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VFD Input Fuses Blew, Suggestions?

12/22/2017 10:46 AM

Hey guys,

Looking to spit ball on this a bit

Large-ish drive about 150 HP motor running on a machine that has two states

State 1: the machine is clutched and product is produced about 340A

State 2: the machine is unclutched and the motor runs unloaded about 80A

On the drive there are input fuses, solid state that are in between the feeders and powering the drive itself.

While the machine was idling yesterday the main MCC bucket opened and those input fuses blew. 700A fuses

I spoke to the drive manufacturer and they are telling me its a problem with the utility.

However, none of our other drives suffered any failure.

Also, we have checked the obvious issues.

Megaohmeter and continuity

line to line and line to ground on both sides and the motor, all read as normal

Any thoughts from the peanut gallery?

Thanks

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Guru

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#1

Re: VFD input fuses blew , suggestions?

12/22/2017 11:11 AM

I always like to go around with a temperature gun or ir screen to check for hot spots while the equipment is running....secondly you can monitor and record the power supply, and possibly the characteristics of the individual incomers....and always check the grounds for true grounding properties....then there is always the possibility of an anomaly that is a random occurrence...then just watching it to see what happens next, if anything, will tell the tale....In any case monitoring the power supply for a while would be routine in case further problems warrant another conversation with the drive manufacturer...eh?

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Guru
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#2

Re: VFD Input Fuses Blew, Suggestions?

12/22/2017 12:12 PM

Were your other VFDs running loads at the time? Do you have a line reactor ahead of this drive? If not, how large is the transformer feeding this service? Are there line reactors ahead of the other drives?

The point I'm working toward is that there is a scenario in which this can happen, depending on these issues. I can explain it after you answer them (and I'm at a computer instead of my phone).

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#4
In reply to #2

Re: VFD Input Fuses Blew, Suggestions?

12/26/2017 9:59 AM

Hi, No line reactor there

The transformer feeding this bus is huge. we have a 46Kv service, im not sure of the power on the transformer but it is large.

Could you please explain some more to me ?

Thanks

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Guru

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#3

Re: VFD Input Fuses Blew, Suggestions?

12/25/2017 3:10 PM

Those fuses are to protect the rectifier components, they are supposed to clear the circuit before the SCR passes enough current (over time) to damage the component.

If your ac line sensing circuit misses a zero crossing, or (their guess) there was enough noise on the line at that particular drive to trick the circuit into missing the timing, one of your SCRs might have fired directly into one of the others already conducting, creating a line to line short circuit. That's why the drive manufacturer blamed the utility, but it could have been quite local to the drive, or may have tricked one and not tricked the others, depending on phase angles and bridge demand at that particular millisecond.

A shot like this could be 1 in a million, or might indicate the need for anti-line notching reactors as JRaef indicates, quite commonly installed when designers fear the worst, especially with multiple SCRs on a common lineup.

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#5
In reply to #3

Re: VFD Input Fuses Blew, Suggestions?

12/26/2017 10:07 AM

Could you explain a bit more in a dumbed down manor?

Im not a drive expert, im usually a protections guy

So there are no line reactors on this setup.

I think i gather what your saying. if you have time to write again that would be great.

What im hearing so far is we are stupid we dont have line reactors. Fortunatley i didnt do the initial engineering. Im just helping clean up the mess.

the very expensive mess i might add

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Guru

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#7
In reply to #5

Re: VFD Input Fuses Blew, Suggestions?

12/26/2017 10:46 PM

When an SCR fires, the current to charge the capacitor is like a short circuit, it can pull the voltage down that the line sensing circuits use to decide when to fire the SCRs.

If the circuit sees what it thinks is a zero crossing, it can then fire one SCR while another one is still conducting, making a short circuit line to line, and popping the protection fuses.

One or two drives can be OK, but adding more on the same power system bus could tip the scales towards needing to add some anti-notching reactors.

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#6
In reply to #3

Re: VFD Input Fuses Blew, Suggestions?

12/26/2017 11:36 AM

After your excellent suggestion, i have done some more research.

At this point im thinking you are 100% correct with the cause.

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Guru
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#8

Re: VFD Input Fuses Blew, Suggestions?

12/27/2017 2:26 PM

While I can appreciate the answer regarding SCR firing, MOST modern PWM Voltage Source Inverter drives use diodes in the rectifier. Those that do use SCRs are only phase angle firing them when power is first applied as a means to limit the capacitor inrush by ramping the voltage. Once the caps are fully charged the SCRs are simply kept gated on continuously.

