3 Phase Buck Boost Transformers

[fifty60]

I am more familiar with single phase buck boost transformers. I understand that to buck voltage to a 3 phase compressor, I will need two single phase autotransformers.

My compressors are 4.9A each. There are 2 compressors so a RATED load of 9.8A. The autotransformers I have access to are rated for 10.9A loads or 21.9A loads. I will have to use the larger 21.9A rated bucking transformer.

The max circuit breaker I can use for this autotransformer is 30A. I want to protect the autotransformer and not have any nuisance trips. Do I size the primary autotransformer OCPD of the secondary load, as long as it is below the max 30A?

Would these calculations work: 4.9A * 2.25 = 11.025 + 4.9A = 20A primary

Also, I see in NEC 450.4 information about OCPD's for the primary. Am I also able to put breakers on the secondary? For example, I have 2 compressors, can I put 10A UL489 breakers on each branch of the secondary?

 

[kingpb]

You cannot ignore the starting requirements. The transformer may be good for the running load but you have to get the compressor started first.

 

[Besoeker]

I am more familiar with single phase buck boost transformers. I understand that to buck voltage to a 3 phase compressor, I will need two single phase autotransformers.

Do you have a three phase supply?

 

[fifty60]

Starting requirements and running requirements. The running load can be several amps above the Rated load and be perfectly normal. This is why I have to use the next size up autotransformer.

Yes it is a 3 phase supply.

 

[Besoeker]

Starting requirements and running requirements. The running load can be several amps above the Rated load and be perfectly normal. This is why I have to use the next size up autotransformer.

Yes it is a 3 phase supply.

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

 

[jim dungar]

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

Two small 'off the shelf' single phase units connected in open-delta often cost less than a single 3-phase buck-boost, based on the 'laws of supply and demand'

 

[Besoeker]

Two small 'off the shelf' single phase units connected in open-delta often cost less than a single 3-phase buck-boost, based on the 'laws of supply and demand'

OK. It's just not something we would come across here so no "off the shelf" units exist.
For the infrequent use we have for them we'd get a unit made.

 

[fifty60]

The 2 transformer manufacturers I deal with only sell single phase units. Does the above method sound accurate for sizing the circuit breaker for the primary side of the autotransformers? As I mentioned, my compressors are slightly to big for the smaller units, so I have to bump up to the next higher size. The max fuse is 30A, but if I can size for the load and use a smaller circuit breaker I believe I would be protecting the transformers better.

Also, I have 2 branch circuits on the secondary. Is it ok to use OCPD's on each branch?

 

[kwired]

Maybe I'm missing something here but the load on the transformer is only the difference necessary for the voltage change and not the current of the entire connected load isn't it?

You are trying to achieve a relatively fixed VA output, but are changing the voltage to do so. In order to keep same VA output the transformer must carry the proportional current to match the change in voltage, the remaining current is delivered directly by the supply and does not pass through the transformer. Current through a buck boost transformer is going to be low compared to what the supply carries or what is delivered to the load.

 

[augie47]

Maybe I'm missing something here but the load on the transformer is only the difference necessary for the voltage change and not the current of the entire connected load isn't it?

You are trying to achieve a relatively fixed VA output, but are changing the voltage to do so. In order to keep same VA output the transformer must carry the proportional current to match the change in voltage, the remaining current is delivered directly by the supply and does not pass through the transformer. Current through a buck boost transformer is going to be low compared to what the supply carries or what is delivered to the load.

That would be my thinking also.
Acme transformer has this link: http://acmefaq.wordpress.com/category/acme-specific/buck-boost/page/2/ which appears to show that no special consideration need to be taken due to the motor load.

 

[fifty60]

Admittedly, still wrestling a bit to understand this...

 

[kwired]

Admittedly, still wrestling a bit to understand this...

For simplicity I will use a single phase application, lets say we have a 1000VA 240 volt load, but only have a 208 volt supply.

1000VA @ 208 is 4.81 amps
1000VA @ 240 is 4.16 amps

If we use an autotransformer to get the desired voltage - we still have 1000VA load that will see 4.16 amps but the supply current ahead of the transformer is 4.81 amps. The difference (disregarding any inefficiency and power factor) of .65 amps is what is flowing in the transformer, not the entire amps of the load. If it were an isolation transformer instead of an autotransformer then the entire load will flow through both sides of the transformer at a level proportional to the total VA and voltage. It works this way whether bucking or boosting the voltage, the difference in current between input and output is what is flowing in the transformer coil.

 

[Smart $]

For simplicity I will use a single phase application, lets say we have a 1000VA 240 volt load, but only have a 208 volt supply.

