Voltage drop in factory

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Rider6x6

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I am having a problem I believe with voltage drop .
i have a 4160 volt service supplying a 7000 hp vfd, on that same service I have a 4160 to 480 volt transformer providing power to several cnc machines . When I rack the breaker in for the vfd I get a low voltage alarm and the lights dim for a split second . I am considering a capacitor bank on the 480 volt side. Any thoughts if this will cure my problem
 
At that size, it sounds like you can afford to pay for the engineering that would tell you exactly what you need.
 
The inrush to the transformer can be ten or more times the full load amps. Are you actually energizing the transformers by racking the breaker in?

Or is the VFD input on and charging the DC bus the moment the breaker is racked in?
Many VFDs incorporate inrush limiting circuitry, but maybe not at 4160V?

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Rider6x6 said:
I am having a problem I believe with voltage drop .
i have a 4160 volt service supplying a 7000 hp vfd, on that same service I have a 4160 to 480 volt transformer providing power to several cnc machines . When I rack the breaker in for the vfd I get a low voltage alarm and the lights dim for a split second . I am considering a capacitor bank on the 480 volt side. Any thoughts if this will cure my problem
Click to expand...
You likely have a Voltage Source Inverter (VSI) MV drive. All VSI drives have a system referred to as a "pre-charge circuit" that limits the capacitor charging current, otherwise that can damage the front-end rectifier components, as well as the capacitors themselves. It's remotely possible that your drive has a failure in that system that is taking place, so you might want to call the VFD mfr for a PM service call to be sure.

But also, a lot of modern MV VSI drives are built to mitigate the severe harmonics that are caused by them and will do so by using a "multi-pulse" front end rectifier design. That system uses a specially wound transformer arrangement to take the incoming 3 phases 4160V and create 9 phase (aka "18 pulse") or 12 phase (aka "24 pulse") output, with each set of 3 phases separated by a 20 or 15 degree phase shift from one to the next. Each set of 3 phases then goes into separate rectifiers and converted to DC, which are all then combined (in various ways) to recreate the 4160V 3 phase output to the motor. So in this case, even though there is a pre-charge circuit for each rectifier, it doesn't actually help with this issue. There is still the magnetic inrush current of the big main transformer input itself when you close that breaker (assuming that you rack it in, THEN close it).

Bottom line, unless there is a problem in the VFD pre-charge circuit(s), there isn't much you are going to be able to do about it now. If the dip is brief, I wouldn't worry about it, it just comes with the territory. If on the other hand is is causing shutdowns and lost production, then someone might want to consider a alternative type of VFD that will not cause that.

PS: Do not add the cap bank, you are asking for more troubles and it likely won't help anyway.
 
For 7000HP, this is likely what the insides of your drive look like, although everyone has their own design differences. The transformer is to the left of center.
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As you can imagine, lots of magnetics going on in there when you first energize it.
 
Rider6x6 said:
I am having a problem I believe with voltage drop .
i have a 4160 volt service supplying a 7000 hp vfd, on that same service I have a 4160 to 480 volt transformer providing power to several cnc machines . When I rack the breaker in for the vfd I get a low voltage alarm and the lights dim for a split second . I am considering a capacitor bank on the 480 volt side. Any thoughts if this will cure my problem
Click to expand...

Is the problem on your 480V system or the MV?
 
If re-energizing the VFD is causing your CNC tooling to shut down, maybe you need to simply make sure that before you re-energize a VFD, you stop the CNC machines first. As I think about this, are you racking the MV VFD feeder breaker in and out often enough for this to be a big deal? Because THAT is something you may need to re-think. It's not good for the VFD to be doing that too often.
 
Jraef said:
As I think about this, are you racking the MV VFD feeder breaker in and out often enough for this to be a big deal? Because THAT is something you may need to re-think. It's not good for the VFD to be doing that too often.
Click to expand...

How else do you turn the VFD on and off?! ;)
 
publicgood said:
How else do you turn the VFD on and off?! ;)
Click to expand...
You command it on and off, but leave it energized unless you have to work on the VFD or the motor. powering it up and down stresses those pre-charge circuits, often just a current limiting resistor and a contactor to short it out after a second. Every time you do that, the resistor heats and cools, heats and cool, etc. etc. Eventually something will fail. As a gross general rule, the components are selected for roughly 5,000 operation cycles (I don't know why, that's just kind of the industry standard). 5k operations for something that is powered up and down once per day is about 13 years, which is roughly the life span of the capacitors anyway, so essentially by the time you need a major overhaul of the VFD, you replace the pre-charge boards too. But if you power it up with every shift change for example, so 4x/day, you will need to replace the resistors in about 3 years.
 
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