Years ago, it was common to see a motor stator on top of a stack of pallets, being burned out in an open fire to remove winding encapsulation resins. We know that is an unacceptable method of winding removal. Even with today’s improved temperature controlled, water suppressed ovens, cores still get damaged. When this happens and a new winding is installed, you’re getting an inferior product with a shortened life and inefficient operation.
When a manufacturer designs a particular motor, the ability of the lamination stack to create a magnetic field efficiently is crucial. If the characteristics of the electrical grade steel or the core plate that the laminations are coated with change, there may be a problem. Core loss testing was designed to identify and quantify these changes from the original design. Core loss testing provides high current at low voltage to simulate operating conditions in the core. Flux, amperes and watts readings, and core dimensions, are fed into the tester's computer system, which calculates watts/lb. (kg) of core loss. With parameters for "Good", "Marginal" and "Bad" cores, the software generates a detailed printed core condition report. Localized damage is found by increasing the excitation level to reveal hot spots within the core. Results are typically shown as “Good”, “Marginal” or “Bad” and Watts per Pound, Ampere Turns per Square Inch, and Power Factor values. Testing can be done with the winding in place or removed. Winding condition, motor flux density, and lamination grade and thickness do not significantly affect the test results, and the entire test takes only about 10 minutes..
We recognize that core loss is a significant cause of wasted electrical energy. Core loss is second only to copper loss in motor windings in causing motor inefficiency. Without core loss testing, it cannot be determined that a motor is capable of operating at rated efficiency after rebuilding. At Evans Enterprises we core loss test everything and as a result don’t rewind damaged cores. If the results of our core loss testing indicate a damaged core we don’t just go ahead and wind it. We have the capability to refurbish and restack the core and re-test to confirm that proper core loss improvement has been achieved. Often, the cost of restacking exceeds the cost of replacing with a new or remanufactured unit. Evans Enterprises can provide recommendations and proposals on options for repairing or replacing. Let Evans Enterprises show you how Core Loss Testing is a crucial part of any complete electrical equipment repair that will help maintain the original design efficiency and keep your equipment running longer.
(Hi Pot) (In accordance with IEEE 95 and IEEE 43)
The DC Hi Pot Test displays insulation faults that may not be shown in and IR or PI test. Conducted at higher voltages, the DC Hi Pot test measures overall insulation resistance to ground and indicates insulation dielectric strength. Industry standards have proven that two times operating voltage plus 1000 volts is an effective level for testing ground wall insulation. The Hi Pot test brings the entire motor winding up to the same voltage. Uniform voltage stress is applied between the winding and the ground throughout the entire winding. The leakage current, in microamperes, is read and plotted against the corresponding test voltage. The resultant plot should be a straight line. An abrupt upswing implies an insulation weakness. An auto ranging trip circuit interrupts the test if the insulation is weak.
(IR/Megohm) (In accordance with IEEE 95 and IEEE43)
The IR Test evaluates the quality of the insulation to ground. The IR test is the oldest and most widely used test for evaluating the resistance of ground wall insulation. While the motor frame is grounded, the tester applies a direct voltage to the windings. The tester furnishes a readout in megohms. A sound winding provides a readout in hundreds or thousands of megohms. Note: Temperature and moisture affect IR readings. For accurate readings, testing is done at a constant temperature above dew point and corrected to 40 deg. C.
A reliable diagnosis of any motor condition requires gathering as much information as possible about the motor. It is necessary to evaluate the motor’s insulation health and how it performs under load. This requires dynamic or on-line testing. We use the latest, state-of-the-art test equipment developed by Baker Instrument Company, the recognized leader in electric motor testing equipment. For dynamic testing, we use the “Motor Performance Monitor” or MPM. The MPM Explorer II from Baker Instrument Company is the latest innovation in plant management and an integral tool in an excellent predictive maintenance program. This equipment utilizes multiple tests to determine power condition, health, load and energy profiles of your motors. The following five test domains describe the functions of each test.
Power Condition Tests: The voltage level, voltage balance and harmonic distortion tests determine the motor’s overall power condition.
The Voltage Level Test: Identifies the incoming voltages the motor “sees” and informs you of over and under voltage conditions.
The Voltage Balance Test: Examines the voltages of each “leg” by calculating percentage of unbalance via NEMA derating.
