Energy costs are a major portion of any plant or facility’s operating expenditures and motors consume a very large part of those expenses. Correct monitoring of motor performance, and making the necessary adjustments, will improve reliability, extend the life of the motor and reduce the overall operating cost of the facility.
Studies show that, after bearing failure, electrical faults are the most common mode of motor failure, so in addition, a structured electrical motor testing regime is vital for ensuring plant reliability.
Vibalign utilises sophisticated PC controlled test equipment to provide accurate testing and fault diagnosis. Test result data is saved in our database, so that a performance trend can be built up over time.
The Main off-line in-service motor testing we recommend to be done on electrical motors on a regular basis is:
- Stator winding resistive imbalance,
- Stator winding insulation resistance (megger checks),
- Polarization Index (PI),
- DC High Potential Test (Hi-Pot Test)
- Step Voltage test,
- Surge test.
Stator winding resistive imbalance:
The Coil Resistance test consists of injecting a known constant DC current through the winding,
measuring the voltage drop across the winding, and calculating the coil resistance using Ohm’s law. If a coil is shorted somewhere in the interior of the winding, the resistance will be lower than normal. This lower coil resistance can be compared to previous measurements of the same coil, measurements of identical coils, or to the motor nameplate value in order to identify a bad coil. Low values indicate shorts, less turns, less cross sectional area. Measured values that are higher than normal can indicate loose, corroded connections or opens. The measured resistance is affected by the variation of copper conductivity with temperature.
Before comparing two different measurements, correct the measured resistance value to a common temperature, usually 25° C per IEEE 118.
Performing Resistance tests on the same motor over time provides early warning signs of motor connection problems. Motors operated in conditions that allow corrosion, contamination, or other physical damage may show initial warning signs of motor failure.
Stator winding insulation resistance (Megger checks)
The Meg-Ohm test consists of applying a DC voltage to the windings of a machine after isolating the winding from ground. According to IEEE 43 the test, voltage is usually near the operating voltage of the machine.
The intended purpose of the Meg-Ohm test is to make an accurate measurement of the
insulation resistance of the ground wall insulation.
The Meg-Ohm test is best used for finding ground faults and the level of moisture or particulate contamination.
Polarization Index (PI)
The Polarization Index Test (PI test) is best used for determining if the winding is wet or is contaminated. The PI test is performed in order to measure quantitatively the ability of an insulator to polarize.
The PI test is typically performed at 500, 1000, 2500 or 5000 Volts. This depends on the
operating voltage of the motors being tested.
The duration of the test is 10 minutes. The PI value is calculated by dividing the insulation resistance at 10 minutes by the resistance at 1 minute.
In general, insulators that are in good condition will show a high polarization index while insulators that are damaged will not.
The Dielectric Absorption (DA) test is essentially a short-duration PI test and is usually
intended for smaller motors. Larger motors whose insulation does not easily polarize are
also good candidates for the DA test. In the situation where PI ratio may not be meaningful, the Dielectric Absorption (DA) is widely used. The DA is the IR value at 3 minutes divided by the IR
value at 30 seconds
DC High Potential Test (Hi-Pot Test)
The DC High-Potential (HiPot) test consists of applying a DC voltage to the windings of the machine, same as a Meg-Ohm/PI test, but at a higher voltage. The intended purpose of the DC HiPot test is to prove that the ground wall insulation system can withstand a high-applied voltage without exhibiting an extraordinarily high leakage current. Therefore, the DC HiPot test is often called a proof test.
The HiPot test is considered a mainstay of motor testing.
Step Voltage test
The Step Voltage test is similar to a HiPot in that it looks at the integrity of the groundwall insulation; however, at a less rigorous way. It is performed to a voltage of what the motor typically sees during starting and stopping.
The DC voltage is applied to all three phases of the winding, raised slowly to a preprogrammed voltage step level, and held for a predetermined time period. It is then raised to the next voltage step and held for the appropriate time period.
The Step Voltage test is necessary to insure the ground wall insulation and cable can withstand the normal day-to-day voltage spikes the motor typically sees during operation. Whereas the Meg-Ohm/PI/ HiPot tests are used to detect ground wall insulation weakness, the Surge test is used to find turn-to-turn insulation weakness.
The surge test consists of applying a fast rise time, high current impulse to a winding. This fast-rise time impulse will induce a voltage difference between adjacent loops of wire within the winding.
If the insulation between the two loops of wire is damaged or somehow weakened, and
if the voltage difference between the wires is high enough, there will be an arc between the wires. This arc shows up as a change in the surge waveform.
The surge test is performed with an impulse generator and a display to observe the surge waveform in progress.
All electric motor testing is done in accordance with:
IEEE Std 43-2000: Recommended Practice for Testing Insulation Resistance of Rotating Machinery
IEEE Std 95-2002: Recommended Practice for Insulation Testing of AC Electric Machinery (2300 V and Above) with High Direct Voltage
IEEE Std 522-1992: IEEE Guide for Testing Turn-to-Turn Insulation on Form-Wound Stator Coils for Alternating-Current Rotating Electric Machines
IEEE Std 1415-2006: IEEE Guide for Induction Machinery Maintenance Testing and Failure Analysis
Safe working on low voltage electrical installations
Benefits of Electrical Motor condition monitoring:
- Improved plant reliability
- Reduction in overall operational costs
- Improved efficiency of plants
- Early fault diagnoses enabling more cost effective corrective maintenance.