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Transformer schemes
2

Transformer schemes

Transformer schemes

(OP)
I have three of these laying around:



I've have 208V 3ph and I need 460V 3ph.

I initially thought I'd use use the 0V and the 208V taps daisy-chaining each H3 to the next transformer's H4. Supply 208 to each H3 and use the three H1s for 460V but on closer examination that seems more likely to get me 600V H1-H1-H1.

Suggestions?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

What are you doing with the X side? Without current in the X winding there's no current in the H winding.

RE: Transformer schemes

I would suggest trying to connect H1 to X2 and H4 to X3 and putting your 208 across H3 and H4, per transformer.

RE: Transformer schemes

I suppose you'd also need to connect H3 of 'transformer 1' to H4 of 'transformer 2', H3 of 'transformer 2' to H4 of 'transformer 3' and H3 of 'transformer 3' to H4 of 'transformer 1', as suggested in OP.

RE: Transformer schemes

A wye connection of the X windings X0-X1 and a delta connection of H1-H4 will give 440. I'm not seeing anyway of getting that last 20 volts.

Bill will come along and have some esoteric connection that won't get 460 but will be within 5 or six volts. Just gotta wait for him...

RE: Transformer schemes

You connection should work if you can live with 440 Volts instead of 460 volts.
You are ignoring the secondary windings and using the primaries as autotransformers.
Draw a delta with H4 and H3. Then extend each side to H1.
There is a phase shift. The amount of phase shift depends on the ratio of primary to secondary voltages of the auto-transformer connection. (The X1, X2, X3 secondary is not used and has no effect.)
I have some old text books that show a similar connection (But with much smaller step-up ratios) used on power transformers to intentionally introduce a small phase shift so as to vary the load sharing between transmission lines.
Is that technique still in use, David?
Thank you for your confidence in me, David.
As I was looking at this and composing an answer I kept thinking;
"Get close to 460 Volts with those available voltages? No way.
440 volts is easy, but 460 Volts...?"
Then the light went on.
Think single phase.
On each transformer if you connect the 24 Volt secondary as an autotransformer boost, you will have 464 Volts. That is within the 5 or 6 Volt limit suggested by David.
And further, our standard voltage in North America is 480 Volts and I consider anything between 440 Volts and 480 Volts to be fair game.
Your connection will be 208 Volts in at H4, H3. Connect H1 to X3.
464 Volts will be developed across H4, X1.
Now we have three 208 Volt to 464 Volt autotransformers.
Connect the H4, H3 windings in delta across 208 Volts.
Take 464 Volts from H4, X1.
Ignore the phase shift.
The voltages will still be symmetrical with respect to the system neutral.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

What old book shows this connection?

RE: Transformer schemes

Quote (waross)

I have some old text books that show a similar connection
Not exactly the same. My memory is a little rusty.
The connection proposed for Keith produces mostly voltage change with some phase angle change.There is nothing wrong with a phase angle change in a single stand-alone unit.

For power flow control, the added winding is taken from a different phase.
The result is mostly phase angle change with some voltage change.
The principle for power flow control and some various possible connections for power flow control:
http://pjm.com/~/media/committees-groups/task-forc...

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

(OP)
First off, davidbeach, that was exactly my thinking! I figured Bill would have me configuring them as some sort of zig-zag Scott-T mobius.

I just need to run a 460V motor blower that draws 250mA so somewhere around 460 will likely work fine.

Quote (Bill)

You are ignoring the secondary windings and using the primaries as autotransformers.

Yes that was my hope so these 100VAs would do the trick. Nothing needs the LV sides.

Quote (Bill)

Draw a delta with H4 and H3. Then extend each side to H1.


Quote (Bill)

There is a phase shift.
Don't care in the least. :)

I must confess that I'm forced to test this scheme on bonified 240V delta power.

So I've hooked them up using H4 and H2 instead of H3. When I do that I get 461V between H4 and H1 measured across each xfrmr and 602V between any H1s. This is without the added X1 based boost.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Hi Keith.
Would your 240 Volts actually be 230 Volts? That would explain the 461 Volts.
As for the 602 Volts. Memory is getting rusty.
Usually I am boosting much less than the 208 Volt to 440 Volt jump that you are making. With that ratio of boost, the phase shift adds more voltage than I had anticipated. I have never used a full autotransformer delta boost.
I use either full wye autotransformer or open delta autotransformer. That's what I see in the field also. Please forgive my for neglecting the added phase shift voltage.
How about an open delta autotransformer boost. Two transformers and 440 Volts. Easy to add the extra 24 Volts if you need it.
What is the VA rating of the transformers?

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

(OP)
Hi Bill,

For some at-the-scene reporting!

These are 100VA control transformers.

This is the voltage I've got to test with. Notice I'm using H4 and H2 (not 208V H3)



This is what we get across a single transformer wired up in our full 3 transformer delta scheme.



But, this is what is seen across the delta of the three H1 terminals between any two transformers.



And another bit, is the current demanded by this "unloaded" micro bank of control transformers. .15A @ ~230V. Seems high but maybe not.



Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

I think the 600V+ measured from H1 to H1 on the next phase is because you have a 120° phase shift and resultant sqrt(3) factor on the H1-H2 section of the winding between phases.

RE: Transformer schemes

(OP)
Whoa 111R. So you're saying:

The extra winding between the 220 tap H2 and H1 the 440 tap, which happens to be 220V too, is contributing 220V x √3 = 381V

Adding 220 + 381 = 601V : (H2-H4) + (H1-H2)*√3 = 601V.

Did I get this right? If I did that's interesting news to me.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

I think the original Dy suggested by Davidbeach is the only possibility. A single stack winding cannot be made in to an auto-transformer. Voltage will appear but you will not be able to draw current! For transferring current from one winding to another, the leakage impedance between windings should be low. Consider a two limb core. One limb has two concentric windings H&X . This will work as a HV/LV single phase transformer. If you put the H and X separately on the two limbs( typical representation of transformer in text books), it will not work. You will get no-load voltage correctly. But you will not be able to draw current.

RE: Transformer schemes

Hi Keith.
Draw a sketch and you will see that the voltages are as they should be.
Part of the problem is that your first hand drawn sketch is grossly not to scale.
You show H1, H3 as a little tag.
It is actually greater than H3, H4.
H3, H4 is 208 volts. H1, H3 is 232 Volts.
To make it simple use your present connections.
Then both the energized winding and the extension will both be 220 Volts.
Your 240 Volts delta seems to be only 227.9 Volts.
227.9 Volts x 2 = 455.8 Volts. Measured voltage = 455 Volts.
Pythagoras tells me that the voltage from H1 to H1 will be √7 x 227.9 Volts.
Measured = 604 Volts. Calculated = 602.97 Volts.
More tomorrow.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

The connection proposed by waross is extended delta auto connection. The relation ship for voltages is as below:
Let E1 =voltage acrossH1 E2= Voltage across the delta Em= extended delta voltage. In the case considered all of these are 220V.
E1 is the vectorial sum of (E2+Em-Em(120 degrees shifted)) So the relationship is square E1= 3 Square Em + 3EmE2 + Square E2 (Please see page 267 of Blume's book on transformers)
This gives square root7 x227.9=603V as concluded by waross. But as far as current loading is considered, my earlier statement holds good.

RE: Transformer schemes

prc; Thank you for expanding on the origin of the square root 7 factor.
I accept your information that some transformer configurations are not suitable for buck boost operation.
However, for these connections we are using different taps on the same winding. If that is a problem, then any tapped transformer would have a problem.
I am aware that some winding configurations of small transformers are not suitable for buck-boost operation, but most transformers are suitable for buck boost use subject to voltage and insulation level issues.
I have used many conventional transformers for buck or boost service and I have seen many more in operation with no issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

The safest connection will be two transformers in autotransformer boost configuration. That will give you 464 Volts. This depends on your willingness to stress the 24-120 Volt winding with 464 Volts to ground.

