As summer and storm season nears, I'm wanting to get prepared. I have a Firman 7kw portable gen that I hook up at the pole to power my house when the electricity goes out. It'll run everything just peachy (well, hot water heater, etc) but for some reason but when I try to turn on the AC it runs slow and has a whine to it - I'm referring to the unit outside with the compressor. This is not a matter of having too many loads as I do manage them by leaving the water heater off until it's needed. Voltage and frequency are fine too. Unloaded the freq starts out at around 62.5 but settles down to around 60 with a moderate load (I need to verify as my memory could be off). Same with voltage - around 250, but I believe it has a voltage regulator so it stays pretty close to its set point. What is the reason for the whine and slow running? I'd like to get it sorted out now even though I do have a 20kw stationary that I picked up a while ago - It needs some repairs and I have yet to tear into it but want to know what's going on in case I encounter this issue with the stationary too.
AC unit runs whines/poorly when powered off the portable generator
- Thread starter The_Eric
- Start date
is this an inverter type compressor or multi-stage?
Good question, from what I have seen from other, similar generators I'd guess that it's more of a chunky/blocky sine. I guess I could bring my lab scope home and take a look. After reading many posts from other members saying that their gennys run their AC units fine, I assumed that mine would too.
I don't know much about home AC, but I don't *think* that it's variable speed? The thing is OLD, new in '92 I think.
No clue? I don't know home AC to be honest. It's old though! Like installed around 1992.
if your unit uses either the above mentioned design it may just be portable power supplied by a generator is not clean enough for the compressor to function as intended. 1992 the condenser has gone the great distance in comfort and electrical sense if this puts additional stress and load on the compressor it could definitely shorten the remaining life left on the compressor. I don't know your level of comfort and multi-meters you have available for my nerdy self I would take (3) volt and amperage readings (1) shore power "utility sourced" (1) standby generator & (1) portable generator. compare these readings also when you're hooking up the portable generator where are you seeing the 250V at the generator itself or downstream at the ac condenser disconnect box? I ask this in case it is a distance away generator vs condenser you could be experiencing voltage drop most units can operate just under 200V around 196v depending on manufacturer specification keep this is mind with an older compressor the tolerance window is smaller. it's common for hard start kits to be installed on older units nearing end of life. you could also look at installing a soft start kit but wouldn't be cost effective on an older unit. from my experience I feel its age of the unit and generator old units at LRA can easily beat down a generator under 10KW.
Last edited:
dnewton3
Staff member
It's likely you don't have nearly enough power (7kw) to properly start the "old" (1992) AC unit. Highly unlikely a unit that old is VSD. It would be very helpful if you took some photos of the unit, including the data plate, to assist in deciphering your needs. And is the Firman 7kw at max, or rated load?
Also - I'm a bit concerned about the "hook up at the pole" comment you made. Do you have a means to isolate the home from the incoming power line before powering up the generator? Please don't backfeed the power upstream; utterly dangerous.
Also - I'm a bit concerned about the "hook up at the pole" comment you made. Do you have a means to isolate the home from the incoming power line before powering up the generator? Please don't backfeed the power upstream; utterly dangerous.
I could probably piece together more than enough equipment to be pretty dangerous 
I have a couple of quality meters, an automotive based lab scope and a Kill-a-watt
The genny has its own readout on voltage and freq and I've verified it correct with the meter and the kill-a-watt (freq). I'm also fairly certain that I checked the voltage at the disco box for the compressor and it was basically the same as the display on the genny (within a volt).
As for running the unit, no, I definitely don't. I tried it once with a borrowed genny and it didn't sound happy, then I got my current portable and it too is doing something that the AC compressor doesn't like, so I ixnayed it. The genny will run the squirrel cage in the inside unit just fine in the winter though. That said, I'd like to know why so if I do lose power in the summer I can run it if possible to make sleeping easier.
