size of generator

[Travelover]

The 140 amp draw is momentary. The largest draw is when they first turn it on in the morning. If the tubes cool down (anode and cathode ray) there is the momentary draw once more. Actual imagery is only a flea's worth of electricity at 300 Ma.

I wonder if a soft start device could be used on the XRay machine like is used on air conditioning units to spread the initial surge current over a few seconds.

 

[prichard]

Wonder if it's worth them looking at a new digital xray. May have less inrush and may pay back with lower cost generator and electric bills.

 

[SPYDERLK]

The 140 amp draw is momentary. The largest draw is when they first turn it on in the morning. If the tubes cool down (anode and cathode ray) there is the momentary draw once more. Actual imagery is only a flea's worth of electricity at 300 Ma.

I wonder if a soft start device could be used on the XRay machine like is used on air conditioning units to spread the initial surge current over a few seconds.

Yes. But I suspect that the heater surge is longer. Still, its the high surge level that is the problem for a genny, and it is almost definite that the level could be lowered significantly by appropriately spreading the time of the initial and subsequent short heater cycles.

... I would 1st get an electronics guy to look at the machine schematics and query the xray mfg knowledgably. Heaters are pretty simple, not rocket science, and softer usage would prevent the significant oversizing of the gen that is being proposed to cover the abusive uncontrolled surges.​

 

[buickanddeere]

The heaters all have to be on together and full output . There is no way to trim that 140amp peak.

 

[SPYDERLK]

The heaters all have to be on together and full output . There is no way to trim that 140amp peak.

Sure there is. ... Or, how do you think you know that assuredly?

 

[buickanddeere]

Sure there is. ... Or, how do you think you know that assuredly?

It's an over sized vacuum tube , the heaters all have to be on in order to source electrons. This is not a domestic water heater

 

[CobyRupert]

I'm still confused.
140A is momentary, but is it just a short cycle, or an inrush?

I suspect it is not inrush as this would be hard to measure and assign a value (like 140amps) to. (i.e. You would probably measure a steady state current of something else.)

Let's assume there is a short period of 140 amps as it quickly heats (i.e. longer than ~1/6 of a second). Then the question becomes: Is there also an associated inrush on top of that? (i.e. Are there windings like a motor would have (I imagine a MRI machine probably has); or does machine have a step-up transformer?)

Generator has to be sized for two things: Running load and inrush ((starting load). You have to know both components of what your load is.
If you have large inrush, the generator has to be (over)sized appropriately. It's not just load volts x amps = generator KW(KVA). Motor torque, governor response & generator saturation current limits then come into play.

It is my understanding that the torque response of a diesel motor powered generator enables it to respond better to inrush and it doesn't have to be oversized as much a a LP or natural gas motor would.

 

[KYErik]

Since it is just a momentary surge, any chance that a large capacitor bank could be installed? Sort of like the starting capacitor that helps provide the power to initially start a large AC electric motor. I'm not an expert in electricity, just thinking out loud.

 

[CobyRupert]

Since it is just a momentary surge, any chance that a large capacitor bank could be installed? Sort of like the starting capacitor that helps provide the power to initially start a large AC electric motor. I'm not an expert in electricity, just thinking out loud.

I might be wrong, but I don't think it's been defined what the "momentary surge" is.
If it's an inrush caused by magnetizing an inductive load (think coils, like a transformer or motor), than a capacitor bank can help counter (balance) this inductance.
If it's a resistive load (think heater) that's "momentary" only because it's a short cycle, then added capacitance doesn't help.

 

[SPYDERLK]

It's an over sized vacuum tube , the heaters all have to be on in order to source electrons. This is not a domestic water heater

These are incandescent heaters. Inrush to a 100W bulb is 13A, dropping to 0.8A as the filament heats -- a 16 fold drop. While the cathode heaters in the xray vacuum tube may not exhibit this phenom to such extent, they will exhibit it. Even a 2 fold drop opens all kinds of possibilities.

Since it is just a momentary surge, any chance that a large capacitor bank could be installed? Sort of like the starting capacitor that helps provide the power to initially start a large AC electric motor. I'm not an expert in electricity, just thinking out loud.

This could certainly work. ... More straightforwardly if the heavy momentary can be fed directly by the DC that capacitors store and release so well.

I might be wrong, but I don't think it's been defined what the "momentary surge" is.
If it's an inrush caused by magnetizing an inductive load (think coils, like a transformer or motor), than a capacitor bank can help counter (balance) this inductance.
If it's a resistive load (think heater) that's "momentary" only because it's a short cycle, then added capacitance doesn't help.

Capacitance can be arranged to help either way, but it could become complicated - depending on what machine function is the culprit.

 

[Travelover]

I don't know the qualifications of the posters (may be sterling) but I think the OP should contact the technical side of the XRay machine manufacturer and talk to an engineer about alternatives. This issue has to be a common problem where these machines are used in emergencies or in less developed countries where power is less reliable and / or robust.

