Electrical Testing Archives - Decatur Industrial Electric

Good, Better, Best

When it comes to energized testing of your electric motor with a Variable Frequency Drive (VFD) there are a variety of considerations. In addition to your electric motor health, broad concerns such as drive health and power distribution health should also be considered. To help you prioritize your approach to these considerations we have come up with a Good, Better, Best plan for testing your VFD motors.

Good practice in monitoring your VFD motor is testing from the line side of your VFD. You will be focusing primarily on the distribution system health with a secondary indication of the drive condition. Power Quality (drive input) and Power Circuit fault zones are the primary focus. Faults in either of these fault zones will have a detrimental impact on your electric motor.

Better practice in monitoring your VFD motor is testing from the load side of your VFD. You will be focusing on the motor health with a secondary indication of the drive health. Power Quality (drive output), Stator, Rotor, Insulation, and Air Gap Fault Zones are the primary focus.

Best practice in monitoring your VFD motor is testing both the line and load side of your VFD. You will have a complete evaluation of the line to load side distribution and all available fault zones.

Whether directly connecting to drive and motor with an MCEMAX, testing through MTAP’s, or permanently connecting a PdMAEYE to both line and load side, don’t miss the opportunity to evaluate your drive and motor using a Good, Better, or Best practice.

To see a case study on troubleshooting a DC Drive visit the PdMA YouTube Channel at https://www.youtube.com/watch?v=OFuhtXevJs4&t=5s

Source: PdMA – the Leader in Electric Motor Testing

Fault Zone Series: Part 1 Power Quality

Sticking to the Fault Zone approach to electric motor troubleshooting we are starting this series with Power Quality. “You are what you eat,” is what you were told as a child when your parents were trying to get you to eat healthy foods. Well, the same is true for your electric motor. The motor’s health is based in part on whether you feed it good clean power quality. Many electric motor fault zones are related to heat and Power Quality is no different. Poor power quality makes a motor work harder than designed to deliver the horsepower required by the load. A 100HP motor with poor power quality may require 110HP worth of current to deliver the requested 100HP. This extra current increases the heating of a motor beyond its design temperature and will reduce the life of the motor. Identifying power quality issues like excessive harmonics and voltage imbalances early will allow you to reduce the load of a motor when possible until you can correct the issues and extend the life of your motor. For more examples of Power Quality anomalies and Power Quality standards applied to your electric motor reliability visit MCEMAX Fault Zone- Power Quality on the PdMA YouTube Channel.

Think Transformers.

The rotor of an AC Induction motor is like the secondary circuit of a transformer. It is easy to imagine that any change to the number of turns in the secondary of a transformer, would certainly affect the circuit performance of the transformers primary windings. Well, the rotor bars of the induction motor could be compared to the winding turns in the secondary of a transformer. Changing the conductivity or resistance in any way on the rotor cage of an induction motor would be like modifying the number of turns on a transformer secondary. So, if we consider the induction motor rotor cage as a comparison to a transformer secondary, then what part of the induction motor would be compared to the transformer primary? You guessed it… the stator windings. Any resistance change to the rotor cage of an induction motor will impact the circuit characteristics (inductance) of the stator windings. Therefore, monitoring the stator winding inductance of an induction motor is a great method of trending the condition of the rotor cage.

Credit: PdMA

Electric Motor Testing- Mechanical or Electrical?

Often when evaluating motor current through a simple spectrum analysis plot, frequencies generated by mechanical shaft line components such as belts, bearings, pumps, and fans are present. With enough knowledge about the system, band alarms can be easily created around these frequencies to trigger an alarm in the event of a sudden increase in amplitude. However, sometimes these mechanical peaks coincide with common electrical peaks such as the pole pass frequency which is equal to the number of poles developed in the stator windings multiplied by the slip between the rotor and stator. When this happens it can create a false alarm for an electrical anomaly which is why it is important to utilize multiple indications when confirming elevated pole pass frequencies. Some alternate indications are the demodulated current spectrum, trending of stator inductance, trending motor start times, and performing a rotor influence check. Having multiple indications of an anomaly won’t guarantee you get the source right but it sure increases your odds.

Source- PdMA

Band Alarms for Belt Frequency Peaks

When establishing current demodulation band alarms for belt frequencies associated with a 2-pulley belt driven system, remember both the 1x belt frequency and 2x belt frequency should be taken into consideration. Initial band alarms for the 1x belt frequency are recommended at baseline + 10% for caution and baseline +20% for severe. Initial band alarms for the 2x belt frequency are recommended at baseline +25% for caution and baseline +50% for severe. Based on test results from known defects, analysis of the 1x belt frequency focuses more on the system alignment while 2x belt frequency focuses more on belt condition and looseness.

belt frequency

For more information on establishing band alarms for mechanical defects read the short article “Advanced Spectral Analysis” found on the PdMA website.
http://www.pdma.com/pdfs/Articles/Advanced_Spectral_Analysis.pdf