ESR testing for VFD DC link capacitors.

[Skogsgurra]

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.


Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[OperaHouse]

A method I have seen is to place a 5 ohm (or less) resistor in parallel with the test capacitor with this being fed by a 100 ohm resistor connected to a signal generator @ 100khz. Signal on the 5 ohm is measured with a scope with and without the cap in place. Calculate the resistance as if the cap was another parallel resistor. Over about 20uF the cap value doesn't matter. I remember searching ESR test or circuit and finding a calculated table for this circuit. Creating the other half of the bridge and adding an op amp will make a nice tester. There is an Italian circuit on the web has this with an oscillator made from a single quad amp. I was asked this once before and realized that I had no way to test ESR on caps over 20uF. Been searching ebay for a year to find such an item.

 

[OperaHouse]

I have an ESI capacitance meter that can measure Dissipation Factor (to 4 digits) which is a variant of ESR. Like many meters, this only goes up to 20uF. It is a 4 lead device, but I usually only use two leads on it. It is sensitive enough to easily see the difference between my using a short clip lead and one that is 6 inches longer.

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.

 

[Skogsgurra]

In this case, I had six 6000 microfarad 350 V capacitors. They are connected in two parallel groups with three capacitors in series in each group. So the total capacitance is 4000 uF with 1050 V working volts.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[fsmyth]

Here is what Operahouse referred to:

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).

 

[OperaHouse]

The real question is how easy are these capacitors to get at and disconnect. I played with six capacitors connected in a ring. You can get some indication, but disconnecting them into two strings of three would give a much clearer indication.

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.

 

[OperaHouse]

I recently purchased a Sencore LC101 Z METER that needed repair. Although I have very nice ESI and GR bridges, I was quite impressed with the functionality of this unit for general repair use. Directly reads ESR in ohms, up to 200,000uf, and leakage test up to 1000V. The LC53 & LC75 have about the same features. Can be found on ebay for under $200.

 

[Skogsgurra]

Sounds right!

Is that a Buyitnow price?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[OperaHouse]

Setting your search for SENORE Z will turn up about 2 a week. The last three sold between $100-150. All things come to those who wait and my $ searches barely even break the double digits, usually buying only defective items. These Z meters do seem to bring a premium compared to other capacitor testers. The current listed one with a problem should be avoided.

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.