ESR testing for VFD DC link capacitors.
ESR testing for VFD DC link capacitors.
(OP)
There is sometimes a need to check DC link capacitors in frquency inverters. They are large units, usually consisting of dozens of large high-voltage electrolytic capacitors. Each capacitor has a parallel discharge resistor, which also serves to distibute the potential evenly so that the capacitors see same number of volts.
It is necessary to check both capacitance of this combination and also the ESR.
I use a resistor (used a 45 ohm 1200 W resistor last time) to discharge the capacitors and record the voltage on a fast recorder or digital oscilloscope. Starting the VFD and then stopping it leaves anything between 500 and 1000 V (depending on mains) across the capacitors and the voltage drops slowly as the discharge resistors do their jobb.
Applying the 1200 W resistor for a few tenths of a second results in a preliminary step (caused by ESR) and then a classic discharge curve. Both ESR and capacitance can be calculated from this measurement.
My question: Is there any commercially available device that does this measurement? It would be nice to be able to do it without starting the inverter. Which we have to do with the method described above. It would also be nice to be able to measure the complete unit without taking the separate capacitors out of circuit as is being done by at least one company that supplies VFD:s.
It is necessary to check both capacitance of this combination and also the ESR.
I use a resistor (used a 45 ohm 1200 W resistor last time) to discharge the capacitors and record the voltage on a fast recorder or digital oscilloscope. Starting the VFD and then stopping it leaves anything between 500 and 1000 V (depending on mains) across the capacitors and the voltage drops slowly as the discharge resistors do their jobb.
Applying the 1200 W resistor for a few tenths of a second results in a preliminary step (caused by ESR) and then a classic discharge curve. Both ESR and capacitance can be calculated from this measurement.
My question: Is there any commercially available device that does this measurement? It would be nice to be able to do it without starting the inverter. Which we have to do with the method described above. It would also be nice to be able to measure the complete unit without taking the separate capacitors out of circuit as is being done by at least one company that supplies VFD:s.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
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RE: ESR testing for VFD DC link capacitors.
RE: ESR testing for VFD DC link capacitors.
I tried using a number of low ESR caps in parallel to make approximately 15uF. I made this into a test lead which is in series with the test capacitor. The meter is happy because it sees no more than 15uF and it will give you a relative indication of the ESR. This can be compared with known good capacitors and/or some test resistors. I tested a number of caps and differences could be easily seen if your meter only had three digits to display Dissipation. This would be a quick and cheap way to test these.
RE: ESR testing for VFD DC link capacitors.
There are discharge resistors that also serve to make voltage drop across the capacitors equal. Would that also be possible to test with your meter?
BTW, what happened to my previous post in this thread? It seems to have disappeared after only a few hours.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: ESR testing for VFD DC link capacitors.
http://octopus.freeyellow.com/99.html
A bit better, but requires construction:
http://members.shaw.ca/swstuff/esrmeter.html
Fully built and calibrated:
http://members.ozemail.com.au/~bobpar/esrmeter.htm
I have not used any of the above, since I have an
old GenRad cap tester that works well, but have used
the same methods that the above testers use (in a
local setup).
RE: ESR testing for VFD DC link capacitors.
I have liked the results of the capacitor in series with the ESI 475 capacitance meter I use and have made up a permanent test lead. I tend to like tests that are real world and envision a 6V 10A transformer in series with a 12VDC source with lots of caps to make it low impedance. This would put a couple of amps through the capacitors with a bias voltage. Measuring the current would give a good relative indication of capacitance and resistance with some experience. With the entire bank being fed this way, you could also measure the AC voltage on individual capacitors.
RE: ESR testing for VFD DC link capacitors.
RE: ESR testing for VFD DC link capacitors.
Is that a Buyitnow price?
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: ESR testing for VFD DC link capacitors.
This ESR does use a different patented test method by sensing the initial voltage across th capacitor. This represents the resistance before actual capcitor charging starts. Interesting gimick method, but cap has over 1uF to work. These also require a test cable of about 68pF in order to zero meter.