Demystifying HVAC Drive Anomalies

0708_maintnenancelog_img11A new handheld test tool makes high-end electrical troubleshooting easier than you thought.

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

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 bleed-throughs on a gate. "Of course, a transistor is basically a lightning-fast 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."

Multitasking problem solver
Vogel thinks it's important that a 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 commentary 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 from a number of sources—sine waves on the VFD inputs and outputs, current and voltage—and compare it, so that I can derive a power factor for the circuit," Vogel illustrates. Vogel is able 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."

With the weather patterns and lightning in Florida, Vogel explains, it's not uncommon for line voltages to rise and fall precipitously. "We were working on a current source drive that I wanted to retrofit, because it had taken a hit from a lightning bolt." At the time, technicians suspected that the drive had been damaged beyond economical repair, and they decided to replace the drive itself, but not the main feeds.

0708_maintnenancelog2Shortly after the drive began to ground-fault and failures in the building's electrical distribution network began to show up, the customer asked Vogel to come out. After doing some low-level diagnostics—throwing amp clamps on the wires and comparing phases and phase draws—he placed a ScopeMeter on the system and discovered there was a lot of line notching (Fig. 1) going on.

Vogel explains that nonlinear ac loads—loads in which voltage and current are out of phase—create harmonic distortion (see Fig. 2 on the following page). Examples of nonlinear loads include welders, VFDs and battery chargers. Distortion is a result of the non-sinusoidal waveform the drive generates, Vogel notes. He goes on: "Any time you have long conduit runs, the wires create magnetic fields around themselves. With harmonic distortion, current is actually reflected back into the wiring. It becomes a self-sustaining loop. (As shown in Fig. 1), that's what we call line notching. As you switch the ac current on and off, it's the equivalent of opening and
closing a valve on a water pipe very fast, causing pulsations
in the flow. Line notching is the electrical equivalent
of that phenomenon."

Circling back to the original problem, Vogel notes that the high current for each of the three phases had led the original installers to use four parallel conduits for each phase. In such a configuration, a smaller conductor for each phase would typically have been run down a single conduit, with multiple conduits going to the equipment and each smaller conductor terminated on a terminal block for its appropriate phase. But instead, the installers had run feeds A and B in one conduit, B and C in another and C and A in the third. "The drives were passing almost 42 A to ground, causing them to trip on ground faults and over-voltages," Vogel says. "Of course, with the phase conductors running through conduits and the sheer number of conductors (sixteen 500 MCM runs), they were concealed, and nobody had thought to look further."


A better look at power factor
Vogel recently was called on to solve a power factor problem in a large commercial building where a number of aging 250 hp chiller motors were in service. In high ambient weather conditions, these old chillers would load up, and Vogel could use ScopeMeter to see the phases moving farther and farther apart (Fig. 3).

As the chill water temperature came down, the power factor would drop nominally from about 0.7, which was acceptable, to about 0.32—the lowest Vogel had ever seen it. Then, as he ‘staged' the equipment downnamely, drives on the cooling towers, drives on the primary loop pumps and drives on the primary chill water system—the phases would come back in sync and the power factor would rise again.

"You can view readings on the meter," Vogel notes, "but you don't understand what's causing the powerfactor drop until you look at the waveform itself." As he tells it, you can see the field collapsing as the motor winds down, and you can see the current and voltage phases come closer to being in sync (Fig. 4). As the power factor comes back and approaches 1.0, it's fascinating to watch, and the customer is more likely to understand the problem. More importantly, it helps one understand how to correct the problem.

Capturing the benefits
One of Vogel's new projects is to install power-factor correction capacitors on an MCC (Motor Control Center) panel at a utility customer's site. The capacitors will be installed in parallel with the connected circuits. This is not just about improving power factor, but about keeping costs in line.

Many electric utilities charge building owners a penalty for low power factor. (One utility, for example, charges building owners $0.14 per kVAR hour when power factor drops below 0.97.)

According to Vogel's calculations, with an added 65 kVARs of capacitance, it's about a $200,000 proposition to add these caps. The customer is running two 800-ton machines fully loaded during the peak of summer in 95 F Florida heat and 90% humidity.

0708_maintnenancelog5Essentially, the customer's air conditioning plant is running at 100% electrically, but not mechanically, Vogel points out, noting that the customer's electric bill varies from $50,000 to $60,000 a month. "We determined that, if we can increase the power factor on this panel to 0.85, the customer's electrical consumption will drop by almost one-third. That correction, considering the utility's high power consumption, will give them a payback period of less than one year. And, they could get additional capacity without any work on the mechanical system!"

ScopeMeter, Vogel says, is what identified the problem. "We took it to the customer and said ‘Hey, as we stage these motors down, as we shut things off, your power factor starts to rise again.' First, we measured the signal on the MCC panel, and then we measured the signal on their main power panel. We set the same function up on the chiller plant, and we could see the power factor clean up."

Vogel confirms that everyone now understands the nature of the problem, explaining that he had directed the customer to an Internet site where he could calculate his own energy savings from improving power factor. "Next, we stood by as the customer observed the current dropping with modifications to the panel, not to mention that they started to see immediate reductions in total kilowatts used. Down here you can't beat the heat, but you can make it a little bit more palatable."

"You'll laugh, but I'm a union pipe fitter by trade," Vogel says, "and here I am doing high-end electrical troubleshooting. The ScopeMeter has taken my trade in a whole new direction." MT

Hilton Hammond is product manager for Fluke Precision Measurement. A technical expert on ScopeMeter test tool products, LCR meters and video test equipment, he has worked for Fluke Corporation for nine years. Originally from South Africa, Hammond began his career in calibration. Telephone: (425) 446-5381; e-mail: hilton. This e-mail address is being protected from spambots. You need JavaScript enabled to view it

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