EBME Forums Forums Maintenance Technical Assistance X ray equipment and mains voltage variation

Hello

Silly question time.

How does well does modern X-ray equipment cope with varations in mains voltage?

Would you expect any problems if the voltage changed between the allowable voltage range of +10 % to -6% of 230v?

Lee

I should imagine that modern x-ray equipment copes just as well as the ancient stuff did. I hope so, any way.

Especially as anything of any size is powered off 3-phase.

Meanwhile, what does the spec (of the x-ray equipment) say?

Most modern solid state units are in fact transformerless switch mode units. The incoming mains is rectified and charges a capacitor, and the voltage across the capacitor is then chopped to give a HF waveform that will be transformed up to the high tension voltage, which is then rectified to give DC for the tube.
The output voltage is controled by a feedback loop, and because the frequency is high, the degree of control is very good. You would expect to get much more accurate control of kV that would be possible using the older three phase technology.
Just think of how the charger for your laptop is able to provide an accurate output voltage without the need to change taps for voltage or frequency like you did in the past.

Hello Geoff,

The Trust is talking about installing "Power Perfectors" which reduce the on site voltage to 220V pro rata for 3 phase.

When this was mentioned to the service Guy from the company that looks after our X-ray kit he said that all of the units would need to be recalibrated etc when the voltage was reduced, considering the supply companies quote a voltage range of +10% -6% and the new voltage will be with in that range it seems a little strange to me that the output of the equipment would change.

Lee

Hello Lee

Personally, my immediate reaction to that is that both are talking b******s!

Have we not discussed the Power Perfectors on here before, only to decide that they are a complete waste of money (taking years to break even, and all the rest)?

Actually, the x-ray guy may have a valid point, at least in theory. But with all the safety (ALARA) stuff built in to x-ray kit these days, I really doubt if anyone would be able to point to any variation in output of the "deadly rays"!

I wonder if the supply companies err towards the upper range of the voltage limit to make money. Power is V-squared over R, so for any small increase in voltage the power supplied and hence the amount charged to us is squared.
Robert

Eh? Power is also IxV. So, if the load remains the same, if the voltage supplied goes up, the current must fall.

Anyway, how can the generating company wind up the voltage? See Faraday's Law et al.

If the resistance is constant as the voltage goes up the current goes up hence the power goes up.

Things do not draw power they are supplied with power which is a consequence of the voltage supplied and their resistance/reactance. (In standard constant voltage situations before someone mentions constant current supplies)
Robert

Oh dear. More revision required, I'm afraid, Robert.

Let's start with Ohm's Law!

Meanwhile, here's an interesting link.