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120V L-N / 216 L-L to 120V-240V 4

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Sn00ze

Sn00ze

Electrical
Jan 16, 2013
176
Hello,

We have a service that is measured at 216V L-L. We are to buy a step-up transformer to power up the AC system of a 25/33/42MVA TX.

The fans and OLTC motor are rated for 240V. The lead commissioner measured 216V from the supply and is saying the motors would probably get closer to 250V.

Since its for AC i don't supposed voltage drop is a big factor. The run will be 45m.

My question is: Is that 8V difference a big deal? I was always taught that motors are supposed to run +/-10V% anyway.

Also, if the Step-up TX is meant to be 208v to 120/240V. is this 8V diff somethign to worry about ? should i look for a 216V-120/240V TX?

I would appreciate insight in the matter, thanks!
 
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Go back and recheck your ratings.
Your profile shows you as located in Calgary Alberta.
Standard motor voltages in North America are multiples of 230 Volts.
Standard system voltages are multiples of 120 Volts plus 208 volts, 277 Volts and 347 Volts.
Your measured 216 Volts is nominal 208 Volts. Consider it as 208 Volts as the allowable voltage variations will be based on 208 Volts.
230 Volt rated motors are commonly fed from nominal 240 Volt systems. The difference between the 240 Volt system voltage and the 230 Volt motor rating allows for voltage drop.
You need a 208 Volt to 240 Volt step up transformer.

The most economical solution is to use buck boost transformers.
A 240:480 Volt to 16:32 Volt buck-boost transformer will boost 208 Volts up to 236 Volts. Close enough for standard 230 Volt rated motors.
On your 216 Volts, the same transformer will develop 245 Volts. Close enough to the nominal system voltage of 240 Volts and still within tolerance for a 230 Volt rated motor.
Two of these transformers in open delta will develop 236 Volts or 245 Volts three phase.
Give us a listing of the motor ammperages and we can help with sizing.
SECTION 2
93
SECTION 2
BUCK-BOOST TRANSFORMERS
© Hammond Power Solutions Inc.
Data subject to change without notice.
Buck-Boost transformers - Questions & answers
1. What is a buck-boost transformer?
Buck-boost transformers are small single phase transformers designed to lower (buck) or raise (boost) line voltage from 5-20%.
The most common applications for buck-boost transformers include boosting 208 volts to 230 or 240 volts for air conditioning
systems, boosting 110 to 120 volts and 240 to 277 volts for lighting applications, heating systems and induction motors of all
types. Many applications exist where supply voltages are frequently above or below nominal.
Buck-boost transformers are conventional low voltage, single phase distribution transformers, with standard primary voltages
of 120, 240 or 480 volts, and secondary voltages of 12, 16, 24, 32 or 48 volts. They are available in sizes ranging from 50 VA to
10,000 VA. The primary and secondary are wired together to form a single-winding autotransformer. Utilizing the additive and
subtractive polarity, small amounts of voltage are either added or subtracted from a distribution circuit.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Thanks for the answer Bill,

The rep recommends : Q005LEKF, Q007LEKF, or
[URL unfurl="true"]http://www.hammondpowersolutions.com/files/HPS_Catalog_Encapsulated_Distribution_Section8.pdf[/url]


[URL unfurl="true"]https://www.galco.com/techdoc/hamd/c1f005ges_wd.pdf[/url]

AC Study shows we need 3.8kVA so a 5kVA transformer is the next standard size.

this is the motor rating that i got this morning from the field guys.

I don't see a range, i imagine the standard they used "IEC60214-1:2003" would shed some light in the matter, but of course none of those standards are free to look at.
 
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Yes, why do you ask itsmoked?

 
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Single phase motors are so much less reliable than 3-phase whereas that transformer will be expected to last forever. I'm just surprised is all. I guess single phase allows a single local transformer to supply it but there again two transformers would be more dependable than one single phase motor... Probably an initial cost rational (irrational?) behind it.

Keith Cress
kcress -
 
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240V single phase fans are very common for transformer cooling.

No worries about phase rotation. Plus some substations, particularly subtransmission without MV available, have only single phase station service.
 
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3.8 KVA at 240 Volts = (3800VA/240V) = 15.8 Amps.
With an auto-transformer solution that 15.8 Amps will only flow in the 32 Volt winding. So you would need 15.8A x 32V = 0.494 KVA. Add 25% = 0.617 KVA
The next standard size is 0.75 KVA. Compare the price of a 0.75 KVA transformer with the price of the recommended 5 KVA transformer and you will see why the sales rep wants to sell you the 5 KVA transformer.
A couple of anecdotes from Central America.
When I was south of the border, I was in a country where it was common for any electrician or contractor buying a transformer to get an under the table, cash, kick back from the supplier.
I was called in to give a price on installing a transformer to run a 100 HP compressor skid. The available voltage was 240 Volts and the skid was set up for 480 Volts.
My price was based on reconnecting the motor and changing out the O/L relay and the short supply conductors.
They went with another contractor and the transformer. The kickbacks were probably more than my total bid.

I went into a plant where both 240 Volts and 480 Volts were available.
All motors on the 240 Volt system were connected for 480 Volts and used a transformer.
All motors on the 480 Volt system were connected for 240 Volts and used a transformer.
I am sure that the electrician had his summer home almost paid for with transformer kick-backs.
If you are on a fixed price contract consider the auto-transformer solution.
If the customer is paying all expenses plus a commission, by all means go with the 5 KVA solution.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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im sorry Waross, i am not following. Why is there a 32Volt winding?
 
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That's how buck-boost autotransformers are built - one lower voltage winding and one higher voltage winding that can be connected in series to allow boosting or bucking of the high voltage.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Sn00ze; You would wire the transformer like this:
buckboost3_stzqf3.jpg


The bottom short windings would be 16V + 16V.
The top long windings would be 120V + 120V.

Keith Cress
kcress -
 
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waross' post shows you why not: a transformer wired as a boost autotransformer can have a significantly smaller power rating, typically a factor of (a+1) less, where a is the turns ratio of the transformer. It'll pass the needed power, you pay a lot less, it takes up less room, etc. You sacrifice electrical isolation to gain all of this.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
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Yes.
What xnuke said.
And often in power circuits you don't want isolation. If you use a conventional transformer you must then ground the secondary circuit for safety.
Sacrificing isolation is not always a sacrifice.

Bill
--------------------
"Why not the best?"
Jimmy Carter
 
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Using the SC1 you show you'd just wire it up:

Your 216V L1 to H1&H3 and the L2 to H2&H4
Then to the H2&H4 you add X1.

You tie X2 to X3

You then tie your fan load to H1&H3 (L1) and X4.

This way the transformer is only actually transforming, across the magnetic circuit, the power dished out by the 32V part of the winding. Hence the discussed, greatly reduced, (less magnetic core needed) smaller VA rating.

These boost setups tend to be nice and stiff too, more efficient also.

Keith Cress
kcress -
 
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Thank you so much for the information. I feel like i learned a lot actually lol. I owe yas a beer!
 
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