Anyone installed a Soft-Start kit for a compressor?

I know that the starting current (inrush?) for a compressor (air, heat pump, AC, etc) can be several times greater than the run amps and even the locked rotor amps. I am trying to size a new portable backup generator but am having a hard time justifying one large enough to start any compressors or machinery motors. The soft-start kits I have seen advertised are supposed to control/moderate the starting current so that a portable generator can safely start the motor. Anyone know anything about these. Have you installed one? Is there a risk to the motor it is installed on? They seem to come in various price-ranges, too.
In particular, I need something for my geothermal heatpump. The compressor is about to go out of warranty so I am not concerned with voiding the mfg warranty.
Any and all help would be greatly appreciated. Thank you.
Lloyd
P.S. Just regular 250v single phase.

I think a typical induction motor will draw six times normal current when starting.
Soft starting would no doubt reduce this, but you still need to develop enough torque to get the motor up to speed.
I have one plant that starts 5,000 hp motors across-the-line, and they have to telephone the power company to temporarily adjust the instantaneous trip of their breakers when these motors are starting.

When I specify a generator, I have the supplier run a starting program, with worst-case on the motor starting.
Generally, in industry, where possible, the load is shed via various options when starting a large motor.
Air compressors have a pressure relief valve to do the same.

I need to research 240 volt soft starters to give any kind of real answer.
Solid state electronics connected to a generator can require it to be derated, so you may or may not achieve anything using a soft starter.
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A white paper summary with some good info.
There are a number of parameters to consider.

https://www.eaton.com/content/dam/e...-soft-starter-generator-power-ap03902013e.pdf

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Another link.

https://norwall.com/products/generac-sure-start-16-32-fla-soft-start-kit-8008/

I personally would opt for a larger generator, with across the line starting, since sooner or later the electronics in a soft starter will fail.
Solid state devices are very prone to failure during overvoltages caused by lightning strikes, so if you do use a solid state starter, you may want to also use a good surge suppressor.

A "good" surge suppressor is not the one with the highest amp rating, but rather the one that has the best let-thru voltage clamping.
It is the let-thru voltage that spikes electronics and causes failures.
This is the product I use at home, and work municipal projects; it has the best track record in the industry, and is used extensively in Florida, with is the "lightning capital of the world". This product really works, and is small; about the size of a brick.

https://www.surgesuppression.com/

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I was an electrical engineer in a plant where we had 9 compressors running on soft-starts. They ranged in size from 60 to 900 horsepower. We only had failure problems with the 900 Hp soft-start, but the electronics didn't fail - the disconnect failed twice in under 10 years.

How big of a compressor are you trying to start? 50 years ago when I was still a kid on the farm, we'd run our entire dairy operation from a 20KW PTO driven alternator if the power was off. That included a pair of 5 HP motors for the silage equipment, the vacuum pump for the milkers, and the refrigeration compressor for 900 gallon the bulk milk tank. All of that stuff started across the line.

Like GreenTwin said all induction motors will pull about 6 times their full load amps when starting. But that quickly drops off as the motor starts to come up to speed. Before/When I get my big shop air compressor back on-line, I plan on installing an unloader circuit to vent the compressor output to atmosphere for 10-15 seconds while the motor gets up to speed when starting.

Does your compressor have an unloader valve? Or can you fix one to it?
Unloader valve is kept open during compressor starting. This way the motor can run up to the speed with no load.
Then unloader valve is closed and compressor starts pumping.

