Reversing a Stalled 3-Phase Motor 1

[rdub]

I am looking at a grinding system that occasionaly gets jammed and stalls the motor (100 hp, 3-phase, 460VAC). The thought is to monitor the current in one of the motor legs, upon sensing an over current condition, stopping the motor, reversing the direction to clear the jam, stopping the motor again, then after a time delay, resuming forward operation. How much oversizing should I do with the reversing contactor?

 

[edison123]

If you reverse motor after it has stopped, you don't need to oversize the reversing contactor. It can be the same size as your forward contactor.

 

[jraef]

That's correct, but from experience let me add that you must monitor all 3 legs of power. If you get a low level current fault of any sort on that one leg, you don't want the motor reversing on you! I prefer to use a kW monitoring device, preferrably one with pahse loss protection as well.

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[abcd3286]

As mentioned I would monitor all 3 phases. You can use the same set of overloads for foward and reverse. Use a time delay to let motor cool after a jam before starting in reverse.

I think some sort of alarm would be advisable on this.

Food for thought.
OK so it jams. Will the motor have enough power in reverse to clear the jam or will it make the jam worse?
THEN if you can get the motor to reverse it, when you go back to foward how are you going to ensure you do not hit the "same jam" again?

I am not so sure about automating this. I think this is a good thing to be dealt with manually.

Dan Bentler

 

[itsmoked]

abcd3286 has some very good food for thought there...

 

[jraef]

It is done all the time in his type of application, grinders and macerators, in the wastewater industry. Google "Muffin Monster auto reverse" for some intersting light reading on the subject. They use a PLC when you buy them as a package, but I have built dozens of custom controllers for them using a couple of timers (although micro PLCs have now become so cheap it might make more sense). Works fine, except we learned the hard way about only monitoring 1 phase.

BTW, we tried VFDs since they can reverse without contactors, but you have to seriously oversze the VFD to get enough starting torque out of the motor, and usually nobody wanted to pay that much extra. If your application is under 5HP however, it might be worth looking into. You would need a vector drive, and it must be oversized by at least 2x in order to deliver locked rotor torque when you need it. Don't be fooled by the VFD ads that say "full torque capability". That is full running torque, not full starting torque, and grinders usually need full starting torque.

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[rdub]

Thanks all for the advice.

jraef, regarding your first comment, I'm not totally clear on what your getting at here. Is the concern that on a low level current trip, the motor may still be spinning forward when the contactor reverses?

 

[jraef]

No, the concern is that a low level fault which is insufficient to trip the overload relay will trigger a reversing sequence, then you are restarting a motor into a fault. It can also be a problem if you use fuses instead of a circuit breaker. 1 blown fuse increases the current on the other 2 legs. That can trigger a reversing sequence, then you are trying to restart with a single phase condition, also not good for the motor. By using a true 3 phase power monitor, you can have the system ONLY trigger a reversing sequence when all 3 phases see a power increase, but trip out on anything else.

Your control sequence also should have a dead-band built into it so that you delay reversing until the motor is stopped for sure. To do that without a shaft speed sensor, just time it as if the motor were not jammed. The grinder should stop in 1 or 2 seconds anyway, and that brief amount of delay before reversing should not cause any problems.

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[Marke]

Hello edison123

I am curious about your statement "If you reverse motor after it has stopped, you don't need to oversize the reversing contactor." My understanding is that the current does not change significantly even if the motor is spinning at considerable speed in reverse, so why would you consider changing the contactor size except where the start time is very long? If the motor was spinning at rated speed in reverse, you are effectively starting a machine with double the load inertia and if that required you to up size your contactor then OK.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[edison123]

Hi Marke,

I said if the motor rotation direction is reversed after the motor has to come to standstill, then the reversing contactor need not be oversized.

Did I miss something ?

 

[jraef]

I think he means that by saying you don't need to oversize if it comes to a stop first, you are implying that if it IS still spinning, you DO need to oversize.

 

[rdub]

My thoughts about oversizing the contactor was that if the motor stalls, the contactor opens at locked rotor current. If the jam is significant, when the motor reverses, it could potentially still be jammed and now the reversing portion of the contactor opens and is subjected to this high current condition. I'm concerned about the long term effects on the contactor.

 

[Marke]

Hello rdub

The major issue with the contactor contacts are thermal and are due to the I2R losses in the contacts. This thermal effect is dependant on the magnitude of the current and also the total time of the overload. If there was to be extended time, or a high frequency of overload current operations, then an increase in size is advisable.
When choosing contactors, look beyond the rated KW and compare the contactors at an equal number of operations. Some manufacturers boost their KW ratings by reducing the number of operations so watch for this.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[aolalde]

You should be worried about the motor thermal damage, due to extended time operation with the inrush current.

 

[jraef]

These are all valid points for most machines, but again, this type of machine is designed for this. The reversing feature is used because the blades are designed to eject the clogging agent from the impeller when run in the opposite direction. As I said earlier, it is done all the time and is a very successful design.

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[macmil]

I realise that it's not the question you asked, but I would like to point out, as a mechanical type, that there is mechanical equipment on the market that senses the overload and uncouples the motor from the load, thereby avoiding the stalled motor currents. And I'm not talking about a shear pin device. Please feel free to contact me if you would like further information.

 

[jraef]

But again, in a chopper pump / macerator / grinder application like this, the goal is not to disconnect the load, the goal is to clear the clog and resume normal operation unattended. Think of a garbage disposal. If you accidentally drop a dishrag in it, it will tangle and jam. Disconnecting the load means someone needs to disassemble and clean it all out. Not a big deal on a garbage disposal at home, but a task that nobody volunteers for in a wastewater application I can assure you. So these machines are designed like the premium garbage disposals that sense the clog, reverse breifly, eject the rag, then resumes it's forward direction and hopefully chop up the rag the second time. What they will usually do is try 3 times, and if it doesn't work, then shut down and automatically signal the poor unfortunate soul on duty that day.

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