Soft Starter for 5 ton AC Central Unit?

[Augusto Orosco]

I have a Generac 20Kw whole house generator. It has run very well since installed (18 months ago), including stretches of several days after power went off and with A/C running. But last week, the generator has not been able to start the central AC. It 's a 5ton Lenox 18 SEER unit (2 years old, model SL18xc1-060).

Electricians have checked both and everything is working as per specs... but the AC unit has an LRA rating of 152 amps; meaning that although it runs at 32amps or so (RLA?), the rush draw can be as high as 152. The HVAC technician came over and measured the loads, and indeed, on startup it's asking for 152amps, which is OK when PECO is on, but too much for the generator. I don't know why it was working before but it's not working now... wish I had measured the rush load when it was working.

Anyway, looking for solutions I came upon this:

https://www.microair.net/products/ea...230v-ac-motors

In theory, it seems to be the answer to my problem... but wondering if anyone here has any experience with a similar situation and with this soft starters (or any other suggestions).

Thanks!

[Brian Elfert]

I have the RV version of this product. I had problems with the A/C just turning off with this installed, but it turns out the Microair unit was turning the unit off due to low voltage. I had planned to put these devices on all three of the A/C units in my RV to hopefully stop issues with low voltage. It turns out my generator had bigger issues with low voltage that "might" have been fixed with $3000 in parts. The manufacturer claimed it was normal for the voltage from the generator to drop as low as 100 volts under moderate load. I ended up with a new diesel generator that is 50% larger and has voltage regulation that actually works. I can run all three A/C units with power to spare.

The portion of the product that really does the work is the capacitor. I would think you could find a product to do the same thing for less money unless you want the other intelligence like the low voltage shutdown. For one A/C unit the price is not bad. I spent a ton for three of them for three A/C units. I never installed two of them and should list them on Ebay as the new generator doesn't need them.

I would have no hesitation getting one for a home A/C unit so long as you are aware that they could lock out due to low voltage or similar issues. Low voltage lockout is really a good thing to stop damage to the compressor.

[Augusto Orosco]

The portion of the product that really does the work is the capacitor. I would think you could find a product to do the same thing for less money unless you want the other intelligence like the low voltage shutdown. For one A/C unit the price is not bad. I spent a ton for three of them for three A/C units. I never installed two of them and should list them on Ebay as the new generator doesn't need them.

I would have no hesitation getting one for a home A/C unit so long as you are aware that they could lock out due to low voltage or similar issues. Low voltage lockout is really a good thing to stop damage to the compressor.

Thanks, Brian. My generator seems to be able to handle voltage fairly well under normal load (we measured the output with all the normal stuff minus the AC, and it was providing a very stable voltage (the usual 110 per leg, 220-240 total) at 60Hz cycle (also quite stable). Hopefully that's not an issue here; as I feel once the initial rush is controlled, the 100amp generator should have no issues handling the load and voltage.

I don't know much about this stuff, so I am just trying to paraphrase things... but I believe AC units (at least mine) already have a capacitor. So, this soft start piece equipment acts as a second capacitor?

[Brian Elfert]

Yes, there is a second capacitor in these units.

Generators are not supposed to lose voltage as the load increases. The voltage regulator is designed to keep the voltage constant as load increases or decreases. In my case, the generator seemed to have a issue with low voltage as load increased. I replaced the voltage regulator at least three times at $300+ per time. (There was no inexpensive generic regulator that would work.)

[Bill Dufour]

The other option is a "hard start kit". It is an extra start capacitor and a relay that drops it out of the circuit after things are running. Much cheaper then a soft starter. Like under $35, plus installation.
Bill D.

[Doug Dawson]

The other option is a "hard start kit". It is an extra start capacitor and a relay that drops it out of the circuit after things are running. Much cheaper then a soft starter. Like under $35, plus installation.
Bill D.

My experience with hard start kits for a/c systems is that they are put in almost immediately before the system tries to explode and burn your house down, not to prevent that but to ease it along.

[Augusto Orosco]

The other option is a "hard start kit". It is an extra start capacitor and a relay that drops it out of the circuit after things are running. Much cheaper then a soft starter. Like under $35, plus installation.
Bill D.

Thanks Bill... the HVAC tech initially considered it, but said the "hard start" would not help, because the issue is that the generator was not able to produce enough power on start. And we tried with everything but the furnace and the AC on.

To Brian's point, though (regarding generators shouldn't drop voltage)... they did read that the voltage dropped from 240v to 170 when the AC requested all those amps to start; but I thought it made sense as the Generator is just 22K and rated for 100amps? Otherwise, with everything running, except the AC, voltage supplied is quite stable.

