Anyone installed a Soft-Start kit for a compressor?

Lloyd-ss · Aug 6, 2024

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.

GreenTwin · Aug 6, 2024

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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GreenTwin · Aug 6, 2024

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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GreenTwin · Aug 6, 2024

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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ddmckee54 · 2024-12-15T21:31:54-0500

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.

Nikhil Bhale · 2024-12-16T01:19:55-0500

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

Lloyd-ss · 2024-12-16T01:42:28-0500

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

Anyone installed a Soft-Start kit for a compressor?

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