View Full Version : Does anyone trust the Capacitance reading of a Fluke 87

08-10-2014, 05:34 PM
Some possible unintended consequences of the new 100 amp subpanel in the shop or the 30 yr old Fenner drive power supply damping capacitor just plain dried out and gave up the ghost.

Been banging my head for 24 hrs now wanting to get to work on my DRO install but my mill is having issues

When Turn it on every thing boots up just fine

I reference and home all 3 axis just fine

But as soon as I turn on the spindle motors the drives go into voltage fault.

I checked voltages in my RPC every which way from sunday and even added a few more capacitors to get it closer. Don't remember the voltages being quite so far apart +/- 15 vac

I then checked the 60 vac output of the transformer supplying the DC section of the Fenner Drive, 61.5 vac without the motor running, 59.7 vac with the spindle motor running, but the sag recorded by my Fluke goes all the way down to 37.8 vac.

It didn't do this before and I've ran this mill off of this same RPC for years

So next I looked at the Damping Capacitor of the DC supplying the Fenner Drives. I removed it from the board and for lack of anything more capable to test with resorted to my Fluke 87.

Well it reads O.L.

I have 6ea DCM200 7400 uF caps laying about the shop so I do some comparison testing and they all come in at 5.3 on the capacitance function of the Fluke. I'm fairly positive the fluke is designed more for testing starting caps

Do I trust it ??

Drop a 7400 uF in place of the 5400 uF and cross my fingers

08-10-2014, 05:42 PM
Drop it in. They are probably +50 to -30% anyhow.

08-10-2014, 05:44 PM
No ..I'm not even sure my 87 has a Capacitance function.. My older venerable 11 did...but the button film has oxidised so it doesn't work now.

I like my Peak LCR meter .



08-10-2014, 06:20 PM
Drop it in. They are probably +50 to -30% anyhow.

Ya I did already

- no bwanna

08-10-2014, 06:21 PM
Mine seems ok, but its got a fresh cal ticket stuck to it, probably the last its going to get too as retirement removes the need to get a cal every 6 months at 200
I suppose if unattended it would drift but if you have a range of caps you can see if its close!

Paul Alciatore
08-10-2014, 07:59 PM
Would I trust the Flute? Yes, I would, they're one of the best. And you appear to have done the measurement out of circuit as you should have. So, the only remaining question is, what does the "OL" reading mean?

I have used Flutes, but do not have one now so I can't remember. Perhaps you could check the manual to see. I suspect it means an open circuit or a short circuit, both of which would mean a bad capacitor. Or you could just observe the readings when the leads are open and shorted and see which condition gives that "OL" reading.

If by "damping" capacitor you mean filter capacitor, then it should be fairly safe substituting a somewhat larger one with an equal or greater Voltage rating. But I wouldn't go much beyond 2 X either the capacitance or the Voltage rating. And don't go less on either rating.

J Tiers
08-10-2014, 10:13 PM
Do not forget, they are a polarized capacitor.

The fluke probably (I have never checked) uses a method that charges it and discharges it, meaning that polarity will be important. It will read leaky/bad one way.

Reverse leads and try again.

If it costs anyone 200 sterling to cal an 87, you may want to change cal outfits, you are likely being raked over the coals. We get it done at under $100, which is what, around 60-70 pounds?

As for what the OL means... try leads shorted and open and see which does it.

It is also possible that a residual charge is causing that.... electrolytic capacitors tend to have a reasonably strong residual charge. That fouls up the measurement

08-10-2014, 10:43 PM
On the Fluke meter I use, "OL" indicates an open circuit.

OL = "open line" if I recall correctly.

08-10-2014, 10:45 PM
Aren't caps in a RPC non-polarized for AC use? Maybe the Fluke isn't intended for measuring AC caps......

08-10-2014, 10:51 PM
In the well business we used a analog Smipson Ohm Meter set on the 10K scale to check capacitors. It contains a 27V battery which is used to charge the capacitor. You learn to judge the rate of swing of the needle. It will quickly identify shorted closed or open conditions. After the first reading the terminals are shorted or reversed to do the following test.

