April 1, 2014
Electric motors are ubiquitous in the mining industry. Both AC- and DC-powered equipment is used for critical operations such as drilling, ventilation, hydraulics, conveying, crushing, excavation, emergency power and other applications in underground, open pit and surface mining operations.
Keeping these operations running continuously and efficiently, therefore, depends on effective motor maintenance, which, in turn, creates a need for regular and accurate testing. By being proactive in this regard, mining operations can save money by identifying and solving problems without costly production interruptions or time-consuming trips to the motor repair shop.
In the past, however, testing and analyzing of electric motors required the services of highly trained technicians using complex instruments. Today, thanks to the evolution of more powerful, yet user-friendly devices, training and expertise is no longer required to perform highly reliable diagnostics for use in preventive and predictive motor maintenance.
These devices are capable of measuring and analyzing a variety of key motor performance indicators, including winding resistance, insulation resistance, dielectric strength, turn-to-turn and phase-to-phase insulation integrity and strength, impedance, phase angle and other parameters.
Motor stress, failures
The motors used throughout mining are often subjected to harsh conditions, including excessive heat, dust, debris, vibration, and even occasional lightning strikes, all of which can damage windings and hinder motor performance.
One of the key points of failure in a motor, after the bearings, is its insulation system.
Motor windings receive any type of voltage from 120 volts AC to 13,800 volts depending on the size of the motor and the application, up to 15,000 volts in some motors overseas. The copper wires of the windings are insulated from each other and also insulated from the frame of the motor.
As insulation weaknesses develop due to the harsh conditions, the windings in these motors eventually begin to leak current from windings to ground, one winding to another, or from a turn in the winding to the next turn. The result is less efficiency, greater heat build-up, more rapid breakdown of the insulation and, ultimately, the failure of the motor.
By performing tests using a portable analyzer and diagnosing any issues early, maintenance personnel aim to minimize repairs and avoid extensive downtime.
Naturally, as with any portable test equipment used in the field, users must be aware of any potentially hazardous conditions (e.g. explosive gas or combustible dust) that would warrant relocating a motor from the field to the service shop for testing or analysis.
One of the advantages of some of today’s most advanced instruments is that they are both easy to operate and contain powerful features.
The iTIG II is a winding analyzer and motor tester from Longmont, Colo.-based Electrom Instruments that comes with varying options and output ranges from 4kV to 12kV. By adding Power Packs, even higher voltages can be achieved.
A key advantage of the winding analyzer is that it uses a 60 Hz surge pulse frequency, the same frequency as most motors operate at. This high pulse rate provides a sufficient frequency to overcome ionization dissipation and can thus isolate insulation weaknesses with more sensitivity, predicting future faults before low frequency testers and also better simulates motor operating conditions.
One of the most significant ease-of-use features is that this device enables users to enter the surge test voltage, push a button and let the machine run the test independently. Surge waveform ranges are automatically set for all models, which eliminate the need to specify configurations, push multiple buttons or turn dials.
All tests can be done with one instrument; they are available in manual to fully automatic models. Tests that can be performed on this system include Surge Comparison, DC Hipot, Step Voltage, Insulation Resistance (Meg test), Dielectric Absorption (DAR), Polarization Index (PI), Low resistance (Ohms), Impedance (Z), Phase Angle, Inductance (L) and Capacitance (C).
Additionally, motor and customer information can be set up on a PC and transferred to the winding analyzer before tests are done. This eliminates the need for the operator to enter any information in the analyzer. The motor can just be selected and testing started. Most motor and customer information can be entered before or after a test.
For the mining industry, a winding analyzer is a powerful diagnostic and maintenance tool that can reduce downtime and save on capital and maintenance costs by identifying problems early, ensuring the insulation system is operating properly, and verifying that spare motors are ready to be used when needed.
Article courtesy of Electrom Instruments. The company can be reached at 800-833-1881 or electrominst.com.