bagged2drag

MCR for motor control and other AC items

9 posts in this topic

I am looking into some insight.  We have a piece of equipment that came into our facility provided by a 3rd party, but brand new.  We recently ran into some electro-mechanical problems (DC brushed motor went bad, armature shorted to ground).  The motor still works because of how it failed, but it caused the dc controller to fail in the on state.  We learned that the drive was not tied into the safety circuit in any way so even though the control power was off and e-stop was pushed, the motor still turned on and ran even though it wasn't getting the input to "run."

Looking at the electrical prints, I see that the motor control, along with numerous other components on the AC circuit, were not tied into the safety circuit in any way (no mcr, and in some cases(such as vibratory feeers), not even a control relay driven off the plc).  

My question is this:  what are the requirements for safety in these regards, and what would you think is the best way to approach such a problem?   Of course, I would expect all motion to stop with an e-stop regardless, but should this be done on a safety rated  level, or just single channel components?

I err to the dual channel path myself, and have also chosen to shut down the equipment until the vendor fixes it, but that comes to a huge loss to our business.

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I would typically disable the Power to the DC drive thru a safety relay. you may need to check the current and volts of your safety relay. seen this before, contact me if you need help.

Mike

 

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Thank you.  That is somewhat the direction I expected as well, but I wanted to verify.   The vendor which built the equipment wants me to just wire in to an existing control relay which is currently tied in to a PILZ safety relay.   The control relay they want me to run the power through is a Siemens 3RH2911-2GA31.  Although pulled in from the safety relay, this contactor itself is not a safety rated device.    This would "get the job done," but I am not sure his recommendation is correct.  I have the understanding that the power should be controlled from  a safety rated contactor.   

 

 

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My other question: if his method is satisfactory, Is it ok and normal to wire in both AC and DC power on the same contactor?  I know physically it is possible without issue, but is it considered safe practice?  He wants me to tie the AC power that supplies power to the dc controller.

 

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Acceptability of the control method depends on the level of the hazard determined by a proper risk analysis.

 

My opinion:

If it's impossible to get hurt by the hazard, then by all means use a PLC controlled relay or contactor that's rated for the load voltage and current.  If there's any risk of injury, then use redundant safety-rated contactors rated for the load voltage and current and properly integrated into a safety-monitoring relay circuit. 

I'm not aware of any problems mixing AC and DC voltages on a single contactor as long as the contactor is properly rated.

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Thank you Joe.  This is an area in which our operators have direct contact to the walking beam indexer when the machine stops.  Your statement reaffirms what I thought; it should be protected with a safety rated contactor.  

 

Currently, the machine builder wants me to tie in to the control relays in place (siemens contact adder p/n 3RH2911-2GA31 on a relay siemens p/n 3RH23712FB40).   From the info I see, neither of these are safety rated (they do get 24v control power from a safety relay though - no 110v is tied in anywhere).  Does that constitute being safety rated then?  I would think, no, but maybe I am wrong here.   The one is a force guided relay.  documentation from Siemens doesn't indicate either is safety rated.  

 

 

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I didn't find safety rating information on those part numbers quickly, which isn't surprising with their website.  I would reach out to your local distributor to verify that it's suitable for this application.  If it is, you still need 2 redundant contactors in series controlling the load with aux contacts from both being monitored by the safety relay.

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I am reaching out to both local vendors and Siemens directly to get a definite answer.   I looked at the electrical schematic for the relay.  It shows only a single relay, not redundant as one would expect for safety rated.   It also has a mechanical means in which to energize the relay manually, which I thought to be a big no-no for anything safety rated.   

Thanks,

 

Relay.JPG

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The Allen-Bradley safety-rated contactors we've bought are sealed so they can't be manually actuated.  We have some machines claiming to be built to European standards that have Siemens contactors used in redundant safety applications that have an accessible indicator on the front that may be able to be pushed to activate the contactor; I've never tried it.

To get redundancy, you would add a second contactor in series with the one you have (if it's suitable) or replace it with 2 properly rated contactors.

 

 

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