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Joined: Apr 2005
Posts: 4
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patb Offline OP
Junior Member
I have a customer that recently installed several high end ovens in their stores. They teach cooking and the store managers are complaining that the oven isn't holding the temp. They also say the oven takes too long to pre-heat. The service going to the appliance is correct. After speaking to the manufacturer their telling me the oven needs a constant 234Vac or more to heat properly.

How do I size the buck/boost properly for a 240Vac 60amp single phase appliance?

Joined: Jan 2003
Posts: 4,391
I
Moderator
It will be bigger than you may need it if you size it to the breaker.

You could size it to the KW of the oven.

I am guessing you want 208 to become 240?

Using the 60 amp breaker and boosting 28 volts I come up with a 10 to 12 kVA transformer.

I got that from this chart.

http://www.transformeronline.com/files/buck-sizingtables1.pdf


[This message has been edited by iwire (edited 04-12-2005).]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
Joined: Feb 2003
Posts: 939
F
Member
the only way i can tell you the correct size is tell me how many KW do this oven or stove set up and what is supply voltage and required voltage it need to be run


if your supply is 208 volt then you need a boost and buck transformer.

for straght 240 volt really it dont need the buck and boost transformer unless your wiring to the stove or oven is too small or weak connection somewhere.

Merci, Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

Joined: Mar 2001
Posts: 2,056
R
Member
go here:
Make sure you follow the "Selection Steps".
You will end up with a Buck-Boost Transformer with a kVA rating much smaller than you might think. A buck-boost that is connected as an Autotransformer has up to 10 or more times the capacity tha it does when connected as an isolation trasformer. http://www.sola-hevi-duty.com/products/transformers/BuckBoost/

Joined: Jan 2005
Posts: 354
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pdh Offline
Member
If the supply is nominal 208 volts, beware the phase angle since this is coming from a 3-phase source. The big problem with this is that while you are raising the L-L voltage you are also raising the L-N voltage. As you approach 240 volts for L-L, you are getting as much as 132.2 volts L-N because the phase angle will still be about 126 degrees (much closer to 120 than 180).

Can the L-N electronics of the oven safely handle that?

Joined: Nov 2003
Posts: 16
T
Member
What is KW of oven? How many ovens?
Sometimes it's cheaper to buy a used 208-240v transformer than it is to buck boost. Check the secondary/reseller/used market for transformer deals.

Joined: Apr 2005
Posts: 4
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patb Offline OP
Junior Member
The oven rated for 240Vac at 8.9kw.

I am planning on using a 1.5kva buck boost transformer with a 120/240 input and 12/24 output.

Joined: Jan 2005
Posts: 354
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pdh Offline
Member
There are a number of ways to wire a buck-boost transformer for this. You're going to get some crazy voltages whichever way you do it because of the neutral and 120 degree phase angle.

If you wire the 240 volt primary (both 120 volt windings in series) to the 208 volt supply, then each 12 volt secondary is only giving you 10.4 volts giving you 228.63 volts for L-L when both are wired in. Since the phase angle will be off by 30 degrees, L-N will be 129.1 volts. That seems a bit high to me, but it is within 10%. Still the L-L is low so the heating on L-L elements won't be a whole lot better.

You cannot get a true 2:1 ratio between L-L and L-N until the phase angle is 180 degrees. And a simple buck-boost transformer isn't going to change it much (about 4.6 more degrees in this case).

Note, the above calculations (including vector arithmetic for the mxied phase angles) are based on square root of 3 giving 207.8460969 volts in and 10.3923 out from the transformer. Actual results will vary by as much as the supply varies.

Joined: Mar 2001
Posts: 2,056
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Member
patb,

The "Sola" model # HS20F1.5A (16/32 volt secondary) will provide 236 volts from a 208 volt supply, when connected as an autotransformer.
A buck-boost with a 120/240 primary and 12/24 volt secondary will only deliver 229 volts, with a 208 volt input.
Double check the selection steps from the website posted above, and you'll see what I mean.



[This message has been edited by Redsy (edited 04-15-2005).]

Joined: Dec 2002
Posts: 308
E
Member
You may here

gohttp://www.acmepowerdist.com/products.asp?PAGE=prod_landing.html

and find your transformer.

I have done the same for a cooking appliance and it has worked out great.

Edward


Thanks
Edward
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