3 Phase Buck Boost Transformers

[kwired]

The secondary sees 100% of the load current, but not the full load voltage. So it's only the load current times the voltage difference between in and out.

Still have some confusion here. I went to buckboostcalculator.com which selects units for you based on information you provide and gives you a Square D model(s) transformers and connection diagrams.

With OP's information I come up with needing two 150SV82A units - which would be .15 kVA units connected in an open delta fashion.

Nowhere in product information do I see max overcurrent protection as OP mentions for whatever he has. So what is/how is current rating determined for an auto transformer. I'm guessing kVA rating divided by voltage of any particular coil in the circuit. so the 480 volt portion of a .15 kVA can carry .31 amps, and the 24 volt buck winding can carry 6.25 amps. Now it gets more complicated with three phase with the phase angle involved.

 

[Smart $]

Still have some confusion here. I went to buckboostcalculator.com which selects units for you based on information you provide and gives you a Square D model(s) transformers and connection diagrams.

With OP's information I come up with needing two 150SV82A units - which would be .15 kVA units connected in an open delta fashion.

Meaningless unless you tell us what values you input. Seems a little small given the voltage difference is 24V and the load current is 11A. That'd be .264kVA

Nowhere in product information do I see max overcurrent protection as OP mentions for whatever he has. So what is/how is current rating determined for an auto transformer. I'm guessing kVA rating divided by voltage of any particular coil in the circuit. so the 480 volt portion of a .15 kVA can carry .31 amps, and the 24 volt buck winding can carry 6.25 amps. Now it gets more complicated with three phase with the phase angle involved.

I've never sized OCPD for a buck/boost (EE has)... but I'm assuming rated full load current is the amount on the primary conductors which would cause the transformer to realize its kVA rating...???

 

[kwired]

Meaningless unless you tell us what values you input. Seems a little small given the voltage difference is 24V and the load current is 11A. That'd be .264kVA


I've never sized OCPD for a buck/boost (EE has)... but I'm assuming rated full load current is the amount on the primary conductors which would cause the transformer to realize its kVA rating...???

I did mention it called for two .15 kVA units, do we need to use a 1.73 factor because this is three phase?
Input values were
Available voltage - 504
voltage change - 5%
buck voltage - 480
load current - 5.65
calculate from KVA - 4.7 - somehow I messed up here as this was not what I intended to enter but did not notice that is what was submitted:roll: But that result ended up with the transformers I mentioned.

ran again with 9.1 KVA of load - it came up with two .500 kVA transformers load current of 10.95 amps. same connection diagram just larger units.

here is the connection diagram given for the application: http://buckboostcalculator.com/diagrams/diagram5.gif

 

[Smart $]

I did mention it called for two .15 kVA units, do we need to use a 1.73 factor because this is three phase?
Input values were
Available voltage - 504
voltage change - 5%
buck voltage - 480
load current - 5.65
calculate from KVA - 4.7 - somehow I messed up here as this was not what I intended to enter but did not notice that is what was submitted:roll: But that result ended up with the transformers I mentioned.

ran again with 9.1 KVA of load - it came up with two .500 kVA transformers load current of 10.95 amps. same connection diagram just larger units.

here is the connection diagram given for the application: http://buckboostcalculator.com/diagrams/diagram5.gif

That's better! I went to the site myself and came up with the same result as your second attempt, though I used 9.15 as the kVA.

 

[kwired]

That's better! I went to the site myself and came up with the same result as your second attempt, though I used 9.15 as the kVA.

My next question gets back to what the OP is asking about current rating of the transformer(s).

OP has a total connected load of about 8.15 kVA, which is about 9.8 amps 480 volts 3 phase, we only need two .500 kVA transformers to make our conversion, last I knew .5 + .5 was not 8.15 or anywhere near that. Not all of that power is flowing through the transformers, or they are not rated the same way as an isolation transformer is rated. I would assume the .500 kVA rating means if you configured it as a 480 x 24 step down then you could supply .500 kVA @ 24 volts which is about 20.8 amps. .500 kVA @ 480 volts is a little over 1 amp. The bulk of the current has to be flowing in the 24 volt section of the auto transformer but is only 24 volts across the coil......and I think I am starting to realize the answers to my questions, this autotransformer is nothing but an impedance in series with another load in a way, that coil should be able to handle 20.8 amps of current but will always have a drop of 24 volts across it in the configuration we have.

 

[kwired]

Getting back to OP sounds like his larger kVA transformer is closest to the one I have been discussing (.500 kVA) and a 20 amp overcurrent device should be just fine, but could be 125% higher than his rating of 21.9 which next standard size up would be 30 amps.

