Forums

Multimeter Question

Started by Rob Snyder December 16, 2011
So, I think I learned the hard way this morning that you're not
supposed to measure the voltage of an AC motor across the motor's
terminals while it is running (sparks flew, fuse blew). Am I correct
in that I shouldn't have done this, or is it safe to test the motor
this way - implying that something else happened?

I don't often deal with AC circuits so it's all a bit of a mystery to
me. I had a motor that wasn't running and I was just trying to see if
the control unit (this is a pellet stove, I was testing one of the
auger motors) was at fault by checking to see if it was trying to turn
the motor on.

Thanks!

On Fri, 16 Dec 2011 09:47:36 -0800 (PST), Rob Snyder
<robert.l.snyder@gmail.com> wrote:

>So, I think I learned the hard way this morning that you're not >supposed to measure the voltage of an AC motor across the motor's >terminals while it is running (sparks flew, fuse blew).
I can't see a problem with that. When measuring voltage, the multimeter's internal resistance is very high, so it will not be able to cause a short. Any chance you had your multimeter accidently set for current measurement in stead of voltage? Some multimeters need the positive test lead to be moved to a different socket for current measurements. These will cause a short even if the dial is set for voltage measurement. -- RoRo
On Fri, 16 Dec 2011 09:47:36 -0800 (PST), Rob Snyder
<robert.l.snyder@gmail.com> wrote:

>So, I think I learned the hard way this morning that you're not >supposed to measure the voltage of an AC motor across the motor's >terminals while it is running (sparks flew, fuse blew). Am I correct >in that I shouldn't have done this, or is it safe to test the motor >this way - implying that something else happened? > >I don't often deal with AC circuits so it's all a bit of a mystery to >me. I had a motor that wasn't running and I was just trying to see if >the control unit (this is a pellet stove, I was testing one of the >auger motors) was at fault by checking to see if it was trying to turn >the motor on.
Any chance that the meter was setup for current (amperage) measurement? -- Rich Webb Norfolk, VA
On Fri, 16 Dec 2011 09:47:36 -0800, Rob Snyder wrote:

> So, I think I learned the hard way this morning that you're not supposed > to measure the voltage of an AC motor across the motor's terminals while > it is running (sparks flew, fuse blew). Am I correct in that I shouldn't > have done this, or is it safe to test the motor this way - implying that > something else happened? > > I don't often deal with AC circuits so it's all a bit of a mystery to > me. I had a motor that wasn't running and I was just trying to see if > the control unit (this is a pellet stove, I was testing one of the auger > motors) was at fault by checking to see if it was trying to turn the > motor on.
It should be just fine. Like Robert and Rich, I suspect you had the meter set up for current measurements. Or you had some other fault that was causing a short. If the meter is rated for AC at line voltage (and most multimeters are) then it should work just fine. -- My liberal friends think I'm a conservative kook. My conservative friends think I'm a liberal kook. Why am I not happy that they have found common ground? Tim Wescott, Communications, Control, Circuits & Software http://www.wescottdesign.com
"Rob Snyder"
> > So, I think I learned the hard way this morning that you're not > supposed to measure the voltage of an AC motor across the motor's > terminals while it is running (sparks flew, fuse blew). Am I correct > in that I shouldn't have done this, or is it safe to test the motor > this way - implying that something else happened? > > I don't often deal with AC circuits so it's all a bit of a mystery to > me. I had a motor that wasn't running and I was just trying to see if > the control unit (this is a pellet stove, I was testing one of the > auger motors) was at fault by checking to see if it was trying to turn > the motor on.
** You do not say what multimeter you have - which is asinine. Most of the cheaper meters ( analogue and digital display ) and most older models are quite unsuitable for testing AC mains supply voltages and high voltages in general - cos they are simply not safe. A safe meter has at least got plugs on the leads that are shrouded, adequate fusing in the current ranges and a battery compartment that cannot just pop open. It also does not have sockets on the top you can plug small components into. It must also be free of serious defects when used with high voltages - like the display freezing when you apply a probe. Hobbyists tend to own meters that have some or all these safety problems. ... Phil
>Rob Snyder wrote: >>[...measuring] the voltage of an AC motor across the motor's >>terminals while it is running (sparks flew, fuse blew) >>
Phil Allison wrote:
>[...]A safe meter[...]does not have sockets on the top >you can plug small components into. >
Running down my own list in my head, I'd overlooked that one. Now I wonder *where* the sparks were (at the far end of the leads, or at the meter).
On Dec 16, 6:53=A0pm, JeffM <jef...@email.com> wrote:
> >Rob Snyder wrote: > >>[...measuring] the voltage of an AC motor across the motor's > >>terminals while it is running (sparks flew, fuse blew) > > Phil Allison wrote: > >[...]A safe meter[...]does not have sockets on the top > >you can plug small components into. > > Running down my own list in my head, I'd overlooked that one. > Now I wonder *where* the sparks were > (at the far end of the leads, or at the meter).
So yes, as much as I was about to swear that something else was going on, it was simply that I had the red lead in the slot for measuring current, not voltage. Sparks were at the motor, not at the meter. It's an old Radio Shack digital multimeter... at least fifteen if not twenty years old, but has always worked well. Amazingly, I did not do any permanent damage to the meter, the motor, or my person. Many thanks for all the replies, and for forcing me to go back and validate what I was doing rather than blame something else. Lesson learned.
On Fri, 16 Dec 2011 18:32:39 -0800 (PST), Rob Snyder
<robert.l.snyder@gmail.com> wrote:

