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[bkenobi]

This is not a HA troubleshooting question per se, but it is a tool that I use for HA troubleshooting, so I'm going to ask my question anyway.   

I have a Craftsman DMM that has the capability to measure 20A AC current.  It has a thermal fuse inside that allows it to tolerate the maximum of 20A for 30 seconds every 15 minutes.  Basically, there is some kind of bimetal strip inside that opens if too much current is sent through the DMM for too long.  It then takes up to 15 minutes to cool down and reset.  I've used this DMM for years and it's been great.  But, I've never used it for current measurement.  I have a chipper that I'm replacing the power switch on and I'm trying to determine the current draw during startup.  The old switch was a 12A thermal breaker that kicked off after a minute or so of being bogged down and would take 15-20 minutes to reset.  I purchased a new 12A switch that was supposed to be identical, but it's tripping after 2-3 seconds.  Rather than continuing to throw money at this old chipper, I figured I'd determine the actual current draw so I could see what switch I need.

I borrowed a kill-a-watt a while back and found that the chipper pulls 9A when running and not chipping.  If it's chipping and bogs down, it will pull up to 13.8A.  But, since the kill-a-watt averages over ~15 seconds, I don't know what the peak startup current is.

Now, back to the DMM.  I hooked it up in series with the switch and the chipper won't turn on.  If I pull the DMM out of the circuit, it will turn on correctly.  When I try to use the DMM, it flashes the fuse icon.  I guess I'm wondering if a bimetal fuse can fail?  The only other time I used it was when I was testing my old furnace.  I did touch the wrong location and throw sparks one time and it did turn off the DMM temporarily.  I've seen no issues with it since, but I wonder if the thermal fuse somehow got borked.  I'll probably open up the DMM to see if anything looks toasty, but I was hoping someone with more experience could provide some basic guidance if I'm obviously doing something wrong.  My only other option is to 1) buy a new DMM, 2) buy a current transformer, 3) buy some kind of kill-a-watt that doesn't average over so long, 4) scrap the chipper and get a new one.

[bkenobi]

Fwiw, I opened the DMM and found that it isn't protected by a thermal fuse, it's a standard 20A slow blow axial ceramic one.  Although it gives no visual clue that it's burnt, no conductivity is an effective sign.  It actually looks very similar to the one used in the smarthome 10A filters.  I should be able to simply swap in a new one and be back up and running.  I don't know if this DMM will help with my chipper current problem, but at least I should have my DMM functioning again.

[Brian H]

I believe the 20A for 30 seconds every 15 minutes. Is a requirement for the internal wiring and 20A shunt used to measure the current. So the shunt and wiring don't get too warm.
I recently bought a True RMS meter and it has a similar type rating for the high amperage scale.

[bkenobi]

FWIW, I decided that my DMM is not capable of measuring the startup current anyway.  My meter isn't true RMS capable, so it wouldn't be able to see that spike.  I bought a couple new fuses off ebay for $1/ea and will fix the meter, but I'm thinking using the motor markings would be better.  I found an article that explains how to calculate the current:

http://www.ehow.com/how_7791777_calculate-motor-inrush-current.html

and the table it references:

http://www.engineeringtoolbox.com/locked-rotor-code-d_917.html

The only problem is I'm not sure where this code is on my motor.  I think it's "A" based on the tag, but I could be wrong.

[Brian H]

Is the motor reversable?
I see a AC capacitor listed on the rating plate.
Probaby a starting capacitor that would effect the starting current, but could less likely be a run capacitor.

[bkenobi]

There is a huge capacitor under the wiring box hanging off the motor.  It's probably 8-9" long and 1-2" diameter.  Would I need to find the model info on that?  This chipper was mfg in W. Germany, so I assume it can be converted to run on 220 if that matters.  From what I see the motor is rated for up to 240V at 60Hz and the 1.15kW is equivalent to 1.5HP.  I don't know what the rest is, but I understand that there should be some kind of letter code that would indicate what the locked rotor current is.  If the code is "A", then the mean value is 1.6 which means 11x1.6=17.6A.  That would make sense since the chipper has been run on a 20A circuit and has not tripped the main panel breaker yet the new switch rated at 12A does trip.

