digital speed pot on vfd failing

[mwuniseal]

We have a dart digital speed pot with a lenze vfd that is giving us problems. At least once a month the digital speed pot is failing. The incoming supply is 480 3 phase. We have a 1 kva 480 to 120 transformer to supply control voltage. The drive, transformer,and speed pot are all in one cabinet. After the digital drive fails, we remove all voltage to cabinet, after several hours we reapply voltage and everything works fine. We believe the speed pot is failing due to noise off of the drive.

 

[petersonra]

I doubt it is noise. It seems more likely to be a heat related failure if you turn it off, let it cool down, and then it works OK. Might be in the Dart unit, or the VFD.

Is the cabinet they are in ventilated?

 

[gar]

110217-1752 EST

What is a digital speed pot. Pot is usually an abbreviation for potentiometer. A potentiometer is a fixed resistance with an adjustable tap on the resistance. Is this the device you are referring to, or some other kind of a device?

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[Jraef]

110217-1752 EST

What is a digital speed pot. Pot is usually an abbreviation for potentiometer. A potentiometer is a fixed resistance with an adjustable tap on the resistance. Is this the device you are referring to, or some other kind of a device?

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Digital replacement for a speed pot.

Sounds like heat to me too. Noise would be consistent.

Typically a VFD is rated for 40C (104F), that Dart Digital Pot is rated 45C (113F). If you have a VFD, a digital device like this and a 1kVA transformer in a sealed box, I'm suprised they lasted this long.

 

[gar]

110217-1959 EST

Thanks Jraef:

So it looks like it might actually be a digital pot.

For background the following is a good comparison of some broad aspects of a digital pot. http://www.maxim-ic.com/app-notes/index.mvp/id/3484

I found it looking for maximum resolution of a digital pot. Maxim mentions a maximum of 1 part in 256, 8 binary bits. This was a guess I had from prior knowledge of these devices.

If this Dart thing actually has a digital pot inside, then a high impedance DVM can be used to check the output voltage of the pot. If this is done before and after the failure occurs, then any substantial change will be an indication of a pot failure or excitation voltage to the pot. I am not able to access details on the Dart device, Adobe is failing.

It does appear the DP4 requires AC power. I do not know if the pot is isolated from ground, but it should be. But by how much is the isolation rating? In addition to AC power the the pot has to receive the voltage to be divided across the outer two pot terminals. Then one of these terminals can be called the reference, and the third pot terminal would be the equivalent to a mechanical pot slider terminal. Depending upon the type of drive the pot is connected to the DC voltage applied to the pot could be floating on a high AC voltage, like 120 to 240. So if you measure DC voltages on the pot terminals you may have to be very careful.

mwuniseal: In summary:

If the pot output voltage measured with a high impedance meter is unchanged from before to after the system failure, then the pot is not at fault.

If there is a change, then it may or may not be the pot.

.

 

[gar]

110217-2038 EST

Following is another Maxim note on the comparison of DACs and Digital POTs. Big resolution difference.

http://www.eetasia.com/ARTICLES/2007AUG/PDF/EEOL_2007AUG15_ACC_CTRLD_AN.pdf?SOURCES=DOWNLOAD

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[Cow]

Typically a VFD is rated for 40C (104F), that Dart Digital Pot is rated 45C (113F). If you have a VFD, a digital device like this and a 1kVA transformer in a sealed box, I'm suprised they lasted this long.

I agree. An easy way to verify this is to leave the cabinet door open while it's running and see if the pot quits again.

 

[Jraef]

I agree. An easy way to verify this is to leave the cabinet door open while it's running and see if the pot quits again.

Simple but effective. Might be tricky in our new Arc Flash world though.