Soft Starting for air conditioners with Micro-Air

[rgano]

My smallish vessel has a mere 3.5 KW Nextgen generator (max current capacity of 31 Amps), but it also sports two Dometic air conditioners, a 13.5 KBtu rooftop unit air-to-air and a brand new water-cooled Dometic 10 KBtu Turbo Unit (replacing the rusted out 7 KBtu Dometic unit).

Between them, the two AC units have a combined locked rotor amperage of 103 Amps, one 62 and the other 41 Amps. The combined running amperage is about half of the max Amps of the generator, and the generator can start and run either. I have never tried to run them both, mostly because the generator, probably being carboned up from historic underloading, seemed to choke on anything more than one AC, if I could even get it started. The generator is currently at the manufacturer for a rebuild.

When it returns to the boat, I am going to want to run both ACs on the generator, and while it is an extremely low probability that both these units would try to start their compressors simultaneously, I worry that if the larger unit is running at its 9-Amp continuous draw, and the smaller one wants its 41-Amp LRA to get going, the generator breaker would be overloaded.

I did some research on soft starting equipment for the two ACs and found that these units can reduce compressor LRA by 60-70 percent, making it possible for me to power both my ACs with my little generator. I also talked with a guy who uses a Dometic Smart Start unit in combination with a 2 KW Honda portable generator to run his 16 KBtu marine AC.

Dometic sells their "Smart Start" unit for over 500 bucks. I would need two. When I queried Dometic about using one on my RV style rooftop AC, I got back the response that the Smart Start is only suitable for their Marine ACs. I doubt that.

More research turned up an "Easy Start" unit made by a company called Micro-Air. As far as I could tell from the literature and from watching a Youtube video of them installing and testing one a rooftop AC with the same capacity as mine, the Smart Start and the Easy Start do the same thing, dramatically reduce compressor LRA, and the Easy Start is priced at about 300 bucks. Micro-Air will also sell you a circuit board-only unit for about half that price. NOW I was starting to take more interest since all I would need would be a 12-dollar capacitor and a watertight box for each board in order to make the boards ready for installation. The four wires necessary to connect the boards to the ACs connect to the boards with push-on spade connectors.

Cutting to the chase, I found a fellow selling two new circuit boards he decided he did not need for a very attractive price, and I finished installing them today. While I will have to wait to run them on the generator, I can already tell from the reduced compressor turn-on noise that the units are very softly starting.

Throughout this whole process, I was in contact with Micro-Air to verify various and sundry questions this electrical novice tends to generate, and I got prompt email answers day and night, Sundays included, which included requested connection diagrams for both of my specific ACs, even though there are diagrams for many on their website. I simply cannot say enough about Micro-Air support.

 

[DavidM]

I am surprised at the running current of your two A/Cs much less the starting current. I have a 12,000 btu marine A/C running on a NextGen 3.5 KW unit. It starts fine and runs at about 12 amps. It would seem that another roof top RV unit should draw about 14 amps and this would put an excessive continuous load on the NextGen.

The Micro-Air gets mixed reports from the RV world, some reporting that with one installed a Honda 2000 won't start a 13.5 btu roof top unit.

I am surprised that Dometic says their Smart Start won't work with other brands. When they first came out they advertised that it would.

But in any case, I think trying to run two A/Cs with the NextGen 3.5 is looking for trouble.

David

 

[rgano]

If the rebuilt Nextgen's exhaust exhibits a tendency to smoke while running the two of them, we'll know about overloading, but the new10 KBtu unit is a new tech by Dometic and should have low running amps. I have looked at the nameplate data on the two ACs and don't see that 15-18 running Amps is excessive.

 

[HopCar]

I had a similar problem many years ago and solved it with just a Supco Hard Start Capacitor for less than twenty bucks.

I was trying to run a 5K btu unit with an LRA of 28 on a Honda 2000i. Running load was about 600 watts but the starting load was over 3000 watts.

After installing the capacitor, I could start and run it with a Honda 1000i.

I recently had a conversation with a fellow at Practical Sailor Magazine. I suggested they do an article on how big a generator you need to run an air conditioner. He seemed to think it was a good idea.

 

[C lectric]

Soft start often means reducing voltage and then ramping it up to get a motor turning. Or it could be PWM meaning a full voltage but short pulses of increasing duration to develop full torque Often refrigeration units are under full or near full load from the instant they start meaning a soft start won't get them rolling untill the amperage is fairly high. Sometimes work and sometimes not.

