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gkarsandas (Electrical) (OP)
19 Nov 07 19:48
Can it be possible to have a soft starter on reciprocating compressor having 15 hp motor?
The control of the compressor motor is on pressure switch.
itsmoked (Electrical)
19 Nov 07 20:01
Yes certainly.  It needs to be rated correctly.

You need to make sure it's protected from ever short cycling.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

jraef (Electrical)
20 Nov 07 1:40
I agree.
I'm curious though as to why you would question it? Did someone tell you that it couldn't work?
gepman (Electrical)
20 Nov 07 14:46
It seems like a small motor for a soft start unless you have a small electrical service.  I think the big issue would be if the air compressor starts unloaded, partially loaded, or fully loaded.  I would think that it is possible that the starting torque of a fully loaded machine could be greater than the rating of the soft start or if you obtained a soft start large enough then there wouldn't be much difference that starting it full voltage.  Following is a quote from a Champion air compressor brochure:

13 Loadless starting
Positive acting, governor-type centrifugal unloader assures longer motor life by allowing the compressor to start unloaded every time.
itsmoked (Electrical)
20 Nov 07 14:50
I would bet almost ALL compressors over 3HP start unloaded.

I know the smoke comes pouring out of our compressor within about 3/4 second if we try to start it unloaded.

I bet gkarsandas has a compressor that is dimming the lights every start and is trying to mitigate that.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

gkarsandas (Electrical) (OP)
20 Nov 07 15:53
I installed ABB make Type PSR 25-600-70 soft starter.While taking trial,motor is stalled ie standstill and making huge humming noice for few seconds and later on it starts. I am worried motor can get damaged(burnt).
Yes compressor motor has to starts at low set point pressure.
Seeking your advice.
itsmoked (Electrical)
20 Nov 07 16:39
Hi gkarsandas.

Easily the biggest killer of compressors, the world over, is a failed unloader.  The small innocuous device will allow connecting rods to bend and break, crankshafts to snap, motors to smoke, contactors to remove themselves from walls, and production to stop.

With a properly working unloader, starting a compressor motor is nearly trivial.  The motor only has to spin up the flywheel inertia.  With a malfunctioning unloader... Most compressors remain stalled until something lets go.

It sounds to me like your unloader is not doing its job.

The SS starts its cycle but since the compressor is improperly loaded the motor can not even begin to shove the pistons into a cylinder already filled with pressurized air.  So the the SS continues to ramp the power, (big humming), while the motor's torque builds thru the start cycle.  At some point you get lucky and the piston is forced thru Top Dead Center,(TDC), the inertia built up after that TDC is enough to carry the system thru the next compression stroke and so the compressor continues to run.

Your unloader is bad.  You must fix it.  The very next humming start could easily be the last one that motor will ever manage.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

DickDV (Electrical)
20 Nov 07 17:04
Or, to say it another way, the load determines how soft you can start.  If the load rolls away from a stop easily (as in an unloaded compressor), a very soft low-inrush-current start can be made.

On the other hand, if the load has a very high break-away torque or has very high torque on the first compression stroke (as in a compressor without unloading), that high torque sets the softness of the start.  So, for example, if the motor requires all its overload torque just to get past that first compression stroke, you can buy a softstarter but you won't be doing any softstarting.  You will have to set the softstarter for full starting torque and immediate ramp up to speed.  That would be the same as an ordinary magnetic starter and a good example of how to waste money!
LionelHutz (Electrical)
20 Nov 07 17:12
That ABB starter only controls 2 phases of the motor. So, it always causes a current imbalance. The more you try to make it soft-starter (lower current and voltage) the worse the current imbalance. If you increase the initial voltage it will help the imbalance but you also don't get a very soft starter. The current imbalance then leads to the motor starting very roughly.

So, in a nutshell, it's a fairly useless soft-starter that gives other good soft-starters that do control all 3 phases a bad name.

If you properly install (proper programming is necessary too) a good soft-starter that motor will start as smooth as smooth.

gepman (Electrical)
20 Nov 07 17:15
I agree that most compressors would have an unloader however I have recently realized that we are dealing with people from all over the world with this forum(I had a USA narrow mindset previously) and that especially in some developing countries there may be some cheaper Chinese or other similar equipment.  Therefore I didn't make the assumption that it had an unloader.  On the Ingersoll-Rand T30 compressor it has an unloader starting at 10hp, other manufacturers may skimp and start higher.  See attached file.

The compressors sounds as if it has stalled.  If it has an unloader it needs to be fixed.  If it doesn't I wouldn't use a soft-start.

I start some reciprocating pumps without an unloader and don't have any trouble
jraef (Electrical)
20 Nov 07 17:30
I doubt this is an issue with 2 phase SCR control, that would just cause a rough start. He says it does NOTHING but hum until it just starts. I'm with itsmoked, it's likely a mechanical load issue. Probably the delayed start is taking place when the bypass contactor pulls in at the end-of-ramp. In other words, the soft start is not able to soft start a loaded compressor. (is there an echo here?)

By the way don't keep starting it this way, it's highly unlikely that those bypass contacts are rated for DOL starting.
LionelHutz (Electrical)
20 Nov 07 23:28
Yes, the unloader will make it so that a soft-starter won't work right at all.

OK, if it struggles for about 1/3 rotation or so and then blasts up to full speed it is the unloader. gkarsandas's description of the start basically said the motor made a huge noise for a few seconds and then starts so maybe gkarsandas can describe the start a little better. The 2-phase controller noise isn't real nice and saying it then starts doesn't at all describe how, ie full voltage, nice ramp, struggles for the first compression stroke.

The easiest way to check the unloader is to turn off the power and try rotating the pump by hand. The unloader will allow it to rotate pretty easily.
MikeHalloran (Mechanical)
20 Nov 07 23:38
A working unloader also typically makes an impressive hiss when you stop the compressor.

Mike Halloran
Pembroke Pines, FL, USA

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