buck and boost 1

[panelmaster]

I'm brand new to the electrical business. I'm curious - why do they call it a buck and boost transformer? Also, transformers are usually referenced with the primary being the higher voltage and the secondary being the lower voltage. Can any transformer be wired "backwards" to step up the voltage?

 

[davidbeach]

Buck and boost generally refer to transformers that make relatively small changes in voltage, say between 208V and 240V. From 208 to 240 would be a boost while going from 240 to 208 is a buck.

Transformers don't care whether the power flow is from high side to low side or from low side to high side. Where power flow will generally flow in only one direction, it can be possible to talk of primary and secondary (either could be the high side or the low side), so primary and secondary can be clear or could be rather ambiguous. But to speak of high side or low side will always be clear.

 

[waross]

To add to davidbeach's excellent advice. A boost transformer is often a conventional transformer wired as an auto-transformer. Note:It may need to be rated for buck/boost duty.
A buck transformer may be the same transformer wired either as an auto-transformer or with a "fold-back" connection.
Example; A 3 KVA 480:120V lighting transformer may be wired as an auto-transformer with the windings in series. 480V + 120V = 600 Volts.
This arrangement may now be fed with 480V to produce 600 Volts or fed with 600 Volts to produce 480 Volts. Two transformers may be used in an open delta connection to transform between 480V 3 phase and 600V 3 phase.
We had a customer with one manufacturing plant running at 600 Volts and another plant running at 480 Volts. They often switched machines from one plant to the other. We had quite a few pairs of small transformers running 480V motors on 600 Volts and 600V motors on 480 Volts. (Yes the actual motor ratings were 460V and 550V)
Another option is to feed 480 Volts into the 600 Volt connection for a voltage of 480V x (480V/600V) = 384 Volts at the nominal 480 Volt tap.
Another possible arrangement is to reverse the polarity of the lower voltage winding. Now the output voltage with the example transformer will be 480V minus 120V or 360 Volts. This arrangement is never back fed and is suitable for an input of 480 Volts or lower.


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

 

[panelmaster]

Thanks to you both. Waross' response was a bit over my head, but his example is exactly what our customer wants to do. He needs to test 575V 30 HP motors with a 460V supply. For some reason our regular component supplier is having a hard time finding a suitable transformer.

 

[waross]

Look for dry type, 480:120 or the common 240/480:120/240 Volt dry type transformers rated for buck boost service. (Even if the transformer is not buck/boost rated, it will serve for testing. The test voltage on the 120 Volt winding will be over 1000 Volts.)
Sizing: The 30 HP motor will draw about 30 Amps at 600 Volts. You want your 120 Volt windings to carry 30 Amps plus 25% or 37.5 Amps.
120 Volts x 37.5 Amps is 4.5 KVA. Use 5 KVA or larger transformers.
Control may be on the 480 Volt side or the 600 Volt side, but the 600 Volt side is preferred. If testing with a VFD, use a 600 Volt VFD on the 600 Volt side.
While you read this I'll try to Google up and post the connections.

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

 

[waross]

Look at:

http://www.emersonindustrial.com/en...mers/buck_boost_transformer_catalog_pages.pdf

On page 6, see open delta, diag. E1 Boosting
If the transformers you find are not identical, post the data and we will let you know if they are suitable and try to find (or draw) a diagram.
Basically you need two 5 KVA or larger transformers that will accept 480 Volts and put out 120 Volts. There are several possible arrangements.

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

 

[panelmaster]

Thanks. I studied it, didn't find exactly what I was looking for, but I sent it to SOLAHD tech support. Let the guys who do it evryday figure it out!