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David ( and Frank), is my analysis correct? I don't want to screw the OP up for his machine. If the input 240v amperage needs to be 14-15 ( 4kw ) and the BB transformer is 240-32 ( more common than a 40v change ), it will take two .75kw single phase BB transformers wired open delta to provide about 15 amps of 208v three phase power to the motor. Dave
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Dave, your concept of power, voltage and energy is incomplete. Let's explore.
VA is a unit that quantifies the rate energy consumption. It is the product of voltage (the 'pressure' that drives electrons through the wire) multiplied by amps (the quantity of electrons flowing past a point in the wire per second)
We'll look at a 200 V, 25 A motor as an example. To keep the math a little simpler, a single phase motor. But the principal is identical to 3 phase. For purposes of this discussion, we'll assume the transformer is 100% efficient. Again, to keep the math simple.
First, some facts and conventions that must be kept in mind:
- The primary side is always connected to the supply
- The secondary side is always connected to the load
- V x A = VA
- VA/A = V
- VA/V = A
- VA and W are the same in a resistive circuit. In an inductive or capacitive circuit, the V and A will be slightly out of phase resulting in a VA value that is higher than the W value would be . The theory and math get much more complicated so we'll just stick with using VA because this is and inductive load.
- VA of the primary side will always equal VA of the secondary side (ignoring the inefficiencies)
- Whatever load is being run, the transformer must have the capacity to feed that load. Think of a crane. If you need to lift 1000 Lbs, then a 500 Lb crane is not going to do.
The motor, at full power, is using 5000VA (200V x 25A = 5000VA). But we have a problem because our circuit that the motor will be run on is 240V. The solution is a buck transformer. The secondary will have 200V output and from that, our motor will draw 25A. The primary will be connected to 240V, so let's figure out what the primary current will be. 5000VA / 240V = 20.83A. So the current on the primary side is lower because it there are more volts available to keep the VA at 5000.
So, as you can see, if you're boosting the voltage, the current on the secondary will go down. Conversely, if the voltage is being reduced, the secondary current will increase. Remember, the VA on both side must be the same. Whatever the case may be, the one thing that cannot change is that you need that transformer to be able to deliver the VA load that is needed for the motor.
Now I'll try to address your math above. You need to drop the 240 V supply voltage to 208V (by 32 V) which is a reduction of 13.33% requiring a 13.33% increase in current on the secondary. I'm not following your math at all because you're mixing units and multiplying things that cannot be multiplied. 240 V / 32 V = 7.5 is true, but that has nothing at all to do with VA. The voltage is being bucked from 240 V to 208 V, which is a reduction of 32 V: the voltage is not being reduced to 32V.
Your statement "The BB are not from 240-208 but reducing from 240-32 which is why smaller units are OK." is completely wrong. The buck is from 240V to 208V. If it did reduce the voltage to 32V, your motor would not even start, but would sit the & hum until it overheated.
The last sentence "Another way to size is output volts x 32/1000 or 16x32/1000= .52 kva for each rounded up to .75 for capacity." makes no sense to me at all. I read this as: 208 V x 32 V/1000 is not an equation that can be calculated. You can't multiply volts x volts. I can't figure out what is meant by 16x32/1000.
I'm not great at explaining things, but I hope this makes things more clear. I wonder if you can check the BB transformer labels & post the VA ratings here?
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Frank, I applaud your diplomacy and I am happy to not spread crap on the web- probably too late. I was attempting to translate the info from the Schneider Electric transformer sizing table calculator I googled. I tried to follow their formula and am sure I screwed it up. The calculator does lead me to the two .75 kva transformers for the application but if possible, I will delete the post that balls everything up. Dav
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Dave, I just read David's post & think I see where some of your confusion is originating from.
In an autotransformer, the line between primary & secondary appear somewhat blurred. There really is just 1 winding that has 2 ends and 1 or more tap(s) in between. I'll try to clear up the terminology here. The primary side includes all the windings between where the 2 supply wires are connected. The secondary side includes all the windings between where the 2 load lines are connected. So if it is a boost transformer, the primary only includes the large part of the winding, whereas the secondary includes all the windings. The reverse holds true for a buck transformer. To borrow David's sketch:
Where the 240V side is the primary & the 200V side is the secondary: The small portion of the winding at the top is not the primary, but the entire winding, by definition, is the primary (primary being the winding that is across the supply voltage). The lower portion of the winding is the secondary. An autotransformer VA rating is for the entire assembly, not for just a part of a winding. If you calculate the VA of just that small portion of the winding then you will have an undersized transformer.
So if you do have 2 - .75 kVA transformers running that saw, then you have it severely throttled. The good news is that if you put properly sized transformers in you'll get yourself a massive power boost.
David explains thusly:
"For a load of 2.4kVA we have to have 2.4kVA in (neglecting transformer losses). So 2.4kVA/240V = 10A in. The 10A flows into the primary so the primary sees 10A x 40V = 0.4kVA.
The secondary output current is one fifth of the primary current (turn ratio) or 2A. Secondary kVA = 2A x 200V = 0.4kVA.
The output current is the sum of the input current and the secondary current (because of the series connection) or 12A. 12A x 200V = 2.4kVA.
In general, the required transformer kVA is the output kVA times the ratio of the difference voltage to the series applied voltage, in this case 40V/240V x 2.4kVA = 0.4kVA."
This is correct, except for the issue of terminology & looking at where the ratings are. The portion of the winding that handles the 40V difference is indeed .4kVA, but that's not how a transformer is rated. As I said earlier, the VA rating is for the entire transformer.
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I'm really mixing up KW and KVA and finally realized that part. My saw doesn't show problems but a 7.5 hp 275 v motor that is running on something less probably would need to run a long time to show issues. Can you explain how the various sizing charts work? I've plugged in the input amperage for the OP motor, three phase, % of buck or boost, and output amperage and every chart I've used shows I need two .75KVA transformers. If I understand your posts, I would need two transformers, each sized at 5.5kva for my saw or 4 kva for the OPs.
The OP is looking to buck to 208 while my saw needed boost. I have two 1kva transformers on mine. Thanks for the discussion. Dave
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No worries Dave, this is a complex subject & I certainly had to put my own brain in high gear to keep things straight.Originally Posted by David Kumm
But now there's a new twist. I went to the Schneider site to have a look & they have a different way of rating the transformers than is done here in the great white north. The rating they give an auto transformer is what it would be if used as an isolation transformer (which is the rating of the little winding that makes up the voltage differential. So you were indeed correct when you chose the .75 kVA transformers.
Sorry for all the confusion I've created.
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Confusion always precedes knowledge and I'm happy to learn. I think Acme, and my others are all rated the same. I didn't know there were differences in ratings. Dave
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I was going to mention that DIY questions are not allowed at the Mike Holt forum, they are immediately locked when it's been deemed DIY. Since it's been a while since was here last too little too late. BTW, Electrician Talk has the same policy.
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Electrician Talk does have a DIY section though.
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They lock a DIY question & suggest DIY chatroom, same ownership but different forum.
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