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Old 02-27-2008, 08:28 AM   #1
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Generator Output Question


I have a piece of shop equipment which requires 3 phase sevice, but that service isn't available in my neighborhood. I do have access to a number of generators-diesel and gasoline, but I do not know what output range would sufficiently supply the transformer (specs below).

The company says I can wire the unit for single phase, but it would cost me a couple of thousand for a 50kva 208/380 60 Hz single phase transformer.

Here is the information from the tag on the transformer:

Rex Manufacturing
CAT No. D30BX/K4
3 PH. Isolation
30 Kva 208 primary 400Y/231 secondary
60 Hz Type ANN
5% IMP @ 170 degrees C
Suitable for nonsinusoidal current with a K factor not to exceed 4


please advise me on generator size, type, fuel?
If it matters: The company suggests 180 amp breaker.


Thank You,

James

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Old 02-27-2008, 08:56 AM   #2
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Generator Output Question


I forgot to mention that the tech guys said that i couldn't use a phase converter since it doesn't supply steady and accurate current.

James

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Old 02-27-2008, 10:28 AM   #3
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Generator Output Question


You can use the 3-phase transformer single phase, that way you don't have to use the generator. What I mean is, wire your machine single phase, use the transformer to get the voltage you want. You just won't use one phase of the transformer.

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Old 02-27-2008, 10:32 AM   #4
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Generator Output Question


Information on the equipment power requirements is needed. The equipment won't use the full load rating of the transformer.
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Old 03-04-2008, 03:11 PM   #5
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Generator Output Question


Hi jared,

the unit draws 125 amps. is that what you needed to know? I really don't need a precise number: I just need to know that I will have plenty if i use a ________ Kw generator three phase.


thanks a ton
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Old 03-05-2008, 09:55 AM   #6
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Generator Output Question


My comments in blue.

Quote:
Originally Posted by gimmeda8 View Post

The company says I can wire the unit for single phase, but it would cost me a couple of thousand for a 50kva 208/380 60 Hz single phase transformer.

The above statement doesn't make sense. It will be quite difficult to supply the transformer with a "single phase voltage" of 208 volts.

Here is the information from the tag on the transformer:

Rex Manufacturing
CAT No. D30BX/K4
3 PH. Isolation
30 Kva 208 primary 400Y/231 secondary
60 Hz Type ANN
5% IMP @ 170 degrees C
Suitable for nonsinusoidal current with a K factor not to exceed 4

Please note: The Primary "Line" voltage is 208 volts. The Secondary "Line" voltage is 400 volts. The Primary "Phase" voltage is 120 volts. The Secondary "Phase" voltage is 230 volts.

Transformer Primary current:
I = VA divided by [1.732 x 208]
I = 30000 divided by 360.256
I = 83.3 Amps.

Transformer Secondary current:
I = VA divided by [1.732 x 400]
I = 30000 divided by 692.8
I = 43.3 Amps.

From the above calculations, it can be seen that this transformer is capable of supplying a maximum Secondary current of 43.3 Amps.


please advise me on generator size, type, fuel?
If it matters: The company suggests 180 amp breaker.

Is it for the Primary side of the transformer? If so, the only reason I can for using this size breaker would be to withstand the "in-rush" current (for transformers - 2 to 4 times the Full Load Current).
Quote:
Originally Posted by gimmeda8 View Post
I forgot to mention that the tech guys said that i couldn't use a phase converter since it doesn't supply steady and accurate current.

Since this was mentioned by the "tech guys", it would therefore be advisable to only use a generator that has a proven high quality sine wave output & also very good voltage regulation characteristics. These 2 things will be almost impossible to find in any Portable generator. Conversely, almost all Standby generators will have these characteristics. Standby generators are more expensive than Portable generators.

James
Quote:
Originally Posted by gimmeda8 View Post
Hi jared,

the unit draws 125 amps. is that what you needed to know? I really don't need a precise number: I just need to know that I will have plenty if i use a ________ Kw generator three phase.

I am confused again. The calculated maximum Secondary output current of the transformer is 43.3 Amps.


thanks a ton

James, there is an inconsistency with your information:
The maximum current output of the transformer is 43.3 Amps but you said that "the unit draws 125 Amps". Please verify.

To answer your generator sizing question, you can rate it according to your load but it sounds like your load is "harmonic". This is why the transformer has a "K" rating. Under these circumstances, I would increase the size of the generator by a minimum of 20% to allow for the extra heat generated in it as a result of being connected to a non-linear load.

For example, if your load is 3 phase (400 volt) & draws 40 Amps per phase, the following calculation can be used:

VA = 1.732 x 400 x 40
VA = 27712 or 27.7 kVA.
Increase this value by 20% - 33.25 kVA.

