Softstart reversal on pump 1

[mil3]

This is an interesting topic that I can answer 100% either way.
Here is the situation:
100Hp, ertical Hollow shaft motor running a pump with a softstart attached.
What happened to the above installation is that the mechanical anti reversing ratchet mechanism on top of the motor sheared right off for some reason. I wnet in to chekc out the damage and found the following:
The "pole caps" outside on the power pole has been tripped/opened. Not sure how these work.
The incoming fuses were still intact and good. The softstart had shorted SCR and control board damage.
The enduser wants to blame someone and is pointing to the softstart and is blaming it for running the motor in reverse and shearing the anti spin back ratchet mechanism.I just can't buy this at all as the softstart was not connected inbcorrectly. Phase rotation was correct and it ran for numerous months just fine. besides, if this softstart ran in reverse all softstart manufacturers can make a lot of money selling full speed reversing units without having to add the extra SCR's.
Can anybody shed some light on the above if they can? I hope the info is enough.

 

[Marke]

Hello mil3

I have witnessed situations where a soft starter that has been damaged operated with one SCR not conducting at all and caused high current pulses. These were accompanied by a ferky movoemnt of the rotor in both directions. The rotor did not run continuosly, but jerked backwards and forwards. I also know of situations where missfired SCRs have broken couplings due to a similar behaviour. I wold not discount that the soft starter has failed in a manner that could cause positive and negative torque transients.
Not, this is not normal behaviour, it is an extreme case but I have seen it, particularly on old starter designs.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[Drivesrock]

An explanation I have - similar to the above - would be that you were missing a phase to the motor for some reason. The motor would then possibly rotate in either direction at start especially if you have a reverse flow of water thru the pump to help with the wrong rotation (but then the back stop should allow this). I have witnessed this. So it seems the softstarter could be the culprit

Best Regards

Drivesrock.

 

[mil3]

Thanks Marke. I've seen the oscillation before aswell, however just didn't think it could be at such a high torque to actually sever the anti spinback ratchet machanism. The motor manufacturer indicates this ratchet to withstand the motor power and is a very robust device (however i am not mechanical and don't know the design of these devices). I'll leave it to the experts.
The strange thing to this whole installation is that the pole caps (kinda like a breaker) outside on the pole was open, but the fuses were still intact. How to explain this boggles my mind.
Marke...My previous thread for info was removed so I couldn't review your response. Thanks anyway.

 

[itsmoked]

The pole fuses could be running near their maximum ability while running other customers and the rest of your facility then along comes this fault that doesn't exceed your fuses but pushes the pole fuses over the hump.

 

[jraef]

mil3,
I have just come off of a similar project, albeit 700HP 4160V starters on VT pumps. Any chance it was a GE Vertical Motor? I just found out that the backspin ratchet they use has a history of that kind of failure when used with reduced voltage starters and drives (GE doesn't make it, but they use it almost exclusively on their vertical motors). It uses a set of vertically oriented rods or pins with tapered ends that drop down into holes on a disk attached to the shaft. The taper on the pins allows the disk to spin one way, lifting the pins part way, then a centrifugal device takes over and keeps them out of the way as long as the pump shaft is spinning. When the shaft slows, the centrifugal device lowers the pins and they drop back into the holes. If the shaft trys to spin the other way, the tapers do not lift them up, so the shaft never spins and of course the centrifugal device never takes over.

What we determined was happening was that due to minor incoming power glitches, the start signal was dropping out so the soft starters were shutting off and then restarting 5-10 seconds later (2 wire control system). They were not reversing, but the motor slowdown that occurred while the soft starter re-ramped was allowing the ratchet pins to drop, then when the soft starter output caught the motor on the fly and reaccelerated, they apparently couldn't get out of the way fast enough. This assessment came from GE themselves, not us. They admitted having the same trouble with almost all forms of reduced votlage starting because when reset, they apply low voltage to the spinning motor and take time to regain full torque and reaccelerate. They don't have that problem with Across-the-Line starters unless the off time is of just the right precise amount of time, which so rare as to be unimportant. On my project, I just turned on a backspin timer feature for 2 minutes and the problems have gone away. A GE tech said that their instructions used to spell that out as a requirement, but couldn't find it in his literatire so he thinks maybe it was lost in updates somewhere and promised everyone that they would add it back in.

Other than that, Marke and Drivesrock's answers are about the only possibility, but as you noted, it is dificult for that type of failure to create enough torque to break a backspin ratchet. It never happens when running because of course, a failed SCR is essentially the same as a fully gated SCR. So by happening only when it starts, there is no way for it to create any more than locked rotor torque (if even that) on the shaft, and a backspin ratchet is specifically designed to withstand locked rotor torque from a dead stop (or should be). Soft starters are not like VFDs where motor reversal can be done electronically without adding poles. You would need to have 5 SCR stacks to do that, so it would be fairly obvious.

BTW, in the GE scenario above, the blown SCRs may easilly have been a result of the locked rotor when the pins jammed.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[mc5w]

Sound to me that you need an undervoltage relay with fast dropout and either manual reset or a power on time delay when using automatic reset. This cannot be done by installing a simple on delay relay in the control system as an on delay relay will not necessarily discharge its timing capacitor during a voltage dip.

Your blown SCRs could be the combination of both lightning and the pins sticking in the ratchet. If your check valve operates correctly you should not have a problem with reverse water flow reversing the pump. However, on a large enough pump a check valve that closes immediately on reverse water flow wil create damaging water hammer.

-----------------------------------------------

I know that Duquesne Light Company ( Pittsburgh, PA ) requires that manual motor starters use a magnetic holding coil that prevents a restart when power is reapplied. Most automatically started motors are required to have some type of power on time delay. About the only motor loads that they allow to immediately restart when power is reapplied are well pumps and attic fans. Room air conditioners will try to restart right away but will eith trip the internal overload or blow the branch circuit fuse.

This is partly because Pitssburgh gets a lot of lightning because of the mountain uplift. Hence, power is guaranteed to be shut off at least once every 2 thunderstorms to stop insulator flashovers. Duquesne Light Company's 14,400Y24,940 volt distribution system is set up for strictly manual reclosing ( via remote controls ) of circuit breakers and oil section switches because thundercells tend to hit a distribution circuit with multiple lightning strikes. That is, God tends to go after a distribution circuit with a Thompson or an Uzi or a MAC 10 which would exhaust an automatic recloser and you would end up with manual reclosing anyways. Pittsburgh also gets snow in June at moderate elevations and it melts as soon as it hits the ground - a bit opposite of freezing rain.

Here is Ohio we do not get the lightning cluster phenomenon and lightning strikes tend to be a single bolt at a time. Hence, automatic reclosers usually work.