Note: A non-balanced voltage condition causes negative sequence currents within the stator, resulting excessive heat. The voltage balance test determines the condition of the incoming line voltages.
The Distortion Test: Examines the total harmonic distortion of the three phases to ground. (Harmonic distortion causes excess heat in motors, and can create problems in other electronic equipment within a plant.)
Motor Performance Tests: The service factor test, overcurrent test and efficiency test observe your motor’s performance and helps identify money saving possibilities.
The Service Factor Test: Identifies any thermal overloading in the motor and divides the estimated percentage load into the NEMA derating factor. This test identifies how close the motor is operating to its effective service factor.
The Overcurrent Test: Compares each phase and totals with nameplate data. The overcurrent test determines if your motor might be drawing more than its rated current on one or more phases, causing excessive heat and decreasing the life of the insulation.
The Efficiency Test: Determines the operating efficiency of your motor. Reduced motor efficiency costs money and wastes energy. This test compares the operating efficiency with EPAct reference, giving statistical feedback on the quality of the tested motor with respect to achievable peers.
Motor Load Tests: The rotor bar test and operating condition test evaluate the overall condition of your motor.
The Rotor Bar Test: Inspects the relative amplitude of the rotor bar sidebands and assesses the rotor cage signature. Broken rotor bars cause excess heat, decreased efficiency and shortened motor life.
The Operating Point Test: Compares the torque, speed and average stator current values to previously stored data and alerts you to deviations from normal operation.
Energy Assessment Tests: Compares the performance of the tested motor with similar motor designs in the MPM Explorer II and aids in deciding when a motor should be replaced.
Motor Load Tests: The torque ripple test and load history test.
The Torque Ripple Test: Warns if the motor’s load requires oscillating or varying torque. The torque ripple test identifies the ripple that the load is requesting, resulting in a percentage of a ripple to steady state operation. This test shows the load stress imposed on the motor.
The Load History Test: Compares current load levels to previously recorded levels.
Along with the results of the above testing, we also record the exact RPM of the motor, the total horsepower the load is requiring, kilowatts of energy consumed and the percentage of nameplate load being demanded from the motor. This service is offered by Evans Enterprises, Inc. to prolong the electrical life of your motor and its operating machinery.
The PI Test provides a more definitive evaluation of the ground wall insulation. In this test the direct test voltage is maintained for ten minutes. Resistance readings are taken at one and ten minute intervals. Results are derived by dividing the ten minute reading by the one minute reading. The PI test takes into account that insulation molecules polarize during the test, causing the resistance reading to increase gradually. A clear decrease in PI readings over time indicates a loss in insulation integrity. The PI ratio provides a standard value to compare the integrity of the motor insulation regardless of varying external conditions. The PI Test compares insulation performance over time under changing circumstances
Experts say that many winding failures actually start as a turn to turn, coil to coil, or phase to phase short. The heat built up from one of these types of shorts can result in an insulation failure or ground fault. Therefore, ground faults are often blamed for most motor failures. By detecting pre failure conditions, catastrophic failures can often be prevented. Surge Comparison Testing is one of the newest motor insulation tests and actually analyzes the condition of windings by detecting insulation flaws that cannot be done by any other method. The test instrument applies brief pulses of high voltage, low current energy simulating the high voltage spike, without the high current that is seen at startup. The pulse creates a resonance in the winding that appears as a damped sine wave on the display. A good winding is indicated as a stable waveform. A faulty winding waveform shifts or changes abruptly during the surge test as the test voltage is increased. If the surge waveform remains constant up to the specified test voltage, the winding is sound.
When two windings that are supposed to be identical are compared, the waveform resulting from the test should be identical. If they aren’t, there is a problem.
Most shorts cannot be repaired without winding the unit. However, if you know the windings are in trouble, you can opt to rewind the unit before a catastrophic failure occurs. Surge testing is also a useful tool to confirm that windings are connected properly before being encapsulated which saves time and money. If your current motor rewind shop doesn’t include surge testing as a standard procedure, call Evans Enterprises, because we do.
The Winding Resistance Balance Test measures the phase resistances of a motor to determine if any unbalances exist between phases. For example, on a 3-phase motor, comparisons of phases 1 and 2, then 2 and 3 and finally3 and 1 would occur. Unbalances are quantified by differences in these measurements.