Archives for February 2019

What a Nuisance!

Most circuits will have instantaneous, short term, and long term over load protection. If the circuit includes instantaneous trip devices which react in less than a single AC cycle such as magnetic-only circuit protectors, this could be the cause of the nuisance tripping your plant operators have commented about.

During a motor start-up there are two components that make up the starting current, the instantaneous peak in-rush and the locked rotor current. The instantaneous peak in-rush is the momentary transient that occurs immediately (within half an AC cycle) after the contacts close. Locked rotor current follows the instantaneous current through acceleration to steady state.

The M-Series EMAX performing a six channel 3-phase In-Rush/Start-Up test using the latest MCEGold^® software displays both current and voltage transients to assist you in determining the possible cause of these random trips. After completion of an In-Rush test, all three phases of RMS current will be displayed as well as the instantaneous peak current. Comparing these values to the trip settings of the circuit’s over current protection allows you to identify the source of the trip as motor or circuit protection driven. For guidance on increasing the instantaneous trip level to avoid nuisance tripping refer to Section 430 of the latest National Electrical Code.

To see a variety of case studies involving electric motors visit the PdMA YouTube Channel at:

https://www.youtube.com/channel/UC-cUONWaudkKReNwC0PPXMQ

Meet Zach Miller, Field Service Technician

This month we are spotlighting one of our field service technicians. Zach has been with Decatur Industrial Electric for slightly over a year. During that time, he has learned many facets of the business including overhead crane and hoist inspections, vibration analysis, and alignments. He takes a great deal of pride in what he does and enjoys being out at customer sites helping with their reliability efforts. Like many of our team members, when not at work Zach enjoys building things….However, unlike most of our team members he doesn’t rebuild cars. He builds computers!

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