 

[buickanddeere]

Undeveloped contries often use cobalt 60 as a gamma source to X-Ray with . Problem is that sooner or later the X-Ray machine is sold for scrap and the cobalt 60 source ends up in somebody's pocket, shop shelf or mixed in with the scrap metal.
Cobalt 60 is a very high energy source . It make an erie blue/purple glow in the used fuel bay shielding water.
The total cost of a 50KW generator installation vs a 20 KW generator installation is minimal and certainly cheaper than Jerry Rigging something to make a 20KW limp along.

 

[SPYDERLK]

The total cost of a 50KW generator installation vs a 20 KW generator installation is minimal and certainly cheaper than Jerry Rigging something to make a 20KW limp along.

That 30KW surplus capability will be wasted 99% of the time, and conceivably cause some cold stacking. A soft start of the heaters may be as simple as pre warming them on 110V for a few seconds before switching 220V to them. - Simple relay function, and may make a 25 - 30KW gen quite sufficient.

 

[SPYDERLK]

I don't know the qualifications of the posters (may be sterling) but I think the OP should contact the technical side of the XRay machine manufacturer and talk to an engineer about alternatives. This issue has to be a common problem where these machines are used in emergencies or in less developed countries where power is less reliable and / or robust.

Mm-hmm ...​

Yes. But I suspect that the heater surge is longer. Still, its the high surge level that is the problem for a genny, and it is almost definite that the level could be lowered significantly by appropriately spreading the time of the initial and subsequent short heater cycles.

... I would 1st get an electronics guy to look at the machine schematics and query the xray mfg knowledgably. Heaters are pretty simple, not rocket science, and softer usage would prevent the significant oversizing of the gen that is being proposed to cover the abusive uncontrolled surges.​

 

[buickanddeere]

That 30KW surplus capability will be wasted 99% of the time, and conceivably cause some cold stacking. A soft start of the heaters may be as simple as pre warming them on 110V for a few seconds before switching 220V to them. - Simple relay function, and may make a 25 - 30KW gen quite sufficient.

120V on heaters designed for 240V will only produce 25% of the design heat.
A spark ignition engine will operate at part load without wet stacking , a diesel will wet stack.

 

[arrow]

To heat the wire for thermionic emission to occur, rather than use high voltage which would push electrons through the filament faster, what is needed is high amperage which results in millions more electrons trying to crowd their way through the filament, thus causing great friction.
The wires in the cathode are thin in order to create greater friction. It only takes seconds for this wire to heat but the amp draw is enormous.

Two currents are used in this machine (and all others) After high amps do their job, high voltage takes over to produce the actual imagery.
Indeed this is a digital machine.

 

[SPYDERLK]

120V on heaters designed for 240V will only produce 25% of the design heat.
A spark ignition engine will operate at part load without wet stacking , a diesel will wet stack.

Yes, but the heat would cause the resistance of the heater to rise some. And then hitting it with 240 would not cause so great an inrush. ... Plus, it would be barely more difficult to use a rotary switch to step through the outputs of a multi tapped transformer starting at whatever voltage and then up in several steps from there to the full 240.

 

[SPYDERLK]

To heat the wire for thermionic emission to occur, rather than use high voltage which would push electrons through the filament faster, what is needed is high amperage which results in millions more electrons trying to crowd their way through the filament, thus causing great friction.
The wires in the cathode are thin in order to create greater friction. It only takes seconds for this wire to heat but the amp draw is enormous.

Two currents are used in this machine (and all others) After high amps do their job, high voltage takes over to produce the actual imagery.
Indeed this is a digital machine.

These are incandescent heaters. Inrush to a 100W bulb is 13A, dropping to 0.8A as the filament heats -- a 16 fold drop. While the cathode heaters in the xray vacuum tube may not exhibit this phenom to such extent, they will exhibit it. Even a 2 fold drop opens all kinds of possibilities.

,,,,,,,,,,,

 

[buickanddeere]

Yes, but the heat would cause the resistance of the heater to rise some. And then hitting it with 240 would not cause so great an inrush. ... Plus, it would be barely more difficult to use a rotary switch to step through the outputs of a multi tapped transformer starting at whatever voltage and then up in several steps from there to the full 240.

Why go complicated with a hand operated mechanical switch and having to train everyone to operate it ? Just use a VFD to ramp up voltage and current .

 

[SPYDERLK]

Why go complicated with a hand operated mechanical switch and having to train everyone to operate it ? Just use a VFD to ramp up voltage and current .

I didnt say anything about a hand operated switch. Rotary switches can be easily automated by instituting timed steps. The control would be designed to the need, and the warming sequence would be transparent to the user other than a longer warmup. Software may also have to be fooled in a microprocessor controlled machine. ... The person modifying would have to pay attention and employ the most suitable methods. Employing a rotary switch in the design is just one possibility.