As far as starting current I kind of think the autotransformer would be somewhat of a current limiter during high current events such as motor starting and lower value overcurrent protection may hold during starting where it may not in other applications

 

[Smart $]

Getting back to OP sounds like his larger kVA transformer is closest to the one I have been discussing (.500 kVA) and a 20 amp overcurrent device should be just fine, but could be 125% higher than his rating of 21.9 which next standard size up would be 30 amps.

As far as starting current I kind of think the autotransformer would be somewhat of a current limiter during high current events such as motor starting and lower value overcurrent protection may hold during starting where it may not in other applications

In the OP he said his actual current is 4.9A ea. ? 2 units... so the 10.9A model should handle them... as far as running current goes. However, if he needs a 20A OCPD or greater to start them, 125% of the transformer's rated full load current input has to be greater than 15A... which means he'd have to use the model rated for 21.9A.

And to answer his original question about OCPD on the load side, it is required that each motor has separate OCPD (or rather SC/GFP). That's an NEC requirement, so not necessarily one for multiple motors incorporated into one piece of equipment, but probably wise even short of any other standard requiring separate OCPD.

 

[kwired]

In the OP he said his actual current is 4.9A ea. ? 2 units... so the 10.9A model should handle them... as far as running current goes. However, if he needs a 20A OCPD or greater to start them, 125% of the transformer's rated full load current input has to be greater than 15A... which means he'd have to use the model rated for 21.9A.

And to answer his original question about OCPD on the load side, it is required that each motor has separate OCPD (or rather SC/GFP). That's an NEC requirement, so not necessarily one for multiple motors incorporated into one piece of equipment, but probably wise even short of any other standard requiring separate OCPD.

The Square D selector selected the .500kVA units, but looks like the application is just barely over limits for smaller units, I entered 9.0 kVA instead of 9.1 in the product selector and it came up with .250 kVA units.

 

[hurk27]

So why would you want to use two single phase transformers?

Common buck/boost transformers are nothing but isolated line voltage to low voltage transformers with split windings on both sides, the common voltages for the high side are 120/240 or 240/480, and the low voltage side are 12/24, 16/32 or 24/48, the latter are most common with the 240/480 high side although you can special order the 24/48 with a 120/240 volt primary.

I have used the 120/240-16/32 volt ones rated at 1.5kva to build many 13.6 volt/ 100 amp power supplies or battery chargers for used to power ham gear or other high amprage mobile equipment so it could be operated from 120 vac.

there are 8 basic ways to wire them, in wiring each winding in series or parallel or both to achive the in and out voltages to buck or boost.

Since the transformer only needs to handle the percentage of voltage change for the most part all you have to do is multiply the kva by the voltage percentage change to size the correct size transformer, lets say we have a single phase 10kva load, and we are changeing the voltage from 208 to 240 since the primary is 240 volt and you are only putting in 208 you will only get about 236 out but close enough, this gives you about a 10% change in voltage, so for a 10kva load you can use a 1kva buck/boost transformer which is a big savings over an isolation transformer.

Here is a link to a PDF that has some good info in buck/boost transformers:
Jefferson Electric Buck-Boost Transformers

 

[fifty60]

Should a compressor phase monitor that monitors phase sequence, phase failure, and phase unbalance operate correctly with the buck boost transformer installed? I do not see any reason why it would not, as long as the phase monitor is set for the bucked voltage. The only paremter I could see tripping would be the phase unbalance or phase failure. It should not be an issue if the monitor is set for the bucked voltage level.

 

[kwired]

Should a compressor phase monitor that monitors phase sequence, phase failure, and phase unbalance operate correctly with the buck boost transformer installed? I do not see any reason why it would not, as long as the phase monitor is set for the bucked voltage. The only paremter I could see tripping would be the phase unbalance or phase failure. It should not be an issue if the monitor is set for the bucked voltage level.

If it would happen to monitor voltage to neutral it may not like the voltage to neutral on the ungrounded conductor that is not transformed when using an open delta buck boost setup as it will still be same potential to neutral as the supply voltage side, but otherwise I don't see a problem if it only monitors the three incoming ungrounded conductors.

 

[fifty60]

I also am using a single phase bucking transformer for a single phase open wire nichrome heater. I am bucking the voltage from 508V to 480V. Is there enough inrush generated by the autotransformer to warrant switching from the B-curve breaker I would normally use to a D-curve breaker?

 

[fifty60]

I am not finding very much information for the inrush current of an Autotransformer vs a standard isolation transformer. I would prefer to use the quicker tripping circuit breakers for the resistive loads, but not sure if I will have problems when I add the bucking transformer.

 

[GoldDigger]

The inrush current for an autotransformer will be comparable to the inrush current of an isolation transformer wired for the same purpose.

Tapatalk!

 

[hurk27]

Inrush will only be for the size of the percentage of boost or buck, if you use a .5kva transformer then your inrush will be the amount it takes to energize only the .5kva, I have never had a problem with inrush in a buck/boost install.