The reason for the question is that I replaced the switch and it's tripping anyway.  The old switch would take a while to trip when bogged down (as is appropriate).  However, the new one trips immediately.  If it's actually pulling 18A at startup, it still doesn't explain the switch cutting off as the performance chart in the datasheet indicates a current of double the rated load taking 20 seconds.  It takes a load of 4x to trip at only 2 seconds.  I'm planning on just buying a higher rated switch (or even one that would not be effective as a breaker and just rely on the house breaker).  I just don't want to keep buying new components and throwing time and money away.

[bkenobi]

I borrowed a clamp meter from my dad this evening.  It claims to be able to report average RMS (average of the sine wave), so I am sure it would not be able to report the true maximum current.  What it does tell me though is that it sees a peak of 48A for long enough to report it (don't know what the sampling rate is).  I'm surprised a draw that large wouldn't trip the main panel breaker.

But, what it also means is that 48/12=4x the rated current.  The switch data sheet says at 4x, the switch will trip somewhere between 2 and 5 seconds.  Now, I don't know how long that peak exists for since I haven't checked yet, but I'm thinking that if I went with a 20A switch, I'd be looking at a factor of 2.4 which should make it 5-12 seconds before tripping.  I'll have to see how long the peak is first though.  Also, I might as well check the other 2 brands I located to see if they would do better.

[Brian H]

Seeing if you can get a closer start up current reading. May assist in your choices.

You may also want to see if the capacitor is OK.
I could not find enough information on the motor. To see if it was a starting cap or a running cap.

[bkenobi]

I don''t know anything about motor capacitors.  I just reviewed the wikipedia article and the only thing I can say is that there are no visible markings and there is no oil leaking that I can see.  I did read one post a while back where someone was looking for a cap for a chipper (IIRC), but there was no info about a source for a new one.  How would I test Its function or find a replacement?

[Brian H]

You could do a test with a Multimeter on Ohms.
Disconnect one side of the cap or both if easier.
Make sure the cap is discharged by shorting it. In case it has a charge on it.
Test with the meter. Should start to show a low resistance reading and then slowly go back to infinity as it charges.
If the meter has a capacitance function. The photo shows 40 uF.

[bkenobi]

yes it does.  Now that I know capacitance could be listed, it says 40uF at 240V.  there was a small wiring diagram included in the switch box.  looks like it shows the cap but I don't read electric diagrams to know what the nomenclature is.  I can see the breaker switch but not sure about the rest.

It looks like the faded text above "main w" is "aux w".  I assume that means that there is a second winding that is run from power through the capacitor.

[bkenobi]

My good DMM is still pulled apart waiting for that 20A fuse.  It can measure capacitance, but that will have to wait.  I pulled the capacitor off the motor to see if there were any hidden markings...nope.  I used my cheapo DMM to test resistance as suggested.  The resistance increased when I had the leads on one way and decreased when swapped.  It was a HUGE pain getting the leads back in the terminal block, but now that it's back together, even the old switch is tripping after just a few seconds.  I'm about ready to collect the scrap rate on this thing and get something different!

[bkenobi]

Are capacitors prohibitively expensive and/or unobtainium?  I was thinking that there are only 2 other things worth trying.

1) Add conductivity grease to the terminal strip so I know contact is good.
2) Replace capacitor.

The first one is easy and something I should probably do anyway.  The second could be expensive.

[dhouston]

Are capacitors prohibitively expensive and/or unobtainium? 

It depends on the size and type of the capacitor. Is there a label with capacitance, voltage, model #?

[Brian H]

I tried following your second photo but it was not clear if the capacitor is starting capacitor.
If it is a starting capacitor and the internal start switch is stuck On. The motor will draw excessive current.

Dave, Photo in earlier post. Shows the motors label. 40uF 240 Volt. I would say AC type.
Motor is of German decent. Along with most of the writing on the label and diagram in a later post.