3 Phase Transformer

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fifty60

fifty60

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My experience has mainly been with single phase transformers. Is there anything else I have to take into consideration when sizing 3 phase general purpose transformers?

I have a single 1HP motor load at 230V and 4.2A. I get 966VA, so I am thinking a 2KVA three phase transformer will be exactly what I need....?
 
fifty60 said:
My experience has mainly been with single phase transformers. Is there anything else I have to take into consideration when sizing 3 phase general purpose transformers?

I have a single 1HP motor load at 230V and 4.2A. I get 966VA, so I am thinking a 2KVA three phase transformer will be exactly what I need....?
Click to expand...
A 2KVA three phase transformer would do the job just fine if you replace that 2HP motor with a three phase motor. :)

To use a three phase transformer to drive a single phase load you need to look at the current which is flowing on the terminals of the transformer which you are actually using, and then realize that the individual delta or wye windings of the transformer cannot carry much more current just because the other two phases are not being used.
Yes, a rule of thumb is that you can get at least 50% of the rating, but that is nothing to stake your equipment on.
Will the motor be attached line to neutral or phase-to-phase? That makes a big difference.
 
GoldDigger said:
A 2KVA three phase transformer would do the job just fine if you replace that 2HP motor with a three phase motor. :)

To use a three phase transformer to drive a single phase load you need to look at the current which is flowing on the terminals of the transformer which you are actually using, and then realize that the individual delta or wye windings of the transformer cannot carry much more current just because the other two phases are not being used.
Yes, a rule of thumb is that you can get at least 50% of the rating, but that is nothing to stake your equipment on.
Will the motor be attached line to neutral or phase-to-phase? That makes a big difference.
Click to expand...
Where'd you get 2HP and three phase transformer to drive a single phase load?

I read the OP to say a 1HP 3? motor...

@fifty60... a 1HP 230V 3? motor at 4.2A calculates out to 1746VA (using 240V). You forgot to factor by sqrt(3) for 3?.
 
Yes 3~ motor. Sorry I should have made that post a little more clear. Great information though Goldigger, I appreciate it.

So I have to factor by the square root of 3 for the 3~ transformer, that makes sense. I get 1673. Do you think a 2KVA transformer would be more than enough? If I multiply 1673VA by 1.25 and I get 2091VA.

Nameplate is actually 3.4A as well, so the VA requirement would be lower. I get 4.2A from NEC table 430.250...
 
fifty60 said:
Yes 3~ motor. Sorry I should have made that post a little more clear. Great information though Goldigger, I appreciate it.

So I have to factor by the square root of 3 for the 3~ transformer, that makes sense. I get 1673. Do you think a 2KVA transformer would be more than enough? If I multiply 1673VA by 1.25 and I get 2091VA.

Nameplate is actually 3.4A as well, so the VA requirement would be lower. I get 4.2A from NEC table 430.250...
Click to expand...

I'm not really up to speed on the fine points of transformers, but what about in-rush current when the motor is started? I understand that can be up to 8X FLA or even more, but perhaps 3-phase makes a difference?
 
fifty60 said:
Yes 3~ motor. Sorry I should have made that post a little more clear. Great information though Goldigger, I appreciate it.

So I have to factor by the square root of 3 for the 3~ transformer, that makes sense. I get 1673. Do you think a 2KVA transformer would be more than enough? If I multiply 1673VA by 1.25 and I get 2091VA.

Nameplate is actually 3.4A as well, so the VA requirement would be lower. I get 4.2A from NEC table 430.250...
Click to expand...

gadfly56 said:
I'm not really up to speed on the fine points of transformers, but what about in-rush current when the motor is started? I understand that can be up to 8X FLA or even more, but perhaps 3-phase makes a difference?
Click to expand...
Actual minimum is 1.5kVA, but standard NEMA size is 3.0kVA.

Starting in-rush isn't generally considered a problem as long as the motor isn't started more than once per hour. More than once per hour the recommendation is to increase [the minimum?] size by 20%.
 
gadfly56 said:
I'm not really up to speed on the fine points of transformers, but what about in-rush current when the motor is started? I understand that can be up to 8X FLA or even more, but perhaps 3-phase makes a difference?
Click to expand...
It does not directly make a difference in inrush. That depends on other factors of the motor design than just single phase or three phase.
But for the same inrush current ratio the three phase motor will have higher (and smoother) starting torque, allowing other parts of the design to be different. Not really safe to generalize.
 
Smart $ said:
Starting in-rush isn't generally considered a problem as long as the motor isn't started more than once per hour. More than once per hour the recommendation is to increase [the minimum?] size by 20%.
Click to expand...

Probably no limitation for use with a motor this small, unless the acceleration time is extremely long (e.g. measured in seconds). Larger transformers are not really good when it comes to handling repetitive starts.
Undersized transformers effectively become reduced voltage starters when the load inrush current is high enough to force the transformer into saturation.
 
So then sizing the primary fuse would be 3000/600 =5

5/sqrt 3 = 2.9

2.9 * 1.67 = 4.8 = 5A fuse?
 
fifty60 said:
So then sizing the primary fuse would be 3000/600 =5

5/sqrt 3 = 2.9

2.9 * 1.67 = 4.8 = 5A fuse?
Click to expand...
600V primary?

It appears your calculations are correct. Didn't actually verify. However, it appears you are looking at primary-only protection. I was thinking there is a provision in the code for a transformer supplying a single motor but can't find it at present. All that I could find was an exception that permitted a single phase two-wire transformer secondary to be primary-only protected. As such, it appears you must have secondary protection for transformer supplying a single 3? motor. Perhaps someone else knows otherwise...
 
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