SMC-3 Starting into freewheeling load

Greg Fowlds

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Join Date
Apr 2004
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Currently, Waiheke Island, NZ
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Anyone know the effect of starting a fan with an Allan-Bradley SMC-3 softstarter when the fan is freewheeling in reverse.
A)Would it "lock" the motor then ramp up.( Stop the motor with very short deceleration time)
B)Would it allow the fan to ramp down first then ramp up.(Under some sort of control)
SMC-3 is set as SoftStart.
Searched on AB site and in documentation and nothing about starting into a freewheeling in reverse load.
 
I'm assuming the fan is not spinning very fast...as the voltage spools up (building torque) the load will stop spinning and then come back the other way. This is done all the time.

Nick
 
Spinning around 10-15% in reverse.

Have had a fan that lost some blades, as in bent over and broken off at startup.

There is some argument that the motor spinning in reverse act like a generator and when the SMC comes on line it violently throws the motor into forward or to zero speed.

Not an issue with a VFD but possibly with an SMC.
 
You usually don't get 'fly-catching', or other such odd names that describe syncing to a spinning motor, unless it is specifically stated. From what I have see in the past soft starts usually don't have this.


If you aren't already doing this you might want to try and operate the soft start in current limit mode. That should limit the available torque from the motor.

If this is a concern ideally you should look into a drive that provides a fucntion to sync to a spinning motor. Many of the drives on the market today will do this. I just find it a little hard to believe that the motor could damage the fan blades without damaging itself or another powertrain component at the same time.

Keith
 
Definitely talk to A-B about this. I would bet dollars to doughnut holes that they won't reccomend doing this.

The fan spinning backwards (or forwards for that matter) will try to act as a generator. In some cases this can result in a torque that is several times nominal locked rotor torque. I once saw a 700 hp motor snap a shaft when re-started while already spinning forward at close to full rpm. Not pretty. I understand that the purely random degree of misalignment between motor phases and supply phases is the culprit.

If you are truly only spinning at 10% reverse you might get away with it because the kinetic energy is low. This is especially true if it is a belt drive, as belt slip can reduce effective torque. I'm not sure you can guarantee that the speed will always be that low, though. You should talk to the supplier to get their take on potential equipment hazards to the fan and the starter both from torque and from back EMF.

VFDs, unlike soft starts, can "catch a spinning motor" by aligning hte supply power to the motor with the actual rotation. This is standard on most VFDs. This, or a backstop, might be reasonable precautions for this application.
 
I guess I will have to go with checking with allen bradley too just to be safe, but I fail to see how you can get torque without current. The entire point of the soft start is to limit current at start up. A motor free wheeling backward at 50% of it's normal speed would see an intial start up similar to starting up from a standstill only at 150% of the normal operating frequency (90hz), no?

I am not pontificating here by the way, I am curious, maybe I am dead wrong.
 
I don't know the theoretical background, but I absolutely guarantee that if you have a motor free wheeling close to full load rpm and connect it across the line you will on occasion get very high torques. I have seen it snap shafts and I have seen it shear couplings.
 
Tom Jenkins said:
I don't know the theoretical background, but I absolutely guarantee that if you have a motor free wheeling close to full load rpm and connect it across the line you will on occasion get very high torques. I have seen it snap shafts and I have seen it shear couplings.

I have seen some similar things. In this situation the motor is not nor ever will be a generator because it has no form of excitation to produce electricity.

I assume (at least in this case) the wind is making the fan turn backwards. The wind can easily turn a fan in speeds equal to or greater than those obtained thru the motor.

You know there is relationship between speed and torque; therefore if it is rotationg (for whatever reason) in reverse, when you apply power you will have toque in direct opposition to each other. If the applied power (torque) is significantly greater then the rotatotional force the sudden change could shear or at least bend parts.

In the 60's I was driving a Chevy with an automatic tansmission and floor mounted shifter, doing about 80 when someone hit the stick and knocked it in reverse. The driveshaft did not survive, thank goodness it was the driveshaft though.

JUST IN CASE: I know many will think "You can not do that with an automatic." Most of the cars, with automatics, before the mid 60's could be pushed or roll started AND could be placed in reverse while in motion. I had a 56 Crown Victoria that had a 6v electrical system and a bad starter, I just rolled started it, never did get a starter for it.

As suggested, obtain all the data you can on the fan i.e. speed of rotation (in reverse) etc then call RA to see what they say.
OR
You could add a brake to prevent it from turning when not powered.
OR
Install louvers that are only open when the fan motor is powered or air blows the proper direction.
 
Thanks for the input guys.:nodi:

The situation with the fans in the road tunnel is very similar.
(As per PDF link above on page 22)

Will continue to research these questions with AB.
I would have expected to read something in the manuals about NOT starting into a rotating load with the SMC-3.
 
I'm operating cooling tower fans with AB SMC-3 soft starters. They are configured for soft starting with a 10 second ramp. Now these aren't huge fans, 15hp direct drive, but sometimes they are freewheeling in reverse. I've never had a problem with it.
 

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