Furnaces and THD with generators

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jes25

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Location
Midwest
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Electrician
Hello all,

Our company has an HVAC department in addition to an electrical one. We often have an issue with our high end furnaces not running off generators. One solution I thought of was to install a 1500 watt UPS in front of the furnace, but obviously that's not what a UPS is intended for. My question is would the ups idea work and are there any better options? This is a residential application.
 
Perhaps it's only 24VAC that needs low THD for powering the sensitive electronics on control boards. So maybe a low THD UPS for just the 24VAC portion would work. Then the UPS wouldn't have to support the high current draw of the blower motor.
 
Yes we get a fault code. I've done the research on it and the issue is the Total Harmonic Distortion is too great on portable generators. In other words, the THD listed for most generators is out of spec with Lennox furnace. An inverter generator or large standby do work. But often people have a portable gen set up established and then the hvac guys will change the furnace creating this issue. And a larger inverter generator is quite expensive.

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jes25 said:
One solution I thought of was to install a 1500 watt UPS in front of the furnace, but obviously that's not what a UPS is intended for. My question is would the ups idea work and are there any better options? This is a residential application.
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Depending on how much THD the generator is putting out, the UPS may not recognize it as a valid source and then limit the furnace runtime to the amp-hour rating of the battery (15 minutes or whatever you selected) at that load.

This is what happens to my UPS that supports my VOIP equipment at my home when on generator. :ashamed1:
 
ron said:
Depending on how much THD the generator is putting out, the UPS may not recognize it as a valid source and then limit the furnace runtime to the amp-hour rating of the battery (15 minutes or whatever you selected) at that load.
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Good point. To address this issue, some UPS units allow you to lower their sensitivity to distortion and frequency error before they disable the AC input and go to battery-only mode.

https://www.apc.com/us/en/faqs/FA156514/
 
190606-1056 EDT

jes25:

A low pass filter is a circuit that impedes and shunts higher frequencies than your desired frequency. Your desired frequency is about 60 Hz. Harmonics are multiples of your base frequency.

It is important to determine whether the problem is in the motor driver or in some other area that tells the motor to run or how fast.

The motor driver is high power and a low pass filter might be best here.

This control circuitry is most likely DC and the best solution might be to increase the capacitance in its DC supply to lengthen its time constant.

Both these areas could be at fault, but my guess is that it is the motor driver.

A scope would allow you to look at the generator output and see what it looks like.

.
 
drcampbell said:
Next guess would be a ferroresonant transformer.
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Just don't plug a ferro transformer into an inverter or inverter generator. I still remember well warning a more senior coworker against plugging a Sola constant voltage transformer into an inverter. But he insisted, and soon we were both stepping back because somehow it caused all of the smoke to leak out of the transistors. ;)
 
In addition to the hazards associated with a ferro-resonant constant voltage transformer fed by a waveform with high THD, there is another problem which applies even to standard sine wave generators: The "stabilized" output voltage is frequency dependent. A 1% change in frequency will cause a 2% change in output voltage. Higher frequency (within limits) causes higher output voltage. It allows you to use a voltmeter to monitor generator frequency. :)

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Makes you wonder about the waveform from cheap standby generators like Junkerack. Most people have expensive electronics these days and that furnace control is another example. You would think that manufacturers would jump on the generator/inverter bandwagon. Money to be made there.

-Hal
 
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hbiss said:
Makes you wonder about the waveform from cheap standby generators like Junkerack. Most people have expensive electronics these days and that furnace control is another example. You would think that manufacturers would jump on the generator/inverter bandwagon. Money to be made there.

-Hal
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Um, they have. Prices on inverter generators have come way down in recent years especially with Harbor Freight now a big player in the generator game.
 
190695-2105 EDT

jes25:

Thinking about your problem my thought is that experiments are in order. The best starting place would in your shop. What I describe as a bench test where you have tools and equipment to test the system.

If we assume the problenm is localized in a portion of the furnace system, then how do we isolate that one area.

First, I would separate the power input to the motor driver from the control circuitry.

Second, find an AC source on a different circuit than where the motor driver is driven. Connect the control to this circuit to minimize interference form the motor circuit. A good suggestion is one on the other AC phase.

Set up the generator to supply the motor driver. So on the loss of AC power to the motor driver the auto transfer switch changes to the generator and starts the generator. Does failure of the furnace occur at this time? If not connect an induction motor, like a refrigerator, to add starting current load to the generator. You can usually hear the generator momentarily bog down from this starting current. This produces both voltage and frequency dips. Possibly overshoot on recovery. Does this cause failure? Many trials may be necessary.

Do the same kind of test on the control circuit.

Repeat with both the control and motor circuits switching to generator.

If you can simulate the problem, then where does it appear to occur, and under what conditions?

These tests may have to be run in a customer's location, but harder to do.

When and if you can simulate the cause, then what is needed to prevent failure?

We are not where your problem is, and thus, we can only guess at possibilities.

.
 
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