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Vibration Information for rotating machinery applications.


Electric Motors: Static/Offline Testing

Friday, February 24, 2012

Electric motors are a key component for running machines in most industrial environments. Therefore it is important that they are properly monitored to avoid unwanted failures or shutdowns.

Typically, there are two testing categories for electric motors:

1. Static or offline tests

2. Dynamic or online tests

This week I will be discussing some offline tests that are used for monitoring electric motors. Offline tests, as the name suggests, take place while the machine is not operational i.e. it is “offline”

Offline testing can be performed during equipment shutdowns, or before reconditioned motors are put back into service. The main objective is to test the integrity of the motor winding insulation. Offline tests can detect situations of contamination, overload and voltage issues that may impact the insulation.

Offline testing includes the following:

1. Surge testing

2. Winding resistance tests

3. High potential testing

4. Polarization index test

Surge testing applies increasing voltage pulses into a single phase of the stator. The other phases are held to ground. The voltage pulses increase from 0 to the desired target and generate a waveform. A shift in the waveform frequency as the voltage is increased will indicate weak insulation

Winding resistance is tested by applying a set DC potential across the winding. THe amount of current leaking across the insulation is measured and converted to a resistance. The winding is typically charged for 60 seconds to allow the insulation dipoles to become adequately excited and polarize. This test will cause short circuiting if the insulation is damaged in any way. It is important to select an appropriate voltage for this test as excessive voltages could damage the insulation.

High potential testing determines if the electrical insulation between two electrically isolated components is adequate to face any overload voltage conditions. A high voltage is applied across the two components being tested, and current is measured to detect the amount of leakage in the insulation.

In a polarization index test, the insulation resistance is measured after 1 minute and after 10 minutes. The ration between these two values (value at 10 minutes divided by value at 1 minute) is known as the polarization index. When a DC current is applied to the insulation, its atoms become polarized and its resistance increases. A higher polarization index is considered to be better. Before running this test, it is important to completely discharge the windings to obtain accurate readings.

In my next article I will be discussing online tests for electric motor analysis and monitoring.

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