Which is the correct way?

[2rlp]

Hi,



In a Direct On Line circuit for CW and CCW operation of a 3 phase motor, what is the right way to interlock the contactors.


1) Use a NC contact from the CW in series with the CCW coil and vice versa? or


2) Use a mechanical interlock between the 2 contactors

 

[Tinine]

I guess it boils down to the consequences of shorting phases. Blowing a local breaker is one thing but that doesn't always happen.

Personally? Electrical interlocks. I rely on dry contacts for e-stops and overtravel limits so why not for interlocking contactors.

Craig

 

[parky]

I always do both, just incase, mechanical ones are very reliable but always use electrical ones as well.
Scenario's some aux contacts are extras fitted & can be removed or not clipped in properly so did not operate(had this once).
Mechanical ones can break not seen it myself but heard of a couple of instances where they failed.

 

[JesperMP]

We use electrical interlocks only. Don't seem to have any issues with there not being a mechanical interlock.

 

[geniusintraining]

We use electrical interlocks only. Don't seem to have any issues with there not being a mechanical interlock.

+1

The issue with only NC and NO... in theory its perfect but on several occasions I have seen a wanna-be electrician troubleshooter use a screw driver to bump a motor and if the logic (PLC or relay) is wanting to hold the position you will have a direct short (phase to phase) and guarantee something other then the wanna-be is going to pop

 

[Tinine]

+1

The issue with only NC and NO... in theory its perfect but on several occasions I have seen a wanna-be electrician troubleshooter use a screw driver to bump a motor and if the logic (PLC or relay) is wanting to hold the position you will have a direct short (phase to phase) and guarantee something other then the wanna-be is going to pop

Oh so now we bring the real world into it?

The troubleshooter who jumpers out the "safety" relay and then forgets to remove the jumpers?

I hear ya.

Craig

 

[James Mcquade]

We use both.
if the mechanical interlock hangs / fails, you still have the electrical interlocks.
as already mentioned, many times a screwdriver is used to force the contactor on or jammed into place and left in place. It happened where i used to work multiple times.
the mechanical interlock is the main safety, the electrical interlock is the verification and programmed into the code to prevent the energizing of the other contactor. if a contactor fails that has the interlock, maintenance may just replace the bad contactor and not put the interlock back. that has also happened. te only protection you have then is the electrical interlock.
for contactors with 4/0 wire, you need all the protection you can get.
james

 

[joseph_e2]

I vote also for both. I've seen a screwdriver wedged into the motor starter to figure out why the overload keeps tripping. Turned out to be a partial short in the cable feeding the motor. They figured it out by watching for smoke.


When it comes to keeping people from hurting themselves with their stupid, I tend to the "belt & suspenders" method.

 

[dwoodlock]

+1

The issue with only NC and NO... in theory its perfect but on several occasions I have seen a wanna-be electrician troubleshooter use a screw driver to bump a motor and if the logic (PLC or relay) is wanting to hold the position you will have a direct short (phase to phase) and guarantee something other then the wanna-be is going to pop

Younger and dumber me did this and it was unpleasant, would recommend mechanical interlock.

 

[OkiePC]

Be Like Deion

 

[2rlp]

Thankyou all for the insight. I'll follow your advise and use both the methods.

 

[geniusintraining]

Younger and dumber me did this and it was unpleasant, would recommend mechanical interlock.

Your not alone... I learned it from seeing someone else do it, its a one hell of a way to learn that lesson