|
Tell Me More:
Sometimes poor power quality will cause delayed failures that go undiagnosed. For instance, harmonics and flat-topping can cause electronic power supplies and induction motors to overheat, leading to premature failures. And sometimes, if distortion is severe, the unit may not even work.
Power requirements for furnaces
Furnaces, unlike consumer electronics, do not have a built-in regulated power supply (which would add about $1500 to the cost). They rely on a clean, consistent sine wave to power an IFC (Integrated Furnace Control) and establish clock timing and synchronization of integrated circuits.
A simple IFC may only control furnace functions such as turning line or low voltage switches on and off, controlling fan-timing circuits, and monitoring flame rectification circuits. More advanced IFCs also control voltage to igniters, ECM and VFD motors, and variable frequency drives.
While the advanced circuits may be more susceptible to power quality issues, even the simplest IFC may not operate when supplied with poor quality power.
Furnaces have a minimum basic electrical requirement for voltage, frequency and power quality. The accepted standard is
- 120 volts +/-10% (108-132 volts)
- 60 Hz frequency +/-3% (57-63 Hz)
- harmonic distortion <5% THD.
Furnaces supplied with voltages that have harmonic distortion greater than 8% THD, may not operate or will operate with possible damaging consequences for the electronic circuitry and components.
Case in point
One such case involved a sophisticated furnace installed in a residence that also had a standby generator with a combination transfer switch and essential loads sub-panel. The furnace operated reliably throughout an unseasonably cold winter until the power failed and the standby generator was called into action. Under generator power, the furnace locked out on a communications error failure code.
One could assume either that the furnace was substandard, or the generator was substandard. The electrician who installed the generator said he had never had an installation where a furnace would not operate on generator power, so the problem had to be with the furnace.
The HVAC contractor had only seen this problem with portable generators, not with standby generator installations. He also knew that no other furnace did so much to provide the greatest comfort at the greatest efficiency. It had a variable frequency/variable voltage CAI and an ECM blower motor. Both motor circuits converted single phase to three phase power and used multiple "power gulping, sine wave flat-topping" 500 and 1000 µf filter capacitors. The IFC included the power supply and the program for the VFD and the program for the 20 kHz PWM signal to drive the blower motor. It also had a circuit that regulated voltage to the hot surface ignitor. But still, the generator worked, why wouldn't the furnace?
The heating contractor knew he was in over his head. Both the furnace manufacturer and the generator manufacturer were contacted. Each blamed the other. The generator manufacturer did not know why the furnace would not operate on one of their generators. The furnace manufacturer said it had to be a power quality issue and recommended using a power quality analyzer to verify the power supply. Presented with this, the electrician had the power quality checked and reported that it was fine, but had no actual documentation to back it up.
The HVAC contractor knew it would be up to him to resolve this issue. He had several options.
- Install a furnace with fewer high-tech functions and hope it would work.
- Refund the customer's money and pass it off to another HVAC contractor.
- Install a 2200 kVA UPS (uninterruptible power supply) to condition the power to the furnace.
- Replace the generator with one he knew had low THD.
- Acquire a power quality analyzer to learn the truth and be able to document it.
Options one and two were immediately dismissed by the home owners. This was the most comfortable heating system they had ever lived with and were not about to give it up. Option three was not assured to solve the problem. If the power quality is poor, the UPS may want to operate in battery backup mode. Option four would solve the problem, but was expensive and would not really prove anything. Nothing would be learned except to be very afraid to install a furnace when a particular brand of stand-by generator is installed. Option five was a gamble. If it didn't produce the desired results, option four would be the only acceptable way out.
A decision was made. A power quality analyzer would not only document the needed facts on this job, but would be available to verify generator power quality on future jobs before a sales contract was signed.
He chose the Fluke 43B power quality analyzer/logger because of its built-in waveform view, just like an oscilloscope. Presented with a visual representation of voltage and amperage waveforms, anyone (including homeowners) can see distinct differences between clean and distorted power.
In addition to measuring detailed electrical data, the analyzer can also store screens, graphs and waveforms in meter memory and/or transfer those files to a computer for subsequent analysis and reports. The analyzer was expensive, but not nearly as expensive as a replacement stand-by generator.
It worked!
The analyzer measured utility power at 2.8% THD under normal loads…but measured the generator power at 12.8% THD without any loads. The data was saved and sent to the generator manufacturer. Since the generator was less than one year old, and its marketing literature promised a harmonic distortion rate of less than 5% THD, the manufacturer offered to supply a newer model generator with less harmonic distortion.
Anyone who's spoken with engineers knows that they respond to documented, accurate data and facts, not anecdotes or heresay. When we think about it, most HVAC techs are pretty similar. Without the necessary meters, gauges, thermometers, manometers, anemometers, etc., we are nothing more than parts changers who hope to get lucky.
|