That grumble may not be the motor turning but the motor stalled. They do make a noise if they are energized but cannot rotate because they can’t get enough power to do so.

Often touted as an answer is a "Hard Start Capacitor" for units intended to be started on barely adequate for the inrush current generators. They do work. THese bigger capacitors create a stronger torque instantly, almost, to get that motor moving by increasing the phase offset between the run and the start windings. The stronger the torque the faster the motor will start thus getting out of the very high inrush current part of the startup much more quickly.

That will have a huge bearing on what the breaker does. A very high overload of very short duration can often be handled although it may cause the generator itself to bog somewhat. As long as the high inrush is fast enough your A/C and gen. MAY be ok.

Just watch the total current output as I'll bet it shouldn't be more than 75% to 80% of the generator's output capacity. Find out what the generator continuous rating is as it is often less than the so called size name would indicate.

 

[FF]

Your friendly RV dealer has a set of relays that will stop a second unit from starting until 2 or 3 min after one has started.


The delay allows the refrigerant head pressure to drop easing the start load.

 

[rgano]

The Nextgen 3.5 Kw generator in question here has an electrical end capable of 51 Amps. The limit of 3.5 KW is because of the capacity of the Kubota single cylinder diesel running it. So running the unit at even 3000 Watts is not an electrical end issue according to the conversation I had with Nextgen recently. I'd much rather wear the Kubota out for overload than go through the failure I have had with it due to under loading after only 220 hours of use (I DID NOT Own IT FOR MOST OF THAT TIME).

Cheaper hard start capacitors are NOT a substitute for soft start units like the Easy Start because they still have the high-Amp start, which soft start units significantly reduce. Hard start units have applicability in some marginal installations, but not in this case.

I have cruised the RV forums and found great praise for the Easy Start. One thing I learned is that those folks are concerned with higher altitude reducing the capacity of their small portable generators, and soft starting helps them a lot.

 

[sdowney717]

Soft start often means reducing voltage and then ramping it up to get a motor turning. Or it could be PWM meaning a full voltage but short pulses of increasing duration to develop full torque Often refrigeration units are under full or near full load from the instant they start meaning a soft start won't get them rolling untill the amperage is fairly high. Sometimes work and sometimes not.

That grumble may not be the motor turning but the motor stalled. They do make a noise if they are energized but cannot rotate because they can’t get enough power to do so.

Often touted as an answer is a "Hard Start Capacitor" for units intended to be started on barely adequate for the inrush current generators. They do work. THese bigger capacitors create a stronger torque instantly, almost, to get that motor moving by increasing the phase offset between the run and the start windings. The stronger the torque the faster the motor will start thus getting out of the very high inrush current part of the startup much more quickly.

That will have a huge bearing on what the breaker does. A very high overload of very short duration can often be handled although it may cause the generator itself to bog somewhat. As long as the high inrush is fast enough your A/C and gen. MAY be ok.

Just watch the total current output as I'll bet it shouldn't be more than 75% to 80% of the generator's output capacity. Find out what the generator continuous rating is as it is often less than the so called size name would indicate.

I contacted Tecumseh about about my Cruisair system and gave them the compressor number

He said a range of start capacitors were specced for the model number, my plan is if it goes out, I will put in the 88-108 mfd to reduce the amp draw at start the most it can bear.

We have several starting capacitors released for this model.
 
21-25 mfd
47-56 mfd
72-88 mfd
88-108 mfd

So most likely a higher start capacitor can be used, if the unit came with one, or add the hard start kit to do same thing.

Subject, correct value for run capacitor compressor # AK147AT-009
1
Lattuca, John <xxxxx@tecumseh.com>
Wed 6/25/2014, 12:00 PM
You
Inbox
Scott:

Regarding your question on the range of starting capacitors, we sometimes run tests per customer starting requirements, which results in the release of various microfarad ratings.

Your compressor should start easiest with the largest rating possible (i.e. 88-108mfd/330V).

Normally, the acceptable tolerance on run capacitor ratings is +/- 10% (i.e. 31.5 mfd to 38.5 mfd).

If the compressor motor is optimized using the 35 mfd run capacitor, using a 30 mfd run capacitor should not change the compressor amps much.

