Archives for 2020

Why do we measure the insulation resistance of a motor winding? Because the resistance measurement indicates the ability of the insulation to resist the passage of electricity from coil to coil or to the frame of the motor. High resistance above the recommended IEEE minimum usually indicates a good winding, while low resistance may indicate the winding is dirty or wet. When performing insulation resistance tests, it is important to verify your leads are connected at the proper location and that you have recorded the temperature of the windings under test; insulation resistance varies with winding temperature. In MCEGold, if after testing you need to change the temperature, open the test in Test History, double click on the temperature of the test, enter the correct winding temperature, and click Change Temperature. The resistance test results will be recalculated with the proper temperature and ensure you have collected and saved accurate data to the asset test history.

Changes in voltage have a significant impact on current. When it comes to supply voltage variations, starting current and running current respond very differently. A drop in line voltage will result in a lower starting current and a higher running current. A rise in line voltage up to 5% will increase starting current and slightly reduce running current. If the line voltage increases more than 5%, both starting and running current will be higher. As a general rule of thumb a 1% voltage imbalance can create a 7% current imbalance and a motor with a 5% voltage imbalance has a reduced capacity of 25% and should not be run unless the motor is de-rated accordingly.

Visit the PdMA YouTube site at https://www.youtube.com/channel/UC-cUONWaudkKReNwC0PPXMQ to see a variety of case studies from the electric motor reliability industry.

Have you had issues with motors that are stored in your facility? Have you experienced poor meg readings and/or bearing issues?  Here are some tips for keeping your motors running optimally:

1. If your motor is in an ambient above 140ºF, it should be in an enclosed ventilated area.
2. Does the temperature of your motor get below an ambient below 40ºF?  Heaters should be installed and energized.
3. Is the area your motor is in experiencing  large swings in temperatures? You could be collecting moisture.  
4. Rotate shafts bi-monthly.
5. Megger motors bi-yearly.
6. Grease ball bearing motors yearly.
7. Sleeve Bearing motors: Install oil while in storage and drain when moving.

If you have any questions, please feel free to call your Account Manager, or call 217-428-6621

Decatur Industrial Electric offers storage for motors and the staging of new motors for projects! 

Often when we hear technical terms involving power quality such as total harmonic distortion, common coupling, and harmonic voltage factor our eyes glaze over and we throw up the white flag. Yes, there are some impressive algorithms involved in some of those terms, but in the maintenance and reliability world we can look beyond the details to the end game which is heat. Heat has been identified as the number one killer of electric motor insulation and the bottom line is poor power quality creates excessive heat. High distortion levels in the power being delivered to a motor will result in current flow through the motor that performs no real work. This harmonic current added to the real work current often results in the motor running in an overloaded condition electrically even though the horsepower delivered is at or less than rated horsepower.

When performing voltage and current measurements for power testing or current signature analysis, careful attention must be given to the current probe range selector switch, if available. Many of the clamping current probes offer more than one range like 10 amps and 100 amps. Additionally, these probes are often designed and built with a burden resistor resulting in an mV output from the current probe. If the wrong position is selected on the current probe a relatively large mV output may exceed the maximum input voltage of the test device resulting in irregular or clipped current measurements and displayed signals. So remember, if you are measuring 100 amps on a 10 amp scale you might think you have a motor problem…but you probably have a meter problem.