30 KVA transformer for 20 HP motor

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JoninCO

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Hello, I am new to this forum, have checked it a few times for different questions in the past, and now doing my first post.
I have a customer that has a new piece of machinery that has twin 10 HP motors. The name plate of the equipment states 29.4 FLA. The building voltage is 3-phase with a good 240 to 250 Volts depending on the time of day. The new piece of equipment is only able to be hooked up 480 Volts as per the manufacturer. They are wanting to sell a 30 KVA transformer with the equipment. When I do a calculations on the motor and a 30 KVA transformer, to me it is marginal if it will be adequate for the install.
I figure a 30 KVA will have a FLA of 36.08 (30,000 ÷ 480 ÷1.732) on the secondary side.
Sizing just the fuses on the 29.4 FLA is 51.45 amps, or 60 Amp fuses.
Does anyone have any thoughts on if this 30 KVA will work or do some agree it should be bumped up to a 45 KVA?
Any thoughts on this is greatly appreciated!
 
There is a lot of info missing to fully analyze your situation.
Such as: What are the starting characteristics of the motors (do they employ a soft starter)? Will they start simultaneously? What is the impedance of the transformer?

Since the equipment manufacturer is proposing to supply the transformer, they should know all of this and select a proper transformer.
 
retirede said:
There is a lot of info missing to fully analyze your situation.
Such as: What are the starting characteristics of the motors (do they employ a soft starter)? Will they start simultaneously? What is the impedance of the transformer?

Since the equipment manufacturer is proposing to supply the transformer, they should know all of this and select a proper transformer.
Click to expand...
I would not bet on this one bit. In most cases these kinds of things are decided by the people in charge of sales who are usually pretty much clueless about such things.

I think it is a little marginal too but it'll probably work. If you want a bigger transformer just ask for it or supply your own.
 
There does not seem to be any soft starter on the unit. Both 10 HP motors start and stop at the same time. The impedance of the proposed transformer is 1.9
And I agree with you Petersonra alot of times the manufacturers left hand doesn't know what the right hand is doing. As in this job, the machine product sheet shows specs of 480 Volt and 18 Amps. But when the schematic arrived it showed 29.4 as FLA.
 
Have you investigated the possibility that the motors are dual voltage ??
 
I believe the motors are dual Voltage. The product sheet shows different voltages and the amps per each voltage that the equipment should be able to be purchased as. But my customer was specify told that it was available in 480 Volt only. My best guess, they had the equipment setup for a 480 Volt system, maybe a unit that they previously had sold and was returned and that was the way the manufacturer could unload it. I really don't know or understand why it has to be the 480 Volt only. My customer didn't understand that either when I brought it to there attention.
 
petersonra said:
I would not bet on this one bit. In most cases these kinds of things are decided by the people in charge of sales who are usually pretty much clueless about such things.

I think it is a little marginal too but it'll probably work. If you want a bigger transformer just ask for it or supply your own.
Click to expand...

I wouldn’t bet on it, either.
I should have said “The way it’s supposed to work is the manufacturer knows what they’re doing”.
 
10HP motors should be no more than 14A at 460V, so the 29.4A load rating seems legit, with 1.4A for control power and such. But remember, that is FULL LOAD amps on the motors, not actual running load amps. They might be basing this transformer recommendation on the known actual running load amps of the motors based on what the machine does. Their fallacy however is that the STARTING current for those motors will be based on the nameplate FLA and under sizing the transformer could result in the load causing a voltage drop on start-up, and if that is too low, may affect the control system, causing a shutdown when you try to start it. If there were VFDs or Soft Starters on those motors however, all of that might be avoided. When motors start Across-The-Line, I prefer to size the transformer kVA at 2-1/2X the motor HP that starts all at once to avoid a VD of more than 5%. So that puts this at 50kVA, but I might consider 45kVA a bare minimum. With 30kVA, I would stagger start the motors.

