Motor Circuit Analysis

The electrical condition of plant equipment is as important as their mechanical condition. Electrical failure can be as catastrophic as the worst mechanical breakdown. Using state-of-the-art equipment we collect and analyze separate parameters to determine motor condition. We test both AC and DC motors and coils.

The Reliability Services Group collects five different types of electrical data. One is off line and four are on line.

Motor Circuit Off Line Testing

MCE Testing (Static Testing) performs off line testing of electric motors. There are five types of off line tests.

• Polarization Index (Insulation Resistance)
• Phase to Phase Resistance
• Phase to Phase Inductance
• Capacitance to Ground
• IEEE Meggar Test

Motor Circuit On Line Testing

The four other types of electrical tests taken are:

• Running (In Rush) Data - Plots actual current vs. time during start-up. Maximum in-rush current can be measured and the time required to accelerate the motor to operating load can also be measured. If there are problems related to the ability of the motor to develop the torque necessary to accelerate the load within a given amount of time, it could show up as a change in the Running profile. For example, if there are broken rotor bars one would expect the motor to require more time to accelerate the load to full speed.
• Current Signature Analysis - Plots current frequency vs. amplitude (in dB). If operating speed is known, the slip frequency can be calculated. These slip frequencies show up as side bands on either side of 60 HZ. The amplitude of these bands are indicative if electrical problems exist, most notably broken rotor bars.
• Eccentricity Spectrum - Calculates the frequency at which dynamic eccentricity occurs. If the number of rotor bars and running speed are known, then the eccentricity frequency can be calculated. The software then plots the frequencies on either side of the eccentricity frequency where dynamic eccentricity would show up. These frequencies are marked with a red X. If there are peaks at these red X's, then there is a measurable amount of dynamic eccentricity. If these peaks increase in amplitude, then dynamic eccentricity in increasing.
• Power/Quality Analysis - Measures voltage, current, power factor, total harmonic distortion (THD), crest factor (CF), impedence, sequence, efficiency and measures imbalances and applies a derating factor to the motor if the applied voltage imbalance exceeds a certain level.

Equipment Used for Motor Circuit Analysis

The MCE Tester, manufactured by PDMA Corporation, is a lightweight, static tester that provides a detailed analysis of motor and circuit condition. This portable unit features diagnostic results that evaluate all five of the motor's fault zones including the power circuit, insulation, stator, rotor and air gap.

The EMAX is a dynamic tester that utilizes state of the art technology to collect data while the motor is operating. This information can be used to evaluate incoming power quality and motor efficiency, as well as rotor, stator, air gap and power circuit conditions. The data can be analyzed immediately or recalled later for trend analysis.