But the principal described (transients causing conduction issues) is still valid for diode front-ends anyway, and this is where the impedance of the circuit comes in.

The front-end rectifies the AC to DC, the capacitors smooth the DC and also store energy for the transistors, which fire into the motor to simulate an AC sine wave. Capacitors charge and discharge virtually instantaneously, that's why they make lousy batteries. So if a motor is running, it is always drawing energy instantaneously OUT of the capacitors, while the rectifier is instantaneously putting energy back INTO them.

All day long there are ringing transients on the line side, most often caused by a "grid switch" events from the utility moving power around, meaning things we have no control of. Diodes (and SCRs used as rectifiers) do not actually conduct continuously, they only conducts once the line side potential in the sine wave is above their "Forward Conduction Voltage" threshold. Whenever the ringing transient causes the line side sine wave to drop, even momentarily, below that forward conduction threshold, that device ceases to conduct, so it is not "feeding" the caps. But if the motor is running, the caps ARE being depleted. So the instant that ringing transient is again higher than the conduction threshold of the next available diode, THAT diode has to make up for the loss and the caps will draw current into the DC bus AT THE AVAILABLE FAULT CURRENT LEVEL. So when the supply kVA is high, the impedance of that circuit is so low that the diode / SCR will attempt to pass far far more current than it is rated for in that cycle and runs the risk of shorting, which will then set off a cascading list of component failures, starting with the capacitors themselves. If you are LUCKY, the fuses blow first.

Why would the other drives survive? Simply put, if their motors are NOT running at the time of the transient event, nothing is depleting the DC bus, so even though the rectifier devices stopped conducting for a fraction of a second, the energy loss in the caps is so low that nothing bad happens. This entire scenario ONLY happens if the motor is RUNNING at the time of the transient.

Adding line reactors adds impedance to the circuit, which slows down the rise time and the ringing effect of the transient in the first place, helping to avoid the loss of conduction in the rectifier components and the host of possible failures that can happen. So even though the transient takes place on the line side of the reactor, the inductive time constant of the reactor makes it so that the sine wave getting to the diodes never drops enough to stop them from conducting. The only time I don't recommend adding line reactors is when the source kVA transformer is smaller than 10x* the VFD kVA, because the impedance of that circuit is already low enough to avoid this phenomenon. That's why I asked you what size it was. You8 didn't supply that information, but assuming for example this is 150HP 460V, the drive is likely rated for around 200A @ 480V, so that makes it 96kVA. So if your source transformer is 1000kVA or larger, you definitely need Line Reactors ahead of this drive to protect it.

*You may see other literature stating this as 20x the VFD kVA rating. That would be for VFDs that have internal DC Bus Reactors, and not all of them do. So the safer bet would be to assume not and use 10X.

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#9
In reply to #8

Re: VFD Input Fuses Blew, Suggestions?

12/27/2017 5:17 PM

Thank you very much for all the info.

the transformer is definatley much larger then 10 * the Kva of the drive

Since we are on this topic. How does one determine the Kva of a drive if the power factor is unknown?

Are you just assuming a Pf of 1 and assume a worst case scenario like your FLA of the transformer?

say 200A drive *480V = 96kva Drive?

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#11
In reply to #9

Re: VFD Input Fuses Blew, Suggestions?

01/08/2018 11:11 AM

The reason you use kVA is because you DON'T need to worry about power factor going into the drive. This is a rule of thumb, so it's kept simple. A (input) x V (line) x 1.732.

And the input impedance of a VFD is tricky. But the impedance of the RECTIFIER alone is virtually nil, hence the issue of the capacitors attempting to charge themselves at the available fault current level. That's why all VFDs have what's called a "pre-charge circuit" that puts a resistor in series with the caps for the first second when you initially energize it, acting as a current limiting resistor. That is then shorted out of the circuit once the caps are charged, basically 1 second or so. So once the VFD is already powered up, that is no longer there and hence the brief transient dip in voltage allows the caps to pull that high current again, unimpeded. That's where the line reactor plays its part.

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#10
In reply to #8

Re: VFD Input Fuses Blew, Suggestions?

12/27/2017 5:35 PM

I guess the other thing im curious about is, what is the input impedance of a DC inverter like this?

Given the fact that the impedence on the input side is really just the SCR?

Thanks

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