| Application Notes: Demystifying Variable Frequency Drive Anomalies With Scopemeter®
Seeing is believing? More aptly, seeing is understanding.If anyone can wring every last ounce of functionality out of a piece of electronic test equipment, it’s Chris Vogel. At Siemens Building Technologies, Vogel has his work cut out for him keeping HVAC systems running at their peak for the company’s large commercial customers during peak Florida weather marked by seemingly nonstop 90 °F temperatures and 95 % humidity. And that’s just one of the challenges faced by technicians at Siemens Building Technologies, which plays a more sweeping role in its customers’ success: ensuring energy efficiency, comfort, protection against unauthorized access, and fire safety year-round for every building or office tower entrusted to it by customers. Vogel, an HVAC technician, becomes energized when discussing the return on investment in his handheld ScopeMeter® Test Tool. “Out at one large site, where we monitor and troubleshoot variable frequency drives (VFDs), component-level repair can often mean the difference between a $20 repair part and a $100,000 repair bill. I know firsthand, because we recently documented that very scenario.” On large VFDs, Vogel uses his ScopeMeter to uncover capacitance problems, transistor firing mishaps, and even bleedthroughs on a gate. “Of course, a transistor is basically a lightningfast switch,” he says. “It switches back and forth between open and closed, and it can sometimes start to break down. When that happens, motors will start doing weird things. For example, at load stage. We’ll actually see the motor banging back and forth as if it is not sure which way to turn.” Storing a slice of timeIt’s important, says Vogel, that the technician be able to characterize VFD problems by capturing a waveform from the offending drive. His premise: A signal is much more telling when presented in a waveform view than in a single, static voltage reading. The signal has a shape and value that may look right at a glance, but could just as easily have a distortion or rough “edge,” or a momentary spike almost too short to be seen. Either problem, or a host of other signal anomalies, would be indistinguishable with just a numeric reading of the signal. “The scope allows me to record information, and from a number of sources—sine waves on the inputs and outputs of VFDs, current and voltage—and, for that matter, make comparisons of current and voltage so that I can derive a power factor for the circuit.” ScopeMeter allows Vogel to store up to 25 permanent records for recall at any time. “Sometimes I will see a suspect waveform and say, ‘here’s what it looks like during this slice of time, but here’s what it should look like.’” With that, he recalls a stored image of the same waveform, recorded when the drive was operating properly. “Storage scopes create a graphical representation of the problem, versus a merely empirical value that a multimeter would show. Of course, with ScopeMeter, we get both. Nonlinear loads aboundWith the weather patterns and lightning in Florida, says… Click here to download the full PDF: Demystifying Variable Frequency Drive Anomalies With Scopemeter (.pdf) » | |