They are:Įlectric Motor impulse testing is an integral part of predictive maintenance of electrical motors.
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What is Done During Electric Motor Testing? There are various kinds of testing done on motor. Some of the winding faults are:įault finding of at least one of the above can save your facility countless hours of shut down and numerous dollars in savings. This creates extra stress and increase in temperature across the fault, which increases until the winding fails. They start as energy crossing an insulation fault like moisture, which sets apart at least one turn. Winding defects occurs due to contamination, ageing of insulation, thermal overload, power surges, damaged wire/materials, and other causes. Many diagnostic tools, such as clamp-on ammeters, temperature sensors, a Megger or oscilloscope, can help illuminate these issues. The other 52% of failures are mechanical faults. The 48% can be again divided into rotor problems (12%) and winding problems (36%). The Electric Power Research Institute (EPRI) conducted a survey which brought into the light that, 48% of motor failures are because of electrical failures. Why is Electric Motor Testing Done?Īfter bearing failure, electrical faults are the most common mode of motor failure, so in addition, a properly planned electrical testing scheme is important for making sure of the plant reliability. But smart companies, understands that without proper maintenance programs, there is billions of dollars of lost revenue through increased motor repair costs, downtime, and waste in industrial and commercial companies. Electric Motor Testing is usually the first thing to be sacrificed when cutting back on operational expenses. This information is based on NEMA standard MG1-14.35.Electrical maintenance programs are designed to increase equipment promptness and uptime while decreasing capital operating cost. Increasing the size of the overload protective device is not the solution in as much as protection against heating from overload and from single phase operation is lost. This introduces a complex problem in selecting the proper overload protective devices, particularly since devices selected for one set of unbalanced conditions may be inadequate for a different set of unbalanced voltages. The currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance. The locked-rotor current will be unbalanced to the same degree that the voltages are unbalanced but the locked-rotor KVA will increase only slightly. The full-load speed is reduced slightly when the motor operates at unbalanced voltages.
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If the voltage unbalance should be extremely severe, the torque might not be adequate for the application. The locked-rotor torque and breakdown torque are decreased when the voltage is unbalanced. To illustrate the severity of this condition, an approximate 3.5 percent voltage unbalance will cause an approximate 25 percent increase in temperature rise. The increase in losses and consequently, the increase in average heating of the whole winding will be slightly lower than the winding with the highest current. In the phase with the highest current, the percentage increase in temperature rise will be approximately two times the square of the percentage voltage unbalance. Temperature rise and load carrying capacityĪ relatively small unbalance in voltage will cause a considerable increase in temperature rise. With voltages of 220, 215 and 210, the average is 215, the maximum deviation from the average is 5, and the percent unbalance = 100 X 5/215 = 2.3 percent. The voltage unbalance (or negative sequence voltage) in percent may be defined as follows: A small negative sequence voltage may produce in the windings currents considerably in excess of those present under balanced voltage conditions. This negative sequence voltage produces in the air gap a flux rotating against the rotation of the rotor, tending to produce high currents. The effect of unbalanced voltages on polyphase induction motors is equivalent to the introduction of a "negative sequence voltage" having a rotation opposite to that occurring with balanced voltages. The voltages should be evenly balanced as closely as can be read on the usually available commercial voltmeter.
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The effect on the motor can be severe and the motor may overheat to the point of burnout. A small amount of voltage unbalance may increase the current an excessive amount.
![motor winding resistance imbalance motor winding resistance imbalance](http://www.pcbheaven.com/userpages/images/med_thumbs/Check_the_windings_of_a_3_phase_AC_motor_2.png)
When line voltages applied to a polyphase induction motor are not exactly the same, unbalanced currents will flow in the stator winding, the magnitude depending upon the amount of unbalance. Three phase induction motors are designed and manufactured such that all three phases of the winding are carefully balanced with respect to the number of turns, placement of the winding, and winding resistance.