Capacitance meter.....

lab · Oct 1, 2008

Hi,
I know this is not exactly a plc question but seeing as alot of sparkies frequent the forum so I thought I'd ask. I have a capacitance meter and I trying to check a split phase motor start cap., The cap is rated for 15.7 MFD 250v , the setting on my meter go from 200 pf to 200 Mf, using the 20 mf setting I get no reading what so ever, but when I change the setting to 2000 micro F, I get a value that is close (16.8) .Is there a reason why when the meter is set to 20 MF I get no reading, and what is an acceptable
range for a good cap +/- 10 %?
Thanks in advance, I know this is long winded
Lab

Mike Dyble · Oct 1, 2008

An obvious question maybe, have you tried the meter on any other caps on the 20uF range.

I think this sort of cap could have tolerance as big as +-20%, tolerance is usually marked

gas · Oct 1, 2008

A cap meter measures by charging a cap with voltage then discharging. The meter must have a good battery, leads and alligator clips.
Check a known good unit first.

lab · Oct 1, 2008

Hi,
I have check the meter with smaller caps, the type mounted on circuit boards , and did get correct reading from them, and I too thought that the battery maybe to blame, I have replaced it with a fresh 9V battery (9.67V).The meter does have leads with aligator clips on them.I'm still at a loss....
Thanks for your ideas so far......
Best Regards
Lab

Mike Dyble · Oct 1, 2008

Measure the leakage of the motor capacitor with a test meter on a high ohms range, maybe it's leaky?

L D[AR2P#0.0] · Oct 1, 2008

It must be a 15.6 microfarad capacitor as your meter tells you.

Here's a 15,000 capacitor at 75V:

rdrast · Oct 1, 2008

Question: What kind of Capacitance Meter? One really, honestly, truly designed as a Capacitor tester? or just a Multi-Meter with a capacitance function tossed in?

The latter really aren't terribly reliable for much beyond PCB level devices. Even there, they still aren't great.

jdbrandt · Oct 1, 2008

rdrast said:
Question: What kind of Capacitance Meter? One really, honestly, truly designed as a Capacitor tester? or just a Multi-Meter with a capacitance function tossed in?

The latter really aren't terribly reliable for much beyond PCB level devices. Even there, they still aren't great.

I agree. The best I've ever seen is a SENCOR Z-Meter.

rdrast · Oct 1, 2008

jdbrandt said:
I agree. The best I've ever seen is a SENCOR Z-Meter.

Wow, there's a name from my past!

I used to work in an electronics shop that had a full array of Sencore test gear... absolutely premium stuff (at an absolutely Premium price!)

lab · Oct 1, 2008

I took a picture of the cap, although the writing in not readable....the casing says "15.7 MF 280AC MAX 60HZ EUC 951-421001" and the meter is a dedicated cap meter....I think the brand is mastech or something cm200 is the model......From what is posted here so far the meter should be set to MF in order to read a MF cap correct? shown in the picture below I have the meter set to 200 microFrad (u) and the reading on the meter comes up as 16.9....I have tried other motor start caps and have the same thing happen.......Is the meter faulty? or am I missing something...I just purchased it off flea bay, I won't be using it regularly but thought it would be a nice addition to my tools.

leitmotif · Oct 1, 2008

rdrast said:
Question: What kind of Capacitance Meter? One really, honestly, truly designed as a Capacitor tester? or just a Multi-Meter with a capacitance function tossed in?

The latter really aren't terribly reliable for much beyond PCB level devices. Even there, they still aren't great.

Are you saying that a Fluke VOM with a capacitance in addition to standard functions is not a good option? I fully realize it may not be possible to stuff all completely reliable test functions into one box. Sorta like a crescent wrench cannot do all jobs.

Dan Bentler

rdrast · Oct 1, 2008

leitmotif said:
Are you saying that a Fluke VOM with a capacitance in addition to standard functions is not a good option? I fully realize it may not be possible to stuff all completely reliable test functions into one box. Sorta like a crescent wrench cannot do all jobs.

Dan Bentler

Perhaps better then nothing, but certainly not a diagnostic you can pull out and be confident in your readings. In general, actually measuring a capacitor accurately involves a lot more then the typical (and very very small) constant current source.

High capacity capacitors (no pun intended) will almost always fail to read correctly, as will most large (either capacitance or voltage wise) devices.

Dedicated LCR meters usually go a bit further, and attempt to measure the unknown device via it's frequency response.

Serious capacitance checkers can measure everything from equivalent series resistance to micro-bridging or perforations between the plates.

John Hawkins · Oct 1, 2008

older equipment

For testing capacitors & motor windinga I use a General Radio L-C-R bridge. 1kHz for caps, 60 Hz for motor windings. Very accurate. BTW
General Radio came up with the "banana plug"

L D[AR2P#0.0] · Oct 1, 2008

MF = microfarad for this capacitor.

(Small m for millifarad)

leitmotif · Oct 1, 2008

leitmotif said:
Are you saying that a Fluke VOM with a capacitance in addition to standard functions is not a good option? I fully realize it may not be possible to stuff all completely reliable test functions into one box. Sorta like a crescent wrench cannot do all jobs. Dan Bentler

Sorry I should have given the operating environment.
For doing quick (ie good or bad or about right) readings on motor capacitors and
I would have volts amps, ohms and farad in one pocket size instrument.

Would the Fluke (with capacitor readout) be a good instrument for an electrician? I think the anser is an overwhelming YES.

As a lab grade instrument the overwhelming answer is NO.

Dan Bentler

Capacitance meter.....

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