However there may be another issue.
100 VA / 440 Volts = 0.227 Amps. That's cutting it close (actually cutting it on the short side of the line) for a 0.250 Amp motor.
The open delta autotransformer boost connection is a standard connection in some industries. I see it a lot in the field.
I am hesitant concerning the 120 Volt wye to 440 Volt delta connection. If you use this connection consider floating the wye point.
Given the unique properties of a four wire wye:delta connection, it won't take much unbalance on the supply system to fry those transformers.
The note allowing use of 115 Volts from the 120 Volt winding, raises a little concern about the origin of these transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

(OP)
That may be the 'safest' Bill but it won't work with what I have.

However.

Your mention of 120V/115 in that screwy schematic had me hunt down the manufacturer's data sheet. This clarified that the the secondary was really 115V and the primary side is actually 208/230/460


Here's what works:

Auto-transformer using everything.
The secondary 115V winding is added to the bottom of the primary HV part of the transformer which is the first 230V half.

This gives 115 + 230 = 345 (input ratio)


Then this 345 has added the rest of the original primary which is another 230:
345 + 230 = 575 (output ratio)


Our turn-up ratio then is 575/345 = 1.67

Those are the hard winding ratios.

Now we supply merely 208V to the lower half and get the ratio'd result:
208V x 1.67 = 347.4V out of each transformer's H1.

Referring back to your handy √7 Pythagoras rule we see that 460 Volts, the ultimate delta voltage sought for, would be present with interim voltages:
460V/√7 = 173.86V which when doubled:
173.86V x 2 = 347.7V (All from your earlier post)

As for VA it seems roughly, 100VA/230V(transformed part) = 0.43A, needed: 0.25A.

Here's a better balanced diagram:
(NOTE: The circled values are actual applied/resulting voltages while the uncircled are winding ratios.)



Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Let us make things simple:
1) You require 460V 3 phase ( 265.6 V single phase) with 208 V 3 phase ( 120V single phase) input.
Put X winding in star with X3 earthed, H winding in star with 208V tap earthed.
Supply 3phase 208 V to X1. So it is overexcited by 120/115 pu.
At H1 you will get (460-208) x(120/115)x1.732 =455.5 V, nearest to your requirement.
2) Waross, you can use tappings in a winding freely, provided power input is in the primary winding. Generally you cannot make a tapped single winding in to auto-transformer, with primary open. Exception is small control transformers where the entire windings will be layer type, each tapping section forming a layer. So each layer will act as a concentric winding and then you can draw current from auto connection. Please try the winding connection in my first posting. You will get voltage, but you will be able draw only negligible current.

RE: Transformer schemes

(OP)
Thanks PRC. The only issue your elegant solution has is that now that it depends on 100% transformer action the 100VA transformers cannot, theoretically, support a 0.25A load (113VA).

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Sorry to come so late to this thread. But I do not see the problem at all.

I would connect H4 to phase A, then connect H3 to phase B. That will result in 440 V between H4 and H1. Then connect H1 and X3 to add 24 V so you get 464 V between H4 and X2. That is as close what you want as one can hope for - isn't it?

If you then do the same thing with the other two transformers, I think you will get the same result there.

I didn't have the energy to read all the postings in detail. But I see a lot of discussions where phase angles are involved. I see no need for that. Not in this case.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Transformer schemes

Sorry again. I just realized that there is a problem with that connection. Never mind - Just forget.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Transformer schemes

Dammit, I have to find three transformers and test. BBS or, more probable, BBL.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Transformer schemes

(OP)
Hahaha, thanks for the try Gunnar. You're making the same mistake I did on my first cut. If you look at one transformer and get it to put out 460, or there-a-bouts, when you subsequently go across those single transformer 460 terminals between transformers you get ~600V.

Also, if you'd actually read further you'd see these transformers actually have 0-208-230-460 taps unlike the ad-copy pap I initially believed and posted in my OP.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Much ado about nothing, then.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Transformer schemes

VA rating can be quite misleading esp with multi ratio transformers. Better to check with rating plate or manuals to see what is the max rated current of windings.

RE: Transformer schemes

Hi Keith;
A lot depends on how the windings are arranged in the transformers.
Step 1: Take one transformer and connect the available 230 Volts to the 230 Volt tap.
Step 2: Now you should have near 460 Volts on the 460 Volt tap. Use a resistor and see if you can load that tap (460 Volt) up to 100 VA.
Step 3: If you can load the transformer to 100 VA then the windings are arranged to our advantage.
Step 4: Take a second transformer and connect both H4 terminals to A phase. Depending on the voltage available connect either 208 Volts or 230 Volts to the appropriate tap. One transformer to B phase and the other to C phase.
Now you will have 460 Volts from H4 to H1 on each transformer and 460 Volts from H1 to H1.

