09:22 PM
It would probably work as well from an RPC as the motor will.....

Questions that have been "sort-of" answered......... but not with a "nail it" answer.......

1) is this one transformer with three windings on one "core" (a 3 phase transformer), which seems most likely? The previous descriptions leave open or encourage the idea that it is for some odd reason 3 separate in a box..... I suppose that could be the case, but.....

2) Is the motor for-sure 575V only? That seems pretty likely, since that voltage doesn't turn out to be anything very useful when cut in half or taken as the "Y" voltage..... But we don't have a firm answer.

As far as the choice of approaches......if the transformer turns out to be no help, and the motor can't be changed..................

A 240V RPC into the transformer might work...... if it works into a motor it should work with a transformer and then into the motor, but might heat the transformer a bit extra. Besides, all he needs is a motor and a few fairly cheap parts for this to work. The transformer is already there (if it is 240 input).

For sure, a 240 to 575 transformer and another 575V motor set up as an RPC would work, assuming parts good for 575 could be found (caps, relays, switches, contactor). But I suspect the 240V RPC would be many times easier and cheaper to set up unless parts are already at hand. The transformer would have to be pretty beefy to run the RPC plus the lathe......

240 to 575 and a 575 VFD is also possible, but they are a bit less common than even 480V, at least here. In Canada, 575 is less exotic, but the VFD parts all have to change between 480 and 575, since the peak is close to 900V with 575 VAC and a high line condition.... Not the place for a standard 1200V IGBT.

Steve Steven
01-09-2009, 06:50 PM
Well, I tested the transformer today (And yes, it is three coils on a common core). I put in 122VAC (measured with the power applied to the coil, so some current was flowing) and measured the corresponding open terminal voltage and got 340VAC. I assume this is the PEAK voltage, and needs to be multiplied by .866 to get RMS volts. If so, then this transformer will work, as putting in 240VAC should get .866 X 2 X 340=580VAC out, just what I need.

I have started looking for a 3 phase 5HP motor, preferably 1750 RPM, to build a rotary converter.

Thanks all for the assistance.

Steve

J Tiers
01-09-2009, 10:29 PM
Well, I tested the transformer today (And yes, it is three coils on a common core). I put in 122VAC (measured with the power applied to the coil, so some current was flowing) and measured the corresponding open terminal voltage and got 340VAC. I assume this is the PEAK voltage, and needs to be multiplied by .866 to get RMS volts. If so, then this transformer will work, as putting in 240VAC should get .866 X 2 X 340=580VAC out, just what I need.

I have started looking for a 3 phase 5HP motor, preferably 1750 RPM, to build a rotary converter.

Thanks all for the assistance.

Steve

Not sure where either the "peak" voltage, OR the factor 0.866 came from......

If you put in 122 VAC, that was RMS, and if you got out 340 VAC that was RMS also (or at least rms based on an average responding but rms calibrated meter).

And the peak to RMS for any sine wave is a factor of 0.707 (or 1.414 going the other way).

It looks like it is fairly close, but not quite...... You will get a bit high going from a solid 240. And you presumably don't have 240V "Y" either, so the common connection has to go the way of the dinosaurs.

In fact, you seem to have approximately 2X my first case.... meaning that you are set up to accept approximately 208 and produce 575, With a 5% impedance, you will get lower than 575 under load, meaning that it would work off around 220V if re-wired to delta.

Do you have 220 available?

Steve Steven
01-09-2009, 11:17 PM
Yes, I have 220V available, I don't know if it will be 220 or 240 tho. I measured 122 with the 120V connected to the input side of the transformer, so I "assumed" that I would get twice that from the two hot leadsout of the breaker box. I'll measure it tomorrow.

I assume I could run the motor with a little higher voltage anyway?

Thanks,

Steve

J Tiers
01-09-2009, 11:41 PM
Well, your ratio of 340/122 is 2.78:1, meaning that the ideal is around 220, including 5% or so "impedance'.

The 240 x 2.78 gives about 268V, more than a little high.... given 5% "impedance", is still 634V under load, or 10% high. Top end of normal range.

The "impedance" is essentially the percent voltage drop under full load for that transformer. If you don't pull the rating of the transformer, power-wise, it will drop less and output will be higher.

Re-thinking, and remembering that this is an autotransformer, it does look as if it will work OK in "Y".

If you have 220 delta, that is 127V "Y". The 127V gives 127 x 2.78 = 353V "Y" at the high side. The corresponding delta phase voltage is 610, and with 5% impedance gets to 580V delta, which will run your motor.