ELECTRI AL TESTING – of the poten ial failu em des. nm ni vital role thatm

channel and platformdevel pmentm nager electri motor condition monitoring at SKF, describes the vital role that modern electrical testing technologies play in predicting and preventing motor failures

h are i capable of detecti g al chael H rring ,

ng at SKF, describes the rn electrical testing tech M

onitoring the health of an electric motor should entail measurement

of both mechanical and electrical parameters in order to gain a comprehensive assessment of its

operating condition. Vibration signature analysis, thermography and even oil analysis will provide early indications of mechanical problems, but these are unlikely to reveal the onset of insulation failure, phase imbalances, torque

anomalies and the like, which will have a significant impact on motor performance and life expectancy.

Vibration measurement applied in motor conditio ,

n monitoring is commonly

programs and this alone, can identify approximately 70 per cent of the issues that might indicate pending failure, including problems with bearings, unbalanced loads and shaft

misalignment. Identifying the remaining 30 per cent of potential problems requires the use of electrical testing techniques, which fall into two broad categories: static tests, conducted on a motor when it’s not running, and

dynamic tests, conducted on the motor in service.

Some 80 per cent of the electrical

failures in motors begin with a failure of the thin insulation covering the motor winding, otherwise known as the turn- to-turn insulation. As this insulation degrades, the inrush of voltage during motor starting and stopping can cause arcing, further degrading the insulation and creating a conductive carbon path that will ultimately lead to a short-circuit and motor failure.

Static electrical testing of motors uses industry-accepted standards to identify these weaknesses within windings. Testing can be using specialist portable i

such as the SKF Static Motor Analyzer Baker AWA-IV, which removes operator


nstrumentation automated the motor

variations in test sequencing. logies

play i predi ting and preventi gm tor fai ures error and inconsistencies arising from The test sequence is defined by the

measure winding resistance a AWA-IV software and is confi

gured to nd verify

Modern instrumentation and specialist services mean there really is no excuse for companies to leave the health of their critical motors to chance

Modern inst strumentation

and specialist services mean there really is no

critical motors to chan leave the health of the excuse for companies

rs to

ce ir

the dielectric strength of the insulation systemvia a series of specific test routines. One of these is the DC step voltage test, carried out over five or more incremental voltage steps, with the leakage current being plotted for each. Clean, dry insulation will show a linear plot, while insulation weakness or contamination is revealed by a non- linear increase in leakage. A surge test is also carried out to detect weak turn-to- turn and phase-to-phase insulation. Such weakness can be present without the motor displaying any signs of a problemduring operation, yet the winding resistance and megohmtests cannot identify such problems. Static test results are clear and unambiguous, and produce a specific result that requires little interpretation.

Static testing should also be performed annually on spare motors that are being held in stock. These motors often go unused for many years, yet are held solely for use when a production motor fails, or requires replacing. If the

condition of a stock motor is unknown prior to installation, there is a risk of failure during production.

Dynamic motor testing involves the measurement of the three phase voltages and currents which are then processed via a set of algorithms to assess the quality of the power supplied to the motor certain issues within the motor, such as cracked and broken rotor bars, and the condition of the driven load. It can be undertaken using instrumentation such as the SKF Dynamic Motor Analyzer - EXP4000, which is capable of multiple tests to determine power condition, motor health, motor load and energy


also able to monitor and percentage load

profile. As many as 40 parameters are monitored and recorded, including

voltage, current, frequency, torque, speed

the performance of . The EXP4000 is

inverter driven motor applications. Also worth noting is motor current

signature analysis, which, unlike dynamic analysis, measures only the phase currents to detect cracked and broken rotor bars and other mechanical issues. Also performed with the EXP4000, this test is normally carried out when the motor cannot be stopped to allow

voltage connections to be made, or in the case of high voltage motors, when low voltage secondary connections are not available in the panel to allow safe connection of the equipment.

While these tests are relatively easily conducted by trained in-house staff, the task can be outsourced to specialist providers such as SKF, fielding

engineering teams to undertake surveys of plant-wide industrial motor

installations. Comprehensive reports are made available to customers advising themof any actions or remedial work that may be required.

And for those sites that are either difficult to reach or pose a hazard to personnel, SKF can also offer remote dynamic motor monitoring based on the SKF Online Motor Analysis System– NetEP, which is capable of monitoring up to 32 motors, 24 hours a day, seven days a week, with data being sent directly to SKF for analysis. Multiple NetEP devices can be networked, to increase the number of motors being monitored. Modern instrumentation and specialist services mean there really is no excuse for companies to leave the health of their critical motors to chance.

SKF (U.K.) www

ww. /AUTOMATION AT /AUTOMATION T: 01582 490049 T: 01582 490049

com on cause ofm tor failu e. Yetm organisations perform only the very basic of electrical tests, which are incapable of detecting all of the potential failure modes. Michael Herring, channel and platform development manager, electric condi

After beari g problems, electrical faults are the most common cause of motor failure. Yet many organisations performonl electri al tests,

After bearing probl the very basic of electri al faults are them st

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