Regards
Nikhil

Thanks @ddmckee54 and @Nikhil Bhale
Well, my application is way below 900HP. Its for a Geostar Aston geothermal 4 ton heat pump. There are two 260 foot vertical wells with a closed loop coolant system. Normal temp of the coolant loop is about 55 deg F with a 4 or 5 degree temp rise (fall) across the unit depending on the season. I just took the time to look carefully at the name plate on the system, and look the unit up on the GeoStar site, and it is listed as 21.1 RLA and 104.0 LRA. But I noticed that it had a foot note on the nameplate that said 37.0 LRA with "GeoStart". It did not come with the GeoStart option but I found it on the GeoStar website and the unit looks just like this Micro-Air Easy Start unit for about $350. When I get a new back-up generator it could be of a reasonable size and price if the soft start was installed because that is the only big starting load I have. The Micro-Air soft start seems to be a good unit. The system has just gone out of warranty (but I do have a seasonal check done twice a year), so I could install the soft start myself. The same service tech has been coming here for 10 years and he told me what steam humidifier to buy and install on the heat pump (which I did) so I will ask him about the Micro-Air unit when he is here for next the next visit. He understands about saving money with DIY, LOL.

https://invertersrus.com/product/mi...qaXPMTV8PPrveXmgUm1mpAWfewEBQheBoCRKMQAvD_BwE

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Lloyd-ss said:

I know that the starting current (inrush?) for a compressor (air, heat pump, AC, etc) can be several times greater than the run amps and even the locked rotor amps. I am trying to size a new portable backup generator but am having a hard time justifying one large enough to start any compressors or machinery motors. The soft-start kits I have seen advertised are supposed to control/moderate the starting current so that a portable generator can safely start the motor. Anyone know anything about these. Have you installed one? Is there a risk to the motor it is installed on? They seem to come in various price-ranges, too.
In particular, I need something for my geothermal heatpump. The compressor is about to go out of warranty so I am not concerned with voiding the mfg warranty.
Any and all help would be greatly appreciated. Thank you.
Lloyd
P.S. Just regular 250v single phase.

Click to expand...

Soft start systems generally are applied to three phase systems. They are also used with systems that can unload while starting. Not so sure it will help on a single phase system. When its all said and done the incremental cost of a larger generator is in the prime mover and that is where I would put the dollars rather then modify to accommodate a smaller generator. The three phase system allows ramping to running rpm via a variable frequency drive not sure how you do that in single phase. One of the issues with compressors in AC systems that they are generally loaded on start

Nikhil Bhale said:

Does your compressor have an unloader valve? Or can you fix one to it?
Unloader valve is kept open during compressor starting. This way the motor can run up to the speed with no load.
Then unloader valve is closed and compressor starts pumping

Click to expand...

Most single phase machines have an unloader bottle - a reservoir that exhausts when it reaches pressure. Sometimes this reservoir is just the volume of the discharge pipe between the compressor and check valve entry to the pressure vessel.
This volume discharges when the motor is switched off by the automatic pressure control switch - that's the hiss you hear each time the motor stops.
So when the machine restarts it's pumping into a (small) zero pressure reservoir to help ease the motor loading - however this small volume builds pressure before the motor is fully up to speed.
I had to increase the size of my unloading bottle to get the machine to start without tripping.
Prior to that I fitted a manual valve which I opened - allowing the compressor to exhaust until it was fully up to speed - I then manually closed the valve.
But that didn't help on auto-restarting. So I fitted a bigger reservoir.
https://www.homemodelenginemachinist.com/threads/small-compressor-stall-stopper.23906/#post-258384
Might work for you.
Regards Ken I

A soft start is sorta like a variable freq. drive - at least at first. The ones that I am familiar with have 2 modes of operation. The ramp mode will ramp up the motor speed at a fixed rate until they get to full speed, or they hit the time limit, then they pull in a contactor and become an across the line starter.

Or they use the current limiting mode, where they will ramp up the speed as fast as they can until the motor current hits a maximum level. Once the current hits the maximum they will hold at that speed until the current drops, then start ramping up the speed again. Once they get up to speed, or hit the time limit, the across the line contactor pulls in. It sounds like the GeoStart is using the current limiting method, since they are specifying a maximum Locked Rotor Amps with the GeoStart of 37.0 amps - as opposed to the normal 104.0 amps. It looks like they are limiting the maximum motor current to about 175% of the Running Load Amps.

ddmckee54 said:

Like GreenTwin said all induction motors will pull about 6 times their full load amps when starting. But that quickly drops off as the motor starts to come up to speed.

