OT: What happens to a backwards freewheeling motor when started?

This morning on our cooling towers two fan motor soft starters had shorted SCR faults. We had a large windstorm overnight. Is it probable that the motors could have been freewheeling backwards in the wind. Could turning on a fan that is free wheeling backwards damage a soft starter? How do ya'll normally deal with that kind of a problem.

The SCR's can blow because the motor has a longer startup time - draws high current.
Some controllers have an option to deal with that (fan spinning backwards). It applies dynamic braking prior to startup.

Many years ago, the company I worked for had some deep-well shaft-driven water pumps. Thay had a mechanical device on the fan end of the motor that was centrifugal operated. It released at startup, only if the motor ran with the right rotation. This prevented the shaft threaded coupling coming apart if it was running with the wrong rotation.

Alaric said:

This morning on our cooling towers two fan motor soft starters had shorted SCR faults. We had a large windstorm overnight. Is it probable that the motors could have been freewheeling backwards in the wind. Could turning on a fan that is free wheeling backwards damage a soft starter? How do ya'll normally deal with that kind of a problem.

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Same situation as not letting a motor stop before reversing it. Was not encouraged because it was very hard on starters - not so much the motor - for sure it does it no good - also very hard on driven machinery.

Starting current would be very high and for longer time interval which would increase as "reverse?? fan RPM is higher.
Very likely to blow a soft starter.

How large are the fan motors?
I wonder if a VFD may be the soluton. More recent have ability to set output freq to that of freewheeling motor then apply power to motor and speed or slow as needed. What I am not positive on is if it can match to fan freewheeling in reverse "catch it" then revese it and bring to commanded speed. This is of great interest to me with an electric vehicle here in Seattle starting on a steep hill.

Dick DV have I got the right idea??

Dan Bentler

I don't think the softstarter has a feature to apply braking. Its set for a current limit start of 300%. This is a multi-fan tower cell and we sequence the fans to control cooling capacity. Nothing fancy, just temperature limits and timers. Each fan has a minimun off time and a minimum run time. This particular cell has been operating for four years, this is the first problem.

I briefly considered changing the program to first pulse the soft starter for a second, then turn it on fully, but I think that could actually be worse.

The fans are on 10HP motors.

I actually just read an advertising article about this in the September issue of MRO magazine. Gates Canada has developed an anti-backdriving clutch specifically for this type of application. Here is a link to the supplier's web site:

http://www.gates.com/brochure.cfm?brochure=10936&location_id=4950

All the VFD's I have programmed before with the

"catch it"

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command run on the fly, I call it, was only ever available in one direction.Mainly because you wanted to catch up then free wheel etc, all in the same direction. So If you commanded the VFD to reverse it would not catch up first then stop then reverse but brake decelerate for the programmed time then reverse.
Best bet is an electric brake on the back of the motor, not to stop but to hold like a handbrake.
Still ramp to a stop then Hold.
No electric means braking.

Alaric said:

I don't think the softstarter has a feature to apply braking. Its set for a current limit start of 300%. This is a multi-fan tower cell and we sequence the fans to control cooling capacity. Nothing fancy, just temperature limits and timers. Each fan has a minimun off time and a minimum run time. This particular cell has been operating for four years, this is the first problem.

I briefly considered changing the program to first pulse the soft starter for a second, then turn it on fully, but I think that could actually be worse. The fans are on 10HP motors.

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Alaric

If you stay with soft start and you install a "locking brake" a timer could be used to deenergize and set the brake 5 or 10 second after power is off. That way you would not be burning up the brakes slowing the fan.

The other thing that comes to mind with these being only 10 HP motors is with a VFD drive(s) you could get better cooling by running all the fans but at slower speed. This would work very well on a fin and tube type cooler and depending on setup on a waterfall type cooling tower you would have equal air flow across the whole surface area. You could economize somewhat and drive one half the fans with one VFD and the other half with another VFD. For max reliability one VFD per fan but that may be a few bucks.

The main savings would be power because of the affinity law.
Other savings would be better bearing life - motors would be constantly running and able to sling water away and not let it in the bearings.

Dan Bentler

Catch a running motor...

There are VFD's available that wil catch this motor, bring it to a stop, and restart in the correct direction. Yaskawa, Control Techniques, Leeson, and Teco all make drives I have done this with. I have also seen the "old school" method (used a lot on old woodworking machinery) to use an external DC braking module. Monitor to see if fan is turning, if so, engage DC brake, then restart afterward with softstart. I like the VFD approach myself.

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If all else fails, then dig the directions out of the trash and read them.

Alaric --
There are several companies that make Electronic brakes, like the Emerson/Saftronics DY5:
http://www.emersonct.com/products/soft_starters/dy5_-_electronic_motor_brake/overview.aspx

At pumping stations, and in HVAC systems, we ALWAYS put one of them on, sequenced to apply braking for 5 or 10 seconds, and then energize the VFD/SoftStarter precisely to prevent VFD/Softstarter blowups, fuse clearing, and huge banging noises.

Those DY5 units saved a lot of old, temperamental Graham AC drives from premature death.
It's just a DC Injection brake unit, but they can reduce rotation speed to safe levels before applying a drive.

leitmotif said:

Alaric

If you stay with soft start and you install a "locking brake" a timer could be used to deenergize and set the brake 5 or 10 second after power is off. That way you would not be burning up the brakes slowing the fan.

The other thing that comes to mind with these being only 10 HP motors is with a VFD drive(s) you could get better cooling by running all the fans but at slower speed. This would work very well on a fin and tube type cooler and depending on setup on a waterfall type cooling tower you would have equal air flow across the whole surface area. You could economize somewhat and drive one half the fans with one VFD and the other half with another VFD. For max reliability one VFD per fan but that may be a few bucks.

The main savings would be power because of the affinity law.
Other savings would be better bearing life - motors would be constantly running and able to sling water away and not let it in the bearings.

Dan Bentler

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On second thought if you were to use VFD to eliminate the problem you would have to use one VFD per fan - I do not think the VFD would be able to catch multiple motors.

I don't see any good reason why a freewheeling motor backward or forward would necessarily damage an SCR softstarter. These units have current limit settings which are activated whenever the current exceeds safe limits and, if freewheeling backward, could conceivably result in overcurrent.

I would more strongly suspect a power surge or lightning activity as the cause of blown SCR's.

I would replace the SCR's or softstarters and hope for four more good years.

command run on the fly, I call it,

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usually (on the fly) is an actual command, not for soft starts. I am sure VFD will work in your application.

I have often seen anti reverse ratchet device on pumps, also on conveyor systems that slope up-hill, this would be the simplest fix.
Roy