Higher than expected current draw - Fender Pro Reverb Amp

[Mick Bailey]

My energy meter turned up this morning. The Fender transformer reads a PF of .62 with nothing connected to the secondary, and .78 fully connected and switched on. My plan is to remove it from the amp and take off the end-bells and through bolts, to see if the windings heat up as quickly as the laminations.

[km6xz]

It would be nice to get a collection of capacitance readings for popular transformers so a quick single measurement can tell us whether it is normal or not due to shorted laminations.

On your transformer, losses in the core represent the major "work" being done when the transformer has no load, compared to when there is a load, when there is real work performed, so we would expect the PF to increase when going from no-load to full load.

[Steve Conner]

A PF of 0.62 with no load seems pretty crap.

I don't believe the capacitance idea will work. I'm pretty sure that on most transformers the laminations are only insulated from each other haphazardly in places, by a film of iron oxide. They would probably all read shorted to each other. Indeed in some setups the laminations can be touched, so there would be electrical safety implications if they weren't grounded.

[J M Fahey]

Agree.
Plates *are* shorted at the edges (burrs) and WAY below our instruments resolution.

Then ... what's the point?

Well, it's not the same having 0.1 ohm (edges only touching) that 0.0001 ohms (full surface touching).

And, more important, as-is eddy currents are forced to travel *within* each sheet (even if they can "jump" at the edges, which seems to be unefficient for them, as laminations are perpendicular to flux, while in a solid core, current can travel parallel to flux, or at least in a much more favourable angle to "suck" energy which will later turn into heat.

Of course, for *our* meters 0.1 ohms and 0.0001 ohms "are the same".

[Mick Bailey]

Just had a look through my transformers and here's a Peavey with the stack welded to a baseplate along both edges;

http://music-electronics-forum.com/i...attach/jpg.gif

[Enzo]

yes, Peavey welds on a mounting plate for many of their transformers.

[mozwell]

dont be worried about the lamination edges. It is perfectly ok to weld a transformer together across the laminations. Most mains freequency transformers in welding machines are done this way, as long as the weld is right on the "outside" edge, very little magnetic flux goes through there & while magnetising current goes up a little, its really not of much concern. You cannot weld across the laminations on the inside of any core, ie on the inside of the window where the coil sits, as that weld will act as a shorted turn.

Of much more concern is a temperature of 70C with only the primary connected to the mains.
As others have asked, as you turn the supply voltage up, is there a point at which the curretn starts to rise rapidly, ie at 200V i get 1mA, at 210V i get 1mA, at 220V i get 50mA, at 230V i get 250mA
If you get readings like this, the transformer is saturating, and you will need to use a primary tap for higher voltage.

[Mick Bailey]

It goes up smoothly, with no jumps. The amp was originally wired for 230v, but corrected this for 240v UK mains. I pulled the transformer to check whether the windings or laminations are heating - the laminations heat more than the windings. I've got a lot better figures on the bench with the end-bells and through-bolts removed - down to 17w. If it holds out at that I think I'll leave it and see how it goes. I've reassembled it with new nylon washers and replaced it to check over today.

Just out of interest I measured a Blues Deluxe transformer and it reads 2.2w with a PF of .16 with nothing connected. Big difference.

[km6xz]

Steve, Large transformers are checked with capacitance readings to see how the lamination insulation is holding up over time. If a reading is taken to get an average of the same model transformer, it could be useful on small transformers like used in consumer gear like amps also. They were intended to have insulated laminations, and shorting out will impact performance and heating.

[Steve Conner]

Condition monitoring of large transformers is my day job, and I've never heard of this. We monitor the power factor of the "bushings" (insulators on top of the tank) and in American practice the transformer is taken offline now and again to test the insulation in the windings, but I've never heard of anyone monitoring the insulation between laminations, if there even is any.

[km6xz]

During a visit to the PG&E(the large west coast electricity and gas supplier) test and analysis facility electric power supplier, I saw a demonstration of their use of capacity measurements when doing research in their contract testing lab 15 years ago.

[Steve Conner]

I'll believe it if you can provide a link to a paper describing the technique. Capacitance measurements are certainly used, just not for laminations.

[Mick Bailey]

Rather than insulation between laminations, I'd think of it as resistance. Measuring across the stack, I've got an Orange transformer by me and it measures 126 ohms. A small 12v transformer measures open circuit, A Laney VC30 measures 1.5 ohms, A Trace Elliot measures 12.4k. Those are all with scratching through the varnish to get to bare metal.

If the transformers are vacuum impregnated then I'd expect the varnish to get between the laminations to some degree, as well as many being stamped from oxidised/blued steel. The brief checks I've done on small transformers shows that they're highly variable. I always considered that the laminations should be separated or insulated from each other in an ideal case, but this clearly isn't so with amp transformers. Perhaps the losses are acceptable given the fairly low power consumption and the manufacturing cost of making a 'more perfect' transformer.

[BLS Transformers]

I think I'd still worry if the transformer was reaching 70 C whilst sitting with no load. What's the actual no load current of the transformer and how big is it? If it's 4.5" x 3.75" with about a 2" stack of laminations, I can check one of the 400VA beasties we do for a customer to see what mine read.

PS, whoever put that transformer together shouldn't have used a steel hammer for tightening the stack and putting the final laminations in. It looks awful.

[BLS Transformers]

That'll teach me to read it properly. 230mA it is then...