My electronic module went bad on my Cold Machine and the shop suggested that I
check the resistance of the compressor. When I check the resistance between 2
of the points my meter shows .8 ohms. When I touch the 2 leads of the tester
together I get a .4 reading. Do I subtract .4 from the original reading to get
the true resistance in the compressor? I have 3 different points to to check.
Thanks

Mike

On 23 May 2004 22:33:59 GMT, something compelled
(C1gmlm), to say:

My electronic module went bad on my Cold Machine and the shop suggested that I
check the resistance of the compressor. When I check the resistance between 2
of the points my meter shows .8 ohms. When I touch the 2 leads of the tester
together I get a .4 reading. Do I subtract .4 from the original reading to get
the true resistance in the compressor? I have 3 different points to to check.
Thanks

Zero the ohm meeter before each test. There should be a knob or
something.

In article ,
says...
On 23 May 2004 22:33:59 GMT, something compelled
(C1gmlm), to say:

My electronic module went bad on my Cold Machine and the shop suggested that I
check the resistance of the compressor. When I check the resistance between 2
of the points my meter shows .8 ohms. When I touch the 2 leads of the tester
together I get a .4 reading. Do I subtract .4 from the original reading to get
the true resistance in the compressor? I have 3 different points to to check.
Thanks

Zero the ohm meeter before each test. There should be a knob or
something.

If your meter does not have a zeroing capability, then yes, subtract the
"shorted" reading.

Another approach for such low resistance tests is to use a large power
resistor to pass a current of 1 to 10 amps thru the circuit to be tested,
using a separate 12V battery. If the current is known, most digital
multimeters will read small voltages much more accurately than small
resistances. Do ohm's law: R=E/I so if the current is 10 amps and the
voltage across the device under test is .05 volts, the resistance is .005
ohms. Practically, about 1 amp is fine for most motor winding tests
unless it's a large starter motor etc. You need to know the actual
current you are 'forcing', usually by having a known series "current
limiting" resistor (perhaps 10 ohms rated at 25 watts or more) and
measuring the voltage across IT to calculate the current.

Make sense, or do you need more??

I do this often to measure relay contact resistances, motor windings,
circuit breaker internal resistance, cabling resistance etc.

--
Regards, Terry King ...In The Woods In Vermont

Capturing Live Music in Sound and Images
http://www.terryking.us

Hi Mike,

measuring with an ohmmeter as you`ve done won`t bring any true result,
because the compressor is an electrical motor and has not only resistance.
With your method you should substract the values and you will get .4 ohms as
the motor`s resistance. At 12 V voltage the current of that compressor would
be 30 A. I don`t think, your compressor will have such a high current.

Better check the current of your compressor with an amperemeter. In most
cases current is labeled on motor or compressor. If the current is much
higher than the labeled value, then motor has a problem.

You can`t get the current only from voltage and resistance because every
motor is a generator at same time, that makes it a little bit complicated.

Juergen

"C1gmlm" schrieb im Newsbeitrag
...
My electronic module went bad on my Cold Machine and the shop suggested
that I
check the resistance of the compressor. When I check the resistance
between 2
of the points my meter shows .8 ohms. When I touch the 2 leads of the
tester
together I get a .4 reading. Do I subtract .4 from the original reading to
get
the true resistance in the compressor? I have 3 different points to to
check.
Thanks

Mike

"Jürgen Spelter" wrote in message
...
Hi Mike,

measuring with an ohmmeter as you`ve done won`t bring any true result,
because the compressor is an electrical motor and has not only resistance.
With your method you should substract the values and you will get .4 ohms
as
the motor`s resistance. At 12 V voltage the current of that compressor
would
be 30 A. I don`t think, your compressor will have such a high current.

On startup, it will. Once the motor runs, it generates back-emf. The
difference voltage between supply and back-emf, divided by the internal
resistance, is the resulting current.

Meindert

Try link this is what is used to check my compressor out
http://www.novakool.com/install/friver2.htm#trouble
"Meindert Sprang" wrote in message
...
"Jürgen Spelter" wrote in message
...
Hi Mike,

measuring with an ohmmeter as you`ve done won`t bring any true result,
because the compressor is an electrical motor and has not only
resistance.
With your method you should substract the values and you will get .4
ohms
as
the motor`s resistance. At 12 V voltage the current of that compressor
would
be 30 A. I don`t think, your compressor will have such a high current.

On startup, it will. Once the motor runs, it generates back-emf. The
difference voltage between supply and back-emf, divided by the internal
resistance, is the resulting current.

Meindert