Electrical guys

B is just L di/dt, right? Faradays law. There should be no material properties involved there. Only the geometery - number of turns, size, etc.

Right, the B-H curve gives H inside the material, for a given B.

Jim

================================================== please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com ==================================================

In fact, this is sometimes a useful characteristic- transformers act as transient filters and don't let spikes through due to the core saturation. If you apply double the input voltage (without doubling the input frequency) it will almost certainly kill the transformer, even with no load at all on the secondary.

When you say "efficiency", Ted, I think you're talking about the mass of iron in the core (core size) vs. the VA of the transformer. IME the actual efficiency of the transformer (output power over input power *

100%) will be better at moderately lower flux levels in the core (despite the typical loss figures in watts/pound and more core, the lower flux more than compensates for the extra core..).

Bottom line- at fixed frequency, you can apply the rated *voltage* (or LESS) but NOT MORE to a given transformer.

Best regards, Spehro Pefhany

L is a function of geometry and permeability. So the presence of a core affects L, which in turn affects B.

Gary

For power transformers, there will probably be a design tolerance for the input/primary voltage, but it probably won't be much more than about 5%.

For power transformers in equipment that's intended for international distribution, the input voltage is commonly selectable to meet global line/mains voltages. In these types of equipment, a switching method is sometimes user selectable, other times the internal wiring needs to be changed.

Some power transformers have dual primary windings to accomodate different input/line voltages of 120/240 (for example), even though a voltage selector isn't apparent. These transformers' terminals are wired during assembly of the equipment for the specific line voltage of the destination country. Some equipment can be rewired for the other voltage when there are dual primary windings. For transformers of this design, the output/secondary voltages are the same for either input/primary voltage when wired appropriately.

WB ...................

Unfortunately, by definition a transient is fast, and may well be faster than 8ms, which is the period of half of 60Hz, and this fast spike will be passed.

OTOH, a lot of transformers (such as the side-by-side bobbin style) have some good leakage inductance going, and that'll reduce the current the spike carries. I suppose a RC snubber on the secondary would do a good job of filtering out the junk.

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

Ok, As I understand it, a transformer can operate at 2x voltage as long as the freq is also doubled (assuming the transformer is happy at the increased freq).

Now my question. I have a 208V 3PH motor that I am running on a VFD. I have programmed the VFD to limit the motor voltage to 87% of line voltage (208/240 = .87).

If I run the motor above 60Hz, is it safe to let the voltage also rise above 87%? If everything is linear, it seems like it would be safe to apply 240V at 69Hz. I can create a custom VF curve in the VFD to do this.. Hum.....

chuck

This might be true of air core transformers as used at RF but not iron core transformers except perhaps in substations. For the type of transformer being discussed in this thread, i.e. power transformers for use on 120/240 power distribution in the home or shop, core saturation is what limits the input voltage.

Ted

I meant efficiency. Transformer loss is core loss plus wire loss. In a well designed transformer, these are often roughly the same. If you want to use half the rated input voltage to get half the rated output voltage, wire loss will usually dominate. You could use half the number of turns of twice the X-section area and reduce the wire loss by a factor of four.

Ted

Magnetic core materials have a property termed Rx. The core (resistive)losses go up when a fast rise time current is applied. This is the reason you will find ferrite attenuator beads all over your computer boards.

Kevin Gallimore

Yup, that's how it works. You could also run it up to 200Hz and that'd be like, 1kV? Not like it would survive many seconds of course. >;-)

Tim

-- "That's for the courts to decide." - Homer Simpson Website @

Agreed -- as others have already pointed out, the limiting factor is the core saturation. For purposes of getting the maximum power out of a transformer, the transformer is normally operated not too far below saturation -- just enough so a line surge is unlikely to enter the saturation region and thus blow the equipment's fuse.

For some electronic test equipment (prior to a lot of things being run from switching-mode power supplies, which can often self-adjust to input voltage, as well as be pretty immune to line frequency in the 50Hz/60Hz region at least), there is a switching method which is not immediately obvious.

The power cord (once we got into the European standard connectors) would frequently have the fuse clip molded in with the socket for the power cord. *If* the fuse is changed by sliding a frosted or clear plastic cover from over the fuse to over the socket for the cord (thus assuring that there is no power applied, as the cord must be unplugged to gain access), you have a plastic lever which pulls out the fuse, and once it is to the side and fuse is out, you have access to a small printed circuit card which is presented edge on. This card plugs into a set of contacts in the bottom of the socket, and, depending on its orientation as it is inserted, connects the two transformer windings either in series or in parallel for 120V or 240V -- and some also switch in secondary winding taps, to adjust for different voltages, such as 208V. What you get is determined by which edge of the card goes in first, and which side is up. Whichever way it is in, you should be able to read the selected voltage before the fuse is inserted.

Enjoy, DoN.

Very nearly true - it's the motor back EMF that changes directly with frequency. At rated voltage and frequency this is the supply voltage less the voltage drop caused by the winding resistance.

Since you are only proposing a modest increase in frequency the difference is small and can be ignored.

It becomes dominant if you make a large downward shift in frequency - at low frequencies the winding voltage drop can approach or exceed the frequency dependent back EMF. VFDs correct for this by adding a current dependent voltage boost sometimes labelled resistance compensation.