1000VA @ 208 is 4.81 amps
1000VA @ 240 is 4.16 amps

If we use an autotransformer to get the desired voltage - we still have 1000VA load that will see 4.16 amps but the supply current ahead of the transformer is 4.81 amps. The difference (disregarding any inefficiency and power factor) of .65 amps is what is flowing in the transformer, not the entire amps of the load. If it were an isolation transformer instead of an autotransformer then the entire load will flow through both sides of the transformer at a level proportional to the total VA and voltage. It works this way whether bucking or boosting the voltage, the difference in current between input and output is what is flowing in the transformer coil.

That's not quite right. A buck/boost tranformer is essentially an isolation transformer connected as an autotransformer, so it does have a primary and secondary windings. For your example, the entire 4.16A flows through the secondary and the other .65A flows through the primary.

4.16A ? (240-208) = 133VA on secondary
0.65A ? 208 = 135VA on primary

The 2 VA difference is from rounding errors.

 

[fifty60]

Thanks for the explanation.

This is a bucking application going from 504V down to 480V. My concern is with the conductors that are connected to the primary of the transformer, and the conductors connected to the secondary of the transformer and directly to the load. There are two branches on the secondary, one for each compressor. All of the current being delivered to the load has to go through the primary conductors AND the secondary conductors, right?

 

[kwired]

Thanks for the explanation.

This is a bucking application going from 504V down to 480V. My concern is with the conductors that are connected to the primary of the transformer, and the conductors connected to the secondary of the transformer and directly to the load. There are two branches on the secondary, one for each compressor. All of the current being delivered to the load has to go through the primary conductors AND the secondary conductors, right?

Not exactly. The total VA on each side of the transformer is the same, but since voltages are not the same the currents can't be the same either.

 

[Smart $]

... All of the current being delivered to the load has to go through the primary conductors AND the secondary conductors, right?

In a sense, yes... but not exactly. A more accurate statement would be all the power delivered to the load has to go through the primary and secondary conductors.

 

[fifty60]

So would sizing the primary conductors for the load of the transformer be sufficient? And then sizing sizing each branch fed of the secondary to its respective load?

If there were only one load on the secondary, I would not need to use an OCPD on the secondary. the Confusion that I am having is with the 2 branches on the secondary. Also, with the max main fuse size of 30A. Since my load (compressors) is only 11A, i do not want to size the primary of the transformer to 30A, but would prefer 20A.

 

[kwired]

That's not quite right. A buck/boost tranformer is essentially an isolation transformer connected as an autotransformer, so it does have a primary and secondary windings. For your example, the entire 4.16A flows through the secondary and the other .65A flows through the primary.

4.16A ? (240-208) = 133VA on secondary
0.65A ? 208 = 135VA on primary

The 2 VA difference is from rounding errors.

I have to think about that one a little more. I do know that the VA rating of a buck boost is generally much lower than the VA of the supplied load and is much closer in proportion to the voltage and current change needed for the application. The fact that 100% of the VA for one side would have to flow through the transformer would seem to mean the transformer needs to be larger than they typically are for a buck boost installation, and would make the idea of an autotransformer somewhat useless as it would need to be similar in size as an isolation transformer if it carried 100% of the load.

 

[Smart $]

So would sizing the primary conductors for the load of the transformer be sufficient? And then sizing sizing each branch fed of the secondary to its respective load?

If there were only one load on the secondary, I would not need to use an OCPD on the secondary. the Confusion that I am having is with the 2 branches on the secondary. Also, with the max main fuse size of 30A. Since my load (compressors) is only 11A, i do not want to size the primary of the transformer to 30A, but would prefer 20A.

The transformer isn't really a load, per se. You'd size the conductors for the power they carry, but with little change in current level they'll probably all end up the same size. Let's see...

480V ? 11A ? 504V = 10.5A

Not really sure how to size OCPD for a motor (compressor) circuit with an autotransformer supply. 450.4 says OCPD cannot be more than 125% of rated full-load input current. I assume you'll need it to be sized sufficient for starting without tripping, so you may have to elevate the transformer rating to meet the OCPD necessary.

 

[Smart $]

I have to think about that one a little more. I do know that the VA rating of a buck boost is generally much lower than the VA of the supplied load and is much closer in proportion to the voltage and current change needed for the application. The fact that 100% of the VA for one side would have to flow through the transformer would seem to mean the transformer needs to be larger than they typically are for a buck boost installation, and would make the idea of an autotransformer somewhat useless as it would need to be similar in size as an isolation transformer if it carried 100% of the load.

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.