Click to expand...

I'm an electrical Neanderthal. I had no idea it was this high (6 * FLA). I struggled with this on my new Bridgepoint style mill tapping out the breaker on start. I didn't even consider my 220V circuit was only 15A. Ended up running a new dedicated line at high amp duty & rewiring the panel to accommodate. Sorry for intrusion but a few side questions

- I always assumed you rated the lines & breaker for FLA 'plus a bit' because the start surge was so brief. In fact my motor plate didn't even state FLA, just 'running amps'. If you had to increase the line duty to 6x, that would be considerably larger copper wouldn't it? So what is the appropriate line/breaker sizing procedure?

- my new Makita track saw (typical AC 120V, 12A) has this lovely 'soft start' feature. First I thought something was wrong. Now is this more of a controlled RPM ramp up program? (because there is virtually no load just spinning a blade).

I'm an electrical Neanderthal. I had no idea it was this high (6 * FLA). I struggled with this on my new Bridgepoint style mill tapping out the breaker on start. I didn't even consider my 220V circuit was only 15A. Ended up running a new dedicated line at high amp duty & rewiring the panel to accommodate.

Click to expand...

Most electrical breakers have a "curve" and other ratings you can look up from the manufacturers - typically to accommodate "inrush" for motors and such.

A 15A breaker will hold 15A all day but 20A load might take 20-30 minutes to get it to trip out and 30A might be 10 seconds - this is usually "thermal overload" a heated bi-metal strip causes the breaker to trip - this has thermal inertia and is slow to respond to overload.

A 15A breaker will typically handle 5 - 7 times its rating for a very short period but then a second overload device which is magnetically tripped kicks in more or less instantly.

You can select breakers that are deliberately "slow".

There is also a fault current rating like 10kA (yes you read that correctly ten thousand Amps or higher) - this is the fault current that it can open the circuit safely against - if you exceed the fault current rating the breaker simply self-destructs under fault (dead short conditions).

Assuming your panel is to code make sure a replacement breaker also has the existing fault current rating. Fault current depends on how large your supply transformer is and how much fault current reducing cable, fuses and such are between your board and source.

Example of breaker and a curve chart.

The blue area is the curve (range including manufacturing variance) so anything between 5 to 10 times the rating will cause a near instantaneous trip.
Three times rated current will trip in 10 seconds to 30 seconds (Minimum to Maximum - product variance).

Long answer to a short question - hope it helps.

Regards, Ken I

Lloyd, I might have gone off at a tangent - On re-reading you are trying to size a generator to drive a heat pump ?

You would need to see the tripping curve of the generator's circuit breaker compared to your heat pump's demand curve and not exceed that.

There is a "quick and dirty trick" to limit the inrush by including an in-line resistor or choke and by-pass relay on a timer.

If you are on a 15A x 5 = 75A limit at 220V = 2.93 Ohms. Resistor needs to only handle a tiny fraction of the wattage as long as it's for a very short time before bypassing it (I've done this as a cheap soft start on industrial transformers but it only has to work about 5 cycles) so you are probably better off winding a choke or using the low voltage side of a fairly heavy 220 - 12V transformer in series - use a manual switch as a bypass and anything you have to hand just to suck it and see.
Both of these methods simply limit the inrush and lead to a laboured start - which may or may not be appropriate.

I assume much of your inrush is getting the mass of recirculating water moving and you can't negotiate or fool the laws of physics.

That unit in the link is more like a high transient output inverter rather than a soft starter - which limits both the mains demand current and the maximum starting current (and a bunch of other safety features) but looks like a good solution to your problem.

I suspect your startup may be way longer than airconditioner surge.

Ken I

Each industry has their own "alphabet soup" of abbreviations that they love to use to make them feel special. The HVAC industry uses RLA to define the maximum load that the unit will see while running. I've always considered Full Load Amps and Running Load Amps to be equivalent. (Google and Wikipedia are your friends.)

