What do you know about control reliable circuits?

I need to break the connection to 6 motors, all fed from the same 480V 3 phase, from the same fuses.

When an E-stop is pressed, I need to break the power going to each motor.


With two contactors, and a safety relay, I would typically run the power through the two new contactors and then only pull them in when the safety relay is safe. On the reset, I would monitor the NC on both of these contactors to make sure they do not get welded shut and make it control reliable...

The question I have is this.... If one of the contactors were to get welded shut, wouldn't it just weld both of them? I would assume so because the same current that welded it on one would weld the other...

How do I better this design?



Thank you!

I use safety rated motor contactors like here https://ab.rockwellautomation.com/Motor-Control/Contactors/IEC/Safety.

Usually, safety contactors aren't commuting the load but just isolate it when needed. Yes in a real emergency, it will need to open a loaded circuit but this should not happen on a regular basis to wear out the contacts like a motor starter.
And your tripped safety relay should also unlatch your 6 individual motor starter in order to be able to reset the safety without load and without the motor to automatiquely restart.
By doing so, the chances that contacts weld are very limited.

Then I would be very surprised that this happen simultaneously on 2 série contactors.
What's melt a contact is heat generated. Amps alone or voltage alone aren't making any energy, you need a contact resistance to held voltage + flowing current to heat up the contact at the melting point. If this happen on 1 of the contactor, it will significantly makes the voltage drop on the other reducing the chances to melt the other one...

Same when opening under load, the arc generated is halved on the 2 contactors when both are suppose to be good enough to extinguish it alone.

Very often you have one or even two safety contactors, then a control contactor for each motor. wire all the normally closed contacts in series with the safety relay so if any one of the contactors welds in the safety will not re-energise, providing the safety contactors are rated for the job and not all motors are started at the same time on re-enable the chances are very small that the safety contactor(s) and one of the control contactors will weld in.

Perform a risc assessment.
Based on the risc assessment you decide what safety category must be achieved.
If the risc is not so severe, it may be enough with a single contactor with no safety feedback. If the risc is very severe you may need redundant contactors with extensive diagnostic feedback into the safety relay.

If that is all greek to you, let someone else do the risc assessment and safety design. If an accident happens and someone gets hurt, and your name is on the design of the machine, then you can get into serious trouble.

My philosophy completely, When I did risk assessments, I usually take it to the next level to be sure.
I suggest that anybody who designs safety circuits should attend one of the courses run by any of the leading Manufacturers like Pilz it's worthwhile.
However like all guidelines there are no definitive answers, and I hate that sentence as far as reasonably practicable. The best policy is if possible engineer out any risks, if it is not practicable, then safe systems of work need to be implemented to reduce the risk. Machines can be replaced, life & limbs cannot.

parky said:

My philosophy completely, When I did risk assessments, I usually take it to the next level to be sure.
I suggest that anybody who designs safety circuits should attend one of the courses run by any of the leading Manufacturers like Pilz it's worthwhile.
However like all guidelines there are no definitive answers, and I hate that sentence as far as reasonably practicable. The best policy is if possible engineer out any risks, if it is not practicable, then safe systems of work need to be implemented to reduce the risk. Machines can be replaced, life & limbs cannot.

Click to expand...

There are definitive answers...

Take a look at EN 13849-1 & EN 62061

These are European standards, but complying with those also suffices for 90% of US safety standards, the bits they don't deal with can be found in UL508A

I am aware of these designs and standards, but my question was only this:

If I have two contactors in series, monitor both of their NC to make sure they are unlatched when an E-stop condition occurs, the only thing I monitor them for is to make sure they did not get welded shut...Wouldn't the same current running through one contactor cause both contactors weld shut if they are made to the same spec?

I can prevent the reset from occuring if both are welded shut, but at that point I now have motors running freely.



Thank for the feedback!

Time is Money > I can prevent the reset from occuring if both are welded shut, but at that point I now have motors running freely.


Doesn't each motor have its own starter and or VFD? It seems to me that if one
can prevent the reset, one can also ensure none of the other starters or VFD's
activate either - unless the reset circuitry sees no shorted contactors. Keeping
the motors from running.
Poet.

By the sounds of it you have 6 motors all connected in parallel. from two contactors, this is not ideal and suggest that all 6 motors have their individual contactor and a main safety contactor, all contactors should feed back the N/C contactor to the safety circuit so that in the event of one of the contactors fail then the system cannot be re-started. The main contactor should have a rating greater than all motors combined and the individual ones rated at least to the size of each motor. The biggest problem is if a direct short on one of the motors could cause a contactor to weld. or a mechanical failure in the mechanism i.e. return spring. then it is extremely unlikely that the main contactor will fail at the same time. As I said before it would be prudent to stage the start up of the motors in sequence this will further reduce the initial load currents often responsible for contactors welding after a certain amount of time.

They all have their own contactors, with no safety in the initial design. None of their contactors have NC contacts on them either.

I want to add two contactors after the breaker (that feeds all 6 motors in parrallel) to drop the 480 to all of them simultaneously when an E-stop is pressed.

I could have one to feed power to all the 480 and another to drop the 0V to all contactors, then monitor all of their NC, but the cabinet is so small I can't add the additional contacts on their contactors and still have the door shut.

If the current is enough to weld them closed under normal operation, cabling would set on fire.

Contractors weld shut when either the coil/spring fails. Or a contact pad is worn and it arcs on closing and opening.

The probability of them both welding shut at an identical time is insignificant.

This is also covered with the diagnostic time of the circuit, how often they are actuated and tested.

The safety relay should also remove the control voltage to the contactors on each motor, therefore if one or both the safety contactors welded in then the safety could not reset and the control voltage to the motor contactors will stay isolated. this way the chances of both safety contactors and one or more of the motor contactors welding in is extremely small. The category of any safety circuit needs to be determined so that you conform for example what is the likely hood of loss of limb etc.

Rob... said:

If the current is enough to weld them closed under normal operation, cabling would set on fire.

Contractors weld shut when either the coil/spring fails. Or a contact pad is worn and it arcs on closing and opening.

The probability of them both welding shut at an identical time is insignificant.

This is also covered with the diagnostic time of the circuit, how often they are actuated and tested.

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I think this was the answer I was looking for. I didn't understand how the welding would occur. This makes more sense. Thank you!

parky said:

The safety relay should also remove the control voltage to the contactors on each motor, therefore if one or both the safety contactors welded in then the safety could not reset and the control voltage to the motor contactors will stay isolated. this way the chances of both safety contactors and one or more of the motor contactors welding in is extremely small. The category of any safety circuit needs to be determined so that you conform for example what is the likely hood of loss of limb etc.

Click to expand...

Correct, I drop the coil voltage to both contactors separately via the safety relay and then monitor the two NC's to allow my reset to occur.