Baker D30R, User'S Manual & Operating Instructions

Page: 35 / 142

D30R Users Manual
35
Do Not Touch Test Leads while test is in Progress!
71-023 Rev C 5/8/2007
Principles of Coil Resistance Testing
The coil resistance test is a very simple test to perform and is an immediate
indication of the health of the conductor(s) in a winding. The coil resistance test
consists of injecting a known constant current through the winding, measuring
the voltage drop across the winding, and calculating the coil resistance using
Ohm’s law. If a coil is shorted somewhere in the interior of the winding the
resistance will be lower than normal. This lower coil resistance can be compared
to previous measurements of the same coil, measurements of identical coils, or
compared to the motor name-plate value to identify a “bad” coil.
The measured resistance is affected by the variation of copper conductivity with
temperature. Therefore, the measured resistance value should be “corrected” to a
common temperature, usually 25
o
C, before comparing two different
measurements. MTA for Windows, a data logging application for the D30R,
does this correction. See IEEE 118 for more information on correcting
resistance measurements to 25
o
C.
Since the windings found in many motors have very low resistances, the injected
current might have to be as high as 10 amps to accurately measure the voltage
drop across the coil. One of the difficulties encountered measuring the voltage
drop across the coil itself is, the affect of the contact resistance of the clip leads
used to connect to the motor’s winding. Contact resistances can be comparable
or even greater than the resistance of some coils. The effects of contact
resistance are reduced by using a “four wire” or Kelvin measurement. Baker
testers use this technique.
Resistance Test Display
Fig 3-1: Test Display