I have a couple of quality meters, an automotive based lab scope and a Kill-a-watt
The genny has its own readout on voltage and freq and I've verified it correct with the meter and the kill-a-watt (freq). I'm also fairly certain that I checked the voltage at the disco box for the compressor and it was basically the same as the display on the genny (within a volt).
As for running the unit, no, I definitely don't. I tried it once with a borrowed genny and it didn't sound happy, then I got my current portable and it too is doing something that the AC compressor doesn't like, so I ixnayed it. The genny will run the squirrel cage in the inside unit just fine in the winter though. That said, I'd like to know why so if I do lose power in the summer I can run it if possible to make sleeping easier.
It is the square wave that your generator puts out.
No back feeding is happening.. I respect the linemen out there busting their asses for me way too much!
The unit is a TO7571 I believe with 9500 starting/7500 running. I'll see about getting some nameplate info later
Yup, anything that's not a perfect sine wave makes the compressor fight itself, so it'll run hot and sound angry. Even a conventional single-cylinder, 4-stroke 3600 RPM generator will make your stuff sound slightly unhappy because the non-power stroke of the gas engine still has the load calling for power, so it slows it down slightly. A 2 cylinder V-twin is better but of course as dumb as a harley because the power strokes aren't even. A heavy flywheel or lighter electrical load is better.
That makes sense I suppose. All the loads in my house are 120v, except for the dryer and water heater and those are just resistive loads. The exception to that is my well pump, but it is run from a VFD style controller so I suppose it's less of an issue? But that makes me wonder how other forum members can run their ACs? Little doubt that theirs are newer ones - are they less particular?
Hopefully my 20kw enclosed standby will make clean enough power?
Yes, generac/kohler/cummins output pure sinewave power.
I run my HVAC using one of these.
https://a.co/d/03mQ80Ub
Also if the appliance (hvac, water heater, etc) are 220, they'll run really poorly on 110. You'd need the appropriate 220 circuit on the transfer switch.
https://a.co/d/03mQ80Ub
Also if the appliance (hvac, water heater, etc) are 220, they'll run really poorly on 110. You'd need the appropriate 220 circuit on the transfer switch.
This. While my AC is quite happy running on the generator, the microwave definitely isn't.
4wd
$50 site donor 2026
I would research the VFD question more and not assume anything on that - they are also sensitive to input …
Yeah, if I heard that coming from my compressor I would kill the power and say a prayer that I cut power off fast enough.
Maybe an overstatement but ...
You most likely have a non-sinusoidal output on the generator, and its causing the issues. Non-sinusoidal current would cause excessive current flow through the run capacitor, as well as through the start capacitor. The motor its self will also have a counter rotating torque if the output waveform is bad enough that will cause a slow down, some vibrations and excessive heating of the motor. One of the clearest ways to see this is to get a cheap inverter (Modified sine wave...basically just a offset pulse DC to be honest), and power a cheap motor with it, it'll run slow, hotter and noiser.
Edit: For anyone that wants to get to know more about the topic look up Voltage Harmonics, the ones that produce the counter rotating magnetic field are typically: 2, 5, 8, 11, 14,17, etc... Triplen Harmonics just increase heating but don't produce the counter rotating torque: 3, 6, 9, 12, 15, 18, etc...
Edit: For anyone that wants to get to know more about the topic look up Voltage Harmonics, the ones that produce the counter rotating magnetic field are typically: 2, 5, 8, 11, 14,17, etc... Triplen Harmonics just increase heating but don't produce the counter rotating torque: 3, 6, 9, 12, 15, 18, etc...
Reading many posts in here on generators, is it safe to say that inverters do not have the same issues running appliances and electronics that generators do??
Depends on the inverter. A really good "Pure sine wave" inverter shouldn't experience the issue, if its well designed. I have seen cheap filter stages on these inverters at my old work hit a resonance point ( "Poles and zeros") that destroyed the connected equipment, but that is a odd ball, only ever seen that happen once from a bad filter design.
