Controlling AC contactor by NC & NO push button

[jacklam2048]

Hi，
I cannot find out why people use a Normally Close and a Normally Open push button to control an AC contactor (see figure A), instead of just using a normal ON/OFF switch (say a rocker or sliding switch) (see figure B).

What are the advantages in using NC & NO push buttons ? is it more save ? and why ?

Thank you for all in advance.

Jack

 

[Harald Kapp]

Fig. A makes no sense. S_NC is short circuited by the wiring. It has no effect at all. When you try to open the connection 1-2 by pushing S_NC current will still pass through the wiring from L to L1

What are the advantages in using NC & NO push buttons ? is it more save ? and why ?

I can't see why pushbuttons should be more safe than a switch. One difference between the two is when mains power fails and returns:

• With pushbuttons the contactor will turn off when mains power fails and will not automatically turn on when it returns.
• With a switch the contactor will turn on as soon as mains power returns and thus the load will turn on, too.

Turning the load on when mains returns is undesirable for two reasons:

1. The instantaneous load jump on the grid will be enormous when all loads are on at this time. Take alone the massive number of HVAC installations that would draw an enormous peak current when all turned on simultaneously. This might lead to an immediate next shutdown of the grid.
2. One may not want to turn any (heavy) machinery to turn on on its own. Be it that a defined startup sequence needs to be fulfilled or simply for safety reasons. Imagine a saw that starts running unexpectedly while someone is working on it (maybe trying to find out why it stopped).

 

[jacklam2048]

Thank you Harad !
I didn't think about the points you raised. Thank you very much !

 

[Bluejets]

Wiring in both diagrams is just plain wrong.

 

[Minder]

In Fig A , a N.O. contact on the relay would be wired across the N.O. momentary start PB, this contact would then act as a retaining contact when the contactor is picked up.
It is used primarily in E-Stop -Safety circuits where multiple N.C. Stop PB are often used in the series string.

 

[jacklam2048]

Hi, I have just revised two drawings, I think both options work this time.
But in Fig D, the two button switches (SNC & SNO) have to carry the current of the load so is not preferred (I think). Do you agree ?

 

[Minder]

Normally on a 3ph contactor there is a smaller aux contact for the coil latching circuit and normally only switches the coil current.

 

[Bluejets]

Double up on what Minder said plus........ Fig.D , orange line to A1 should go to T2
AND....L to L2.
all as fig.C
Assuming line voltage control circuit.

 

[AnalogKid]

The two switches could be at opposite ends of a looooooong conveyor belt.

ak