Transformer Inrush Current

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R_HATT10

Member
I have a customer with a 200 A Single Phase Service and he has a 50 KVA,240 (sec.) to 480 (pri.) Single Phase Transformer.

The customer needs single phase 480 V for machinery he has for his shop.

My question is as follows:

Is a 200 A Service enough supply or VA to handle the inrush current of the 50 KVA Transformer?

If my calculations serve correctly (as far as sizing the breaker size for the primary of the transformer), I would need the following primary feeder breaker:

Ampacity of Breaker = Transformer KVA/Transformer Pri. Volts/1.73

= 50 KVA/240 V/1.73 = 120 A

The breaker that is currently feeding the 50 KVA Single Phase Transformer is 100 A (2P, 240 V).

Customer said he closed the 100 A Breaker feeding the 50 KVA Transformer and it tripped instantaneously.

Now, if my calculation of breaker size is correct then this should work, right?

I work for a third party testing agency and it is in my experience that Molded-Case Breakers typically trip around 5-10x Breaker Current Rating.

If the customer's 100 A Breaker is tripping instantaneously, wouldn't that mean he has 500-1000 A (or more) tripping the breaker?

Any help would be appreciated, thanks.
 
V

Volta

Senior Member
Location
Columbus, Ohio
R_HATT10 said:
I have a customer with a 200 A Single Phase Service and he has a 50 KVA,240 (sec.) to 480 (pri.) Single Phase]/b] Transformer.

The customer needs single phase 480 V for machinery he has for his shop.

My question is as follows:

Is a 200 A Service enough supply or VA to handle the inrush current of the 50 KVA Transformer?

If my calculations serve correctly (as far as sizing the breaker size for the primary of the transformer), I would need the following primary feeder breaker:

Ampacity of Breaker = Transformer KVA/Transformer Pri. Volts/1.73

= 50 KVA/240 V/1.73 = 120 A

The breaker that is currently feeding the 50 KVA Single Phase Transformer is 100 A (2P, 240 V).

Customer said he closed the 100 A Breaker feeding the 50 KVA Transformer and it tripped instantaneously.

Now, if my calculation of breaker size is correct then this should work, right?

I work for a third party testing agency and it is in my experience that Molded-Case Breakers typically trip around 5-10x Breaker Current Rating.

If the customer's 100 A Breaker is tripping instantaneously, wouldn't that mean he has 500-1000 A (or more) tripping the breaker?

Any help would be appreciated, thanks.
Click to expand...


Some on this forum are quite knwlodegable about the inrush considerations, and will surely help here, but the first thing to look at is the fact that you say single phase, then use a three phase constant in the calc.

It is either a 50,000 / 240 = 208 volt load, or a 3 phase delta circuit and transformer at 120 amps.

Beyond that, I believe that using a step-down transformer to step-up will have an affect on the inrush as the energizing and energized coil sections are in a different sequence.

Otherwise, you might find a 50 kva unit pulling over 10 x rated on start-up sometimes.
 
quogueelectric

quogueelectric

Senior Member
Location
new york
A 50 kva xformer fed 240v will draw a 208 amp load at full load without inrush. Do not use 1.73 for single phase. This xformer is too large for your service.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
The breaker is seeing the transformer inrush like a short circuit so it is tripping
without any intentional delay.

Volta said:
Beyond that, I believe that using a step-down transformer to step-up will have an affect on the inrush as the energizing and energized coil sections are in a different sequence.
Click to expand...

This is correct.
I would not be surprised to see a reverse-connected transformer draw almost 14x on inrush, although 10-12x is probably more realistic.

It is highly unlikely that you can use a 200A 120/240V (48kVA) service to feed a 50kVA transformer.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090221-0920 EST

Some transformers may be labeled with input and output voltages based of partial or full load. This labeled voltage ratio would then differ somewhat from a calculation base on turns ratio.

On tests that I have performed on a small transformer relative to the source impedance of the supply line a worst case inrush of 40 A resulted. This transformer has a full load RMS current of about 1.3 A or full load peak of 1.9 A. Thus, over a 20 to 1 ratio of inrush to full load current. See photo P6 at http://www.beta-a2.com/EE-photos.html . Toroid core transformers will likely have a higher relative peak inrush current.

The peak inrush can be limited to whatever level you want by inserting an impedance, use a resistor, in series with the primary. After the magnitization of the core has stablized, then short the impedance. For the Signal transformer in photo 6 I use a thermistor in series with the primary and am able to avoid tripping with a moderately fast 2 A breaker.

quo has already told you that your transformer is too big for the service.

.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090221-1225 EST

ptonsparky:

On small transformers it is feasible to use a thermistor and leave it in place without shorting it. Probably below 1 KVA, towards the 0.5 to 1 KVA one would probably want to short with a time delay contact because of the wasted power.

