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I'm using 220v -> 48v 830 W transformer directly connected to the 220v grid and used to operate my PCB drill.

The issue with transformer is that it's making irregular (with 0.1-2 min interval) sudden "BZZ" noises, accompanies with current spike in input coil and voltage spike in output coil.

Before using it as drill power unit, it was powering a linear voltage regulator supposed to regulare voltages up to 60v, but it was burnt out with one of those voltage spikes.

I wonder what could cause such spikes? Defect in transformer or it is normal behaviour?

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Is this a toroid or EI transformer? Your drill it is directly connected to the transformer (AC drill)? Is this "BZZ" noise apears even without load? Can you see any burn sign at the transformer? Please transfer us a better picture... without photo. – GR Tech Mar 6 '14 at 21:02
Answers: 1. EI 2. No, via rectifier bridge (DC drill) 3. No burn signs and no smell Just a simple a bit used power transformer. Nothing special. – setec Mar 7 '14 at 12:24
up vote 6 down vote accepted

Sounds like some kind of arcing within the transformer (perhaps turn-to-turn). If the transformer does this with no load and there is no sign of any other disruption on the incoming power, I would suggest discarding the transformer as it could be dangerous.

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Yes, without load spikes still occur. In fact with load they occur less frequent. – setec Mar 7 '14 at 12:25

It could be a lamination issue - do you have it mounted so that the iron core laminations do not come into contact with something metallic - mounting it on a steel sheet might cause problems. Any conducting material touching/shorting the laminates could have this effect - think mounting brackets, fasteners thru holes in laminates etc..

Transformers are laminated to prevent large eddy currents circulating in the core - if the core was unlaminated solid iron/steel, it would be like one big shorted turn so, laminates are used for this type of power transformer and the laminates are electrically insulated from each other.

Any sign of rust on the edges of the laminates - any physical damage visible?

If in fact the transformer includes a bridge rectifier and smoothing capacitor then maybe the capacitor is beginning to fail - this could be the problem too. But it's still more likely to be an intermittent shorting turn on the primary winding as Spehro alluded to. A shorting turn (and it could be bridging many turns by the way) has the effect of changing the turns ratio so that when the short occurs, the output voltage will rise and the input current will also rise.

It's unlikely to be a secondary turn shorting because this will cause the output voltage to fall slightly and maybe also reduce the input current slightly (load dependent).

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I have checked - there is no real defects visible, or smell, but I'm sure now it's lamination issue in 1st coil, causing arc shortcuts. – setec Mar 7 '14 at 12:27

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