A high resistance or resistance unbalance in the circuit may be an indication of a high resistance connection. Figure 8-3 illustrates for just one type, a bolted connection, some of the possible causes.

Figure 8-3 - Possible Causes of Resistance Unbalance in a Motor Circuit

Areas of concern in a bolted connection are:

• Properly sized lugs
• Proper crimping of the lugs used
• Clean contact area on the wire; buff area if needed
• Properly sized bolts
• Proper torque applied to the bolts
• No nicks in wire where insulation has been removed
• No corrosion of cables where crimping is to take place.

Severe damage can occur, all due to high resistance connections. If not dealt with quickly, they can generate a lot of heat, with the resultant damage not evident because it occurs in closed connection boxes. This can be revealed by periodic monitoring of circuit resistance in the conductor path, a measurement rarely done prior to introduction of motor circuit analysis as it is done with modern testers today. Corrective action should be taken when trends in resistance or unbalance percentage approach or exceed alert or alarm guidelines.

Tip from Motor Electrical Predictive Maintenance & Testing Vol. 3: Technologies for Motor Electrical Predictive Condition Monitoring by Jack Nicholas & Geoffery Generalovic.

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Describes seven technologies for motor electrical predictive condition monitoring, almost all of which have been developed and applied since about 1990. A chapter is devoted to using up to 15 predictive technologies to help refine condition assessments, since no single technology can detect all failure modes in motors.

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