current on an xray machine

[jimingram]

An existing, diagnostic xray machine is being relocated from one clinic to another. I have not worked on a xray machine this big before.
I will need to install a separate switch in the service switchgear (480V). I will also install a 50 KVA transformer in the xray room (no other place to put it). It will be a 1 phase 240V w/ neutral secondary. The neutral will not be used with the xray machine.

The inspector is requiring me to verify the service can handle the new load of the xray machine. I'm confused as to exactly what that load will be.

The specs on the xray generator are:
maximum KW 37.5
KW @ 100KV @ 0.1 sec 30
Momentary line current 345 amps

The xray tech told me the "resting load" would be around 20 amps.

What do I use to determine the load this xray machine will add to the service.

Jim Ingram

 

[gar]

080415-2019 EST USA

I can not answer your question and I have no experience with the device.

First, call the x-ray machine manufacturer and ask what size transformer should be used and its internal impedance.

Some broad scope comments. The peak ampere rating is 345 A whatever this means. We know the x-ray machine has to produce an output voltage in excess of 10,000 V. That is about the lowest voltage at which x-rays are generate. For higher energy x-rays the voltage will be higher.

One reference is http://www.microsemi.com/micnotes/apt9904.pdf for a unit near your size. If I read them correctly they may be referring to near continous duty. You can do other Internet searching.

Continuing to your problem. Is the 345 A a power supply inrush current when you connect power, or is this the current during the 0.1 second on time. For a resistive load 345 A at 240 V calculates to 82.8 KVA, and KW. On the other hand if the 30 or 37.5 KW represents either input power or output power the 345 might not be out of line as a current. Certainly the specifications are not very useful without a prior knowledge.

But let us assume 345 A input for 0.1 second and its 82.8 KVA value, and you have chosen a 50 KVA transformer. Further suppose the duty cycle was 1/10 meaning once every 1 second there was 0.1 second load. This would never overheat the transformer. A different question would be whether the transformer internal impedance was sufficiently low.

If this machine is used to take regular medical x-rays, then I doubt there would be a duty cycle anything close to 1/10. This means a much smaller transformer should work other than for internal impedance.

These are just background comments to aid you in asking questions.

.

 

[steve66]

The numbers you gave don't quite seem to match.

37.5 KW at 240 volts would be a maximum line current of about 156 amps. But you listed 345 amps.

It sounds like the 37.5 KW is a long term rating, and the xray unit actually pulls pulses of 83 KVA (345 A * 240 V).

517.73 says to use the larger of the long term load, or half the peak load. So if I understand the numbers you gave right, you would use the larger of 37.5 KW, or 0.5*83KVA. So I think your load would be about 42 KVA.

 

[ELA]

The specs on the xray generator are:
maximum KW 37.5
KW @ 100KV @ 0.1 sec 30
Momentary line current 345 amps

The xray tech told me the "resting load" would be around 20 amps.



Jim Ingram

How about 30KW @ 100KV for 0.1 sec ?
That would be only 0.3 A at 100KV or 125A at 240V

I would agree that the 345 amps is inrush at turn on.

 

[gar]

080416-1352 EST USA

It is real hard to know what the specs mean. The 100 kV line and 30 KW might be the output power of the of the high voltage power supply. If this is the case then the 345 A input current might be a reasonable steady state value while generating X-rays because there may be substantial inefficiency in the power supply.

What else we do not know is the power factor. If the load on the line is a capacitor input filter then there are current spikes near the peak of the AC waveform. There is relatively little phase shift but increased RMS current compared to a resistive load. If there are some significant inductors then phase shift may be a factor.

.

 

[bob]

The inspector is requiring me to verify the service can handle the new load of the xray machine. I'm confused as to exactly what that load will be.

The specs on the xray generator are:
maximum KW 37.5
KW @ 100KV @ 0.1 sec 30
Momentary line current 345 amps

The xray tech told me the "resting load" would be around 20 amps.

What do I use to determine the load this xray machine will add to the service.

Jim Ingram

Xray machines use very little power. Sometime the MFG specs a large transformer for use with the unit. I have a site listed below that specs the xray machine at 200 VA.
http://www.pearlingtechnologies.com/id17.html.
Check the Xray machine specs and name plate.

 

[steve66]

Xray machines use very little power. Sometime the MFG specs a large transformer for use with the unit. I have a site listed below that specs the xray machine at 200 VA.
http://www.pearlingtechnologies.com/id17.html.
Check the Xray machine specs and name plate.

They may use very little power on average, and when standing idle. But they take large bursts of power for a short time.

My guess is that the machine in the OP takes 345 amps for 0.1 seconds. So that's 83 KVA. Still, the NEC realizes that the burst of current only happens once every few minutes (at the most), so they allow you to size the branch circuit at half that size.

Steve

 

[steve66]

Here is a typical medical xray unit that draws 97 KW while taking a picture (See the right hand side of page 6):


http://www.gehealthcare.com/company/docs/rad/1-136f.pdf