There are areas of concern when trying to control both speed and cost.
End users desiring speed/and or torque control often buy variable-frequency drives (VFDs) to modify existing applications where a standard induction motor is in place. Frequently, they try to control costs by using that existing standard induction motor. Before taking that path, however, it is best to consider a few areas of concern with the approach….
Troubleshooting a motor without historical data to reference for a change in baseline, or a gradual trend up or down can be a difficult task. The IEEE 43 standard (Recommended Practice for Testing Insulation Resistance of Rotating Machinery) states that 100 Meg is the minimum acceptable resistance-to-ground value for a form wound AC motor. So, a 110 Meg insulation-to-ground reading on an installed form wound motor would normally create a cause for concern. However, if you had five years of data on the very same motor that showed only a 1 Meg drop over the five years, your ability to quantify the severity of the low resistance-to-ground would be far better. With trend as your friend, no immediate action would be necessary as the condition is above the minimum and stable. With no history you might feel that this was an imminent failure risk and decide to secure production, resulting in an unscheduled outage, to prevent a potential catastrophic failure that in all likelihood would not have happened. So get your baseline on all assets worthy of your interest and schedule some future tests to establish a trend.
Routine offline and shop stage gate tests should include Phase-to-Phase Inductance measurements. Phase-to-Phase measurements can be useful for diagnosing a variety of situations:
• Poor or incorrect rework such as reversed coil winding leads
• Faults in power cables or main contacts in the power circuit
• Air gap eccentricity problems
• Shorted turns such as stator phase-to-phase and coil-to-coil
• Rotor porosity and lamination damage
• Broken/cracked rotor bars or end rings
For more information read Influence of Residual Flux on the Measurement of Inductance at
http://www.pdma.com/pdfs/Articles/Influence_of_Residual_Flux_on_the_Measurement_of_Inductance.pdf.
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: