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#51362 05/01/11 1:23 AM
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Sage
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Sage
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hello there,i have x-ray machine,150kva 500ma.due to voltage flatuation,im planing to install an AVR.my question is what is the advisable AVR for this kind of unit?
Thanks and more power.


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Super Hero
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AVR ... Automatic Voltage Regulation?

This should not be needed if the unit has been installed (and set-up) correctly.

Take a look into (mains) "line resistance" and (perhaps) "kV compensation". smile

See the bit about "Power-hungry equipment" in this .pdf.


If you don't inspect ... don't expect.
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Sage
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Sage
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thanks sir.i will look into that..


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Sage
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Sage
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we have a big problem in our power system here.during day time voltage power drop from 220 to 208,and at night voltage rise up from 220 to 240 max.x-ray unit stand-by indicator is lit only during night time.


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Super Hero
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As we know, some major x-ray units have automatic input compensation techniques that can handle "reasonable" variations in mains voltage. But those are usually big three-phase machines.

Which type of machine do you have there? A "small" Japanese unit? It may not have any built-in input compensation, apart from the line resistance that is (should be) set up during initial installation. I'm wondering if it would help to adjust this (and then re-calibrate the unit) under "worst case" (low mains voltage) conditions. But all that work may be a waste of time and effort if the machine is then out of cal during "normal" mains supply!

The bottom line is that you may have to live with things that are outside of your control (eg, the utility company). However, if I were you I would check (as far as possible) the power connections coming into the machine (check for tightness, etc. ... but please be careful). smile



If you don't inspect ... don't expect.
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Sage
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Sage
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you are corect sir.small one,japanese unit and connect to a single phase line 220volts.as far as i remember,they checked only the input exposure parameters and calibration.after that no more..
i will check line resistance,with your guidance:)
thanks and GodBless smile


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Super Hero
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When operating a machine at higher output, the input power is obviously correspondingly increased. This gives rise to increased losses due to the resistance of the electricity supply. Mains line resistance is in fact the sum total of the resistance of all cables, terminals etc. back to the power station. In normal domestic situations it may be ignored, but for x-ray units it becomes significant.

For instance, if the total mains resistance was just one ohm, the voltage drop across it would obviously become (that is, according to Ohm’s Law):-

For 1 Amp : 1 Volt
For 100 Amps : 100 Volts
For 250 Amps : 250 Volts

The mains resistance is normally just a fraction of an ohm, but when drawing heavy currents even that seemingly small value gives rise to significant voltage drops. And, of course, the higher the value of mains resistance (due, perhaps, to a poor quality mains supply), the more affect it will have on x-ray output.

How big is your unit (in kilowatts?) ... OK, so (as the Americans say) - "you do the math"! That is, how much current does that 230 volts supply get asked to cough up when the factors (kV, mA, time) are set to maximum?

The normal method of compensating for voltage losses due to mains resistance is to insert extra resistance at the machine’s input to ensure that it operates from a supply whose mains resistance is a known value (within our control). This should be done during (before) initial calibration of the unit at site. In this way we can calculate the voltage loss at various tube currents and arrange that the off-load value is high by an amount equal to the predicted loss whilst on load.

Within the (typical) x-ray unit itself, other compensation circuits attempt to automatically compensate for variations in supply voltage. For instance, voltage stabilization of the filament circuit tries to ensure that supply voltage variations are not transmitted to the filament, causing unwanted changes in tube current.

Meanwhile, in your case I am wondering about the arrangement of the incoming power supply right at (immediately coming in to) the unit. Is it fed from a spur? How far from the distribution panel? Hopefully it will be right next to it. You might also like to make sure that the unit has a really good earth! It's all basic Ohm's Law, really (higher resistance, causing larger than necessary currents, leading to unwanted volt drops beyond the capabilities of compensation circuits to handle etc., etc.). smile


If you don't inspect ... don't expect.
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Sage
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Sage
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Thanks for your assistance sir Geoff.I'v already solve.you are right,that is bitween input voltage and proper grounding.THANKS AGAIN.
GOD BLESS!! smilewink


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