[Andrew Hughes]

Your situation sounds a lot like what I’ve gone through with my electrical service. After adding a 10 hp rpc to start a 3ph motors I saw voltage dips that dimmed the light in my house and neighbors house down stream of me.
What I had to accept was I have soft service. Mostly due to a old neighborhood I live in.
I did get my utility Edison to add a transformer and it did help a lot.
Here some calculations I did some time back that mostly applies to me.
Size the single phase supply (cord wiring, contactor) upstream from the rpc according to the hp rating of simultaneous client motors. Use NEC table 430.250 to get the ampere value for each simultaneous load motor, based on its HP. Multiply the sum by 250%. Size conductors and contactors for this number. Example: simultaneous 7.5 and 5hp load motors, 230v, the entries are 15.2 and 22 amps. Sum 37.2, x2.5 =93 amps. Yow. But doesn’t need to be a NEMA size 3 or 4 contactor. Just needs to be sized for 93+ inductive amps.
Good Luck

[Brian Elfert]

To Brian's point, though (regarding generators shouldn't drop voltage)... they did read that the voltage dropped from 240v to 170 when the AC requested all those amps to start; but I thought it made sense as the Generator is just 22K and rated for 100amps? Otherwise, with everything running, except the AC, voltage supplied is quite stable.

A generator should not drop voltage when run at a typical load. If a generator is overloaded by 50% I would not be surprised by a voltage drop. The voltage regulator can call for more voltage, but there there is nothing more the generator head can supply.

I have a 10K Kohler automatic standby generator at my house in addition to the generator in my RV. The Kohler will start my central A/C unit just fine, but my A/C unit has an FLA of about 1/2 of yours. No idea what the LRA is. I have not tested to see if voltage drops when starting the central A/C.

[Jim Becker]

Wow...I have the same generator at our old property (actually, it's the 22kva version) and it cheerfully starts and runs two 3 ton HVAC systems with no issues. That unit you have really has a startup draw!

Did you ask your HVAC resource if there is a "soft start" solution available and compatible with your system? I cannot imagine you are the only one with this kind of challenge.

[Augusto Orosco]

A generator should not drop voltage when run at a typical load. If a generator is overloaded by 50% I would not be surprised by a voltage drop. The voltage regulator can call for more voltage, but there there is nothing more the generator head can supply.

I have a 10K Kohler automatic standby generator at my house in addition to the generator in my RV. The Kohler will start my central A/C unit just fine, but my A/C unit has an FLA of about 1/2 of yours. No idea what the LRA is. I have not tested to see if voltage drops when starting the central A/C.

Txs, makes sense!

Wow...I have the same generator at our old property (actually, it's the 22kva version) and it cheerfully starts and runs two 3 ton HVAC systems with no issues. That unit you have really has a startup draw!

Did you ask your HVAC resource if there is a "soft start" solution available and compatible with your system? I cannot imagine you are the only one with this kind of challenge.

The link I posted is to a soft start solution that is in theory good for up to 6ton units, and can reduce the rush draw as much as 1/5 of the LRA. But I am contacting both the General reps and my HVAC company to get input from them. It’s still a puzzle to me why it worked fine for more than a year and it suddenly can’t take the AC load anymore. The only new issue was the yearly routine maintenance on the generator a couple of weeks ago, but that mostly involved oil change and the usuals. Of course I mentioned that to the electrician, so he checked everything again with me present (oil levels, voltage and Hz readings , etc).

Thanks for the comments!

[Mike Soaper]

I'd consider having the hvac guys check that the compressor start and run capacitors are within spec. , and also check the contactor's contacts and start relay (if there is one)

[Patrick Grady]

If I follow the sequence of your AC start fault correctly, one suspicious variable seems to be maintenance performed on your Generac. Before I invested time and money in alterations to the Lenox AC start system, I would first focus on the Generac motor. Did the recent maintenance include adjustment on the Generac motor valves? The AC start is pushing the the Generac to its upper limits where exactly correct motor valve adjustment is required to get the motor's max efficiency. I would explain all of your research efforts to the Generac contractor and ask them to return and perform another check/adjustment to the valves. That may be a simple and inexpensive answer to this interesting 'who done it' mystery.

[Tom M King]

My money is on the start capacitor getting a little tired. I've had great luck, and longevity from the Rectorseal Kick Start ones.

[Frank Pratt]

The manufacturer claimed it was normal for the voltage from the generator to drop as low as 100 volts under moderate load. I ended up with a new diesel generator that is 50% larger and has voltage regulation that actually works. I can run all three A/C units with power to spare.
.

It definitely is not normal for a generator to drop that low during moderate loads. You should only see the voltage drop slightly as it approaches full load.

Something you need to be aware of with diesel generators is that they need to run a near full load a good amount of time. Otherwise the exhaust system will gradually clog from particulates sticking to the exhaust pipe because the exhaust temp is not high enough to keep them burned off. I've seen it happen many times.