Transformer schemes 2

[itsmoked]

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

 

[itsmoked]

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

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

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

 

[Skogsgurra]

Much ado about nothing, then.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[prc]

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.

 

[waross]

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

 

[itsmoked]

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

 

[prc]

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.

 

[davidbeach]

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.

 

[waross]

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

 

[davidbeach]

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? ;-)

 

[waross]

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

 

[itsmoked]

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

 

[HamburgerHelper]

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.

 

[waross]

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

 

[prc]

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,

 

[waross]

Agreed, pcr.

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

 

[waross]

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