clearing contacts

[khtelect]

This may seem like a dumb question, but are clearing contacts on a mechanically held contactor built in or do you use the auxillary contacts on the side. Square D is the manufacturer.

 

[jim dungar]

It depends on the contactor.
Which one are you using?

 

[hillbilly1]

They are usually factory pre-wired, normally breaking the common, jumpers are already installed.

 

[jeremysterling]

I don't think its a dumb question.

I asked a similar question regarding the trip coil clearing contacts versus the aux contacts of a SqD shunt trip breaker.

Sorry I do not know the answer to your question about the contactor, but my guess would be that you are alright not using the aux contacts. You could verify this with your ohmmeter.

 

[tkb]

What is a clearing contact?
I have been doing motor control for 30 years and never heard that term.

Is is the same as a holding contact?

 

[jim dungar]

What is a clearing contact?
I have been doing motor control for 30 years and never heard that term.

Is is the same as a holding contact?

Actually just about the opposite.

A holding contact maintains the circuit to the coil even after the pushbutton is 'released' - it keeps the coil energized.

A clearing contact breaks the circuit to the coil even if the pushbutton is 'maintained' - it keeps the coil de-energized.

A circuit with a clearing contact requires some method of being closed (reset), similar to the way a holding contact circuit requires a method of being opened.

 

[tkb]

Well...I guess you are never too old to learn something new.

 

[SG-1]

Would another name for a clearing contact be " interposing contact " ??

 

[dicklaxt]

Actually just about the opposite.


Right On jim,,,,,,,A clearing contact breaks the circuit to the coil even if the pushbutton is 'maintained' - it keeps the coil de-energized.


Example of a clearing contact:

A Pressure Switch in series with the stop button in a motor control circuit,,,,,,,,,,often referred to as a "run permissive" as well.



Example of an Interposing Relay:

A relay controlled by the Pressure Switch mentioned above with one of the contacts of the relay replacing the Pressure Switch in the schematic.The interposing relay would be used most often when an additional function off the relay is needed.ie: one relay contact controls the motor and another contact from the same relay is alarming High or Low Pressure in what ever the Pressure Switch is being activated by.

The interposing relay coil could be a timer and keep the motor on line for a preset time and then would shut the motor down if the high or low pressure condition did not clear.

dick

 

[hillbilly1]

I don't think its a dumb question.

I asked a similar question regarding the trip coil clearing contacts versus the aux contacts of a SqD shunt trip breaker.

Sorry I do not know the answer to your question about the contactor, but my guess would be that you are alright not using the aux contacts. You could verify this with your ohmmeter.

Yes, it is a very good question, As with the shunt trip, latching contactor coils are not designed for contiuous applied power, they will over heat and eventually self destruct. On the large old GE "hammer contactors", the latch and unlatch coils are actually 70 volt rated for the 120 volt models, and 120 volt rated for the 277 volt coil models. This gives them the added "ummph" to open and close large sets of contacts. Once closed, coil power is removed, so you don't have the famous hum of electrically held contactors. This also saves energy because the coil does not have to be continously powered. Also if the control circuit fails, the contactor remains in the last state it was in.

 

[augie47]

I'm sure there are 'exceptions', but every mechanically held contactor I have encountered had factory installed/wired clearing contacts.

 

[dicklaxt]

Well kiss my petunia,I just went back and reread the OP,I sure missed the mechanically held,,,,,,,sorry, other posts have it covered, I like hillbilly's answer but all have merit.
dick

 

[BJ Conner]

Unclear

Unclear

tkb said:
"What is a clearing contact?
I have been doing motor control for 30 years and never heard that term.

Is is the same as a holding contact? "
__________________

I have been in in for 30+ years as well, never head of a clearing contact.
Its always been a Latching Circuit and it commonly uses a 42b contact is used to latch a motor in the run position.
A stop button in a simple motor control circuit unlatches the circuit and stops the motor. A Start button latches the circuit and starts the motor.
If you stop a motor with a "clear" contact do you start it with an "Unclear Contact?
It's unclear to me why a descriptive term like latch got changed to clear.

 

[ATSman]

More Stuff

More Stuff

Just to add to the mix. Clearing contact (also called "cut-out" contact) as I've always undersood means it "clears" the circuit or removes voltage to the coil of the operating mechanism. The mechanism is mechanically latched and the coil, as other posts have noted, is usually not designed for continuous duty. All Automatic Transfer Switches that employ solenoids or motors in the transfer mechanism have clearing contacts or "limit switches" in series with the coil. These switches are usually of the "Microswitch" design with the Form C (1-NO, 1-NC + common) configuration and are actuated by the mechanism once it reaches its full travel.
I have replaced many solenoids in ATSs that have burnt up because, during transfer, they do not reach full travel to lock into the normal or emergency position. This is usually due to lack of maintenance; the grease along with dirt dries out and prevents smooth operation of the mechanism. This contact does not open, the coil stays energized and the motor or solenoid burns up. The motto in our business is "Dried Grease Is Our Friend":grin:

 

[SG-1]

Thanks, cut-out contact I understand & the examples helped.

 

[jim dungar]

It's unclear to me why a descriptive term like latch got changed to clear.

A latching contact and a clearing contact are not the same item.

An operating coil can be made smaller if it is not rated for continuous duty. Clearing contacts are used to ensure that a non-continuous duty coil is not abused. Most shunt trip solenoids and latching mechanisms use non-continuous rated coils. As tonytonon said, the clearing contact is often a limit switch. 30+ yrs in the business and I have never called them 'cut-out contacts'.

 

[SG-1]

A latching contact and a clearing contact are not the same item.

30+ yrs in the business and I have never called them 'cut-out contacts'.

Jim, the old Westinghouse MG-6 Auxillary Relay ( now ABB ) could be ordered as an Operate/Reset (latching ) type. See item 9 on page 2 & discription on page 3.--operating coil cutoff contact-- Not exactly the same words, but very close. On page 7 it is called a coil interrupting contact.


http://www05.abb.com/global/scot/scot229.nsf/veritydisplay/758286f30ed4ce32c1256e7e00345ff0/$File/DB41-803e%20%20%20%20%20MG-6.pdf

You may have to cut & paste, because this did not appear as a link.

 

[jim dungar]

See item 9 on page 2 & discription on page 3.--operating coil cutoff contact-- Not exactly the same words, but very close. On page 7 it is called a coil interrupting contact.

Thanks for the reference.
Old habits die hard. I am pretty sure that 'cut-off' contacts will not stay in the front of my memory.