As to the 480V only:
If the machine was made overseas and uses IEC motors, a 400V 50Hz motor can be used here at 480V 60Hz, no problem (other than the 20% faster speed), because the ratio of voltage to frequency is the same (400V 50Hz = 8:1, 480V 60Hz = 8:1). But for them, a "dual voltage motor" means that the motor is connected in Y for the higher voltage and Delta for the lower, so 400V Y becomes 230V 50Hz Delta, a ratio 4.6.:1 However when it gets here, the motor voltage would need to be 277V at 60Hz (4.6:1), not 230V 60Hz (3.83:1), so you can't use that motor here at 230V because the torque would be severely reduced and the motor would burn up trying to do the same work.
 
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Thanks for the input on it. The start up is my biggest concern. I do not see any issues once it is up and running. I even touched base with a transformer manufacturer technical support and asked what kind of tolerance the transformers have. Can they handle a higher KVA for a short few seconds or when it hits the KVA rating does it stop at that point. The technician did not have a answer and sent it to a higher up to reply. Still waiting on that info..
I'm going to stress to the customer that I still recommend going with a 45 KVA and not the 30 KVA transformer.
 
The manufacturer of the equipment is given my customer a price for a 30 KVA transformer. It is not set in stone that they have to go with that transformer. I also provided a price to them for me to supply.
The motors in this application have to start at the same time for the equipment to work correctly. Two rollers each motor running a roller.
 
JoninCO said:
Hello, I am new to this forum, have checked it a few times for different questions in the past, and now doing my first post.
I have a customer that has a new piece of machinery that has twin 10 HP motors. The name plate of the equipment states 29.4 FLA. The building voltage is 3-phase with a good 240 to 250 Volts depending on the time of day. The new piece of equipment is only able to be hooked up 480 Volts as per the manufacturer. They are wanting to sell a 30 KVA transformer with the equipment. When I do a calculations on the motor and a 30 KVA transformer, to me it is marginal if it will be adequate for the install.
I figure a 30 KVA will have a FLA of 36.08 (30,000 ÷ 480 ÷1.732) on the secondary side.
Sizing just the fuses on the 29.4 FLA is 51.45 amps, or 60 Amp fuses.
Does anyone have any thoughts on if this 30 KVA will work or do some agree it should be bumped up to a 45 KVA?
Any thoughts on this is greatly appreciated!
Click to expand...
The rule I normally use is to size the transformer at 100% of the continuous load + 30% of the intermittent loads plus any future loads decided over by the client.
Say you have a total of 20 HP motor loads (continuous) with no other loads, I am inclined to believe a 30kVA transformer would suffice. That is assuming it is a dedicated transformer for the twin-motors! ((20 HP X 746W/HP)/0.8PF = 18.65 kVA)
 
I'm not familiar with your calculation there for it to be 18.65 KVA? On my calculations I was going off the FLA as per the code book chart. And I have no doubt it would run the continuous load, my concern is start up.
 
Short circuit rating of the starting motors should not be greater that 65 %. of the transformer short circuit calculated. Rule of thumb is a factor of 1.5. Say motor starting load both 10 hp motors at the same time = 20 x 1.5 = 30 kva. I would consider that as a minimum and would be inclined to agree with you and go for 45 kva for what every my opinion is worth.
 
topgone said:
The rule I normally use is to size the transformer at 100% of the continuous load + 30% of the intermittent loads plus any future loads decided over by the client.
Say you have a total of 20 HP motor loads (continuous) with no other loads, I am inclined to believe a 30kVA transformer would suffice. That is assuming it is a dedicated transformer for the twin-motors! ((20 HP X 746W/HP)/0.8PF = 18.65 kVA)
Click to expand...
These are 3 phase motors, you left out the 1.732 in the formula. 32.3kVA, then factor in motor efficiency at 90% (/.9) its really closer to 36kVA.
 
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