Multimeter Question

[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.

Thanks!

 

[Robert Roland]

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

 

[Rich Webb]

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

 

[Tim Wescott]

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

 

[Phil Allison]

"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

 

[JeffM]

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).

 

[Rob Snyder]

On Dec 16, 6:53 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.

 

[Peter Bennett]

On Fri, 16 Dec 2011 18:32:39 -0800 (PST), Rob Snyder
<robert.l.snyder@gmail.com> wrote:

On Dec 16, 6:53 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

 

[Tom Biasi]

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 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.

 

[Fred Abse]

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)

 

[Jasen Betts]

On 2011-12-17, Fred Abse <excretatauris@invalid.invalid> wrote:

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

It says in the manual not to use them on mains circuits.
this is probalbly one of the reasons.

--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---

 

[Phil Allison]

"Peter Bennett"

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.

** The OP has told us the meter is fine and the fuse in the circuit blew.

Lotsa cheap DMMs have no fuse in the 10 amp current range and rely on fuses
and breakers in the test circuit to open.

Such meters are not safe on high current ( ie industrial ) AC or DC
circuits - cos the leads can explode in the operator's face.


.... Phil

 

[Fred Abse]

On Sat, 17 Dec 2011 22:05:54 +0000, Jasen Betts wrote:

On 2011-12-17, Fred Abse <excretatauris@invalid.invalid> wrote:
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

It says in the manual not to use them on mains circuits.
this is probalbly one of the reasons.

I've got an MPEG of a similar meter being tested at VDE. Lots of smoke and
flames. Leads in current position. I'd post it to a.b.s.e. but, since the
OP uses Google, there's not much point.

--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
(Richard Feynman)