Use the 35 mfd run capacitor and the 88-108 mfd starting capacitor to maximize starting ability.

The approved 60Hz voltage range for your compressor is 115V +/- 10% (i.e. 103.5V to 126.5V).

If you have additional questions, please contact us.


Best Regards, John

Scott Downey
Thanks for that info.
It currently has a 30 mfd run and need a new one because it popped.
So I will get a 35 mfd

It has a 47 to 56 start right now and it starts well before it blew the run cap.

Why such a big range on start caps?
Which one will make it start the easiest?

I read a lower value run cap makes the compressor draw more amps.
I wonder why it had a GE 30 mfd in there. It is old run capacitor very tall.

Numbers on it are
GE
72F5046
30uf 370v
6577651

What combination of capacitors do you think would be best on a marina's marginal power? Or it makes no difference?
Sometimes the voltage drops below 110.

Lattuca, John <xxxxx@tecumseh.com>
Tue 6/24/2014, 4:24 PM
Download Save to OneDrive - Personal
Scott:

The run capacitor is 35mfd/370V.

We have several starting capacitors released for this model.

21-25 mfd
47-56 mfd
72-88 mfd
88-108 mfd

The relay released for this model has a pick-up voltage of 170-180V.

Oil Charge: 17 ounces.

Oil Recharge: 13 ounces

If you have additional questions, please contact us.


Best Regards,

John Lattuca
Senior Engineer - Technical Specialist
Tecumseh Products Company
5683 Hines Drive
Ann Arbor, MI 48108
800.211.3427 toll free
734.585.9471 direct
734.352.3771 fax
xxxxx@tecumseh.com

 

[DavidM]

Rich:

It sounds like you are one of the few boat owners who had genset problems due to underloading. So, could you elaborate- symptoms, fix.

And i agree the NextGen genrrator end is stout. As long as you dont get reduced rpm and heavy black smoke under load, the Kubota should be fine.

David

 

[rgano]

When I owned a GB42 with an old Onan 7.5 KW genny, I tried to run the boat at least once every couple of weeks to thoroughly heat the engines and the generator. I usually ran the genny loaded up with the water heater and the stove in the years in San Diego before moving the boat to Florida where two 16 KBtu air conditioners were installed. Then I would usually just power on the two ACs. That worked well for 29 years.

I got this boat with the Nextgen three years ago, and from the git-go, the generator took some cranking to get going. I had documentation indicating the previous owner had had it pulled for "valve adjustment," and he told me earlier this year that after that event is started better but then began to get harder ad harder. Being unfamiliar with its quirks, I consulted with other owners.a mechanic, and even Nextgen, but I came away with inconclusive comments about how it should start. At one point I changed the glow plug, thinking that was the issue, but access being the difficult thing it is on this boat, working on the generator, which seldom was needed anyway, sort of put me off the thing. That and the nerve-wracking ages of grinding it took to get the thing running put me off.

When it did start, I put the then-7 KBtu air conditioner on it, but when I added the 50-Amp battery charger, even when derated to 25% of its output, the exhaust water began to look black along with darkened exhaust smoke. When I told Nextgen all about this on the phone, their comment was that it sounded like the unit had been consistently run under loaded and was all carboned up. The suggestion was that I light it off and run it at 3000 Watts of load for about four hours to burn it clear, but by that time I had disconnected it and pulled it out to ship to them, where it has been for a month awaiting their attention. I tried for the three previous months to get anybody in this town to come look at it.

Before I threw up my hands in disgust, I had hit the starter early this spring, and with a fully charged pair of AGM batteries behind the starter,the Kubota would not budge. Once I pulled it out, it took a lot of effort to roll it over, something Nextgen said should have been a relatively easy thing to do.

As an aside, I posted a year or two back about the failure of the POS 12-volt generator-end cooling fan which in turn burned up the wiring harness because the genny had been installed without a fuse to the fan. Oddly enough, the new harness came with a fuse, but I talked to Nextgen, and they agreed with my plan to replace the fan with a 110-Volt inline fan with brushless motor and a 30-second delay to avoid the starting voltage hash. The generator had to come out to allow me access to install the fan and replace the harness.

So, when I either get my Nextgen back or buy a new one, my plan is to run it with significant load, and thus was generated the idea of soft starting the two AC units to enable me to run them both with the generator, something Nextgen agreed would be OK to do given their running Amp draws.