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Introduction Of NDT Technology

Electrical Motor Evaluator

What is it?

Traditionally, the reliability of the critical motors is usually taken care by vibration monitoring and trending program. Although vibration is an important part of motor reliability, what appeared to be overlooked is the importance of electrical parameters of the motor in helping to determine the condition and health of the motor. The motor is after all electrically wound and driven and therefore, electrical faults such as turn-to-turn shorts in the stator windings, high resistive joints, etc cannot be effective diagnose by vibration methods. In fact, a study done by EPRI/GE provides a breakdown of the types of faults that caused the motor to fail, indicates 47% of the motor failure is electrical related (rotor & stator failure).

The electrical motor evaluator (online/offline motor tester) consists and utilizes numerous parameters and measurements (eg: Resistance to Ground, Capacitance to Ground, Phase to Phase Resistance, Phase to Phase Inductance, Harmonic Distortion, Power Factor, Sequence Currents, etc) to provide the necessary information for in depth diagnosis. Thus, in addition to improve the reliability of the motor, such tester also helps to diagnose the quality acceptance of new or rework motors or alternators.

How does it work?

Electrical Motor Evaluator focus on the following 6 fault zones to help determine the health of the motor:

a.    Power Quality

i. Voltage and current values
ii. Voltage and current unbalance
iii. Voltage and current THD
iv. Voltage and current crest factor
v. System harmonics

b.    Power Circuit

i. High resistance connections
  • (1)    At the starter
  • (2)    At the local disconnect
  • (3)    At the motor connection box
ii. Defective PF correction capacitors
iii. Defective cabling
iv. Defective contact surfaces

c.    Insulation

i. Insulation to ground integrity of the motor
ii. Insulation to ground integrity of the cabling
iii. Contaminant build-up on the insulation
iv. Dielectric strength

d.    Stator

i. Turn to turn leakage
ii. Phase to phase leakage
iii. Degraded or faulty connections within the motor

e.    Rotor

i. Cracked or broken rotor bars
ii. Porosity in cast aluminum rotors
iii. Defective rotor iron

f.    Air Gap

i. Eccentricity

(1)    Dynamic
(2)    Static


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