Inverter Transformer Question?

Question

Can a transformer having a given VA rating at 50Hz handle more power at a higher frequency, say, 300Hz or 400Hz?

I want to build an inverter to run an 11 watt compact fluorescent lamp. Since these devices rectify the supply directly, there does not seem to be any good reason to feed the lamp with 50Hz.

If my understanding is correct, the core has to be able to store up to one half-cycle's worth of energy in the magnetic field (which will then be released out of the secondary winding in the form of electricity) to avoid going into saturation and overheating. So at twice the frequency, it should be possible to push twice as much power through for the shorter half-period without saturating the core. (Actually, probably a little less than twice as much, as there will be more resistive heating; but transformer wire is often slightly thicker than it really needs to be, just because it's less likely to break in the winding machine.)

So can I over-run a transformer this way, or is there some "gotcha" that I've missed?

Answer 1

The equation V = 4.44fNaB determines what is required for a transformer to avoid saturation. V is the voltage, f is the frequency, N is the number of turns of wire in the winding, a is the area of the core, and B is the peak magnetic flux density. The transformer has presumably been designed so that the peak flux density is as high as it can be without saturating the core. For an existing transformer, V/f = a constant. If you increase f, you can increase V so that V/f remains constant. However, the hysteresis losses increase in proportion to the frequency increase and the eddy current losses increase in proportion to frequency squared. In addition, skin effect will increase the copper losses at higher frequencies. Reducing the copper losses by reducing the current would compensate for the skin effect. Reducing the copper losses would tend to compensate for the increased iron losses, but it is hard to say how much a cooler coil will compensate for a hotter core.