The standard voltage in Canada is 600 Volts. There is a lot of 480 Volt legacy equipment.
Many petro-chemical and heavy oil plants use 480 Volts.
When a plant goes into shutdown, standby generators are brought in to supply temporary power. almost all rental generators are 480 Volts. There are 600 Volt rental generators but they are rare.
The point is that up in The Great White North, we have a lot of occasion to convert from 480 Volts to 600 Volts and from 600 Volts to 480 Volts.
The open delta autotransformer boost circuit that I suggest is a well known and frequently used connection in Canada.
The other question, Current rating. The 460 Volt tap will support the full 100 VA or 0.217 Amps. It will supply 0.217 Amps to your fan.
There is one winding connected to the winding. The motor current will flow in the transformer winding. No need to worry about phase angles and/or root 3.
The 208 Volt tap will support 100 VA. That is 0.418 amps. Chances are good that in that small a transformer it was more economical to wind the primary with one gauge of wire rather than taking the labour to stop and make a splice and change wire size.
If you pass step 3 you should be good to go.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

(OP)
Thanks PRC. The actual Micron data sheet and the transformer label are pretty short on info. They only state exactly:
100VA
24V/115V
4.17A/.87A
e.g. They only discuss the secondary.


========================================
Hello back! Bill;
Thanks for that clear recipe. I'll do the load check and see what transpires.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Waross, I could not understand how your experiment will show the current rating. Transformers always can take overload. Even if rating is only 0.23 A on HV side (for 100VA) it will easily take 200 % also, of course with higher temperature rise. As mentioned, there will be ample margin as the same wire gauge might have used for HV winding throughout as at 208 V tapping,the current rating has to be 0.48A to get 100VA.

RE: Transformer schemes

If the full H winding is 460V and the full X winding is 120V, why worry about autotransformer connections instead of just going with a wye-delta? It would have to be fed from a 208V source though and not a 240V source.

RE: Transformer schemes

Hi prc;
Respectfully;
My experiment is not to show the rating. It is to show whether the windings are arranged in such a way as to allow the primary winding to be used as a step-up auto-transformer.
As for the ratings: Yes, I know that transformers will withstand overloads. Using the VA / Volts gives the maximum rated current. The actual safe current may be somewhat higher. AS to PU overloads, A small control transformer may not be able to dissipate the heat as well as a distribution transformer. I expected Keith to make a judgement call as to how much overload he was prepared to push through the transformer.
Hi David;
Respectfully.
There is nothing wrong with your scheme as long as nothing goes wrong.
I have a knee jerk reaction to wye:delta as a result of too many issues with that connection in distribution service when something did go wrong. In a central American Country, there was a country wide fear of "Voltage surges" taking out the residential refrigerators and freezers. The cause was usually not a surge but a single phase condition on a residential circuit with one or more wye;delta three phase services. This resulted in a back feed from the wye:delta bank putting close to 50% voltage on the unfed phases. Try to start a refrigerator on 50% voltage and the thermal protection may save the motor a few times, but not indefinitely.
Aside to Keith. In this application you can probably use David's suggestion if you leave the wye point of the 120 Volts floating. That will avoid the most serious issues.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

In general I agree with your assessment of grounded-wye - delta transformers. But for a 300VA installation, indoors, feed from a service that ought to be reasonably balanced, being used under supervision by someone with a handle of "itsmoked", what could go wrong? winky smile

RE: Transformer schemes

As i suggested, just float the wye point. The chance of damage due to the occasional over-voltage energization transient is small.
And;
"being used under supervision by someone with a handle of "itsmoked", what could go wrong?" Grin
A comment on transformer configurations NOT suitable for auto transformer connection. Thanks to a correction that David provided to me a number of years ago.
In small sizes, a common configuration of dry type transformers was a two legged core. A 240/480:120/240 Volt transformer would typically have a 120 Volt winding and a 240 Volt winding on each leg. If only a winding on one side of the core was energized, you may get open circuit voltage but not much current from a winding on the other leg.
Then I remembered my early days working with constant current street light regulators. The old constant current transformers used a movable coil and allowed leakage flux between the primary and secondary winding to control the current.
Link

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

(OP)

Quote:

used under supervision by someone with a handle of "itsmoked", what could go wrong?
I'm right here yah'know! I see what you're writing!!






Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Don't worry. If something goes wrong, you just have to refill them with some replacement smoke and they should be as good as new.

RE: Transformer schemes

My comments were made with the greatest respect and friendship. And maybe a little misplaced humour.
Oh, did we mention to suggest a non-combustible surface? GRIN

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

Sorry Waross. I misunderstood your experiment. But in case you know the impedance(%) of transformer (from data sheet) you can short secondary and feed % impedance primary voltage to see whether getting rated current in secondary. Probably they may not be checking or giving impedance value for control transformers,

RE: Transformer schemes

Agreed, pcr.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

Hi Keith;
I've been wondering if you ever got that little motor running on those transformers. With the flux leakage depending on the actual physical arrangement of the transformer windings, a shop test is the best way to see if they will do the job.
If the winding arrangement is suitable for autotransformer use, either David's three transformer scheme or my two transformer scheme will do.
Up in the tar patch a lot of plants run 600 Volts. I have seen generator outputs of 480 Volts transformed up to 600 Volts for temporary lighting and power and then transformed back down to 480 Volts for some rental exhaust fans. I forget which was which but one transformation used the two transformer scheme and the other transformation used the three transformer scheme.
Either scheme works well if you don't need a neutral.
Thanks Keith.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

(OP)
Ha! Funny that you should ask! Yesterday about 6:30pm I wired up the machine, everything except the power cord which just needs the lugs screwed down. I bailed right at that point because the guy mentioned he'd not eaten since he'd gotten up at 7AM! I didn't want to further that situation. I'll go back today and actually power it up.

I was pleased to megger the Baldor 7.5HP motor and found it pegged my megger, both the motor and the blower and that, after sitting in a cold shop two blocks from the Pacific Ocean for a couple of years.

What I was most definitely NOT happy about was the this!
Look at this picture of the motor plate. Note the motor voltage rating for the actual motor? Now look at the orange circled part? That refers only to the blower. 460V.




As soon as I pull the cover off to wire the motor up... low-an-behold the damn thing has the same exact offerings as the motor. Very annoying. The entire transformer issue made moot. After all that effort I'm still going to try it. What ailed the Baldor numbskulls the day they created this plate?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Thanks for the update Keith.
Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Transformer schemes

Could someone explain what is significant about what is circled? I am missing the boat.

RE: Transformer schemes

(OP)
Sure.
The plate explains in excruciating detail how this INVERTER motor can be wired for 460 or 230, the currents, including even the Magnetizing current, the motor efficiency and the torque it produces, the Slip Frequency, even the MAXIMUM SPEED, but when it gets down to the blower motor it describes that the BLOWER is specifically 460V, 60HZ, .25 AMPS.

Controls were built to run the motor and the 460V blower. Because it was a 460V blower in a 208V realm the controller grew to twice the size, twice the weight, and took two thirds more engineering time. On installation it was discovered that the plate was BS and that the blower was actually 460/230 capable and to rub salt in the wound, even had all the leads pre-grouped for 230V operation! Very annoying, and a painful lesson that no matter how detailed the motor plate may appear you still can't trust it all.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Outside of a motor, what electrical components does this blower have? Or are there 2 motors in this system? Also confused.

RE: Transformer schemes

(OP)
Hi wroggent. This is a special inverter rated motor meaning it can supply full torque while not actually turning all day. To pull this off the motor comes with a separately powered blower. Note how the motor looks about twice as long as it's base foot? The entire left end is a 1/4hp blower. So yes, two motors - one device.



Keith Cress
kcress - http://www.flaminsystems.com

RE: Transformer schemes

Ah, I see. I have not used or even seen a TEBC motor before. I was thinking the 7.5HP motor was being used to drive a blower. Thanks

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