Transformer Over-voltage...

Status
Not open for further replies.
xformer said:
Are transformers voltage ratings typically +-10%?
Click to expand...

Transformer voltage is determined by the voltage being supplied to it and the output voltage is determined by the primary to secondary ration.
The primary on a common power transformer also includes primary taps which are used to compensate for low or high supply voltage so that a give secondary voltage is provided.

Now, where does this +-10% voltage come from?
 
The most common voltage ratio rating is from 110 or 132 kV respectively to 30, 20 or 10 kV.
 
140703-0744 EDT

A ferromagnetic transformer (iron (steel) core) will work with any input voltage up to some maximum. Basically there is no lower limit. The maximum is not generally a sharp threshold point except for square loop core materials.

At lower input voltages the VA rating will be lower because the transformer is really current limited. This is because temperature rise in the transformer is mostly determined by load current and not VA.

On the high voltage end of the input range the maximum voltage is determined by core satuaration, core material, frequency, insulation material, ambient temperature, current loading, expected life, and other factors. The maximum rated voltage at a specific frequency is empirically determined by experiment, past history, and theory.

Your +10% figure is some experience judgement created by the transfomer manufacturer and industry as whole. In part this will be determined by historical information on failure rates, likely normal system voltage variation probability density curves, and type of core material and transformer construction.

A softer saturation curve will be more tolerant of overvoltage than a square type saturation curve.

An EI core construction will have a softer curve than a tape wound core of the same core material.

With a Variac, sample unloaded transformer, voltmeter, and ammeter you can measure and then plot a curve of magnetizing current vs voltage. When magnetizing current is excessive from excessive input voltage, then total load current has to be lowered to avoid excessive heating of the primary coil to achieve a comparable life time.

.
 
zog said:
I think he may be refering to available tap settings?
Click to expand...

Actually, I am referring to either a single tap setting or the highest tap setting as a reference point, lets say the highest tap setting, can the transformer still handle voltages above the largest tap setting? Is there a % tolerance? :)
 
As a practical matter, one would hope that if there is an allowed tolerance for higher delivered voltage from POCO for a particular nominal voltage, then equipment for that same nominal voltage should tolerate that same upper limit voltage.
But I do not think you will find an NEC requirement for that. UL maybe?
 
xformer said:
lets say the highest tap setting, can the transformer still handle voltages above the largest tap setting? Is there a % tolerance?
Click to expand...
If input voltage is higher than highest tap setting, the tranny would likely be overloaded because of higher than rated voltage on secondary.
 
Last edited:
I agree lower volt input is generally not any issue, so you can probably say -10 or -20% is no problem.

I have seen transformers overheat in their enclosures and melt connectors and hook up wire - ith as little as 5% high input voltage - at near no load.

Thus I would say there is no general safe +% value that can be applied to all transformers. If one is concerned over a particular one they need to get specific answer from the mfgr of it.
 
IEC 60076-1 Power transformers ?Part 1: General Table 1 presents some tolerances as -for instance:
2. Voltage ratio at no load on principal tapping for a specified first pair of windings:
The lower of the following values:
a) ?0,5 % of declared ratio
b) ?1/10 of the actual percentage impedance on the principal tapping
Voltage ratio on other tappings: same pair- to be agreed, but not less than the lesser of the values in a) and b) above.
Voltage ratio for further pairs: To be agreed, but not less than the lesser of the values in a) and b) above
 
There are national standards stating the tolerances. For instance:
BS 7671 Appendix 2 section 14. The UK supply 230 V +10% - 6%
ANSI-C84/1995-2005: L.V. Rated 120/240 max 126/252 min 114/228
Medium Voltage Rated 6.9/13.8 kV max 7.24/14.49 min 6.73/13.46
 
JoeStillman said:
Utility voltage variation is going to be orders of magnitude greater than the manufacturing tolerance of the turns ratio.
Click to expand...

As I understand it Voltages higher than those ratings for the transformer can lead to core saturation. When that happens to the transformer with sease to act as a transformer and the secondary voltage will drop.
Your comments are welcome
 
templdl said:
As I understand it Voltages higher than those ratings for the transformer can lead to core saturation. When that happens to the transformer with sease to act as a transformer and the secondary voltage will drop.
Your comments are welcome
Click to expand...

Since the EMF depends on B- magnetic flux density- and not on H [magnetic field intensity] no influence on EMFprimary/EMFsecondary will occur in the saturation state. However since H will grow up the no-load current will increase and the no-load losses will be elevated.It depends on how high degree of saturation it will be, an exaggerated voltage drop in the primary winding will reduce the EMF and then the secondary voltage too.
 
Status
Not open for further replies.
Top