PdMA® Corporation is an industry leader in the field of predictive maintenance (PdM) and condition monitoring, globally offering electric motor testing instruments. Each week, they publish a tip regarding PdM and overall motor care. Below are five such tips.
Understanding the 5th Harmonic
Nonlinear loads such as variable frequency drives, lighting, and computer systems are a common cause of harmonics on a system. Harmonics on a distribution system can have deteriorating effects on motors by causing overheating or tripping of over-current protection devices. In particular, the 5th harmonic has a negative sequence. Distorted voltage containing the 5th harmonic attempts to drive a motor in reverse, creating a negative torque. Compensating for the torque, the motor in turn draws additional current to satisfy the load requirements which causes the overheating.
So, an overloaded motor may not be the product of a faulty motor or excessive load demands, it may just be a harmonic issue.
Testing Synchronous Machines
When testing synchronous machines, one should consider the control circuitry involved. Baseline current tests of both armature and field should be taken when the machine is operating in nominal condition or when it’s first installed. Analysis of the current data should include a thorough review of the time domain waveforms to ensure there is full wave rectification (i.e. all rectifiers are working) in the field circuit. Baseline voltage and current measurements through a start-up and synchronization should also be performed to analyze the performance of the amortisseur windings and synchronizing regulation. Assuming a constant load, a steady state analysis of an RMS enveloped current on both the armature and field should be performed to ensure there is no abnormal variation in current. Should you need additional information on testing synchronous machines, please contact PdMA technical support for assistance.
Synchronous motors are commonly medium/high voltage machines. For additional test considerations read Medium/High Voltage Testing Application Note.
Winding Resistance and Temperature
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.
What is a Baseline Worth?
Several identical motors were installed at a facility. Upon installation, the motors ran hot and didn’t produce the expected horsepower and all of the motors behaved in the same manner. Instead of performing a Root Cause Analysis (RCA) as to why the motors weren’t performing to specification, the facility proceeded to use the motors as is. Eventually, one of the motors failed and was sent in for repair. It was found that half of the windings (on all three phases) were blackened. The motor was repaired and returned to service only to fail again with the same failure. This time, the motor was sent to a different motor shop for repair. After some investigation, the external connections were found to be incorrect, resulting in only half of the stator windings being used. Situations such as this remind us all that it is important to perform both offline and online baseline testing when installing a motor. This situation could have been avoided if baseline data including running current, running speed, startup time, running temperature, and winding resistance and inductance were available to compare to the motor manufacturer’s specifications. So how much is a baseline worth? In this situation about two motor rewinds and countless hours of installation.
Motor Problem or Meter Problem?
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.
You are invited to submit an Electric Motor Testing Tip of your own and receive a free PdMA mug or hat if they publish it. Contact Lou at 813-621-6463 ext. 126 or email him.[subscribe2]