Try MCA For Motor Condition Analysis

Motor circuit analysis is a de-energized, non-destructive, and comprehensive method for assessing motor condition.

De-energized testing is used in our training factory to explain and demonstrate the concepts to College of Engineering students and companies learning how to implement reliability and maintainability best practices.

In this article, Bill Kruger of ALL-TEST Pro, Old Saybrook, CT (alltestpro.com), discusses motor testing/circuit analysis, since it’s another valuable methodology that should be added to your tool bag of predictive technologies.

Regarding electric motor testing, polarization index (PI) is a measure of how much the insulation system resistance improves (or degrades) over time. While the PI test has been considered the primary method for evaluating the condition of motor insulation, it doesn’t fully evaluate the motor system. Newer testing methods provide a more comprehensive view of a motor’s overall health.

Polarization index

The PI test is a standard electric-motor testing method, developed in the 1800s, that attempts to determine the health of a motor’s winding insulation. While the PI test provides information on ground wall insulation (GWI) systems, typically installed prior to the 1970s, it fails to accurately determine the condition of the winding insulation in modern motors. PI testing applies DC voltage, typically 500 V to 1,000 V, to the motor’s winding to measure how effective the GWI system is at storing an electrical charge. Since the GWI system forms a natural capacitance between the motor windings and the motor frame, the applied DC voltage will be stored as an electrical charge, the same as a capacitor. As the capacitor becomes fully charged, the current will decrease until all that remains is the final leakage current, which determines the amount of resistance the insulation provides to ground.

In new, clean insulation systems, the polarization current decreases logarithmically with time as the electrons are being stored. The PI is the ratio of the insulation resistance to ground (IRG) value, measured at 1- and 10-min.intervals.

PI = 10 min. IRG/1 min. IRG

On insulation systems installed before 1970, PI testing occurs while the dielectric material is being polarized. If the GWI begins to degrade, it undergoes a chemical change, causing the dielectric material to become more resistive and less capacitive, lowering the dielectric constant and reducing the ability of the insulation system to store an electrical charge. This causes the polarization current to become more linear as it approaches the range where leakage current predominates.

For various reasons, the dielectric material, used in insulation systems after 1970, polarizes in less than a minute and the IRG readings exceed 5,000 megaohms. As a result, the calculated PI may not be a meaningful indication of the ground wall condition.

Additionally, since this test creates an electrostatic field between the windings and the motor frame, it provides very little, if any, indication of the winding insulation system condition. A Motor Circuit Analysis (MCA) is a more comprehensive method for analyzing this condition.

Motor circuit analysis

MCA is a de-energized, non-destructive test method. Initiated from the motor control center (MCC) or directly at the motor itself, this process evaluates the entire electrical portion of the motor system, including the connections and cables between the test point and motor.

MCA tools identify:

• ground faults
• internal winding faults
• open/loose connections
• rotor faults
• contamination.

Motor testing using MCA tools is easy and safe to implement, takes less than 3 min., and can be performed by an entry-level technician. PI testing typically takes more than 10 min.

When a low voltage is applied, healthy resistive, capacitive, and inductive circuits should respond in a specific way. MCA tools send a series of non-destructive, low-voltage, sinusoidal AC signals through the motor to measure the response of these signals.

MCA measures:

• resistance
• impedance
• inductance
• Fi (phase angle)
• dissipation factor
• insulation to ground
• I/F (current frequency response)
• Test Value Static (TVS)
• dynamic stator and rotor signatures.

MCA is applicable on:

• AC/DC motors
• AC/DC traction motors
• generators/alternators
• machine tool motors
• servo motors
• control transformers
• transmission and distribution transformers.

Technologies such as MCA identify contamination and insulation-to-ground faults, turn-to-turn, coil-to-coil, and phase-to-phase developing winding faults in very early stages. By combining new technologies, modern electric-motor testing methods provide a much more comprehensive and thorough evaluation of an entire motor’s insulation system.

Two upcoming courses deliver more information. “Motor Circuit Analysis & Electrical Signature Analysis Motor Diagnostic Workshop, Level I,” Univ. of Tennessee, Knoxville (utk.edu), provides three days of MCA (de-energized) training introducing MCA (de-energized low-voltage winding insulation testing). The course presents basic theories and principles necessary to understand the concepts and principles associated with this technology. The “Motor Circuit Analysis & Electrical Signature Analysis Motor Diagnostic Workshop, Level II,” three-day course picks up where the Level I course leaves off, preparing you to troubleshoot all types of electric motors, coils, and windings. For more information, visit rmc.utk.edu. EP

Based in Knoxville, Dr. Klaus M. Blache is director of the Reliability & Maintainability Center at the Univ. of Tennessee, and a research professor in the College of Engineering. Contact him at kblache@utk.edu.

Bill Kruger is the Technical Training Manager at ALL-TEST Pro, Old Saybrook, CT (alltestpro.com). He specializes in the theory and application of motor diagnostics.