Mechanical or Electrical Interlock? 1

[ThePunisher]

I found myself in a debate with someone and perhaps both of us have a point, maybe we see things differently. Hence, I would like to obtain other's comments.

On a reversing starter control, there are two contactors wherein interlocks will must be included to prevent both contactors CLOSING simultaneously. I do understand the electrical interlocks wherein the auxiliary contact of each contactors will be wired to prevent energizing one contactor coil if the other contactor coil is currently energized.

The confusion lies on the momentary pushbuttons (FWD and REV) pushbuttons (see attached diagram). I see this momentary pushbuttons still FUNCTION as an ELECTRICAL INTERLOCK because their contacts are still electrically wired and operate to block or prevent energizing one coil. Although both pushbuttons are mechanically operated, their associated contacts still function as an "electrical interlock". The only mechanical interlock I would consider is the part where both pushbutton contacts are MECHANICALLY INTERLOCKED WITH EACH OTHER.

My friend's opinion is that since both pushbuttons are actuated by mechanical means (finger-pushed), they are mechanical interlock (which I disagree).

Mechanical interlock to me is non-electrical operation and there are no wiring like in the case of "kirk key" applications or an ABB VM5 which have insulated mechanical linkage to block the operating mechanism of a contactor.

Your opinion is highly appreciated.

Thanks you.

 

[dubbo]

Mechanically interlocked means the actual contactors are constructed such that only one can be closed at any time. This can be implemented by a mechanical "latch" or lever of some kind which prevents both contactors being closed at same time. Quite common with reversing motor starters. The interlock via push button wiring is only an electrical interlock regardless of how they are operated.

 

[Skogsgurra]

Agree. The interlocks shown are electrical.

A mechanical interlock is sometimes needed to prevent that two contactors (e.g. a FWD and a REV) are actuated simultaneously. That can happen in certain installations (overhead cranes) when forces from a sudden deceleration like hitting a positive stop could activate both contactors.

Another case is when one of the contactors has welded and is stuck in a partly ON state. Auxiliary contacts for electrical interlock then doesn't work reliably while a mechanical interlock *may* do the job.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

It is my policy to go with both mechanical on the contactors and electrical. In some applications, two electrical interlocks.
A couple of anecdotes to illustrate my reasons.
We had a small three phase electric winch used to lift large band-saw blades from the head-saw to the filing room above.
It was originally controlled by pull ropes.
The pull ropes were inconvenient and a pair of contactors was mounted on the wall to control the winch.
The contactors were electrically interlocked, but not mechanically interlocked.
We had the contactors but we didn't have a reversing contactor available.
The contactors were electrically interlocked.
The control was a pair of push buttons on the end of a flexible cable.
One day the control fell and both push buttons hit the floor together.
Both contactors were energized and before the interlocks opened, the contactors had enough momentum to travel far enough to develop a line to line short.
I couldn't believe it and as I was demonstrating how it couldn't happen, it happened again.
Second issue. An overhead hoist to load logging trailers on trucks.
The contactors were mechanically and electrically interlocked.
The control again was on the end of a cord.
A cold day.
A driver loaded his trailer, raised the hook and then dropped the control in the snow and drove away.
The control had been warmed above freezing by his hands. It picked up a little snow.
The next driver had to hold the button down for some time to lower the hook.
The temperature was marginal and the heat of his hand melted the snow which ran down further into the push button and froze.
When the driver released the button, it stayed down and the line continued to descend. Eventually the line played off the winch drum and started to wind up in the opposite direction. When the winch tight lined at the top a lot of damage was done.
Solution:
The push buttons were electrically interlocked so that the up button could stop the down movement and vice versa.
Moral:
Electrical interlocks are cheap. Many control switches and contacts have both N/O and N/C contacts. Often all it takes is a jumper to add an electrical interlock.
Don't be afraid to use extra electrical interlocks.
(And stay away from aunt Karen when she's been drinking) grin
.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ThePunisher]

Thank you very much for all your comments...it was prompt and quick and it supplemented my perception and even better.

regards

 

[jraef]

That diagram is using the older standard JIC symbology, and those dotted lines depict a machanical interlock. The electrical interlock is inherent in the wiring scheme.

In the bygone days when we used JIC standards, you were required to have both mechanical and electrical interlockS on reversing, 2 speed and wye-delta starters (this is no longer the case, it's suggested, not required). The mechanical was preferred to be between the contactors, but it was difficult or impossible to implement that on some smaller NEMA contactors, so they allowed it to be done on the push buttons themselves. So control mfrs sold mechanically interlocked push buttons where if you press one, it pops the other one up. That was never as inherently safe as having it on the contactors and long ago, mfrs redesigned their contactor lines to always have the ability to be mechanically interlocked. You can still buy mechanically interlocked push buttons, but they are no longer stocked anywhere, they are made to order.
Link

"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[crshears]

If'n I recall correctly...

Our high-school electricity teacher, Mr. Mills, showed us three different ways to interlock forward/reverse motor contactors.

First: each contactor has an arm attached to its solenoid or plunger such that when one contactor is picked up, the other is mechanically prevented from closing.

Second: a normally closed contact in each contactor was wired in series with the start circuit for the opposite contactor. Advantage was that one could not reverse the motor without first pressing the stop button, which somewhat reduced issues from attempting to reverse the rotation of a motor or tool or other load while it was still spinning in the original direction due to inertia.

Third: a normally closed contact in one pushbutton is wired in series with the seal-in contact of the other, so one contactor drpos out before the other picks up [absent any welded contacts]. Disadvantage is that it is completely possible to reverse a motor very rapidly, which may or may not be an issue depending on the connected load.

These can all be used singly or in combination with either of the other two.

CR

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[FreddyNurk]

I can see the assertion that both pushbuttons would have separate contact blocks for each function, thus providing a mechanical means of interlocking the forward / reverse functions, but unless there's also some means of mechanically blocking one pushbutton when the other is pressed I'd still not trust it, nor call it a mechanical interlock. From the diagram, there's no indication that the pushbuttons aren't separate discrete units, and it would be entirely possible to hold one button down with a finger and then press the other one whilst the first is still held.

The diagram doesn't really function differently to interlocking the auxiliary contacts of the contactor to prevent inadvertent energisation, its just in the pushbutton mechanism rather than the contactors.

If it were me, I'd look for mechanical interlocking on the contactors if possible, although that isn't always easy to achieve, particularly in the smaller sizes.