Baker D30R User Manual - Coil Resistance Testing

12/3/2009 | 71-022 V6 

D30R 

21 

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Coil Resistance testing 

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 D12R/D6R/D3R, 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 

Resistance test checklist 

1)

Disconnect the high voltage test leads and move them aside. The tester resistance test 

circuitry is protected internally by relays, which ground the resistance test leads when a 

surge or HiPot test is selected. However, the protection relays are in no way rated for 

any type of live electrical buss or circuit. It is possible to cause severe damage to the 

instrument if the resistance test leads are attached while surge testing, HiPot testing, or 

while line voltage is present at the motor leads. 

2)

Connect the resistance test leads to leads 1-2 of the motor.