Soft Start recommendation for Air Compressor 5

6A is extremely small to need a soft starter. Is there some other reason why you think you need one? Or is that a typo?

You must also more fully describe what you mean by running 2 off of one starter. Do you intend that they both come on and off simultaneously every time? That is the only way that would work, so why not just get a bigger compressor?

In general though, to my mind the best recommendation I can give for which brand of soft starter must start with your location and application. It's counterproductive to recommend a brand that has zero local support for you no matter how good it seems to be. I prefer that someone local, who KNOWS about soft starters, comes out to see your application and installation before recommending a product. They are highly application specific and the vast majority of dissatisfied users are victims of misapplication.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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The reason I need a soft start for such a small amperage it is that it is in service on a boat and the inrush current lags the generator, especially if both compressors are running simultaneously. So if I can reduce the Inrush and have a smooth start power managing will become much more efficient.

Since the compressor is located on a boat, sometimes it is beneficial depending on the power draw at the time, to be able to only run one compressor when the demand for air is low. And two smaller compressors as opposed to one large compressor is necessary because by virtue of having two, a redundant system is in place should one go down at an inopportune time. So to be able to individually control them and have them run either in parallel or independent off of one soft start would cut install costs. But after thinking it over more, I guess that the parameters wouldn't allow for starting with such a broad range of current draw, depending on one compressor running or two.

The local support would be the right way to go, unfortunately since it is being installed on a boat that travels quite substantially there is none. So it would be a product that I would have to order, and I was hoping that someone with experience with various soft starts could turn me on to one that is reliable and appropriate for the smaller application.

Do these compressors have head unloaders? That would be the simplest route. Then just DOL the unloaded motors.

Keith Cress
kcress -

If you DOL start the compressors with unloaders, they will still draw locked rotor current during start, the start time will be reduced but not the start current.

I would suggest something like the IC Electronics SMC3DA5015 would do the trick. Danfoss sell the same product under a different model number.

Best regards,
Mark.

Mark Empson
L M Photonics Ltd

OK, so you have a valid reason for using one and you have 2 compressors that will come on and off independently of one another. You will need a separate starter for each motor. There is no way for a single soft starter to differentiate between two separate motors, it would either start them both or start only one, but not start one and then the other.

Marke's suggestion looks like a simple low cost unit, probably all you need. If you want something that is readily available anywhere in the world, you may want to look at the Siemens 3RW40 series product. It is a "2-phase" soft starter which I am not particularly fond of, but for this application it would be fine. By "2-phase" I mean it only ramps 2 of the 3 phases, which accomplishes the desired effect but can be problematic for high inertia loads that take a long time to accelerate. That will not be the case in your application, all you want is a reduction in starting current.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

After looking at the costs involved, you may consider a larger generator. Another option is to find a diesel engine Guru to see if the generator engine can be tweeked for more power for motor starting.
But I have seen a lot of different motor loads started on small generators. If you are having trouble starting a motor that small on a gen set, you really need a bigger gen set.
Have you considered VFDs? They may be quite economical in that size range and have some advantages over soft starts..

Bill
--------------------
"Why not the best?"
Jimmy Carter

If you have 2 motors you want to control independently then you need 2 soft-starters.

I'm going to go against the suggestions above. I would only recommend a soft-starter with 3-phase control for the lowest inrush start. In my experience, a 2-phase controlled soft-starter produces less starting torque at the same current level compared to a 3-phase controlled soft-starter. A little more expensive unit will get you digital ramp settings including a current limit and a built-in overload.

First of all I want to thank all of you for your valuable input on this matter, it has been very helpful.

In response to Waross, a larger generator would set me back around $60,000 where's two soft starts will be about $1200. And it is only a problem when the boat is under full load and having the generator that is running right now preform in its 70 - 80% max load range is desirable to keep it at peak performance. And since this is the only complication that I have in the system, I think that Soft Starts are the right route to go. I also have installed a manual throttle which allows me to set the injection pump for optimal performance at any load within the range of the generators capabilities.

I've done a little research on various soft starts that all of you recommended and I think that the one I'm going to use is the Siemens 3RW4024-1BB. It is equipped with a current limiter and an overload protector, which was LionelHutz's concern and I took that into consideration. This seems to be the most compact and cost-effective unit available which allows for the necessary safety parameters and is appropriate for my small application. Now it is only a 2-phase controller but I think that with the size of the motors the loss in starting torque wont be an issue. And after looking at VFD's I think that a Soft Start is a slightly better option for my application then a VFD since all I need is the slow ramp-up start to a steady frequency and voltage and then a slow ramp-down stop.

Again, thank you all for the thoughts, information, and help on this matter.

AC Drive 2hp 240vac
Variable Frequency Drive, Motor HP 2, Output Current 8 Amps, Height 5.63 In, Width 4.13 In, Depth 5.91 In, Voltage 50/60 Hz 208-240, Single Phase
SCHNEIDER ELECTRIC
ATV31HU15M2
$408.25

I haven't seen a manual throttle on a diesel governor for a few years now, thank goodness. If you don't have to synchronize, the factory installed governor does an excellent job of controlling the engine.
Reading between the lines, if a boat mechanic has tuned up the diesel governor, you may need to have the dead-rack set up by someone who is familiar with generators.
I have been there several times. The procedure for adjusting the governor on a boat drive engine does not work for a generator governor.
I have seen more than one generator that wouldn't handle full load after a boat mechanic adjusted the governor.
The worst case went on and on.
A friend of mine came to me for help. He was installing a 5 Hp refrigeration compressor in a boat. The generator was about 60 KVA, but up until now, the load had only been a few watts of lighting.
A 60 KVA set should start a 5 Hp motor without even noticing it. As soon as they tried to start the compressor, the generator speed would drop, the lights would almost go out and of course the compressor wouldn't start.
I told my friend, the governor is set wrong. About twice a week I would meet him. Stil problems. No-one would (or could?) adjust the governor.
Finally the owners completely rebuilt the engine. The rebuilt engine gave the same results. They flew in a generator and motor rewind specialist (the boat was based on an offshore island). But he was just an electrical specialist, no engine expertise. Again I told my friend, someone has to adjust the dead-rack on the governor properly.
Finally they decided to pay my rates. By this time I had left the island and the airfare was added to the bill.
I got out to the boat and turned the adjustment screw about 4 turns and the generator has been working ever since.
If you have a generator so large that the next size will cost $60,000 and it won't start two small motors, find someone who knows generator engines and get the dead-rack properly set.


Bill
--------------------
"Why not the best?"
Jimmy Carter