On the assumption that your "unit" draws 125 Amps per phase & since the transformer output voltage is 400 volts, the following calculation applies:

VA = 1.732 x 400 x 125
VA = 86600 or 86.6 kVA. Please note that the transformer used in this case would be rated at about 90 kVA.
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Last edited by elkangorito; 03-05-2008 at 10:05 AM.
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Old 03-05-2008, 11:19 AM   #7
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Generator Output Question


The tech guys are wrong. The cheapest and easiest way to get 3 phase out of single phase is with a VFD. Variable Frequency Drive. It allows speed adjustments, but if you want only one speed (RPM) you can set the drive for the speed (Hz) the machine requires.
All you need is the full load current, HP, and voltage and your good to go. It is true that the output of a VFD is not as clean as utility produced power. So you may want to look at the motors on the machine and see if they are suitable for VFD use. The control power can be independantly fed from your current power source or the VFD depending on the voltage. VFD control power will be 24 volts DC.
If you need help making sure the motors are suitable let me know and I will check for you.

Last edited by J. V.; 03-05-2008 at 11:23 AM.
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Old 03-05-2008, 01:58 PM   #8
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PS.......How can a transformer make single phase into three phase? It can't. However three phase to single phase is a reality.

Check VFD price at www.automationdirect.com and compare the price between the generator.
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Old 03-06-2008, 05:03 AM   #9
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Quote:
Originally Posted by J. V. View Post
The tech guys are wrong. The cheapest and easiest way to get 3 phase out of single phase is with a VFD. Variable Frequency Drive. It allows speed adjustments, but if you want only one speed (RPM) you can set the drive for the speed (Hz) the machine requires.
All you need is the full load current, HP, and voltage and your good to go. It is true that the output of a VFD is not as clean as utility produced power. So you may want to look at the motors on the machine and see if they are suitable for VFD use. The control power can be independantly fed from your current power source or the VFD depending on the voltage. VFD control power will be 24 volts DC.
If you need help making sure the motors are suitable let me know and I will check for you.
J.V. you may have misunderstood what the OPs' situation is.
He has a device that requires a 3 phase supply of 400v phase-to-phase. He has been told by his "tech guys" (reps from the manufacturer/seller?) that this device cannot be supplied by a non-linear load (inverter). I would believe the "tech guys" because not doing so may void warranty.
Therefore from the information that has been given so far, an inverter cannot be used to supply this device. The only possible exception to this rule would be to use a "True Sine Wave" output inverter - quite expensive.
All the OP needs is something that will give him a constant & clean 208v phase-to-phase to supply his "step-up" transformer. Unfortunately the OPs' information (125 Amps) conflicts with the size of the transformer.

Quote:
Originally Posted by InPhase277 View Post
You can use the 3-phase transformer single phase, that way you don't have to use the generator. What I mean is, wire your machine single phase, use the transformer to get the voltage you want. You just won't use one phase of the transformer.

Quote:
Originally Posted by J. V. View Post
PS.......How can a transformer make single phase into three phase? It can't. However three phase to single phase is a reality.

Check VFD price at www.automationdirect.com and compare the price between the generator.
InPhase277
It is obvious that InPhase277 was not suggesting that a transformer can make single phase into three phase. Read his post again. His suggestion will work but it's wasteful.
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Last edited by elkangorito; 03-06-2008 at 05:09 AM.
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Old 03-06-2008, 11:40 AM   #10
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Generator Output Question


Quote:
Originally Posted by elkangorito View Post
J.V. you may have misunderstood what the OPs' situation is.
He has a device that requires a 3 phase supply of 400v phase-to-phase. He has been told by his "tech guys" (reps from the manufacturer/seller?) that this device cannot be supplied by a non-linear load (inverter). I would believe the "tech guys" because not doing so may void warranty.
Therefore from the information that has been given so far, an inverter cannot be used to supply this device. The only possible exception to this rule would be to use a "True Sine Wave" output inverter - quite expensive.
All the OP needs is something that will give him a constant & clean 208v phase-to-phase to supply his "step-up" transformer. Unfortunately the OPs' information (125 Amps) conflicts with the size of the transformer.

It is obvious that InPhase277 was not suggesting that a transformer can make single phase into three phase. Read his post again. His suggestion will work but it's wasteful.
You are right. I did not read the posters question as I should have. I realized my mistake yesterday and planned to appologize today. Thanks
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Old 03-06-2008, 11:50 AM   #11
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Generator Output Question


sometimes its cheaper to modify machine. maybe change the motor...
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Old 03-06-2008, 12:15 PM   #12
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Generator Output Question


I don't know where the line is drawn between DIY and Industrial machine power supplies and controls and generators, but clearly, this issue is way over that line.

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