>On Dec 16, 6:53&#2013266080;pm, JeffM <jef...@email.com> wrote: >> >Rob Snyder wrote: >> >>[...measuring] the voltage of an AC motor across the motor's >> >>terminals while it is running (sparks flew, fuse blew) >> >> Phil Allison wrote: >> >[...]A safe meter[...]does not have sockets on the top >> >you can plug small components into. >> >> Running down my own list in my head, I'd overlooked that one. >> Now I wonder *where* the sparks were >> (at the far end of the leads, or at the meter). > >So yes, as much as I was about to swear that something else was going >on, it was simply that I had the red lead in the slot for measuring >current, not voltage. Sparks were at the motor, not at the meter. It's >an old Radio Shack digital multimeter... at least fifteen if not >twenty years old, but has always worked well. Amazingly, I did not do >any permanent damage to the meter, the motor, or my person. > >Many thanks for all the replies, and for forcing me to go back and >validate what I was doing rather than blame something else. Lesson >learned.
You probably blew the fuse in that current input - the rest of the meter will still work, but if you attempt to measure current using that input, the meter will be open-circuit. -- Peter Bennett, VE7CEI peterbb (at) telus.net GPS and NMEA info: http://vancouver-webpages.com/peter Vancouver Power Squadron: http://vancouver.powersquadron.ca
On Sat, 17 Dec 2011 09:56:16 -0800, Peter Bennett
<peterbb@somewhere.invalid> wrote:

>On Fri, 16 Dec 2011 18:32:39 -0800 (PST), Rob Snyder ><robert.l.snyder@gmail.com> wrote: > >>On Dec 16, 6:53&#2013266080;pm, JeffM <jef...@email.com> wrote: >>> >Rob Snyder wrote: >>> >>[...measuring] the voltage of an AC motor across the motor's >>> >>terminals while it is running (sparks flew, fuse blew) >>> >>> Phil Allison wrote: >>> >[...]A safe meter[...]does not have sockets on the top >>> >you can plug small components into. >>> >>> Running down my own list in my head, I'd overlooked that one. >>> Now I wonder *where* the sparks were >>> (at the far end of the leads, or at the meter). >> >>So yes, as much as I was about to swear that something else was going >>on, it was simply that I had the red lead in the slot for measuring >>current, not voltage. Sparks were at the motor, not at the meter. It's >>an old Radio Shack digital multimeter... at least fifteen if not >>twenty years old, but has always worked well. Amazingly, I did not do >>any permanent damage to the meter, the motor, or my person. >> >>Many thanks for all the replies, and for forcing me to go back and >>validate what I was doing rather than blame something else. Lesson >>learned. > >You probably blew the fuse in that current input - the rest of the >meter will still work, but if you attempt to measure current using >that input, the meter will be open-circuit.
I have seen cheap meters that only fuse the mA position, the 10A or 20A is not fused.
On Sat, 17 Dec 2011 13:26:47 -0500, Tom Biasi wrote:

> I have seen cheap meters that only fuse the mA position, the 10A or > 20A is not fused.
Like those three-buck-fifty meters that Jan and Joerg were recently enthusing over :-) -- "For a successful technology, reality must take precedence over public relations, for nature cannot be fooled." (Richard Feynman)