Just for illustration for a medium size transformer consider a 240 V 25 KVA unit. Full load current is approximately 105 A. A 2 ohm resistor in series would would limit inrush to 120 A RMS or 169 A peak. Power dissipation is 25 KW. Probably not unreasonable to use a 1000 W resistor for 1/2 to 1 cycle.

Another approach is to mount a Hall device sensor in the transformer core to measure the residual flux, then use this to control the timing of turn on of power to the primary. This could be done for very large transformers. The major problem would be the design of a mechanical switch whose variance in closing was within several milliseconds. Probably a variance less than 2 milliseconds. The closing time might be 20 milliseconds. Closing time meaning the time from trigger to making contact. Variance is the variability of that time. At 60 Hz 1/2 cycle is 8.33 milliseconds.

Many hobby type people in the CNC area build their own power supplies with toroid cores and large DC filter capacitors. These have the double problem of magnetic inrush current plus the large capacitive load thru a low leakage transformer. Often times these have problems tripping the service panel 20 A breaker. Typical range mighty be 0.5 to 1 KVA.

I do not know what is done with PFC capacitors.

.
 
R

R_HATT10

Member
I am not sure what kind of transformers you all are working on but, i think you are overthinking the question a bit heavily.

Just an FYI, I back-fed 120 V into the low side (x) of the transformer, given the transformer taps are arranged for 480 (H) - 120 (X). My load is 87 A, I threw the disconnect switch, energized the low side of the transformer with 120 V and... Voila, 480 V out the high side.

No issues of transformer inrush current. Actual inrush current (upon energizing transformer) was 43.0 A - well below 10-14x someone else suggested.

At full load, I could understand the transformer inrush to be evidently higher. For the intents and purposes of this application, the transformer is oversized quite generously to accomodate demand of machinery load (worst case).

Thanks for the inquries, keep studying.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090221-1825 EST

R_HATT10:

I do not believe you have studied the theory of ferromagnetic circuits in detail and understand core saturation and residual flux.

In your example you have not specified the transformer KVA size, how you measured inrush current, how many random times you energized the transformer, and what was the trip time curve of the supply circuit breaker.

.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
ptonsparky said:
I haven't studied them either but I am really curious as to what size breaker/fuse he has on the 120v side of this transformer. 87 amps on 480v side comes out to a 348 on the 120. Am I overthinking this??
Click to expand...

He is wiring as 240V not 120V.
 
G

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
090222-1025 EST

jim:

In post #10 he is referring to a 480 to 120 transformer, and provides incomplete information.

If his 480 V load is 87 A, then the 120 V is current is somewhat over 360 A.


R_HATT10:

A high resistive load on a transformer does not cause a high inrush current. But a bridge rectifier with a large capacitor input filter and starting with zero initial charge on the capacitor will reflect a large inrush current to the primary.

.
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
gar said:
090222-1025 EST

jim:

In post #10 he is referring to a 480 to 120 transformer, and provides incomplete information.

If his 480 V load is 87 A, then the 120 V is current is somewhat over 360 A.
Click to expand...

You are correct I missed that post of the test at 120V.
 
glene77is

glene77is

Senior Member
Location
Memphis, TN
gar said:
090221-1825 EST
In your example you have not specified the transformer KVA size, how you measured inrush current, how many random times you energized the transformer, and what was the trip time curve of the supply circuit breaker.
.
Click to expand...

But he did specify this for a Single Phase system:
""Ampacity of Breaker = Transformer KVA/Transformer Pri. Volts/1.73""
So, there is more "studying" to do.


-------------------------------------------
Gar,
I appreciate your responses.
They are reasonable and to-the-point.
Sometimes, It is like opening a new book.
-------------------------------------------
 
ohmhead

ohmhead

Senior Member
Location
ORLANDO FLA
R_HATT10 said:
I am not sure what kind of transformers you all are working on but, i think you are overthinking the question a bit heavily.

Just an FYI, I back-fed 120 V into the low side (x) of the transformer, given the transformer taps are arranged for 480 (H) - 120 (X). My load is 87 A, I threw the disconnect switch, energized the low side of the transformer with 120 V and... Voila, 480 V out the high side.

No issues of transformer inrush current. Actual inrush current (upon energizing transformer) was 43.0 A - well below 10-14x someone else suggested.

At full load, I could understand the transformer inrush to be evidently higher. For the intents and purposes of this application, the transformer is oversized quite generously to accomodate demand of machinery load (worst case).

Thanks for the inquries, keep studying. Well the 240 volt on that primary will pull 208 amps like cow said ,its not a good thing with a 200 amp service and iam not going to study it any further it will not work unless your load is so small on that secondary side and your just using a 50 kva transformer for a smaller load ? with smaller protection fused on secondary its over kill kva wise ? if you need 50kva thats 50,000 watts not 50 watts think about it ?do you need 60 amps on that load or what is the load ? your single phase not three phase do the math . you must have a tiny load if your trans is pulling just 43 amps Best to ya
Click to expand...
 
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