1
\$\begingroup\$

I was looking at the power calculations of the X-ray machines and found something a bit unusual. As we know the input power must be equal to the output power. Considering an x-ray machine giving an output of 150kVp and a current of 250mA, the output power is 37.5kW. Now considering the input mains side which is 220VAC. As the in and out power is the same it means that the current will be approximately equal to 37.5k/310Vp~=120A. This is a very large amount of current and I don't think the mains provide such a large amount of current. Secondly, I have seen wires at the input sides of the X-ray machine, they also don't seem to have such a high capability.

Also, many people use the time parameter to calculate the power in x-rays but still even for such a small time we will have a large power being drawn.

I have also observed at some places where a load resistor of 10ohm 300W was present in series with input AC in the way shown. I don't exactly remember the model of that machine but I have seen it being connected at input in the way shown below(just a rough diagram). In this case the resistor didn't burn if we consider the calculation above of having 120A and using i^2R it should practically be burned but it wasn't.

enter image description here Kindly help me if my point of view on the input and output power being equal is correct. And secondly, if it is correct then how is the machine able to get such a large current from the mains using normal wires as far as I have seen in medical hospitals? Kindly help me in clearing my doubt.

\$\endgroup\$
6
  • 1
    \$\begingroup\$ You must have made a mistake, I find it highly unlikely that a machine that fits in a room is drawing 37.5kW continuously. Some of your numbers are likely incorrect or misunderstood. \$\endgroup\$
    – Drew
    Commented Sep 20, 2023 at 13:58
  • 1
    \$\begingroup\$ I had supposed these values 150kVp and 100mA. But an x-ray machine has this power. you can check this link to see an example model. \$\endgroup\$
    – kam1212
    Commented Sep 20, 2023 at 14:01
  • 1
    \$\begingroup\$ Ok, so it does have an output of 50kW for ..some period of time. I bet you will find that it's pulsed only briefly. \$\endgroup\$
    – Drew
    Commented Sep 20, 2023 at 14:13
  • 1
    \$\begingroup\$ @TimWilliams my purpose in not to make such a machine. I havent written anywhere that I am making that. Secondly this forum is there to get a query related to electrical engineering solved. It may be a very basic problem for you but I am having a problem in it.If you think its a very basic problem, kindly help me in this \$\endgroup\$
    – kam1212
    Commented Sep 20, 2023 at 15:25
  • 2
    \$\begingroup\$ @kam1212 A beam current of 250 mA is unusually high. More typically, it would be 1000 times less. I'd like to see the specs on whatever it is that you are reading. \$\endgroup\$
    – periblepsis
    Commented Sep 20, 2023 at 15:48

1 Answer 1

Reset to default
2
\$\begingroup\$

Assuming specs are right and based on other systems, a couple of things come to mind:

  • Is it 3-phase, so per phase current nearer 32A (and definitely under 63A, another common rating)?

  • 10R resistor could be for inrush protection of the transformer. Might it be switched out via a relay after a timer?

  • In terms of power, is the X-Ray pulsed? If so, you may find the average power is a lot less than the pulse power, with the HV DC cap charging slowly between pulses. You'd need pulse repetition and duration to work out the average power.

(X-Ray machines at this kind of power are referenced in this paper, but only with 100ms pulse length, so average power << peak power. It also explicitly notes the use of 3-phase, both for lower feed currents, and for smaller smoothing caps )

\$\endgroup\$
9
  • 1
    \$\begingroup\$ It was single phase machine. Also the connection of resistor was in series with no relay behind it. The machine was based on inverter circuit/ square waves and the expose is given for some time. means the load exist for some short amount of time usually ms. \$\endgroup\$
    – kam1212
    Commented Sep 20, 2023 at 14:12
  • 1
    \$\begingroup\$ In that case, slow charging of the HV filter cap, followed by brief "firing" will give a much lower average power, and a 300W resistor could be left in series permanently. \$\endgroup\$
    – colintd
    Commented Sep 20, 2023 at 14:14
  • 2
    \$\begingroup\$ So you mean that the bank then supports such a large current for a small instant of time and the input current just maintains the capacitors to get a charge while the resistor avoids a high current to the capacitor? right? \$\endgroup\$
    – kam1212
    Commented Sep 20, 2023 at 15:47
  • 2
    \$\begingroup\$ Exactly. A 500nF cap at 150KV stores 5000J+, pulses at 10ms level would barely make a dent in the stored energy (so little change in voltage). \$\endgroup\$
    – colintd
    Commented Sep 20, 2023 at 16:00
  • 2
    \$\begingroup\$ But good luck finding a cap with 500nF at such a high voltage rating! In this voltage range capacitance will probably be three to four orders of magnitude smaller. \$\endgroup\$
    – jusaca
    Commented Sep 20, 2023 at 16:22

Not the answer you're looking for? Browse other questions tagged or ask your own question.