J Tiers
08-10-2014, 11:20 PM
Aren't caps in a RPC non-polarized for AC use? Maybe the Fluke isn't intended for measuring AC caps......

In an RPC, sure, but I believe the OP is discussing the filter capacitors in the drives.

The fluke will measure non-polar caps just fine.

08-11-2014, 01:01 AM
I just happened to have a Fluke manual on the shelf in front of me for 114,115,117.
In here it says OL means "The input is too large for the selected range"

08-11-2014, 01:12 AM
I have 6ea DCM200 7400 uF caps laying about the shop so I do some comparison testing and they all come in at 5.3 on the capacitance function of the Fluke. I'm fairly positive the fluke is designed more for testing starting caps

Do I trust it ??

When a meter gives a consistently bad reading as in this case, you can't trust anything it says. If it does not measure any of your batch of 7400uf within 10% or so, then it might be wrong about the other cap too.

I have found that erroneous readings are often the result of a low battery or using the wrong leads.

08-11-2014, 01:45 AM
The cap could be leaky or have a residual charge, as stated. Either way, the Fluke or any meter will have a problem reading it correctly. What I would normally do in this case is test a variety of other caps and see if it will identify them. Your 7400s might also be leaky, but could well be fine in use. That does happen.

08-11-2014, 02:17 AM
Capacitance meters also don't measure ESR, one of the more common failure modes of capacitors (high ESR)

The best way to diagnose a capacitor failure without expensive dedicated ESR meters is put in a known good cap.

08-11-2014, 03:26 AM
Another way to check a capacitor is to charge it up to a voltage close to its rating, and see if it holds the charge for a few minutes. Then for a 7400 uF capacitor you can discharge it (carefully) through a 1k power resistor and time how long it takes to reach 37% of the starting voltage. The time is the RC time constant, and with 1k ohms the time should be 7.4 seconds for 7400 uF. This is probably a better test than using a DMM, because it will determine if it is leaky at the operating voltage, and will give a pretty accurate reading of the capacitance as long as the voltage reading and the resistance are accurate, and an ordinary stopwatch can read to 0.01 seconds although reaction time might give an error of 0.05 seconds, but that's still better than 1%.

You might be able to make a "quick and dirty" capacitance checker using a 9V battery and a 1k resistor and a white or blue LED. The LED will turn off when the available voltage is less than about 3 volts, which is close to the 37%. I just tried this, and I had to put another 1k resistor across the LED. A 2200 uF capacitor took about 2.5 seconds to discharge from an old 9V battery. I tried a 33,000 uF capacitor and a very rough time was 29 seconds. The voltage left on the capacitor is about 4.6V, so it only discharged to 50%, but the load is non-linear, and the two 1k resistors form a 2:1 voltage divider so the LED turns off at about 2.3 volts. There are many ways this could be improved, but it's reasonably good for a rough check, and it costs next to nothing.

08-11-2014, 06:29 AM
The cap could be leaky or have a residual charge,

Didn't think to check if it had residual charge

Either way - the 7400s read consistently so I installed one of them and the problem is still there. Not sure where to go now with this - power quality coming out of the RPC or the rectification side feeding the Fenner Drives or even the spindle motor too since I had to remove it to get the mill into its new resting place.

I think my next move is to take a look for a descent scope on Eboner - this is going way past an 87s capability. That and I'll bring home a meger from work and a couple cans of contact cleaner to clean out and check the RPC and the spindle motor

08-17-2014, 10:17 PM
OK guys - she is purring like a kitten now and making chips

As I suspected (not really) the relationship of the windings make a difference. I got the correct termination information from a kind fellow over at the PM forum and it took off running quiet as a church mouse on Sunday once I hooked it up correctly

08-18-2014, 08:04 PM
I forwarded this link to someone who should know, and I was told an old 87iii might have trouble with over load on that size cap but not a newer 87v.