Our guideline for fusing/breaker settings was 125% Full Load Amps, but you'll need to follow your local electrical code.

The trip curves for fuses/breakers is a whole 'nuther rabbit hole to go down. We wound up going down that rabbit hole when starting up that 900 Hp compressor. When we first tried starting the compressor we had the current limited to 200%, that's where the soft-start supplier recommended we start. Everything was going fine for about the first 30 seconds, we were pulling about 1800 amps just like we expected and the compressor was starting to come up to speed. Then the lights blinked and the compressor went quiet. For some reason we had tripped the breaker in the sub-station, it's a big plant with over 20 sub-stations in it. We tried two more times that day, increasing the current limit each time, then we hit the maximum number of start attempts the compressor manufacturer would allow in a 24 hour period. Every attempt ended at about 30 seconds with the load spiking to around 5100 amps. I did a LOT of searching through the fine print in the soft-start specs before I found the line that said the soft-start would pull in an across the line contactor after 30 seconds. That 30 second limit was hard coded into the soft-start firmware and there was nothing I could do about it. I got with the outfit that serviced the sub-stations and explained to them that I needed as many amps as they could give me for 30 seconds, if we weren't up to speed by then we never would be. They ran the numbers and told me they could give me about 3600 amps for 60 seconds, any more and they would be getting into the trip curve of the main breaker on the sub-station. The next day we upped the current limit to 400% and hit the compressor's start button. We threw 3600 amps at that compressor for almost 28 seconds before it was up to speed, but we got it running.

Gents, thanks for all the insight. I know enough about electricity to generally get the job done safely, but for "special circumstances" I always like a second opinion to add a cushion of comfort.

Yes, I just want to get a portable generator for backup that will safely handle the geothermal heat pump in both summer and winter. The generators are reasonably priced up to a max capacity of about 10kw (41 amps) running and 13kw (54 amps) start. I have propane from a 250gal tank plumbed to the current too-small generator. Home generators larger than 13kw start are too expensive for my budget. But the 10kw/13kw generator will easily handle the heatpump RLA of 21 amps. And with the MicroAir soft start mentioned in post #7, the 37amps startup would also be safe, whereas the standard 104 LRA of the heatpump compressor would likely be a disaster.

Because the heatpump manufacturer offers the referenced soft start kit as a factory option (maybe dealer installed) and the unit came with a 10 year parts warranty, I would think this is a quality soft start unit. My unit just went out of warranty so I think I will buy and install the MicroAir soft start myself. That way I won't be mystified if something doesn't work. I like to keep records and manuals for everything and be able to take over the maintenance and repairs when it makes economic sense. For example, my Samsung dishwasher (which I love) has a drain water-level circuit that gets crudded up every 2 years or so. Replacement parts would cost $150 each time, but I can clean it out thoroughly in about 2 hours for zero cost and it is good for another 2 years. And I know there are plenty more gents just like me on the forum.

So thank you all for the lessons on soft start options. I love learning new useful stuff like that. And BTW,there are 2 small 250watt circulator pumps for the coolant loop, so they don't add much load. It all seems to be the compressor. And also, the power might only go out 3 times year for more than 3 hours, so a big fancy auto-switch generator unit is not for me. I will go with the MicroAir soft start and a 10kw run generator.
Lloyd

I have a Microair soft start on a 2-1/2 ton heat pump. I installed it because I run off a solar power system most of the time and the start current of the heat pump was well beyond what the solar inverter could handle without tripping out. It worked as advertised BUT did not reduce the inrush current as much as they advertise, still a substantial reduction. It has been a few years but as I recall it cut the inrush starting current about in half. I used it for about a year then got a high efficiency mini split heat pump that consumes far less power and soft starts as well.

Your geothermal has a water circulating pump in addition to the refrigeration compressor. Air heat pumps do not have this water pump of course which is a additional load BUT usually a very low power circulating pump which would probably not be a problem.