transformer question

[deetz]

What will happen if I power a 5kva 480v primary transformer on 400v, besides the lower voltage on the secondary.

 

[ScottyUK]

It will derate to 5/6 of its capability at 480v because as use the current rating doesn't change even th hough the voltage is reduced. Does the frequency change from 60Hz to 50Hz too?

 

[Skogsgurra]

Semantics again, I guess.

The problem seems to be in "besides the lower voltage on the secondary", which can be interpreted in several ways. Two of them are:

1. I conncet primary to 400 V (besides the connections on the lower voltage side). What happens then?

2. I connect to primary 400 V. What will happen then? Besides that I get a lower secondary voltage?

I think that interpretation #2 is the right one.

Then, Scotty is correct. You can get the same current as before.

A couple of secondary effects are that the iron losses decrease, so you may load the transformer a little more and still not overheat it. The inrush and the I_sc will be lower.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[7anoter4]

In my opinion,[and if I understood well the problem!] if E1 [480 V] is at opposite phase as E2 [400 V] then the consumer gets 400 V but the difference [80 V] will produce a circulating current through both sources in short-circuit .If they are the same phase then the circulating voltage will be 880 V[short-circuit]. If any other angle will be between the E1 and E2 the resultant will be the vectorial sum/difference and the current will circulate in a closed circuit between the sources impedance.
If the consumer is pure resistive the required power will be V^2/Rcons [so will drop to (5/6)^2 as Skotty said].If the consumer will be induction motor the torque will drop to (5/6)^2=70%[the motor could stop] but the current will be starting current reduced to 5/6 [may be 5 times the rated].

 

[Skogsgurra]

Hey, 7-4?

Where is that post of yours that I answered?

Your post (another one, I think) is now posted later than the one I answered. An act of relativity theory, space-time physics - or what?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[7anoter4]

Thank you, Skogsgurra. It’s very odd, indeed! I did nothing in connection with my post disappearance and I don’t remember what was in there. I think may be due to my rusty English I did not understand the question. I did a sketch now-if this is what the open post intended to ask-as attached and this-the second time-is the answer.

 

[deetz]

Thanks for the replies guys. Yes the hz is different also. This is a machine that has 2 motors on it. The motor nameplates on both just says 60hz, but they are referenced as metric sized motors, so I am hopefully thinking 50hz won't be an issue except for the speed difference. One of them says 220v and 380v, so I am not worried about that one. The other one just says 220v on the nameplate. It is also a 2 speed motor. I am kind of thinking maybe to get a motor that will work on 400v and stick a VFD on it for the speed changes. Any thoughts?

 

[7anoter4]

If I well understood this time, that means you have a 5 kVA transformer primary rated 480 V [and 60 Hz] and you intend to use it to supply 2 motors: one of 380/220 and a second of 220 V [only] supplying the transformer with 400 V [50 Hz]. We don't know what the transformer secondary rated voltage was. However, it seems to be 3 phase system.
Usually the secondary rated voltage of a 480 V transformer is 240 V. So will be 240/1.2=200 V- the new voltage. It is only 200/220[90.9%] but it could work.
If they are lightly loaded it looks fine.