Looking for assistance with a soft starter

[Pedro5000]

Hi, new user here.  As the subject line says, looking for help with a soft starter for a project.  Not to worry, I'm no bot and I'm not here to spread delicious breakfast meat around the forum, this is a legitimate request for assistance.   

My problem is this - I am working on an automotive project that will require that I either move the air conditioner to a non-standard location or do without.  The only thing I'm going to have room for on the front of the engine is a high output alternator slung low on the passenger side as the rest of the area between the engine and radiator will be taken up with a couple of turbochargers and the required piping to run them.  This is the compressor I plan to use, and the link goes to a page where it shows a compressor for sale.  I've also already verified that this compressor will provide enough cooling to keep me happy.  I just need to work out this one problem.

https://www.vmacs.net/products/12vdc-electric-ac-compressor-for-e-6100-e-9725-units-rd-2-7777-0p

My problem is an AC compressor cycles regularly, and this particular one pulls 100 amps to operate.  As you can imagine, a 100 amp load on a 12V system can play some havoc with everything else on there.  The other thing, I'm not sure if there will be an inrush current involved on this.  I've been searching all over the place, and every time inrush current is discussed it's always on AC stuff, not DC. 

So, what I'd like to know is, do I have to worry about a current inrush problem here?  And, can anyone recommend a soft starter for this unit that works automatically and isn't huge?  I don't need anything fancy, just something that will start the compressor without blowing a fuse or causing the rest of my system to take a dump.  A 1-2 second ramp-up speed would probably work fine for this, and I'm not concerned with shut-down as the battery should be able to absorb the load.  Of course, if anyone can recommend a 12VDC R134a compressor that pulls less juice while providing at least 12K BTU of cooling power I'm all ears.  I've been researching this issue for quite a while and so far this has been my best find.

Thanks in advance for any assistance.

[fourtytwo42]

Hi Pedro, welcome  IMOP the compressor may not start at all with any kind of soft start as this will reduce the starting motor torque that I assume is required to overcome high pressure and/or inertia. Supplying inrush is exactly what the car battery is there for. To minimise inteferance with other systems connect the compressor with it's own wiring and fuse as close as practicable to the battery terminals, avoid sharing other vehicle wiring. Hope that helps 

[Digorion]

Hello!
For clarity, what type of alternator you want to use with this, and what type of battery? And what kind of vehicle? 100 amperes constantly is a very high load for a 12v vehicle electrical system, especially when idling. This will most likely be your main problem, not inrush current.

[Gregg]

An unloader valve would be far superior to a soft start.  A properly designed automotive electric AC compressor should already have this feature. 
Basically an unloader allows the motor to reach full speed before it starts actually pumping the refrigerant.  It could be as simple as a centrifugal mechanical link that keeps one of the check valves open or it could be some sort of bypass valve.  With little load, the motor will spin up quickly negating any need for a soft start, it should be less taxing on the battery than the engine start motor.
However the AC compressor running without the alternator maintaining the battery, the battery will be flat in a hurry.  Suppliers of automotive electric AC compressors probably have an off the shelf solution.

[Pedro5000]

Thanks, all.  The truck is a small Dodge pickup, and it will be getting a turbocharged V8.  I will also be running an alternator sufficient to feed everything at idle, something in the 350 to 400 amp range.  I'll have room for the alternator, just not any other accessories.  Also, this isn't intended as a no-idle air conditioning system, so I don't have to worry about battery drainage due to the engine being off.

The manufacturer finally got back to me and advised that the 100A is for their entire system, and the compressor only pulls 60A, which helps a lot.  They also said that it won't tolerate any sort of soft starter, so I'm thinking that you two are on the right track now.  I'll probably set it up to run a dual battery setup wired in parallel.  Anyone know of a way to isolate the HVAC compressor from the rest of the truck's electrical system without affecting battery charging or causing an uneven electrical discharge, or am I worrying about nothing here?  The compressor will be mounted in the bed next to the battery (or batteries.)

[Gregg]

A continuous duty solenoid switch like this will do the job: https://www.amazon.com/Cole-Hersee-24106-Grounded-Continuous/dp/B001FQL43U
Buy a genuine made in USA one if you want reliability.

[Pedro5000]

And my problem is SOLVED!  The manufacturer just now got back to me and advised that the on-board electronics include a 10 second soft start, which is why anything else in front of it would cause problems. 

Gregg, no longer needed since it's got the soft start built in, but I'd have been looking hard at using that otherwise.  10 seconds from 0A to 60A is practically no load at all.

I appreciate the responses folks, you are ladies and/or gentlemen, and scholars. 

[bdunham7]

My old Dodge Cummins diesel had an electric intake air heater that drew 175 amps and it would cycle when the engine was idling cold.  You would notice it, but it wasn't a big deal.  If you have dual group 27 batteries (regular ones, no fancy AGM or supercap crap, just good old lead/acid) and an alternator that big (why??) you should have absolutely no issue with a 60A cycling load as long as your low speed fuel map and idle control system are well hashed out.