Push Button Station on a Motor Controller

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Dennis Alwon

Dennis Alwon

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Why do we use momentary contact buttons for a stop/ start to control a contactor. Why not just an on/off switch or maintain contact. It seems the momentary setup adds an aux relay that controls the power-- what advantage is this to a maintain contact.

I have a pool contactor that someone has bastardized and it now works with a toggle switch mounted on the door. This is a 3 phase contactor running a 10 hp pump. I don't see the advantage here.
 
One of the biggest reasons is to prevent automatic restart after a power failure. It also makes the addition of other controls easier.
 
Dennis Alwon said:
Why do we use momentary contact buttons for a stop/ start to control a contactor. Why not just an on/off switch or maintain contact. It seems the momentary setup adds an aux relay that controls the power-- what advantage is this to a maintain contact.

I have a pool contactor that someone has bastardized and it now works with a toggle switch mounted on the door. This is a 3 phase contactor running a 10 hp pump. I don't see the advantage here.
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In many applications it is desirable for safety or other reasons to not have equipment restart if power is interrupted, hence the age old practice of momentary switch and holding contact. In some cases, like your application, a maintained switch might be fine. However, when I find a motor starter that has been hacked, it often has left the OL protection inoperative due to people who have no understanding of what a starter does and just inadvertently turn it into a glorified contactor.
 
Dennis Alwon said:
Why do we use momentary contact buttons for a stop/ start to control a contactor. Why not just an on/off switch or maintain contact. It seems the momentary setup adds an aux relay that controls the power-- what advantage is this to a maintain contact.

I have a pool contactor that someone has bastardized and it now works with a toggle switch mounted on the door. This is a 3 phase contactor running a 10 hp pump. I don't see the advantage here.
Click to expand...

I agree with Don on the automatic restart thingie.
I'd like to add though that many of the equipment that has momentary contacts come packaged with limit switches. It makes it a lot easier to work in all the limit switches and what not. Having momentary contacts also allow for parallel wiring of the controls as well.
Let's say I have a piece of equipment that I want to control from 3 different locations around the machine? Am I gonna install 3 way switches on it????
And last, how else are you gonna have two buttons on the controller?
I don't know, having only one button (or a toggle switch) for on/off for some machines seems a little hazardous to me. I like the "BLACK IS ON/RED IS OFF" buttons. Faster human response time in an emergency????
 
Strife said:
And last, how else are you gonna have two buttons on the controller?
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In the case of the swimming pool they wanted to add a safety switch remote from the motor that just was an emergency off. Easy enough-- switch is run in series with the other switch. It depends on the application I guess. I was thinking solely from the swimming pool perspective and I am not sure what the worry would be if the motor came on after an outage since the pump runs 24/7 during the season anyway.
 
Dennis Alwon said:
In the case of the swimming pool they wanted to add a safety switch remote from the motor that just was an emergency off. Easy enough-- switch is run in series with the other switch. It depends on the application I guess. I was thinking solely from the swimming pool perspective and I am not sure what the worry would be if the motor came on after an outage since the pump runs 24/7 during the season anyway.
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In commercial applications, a shut down switch is required. Has nothing to do with controllers though:)
In fact, I wire these shut downs right between the coil and everything else.
 
The common maintained On/off switch normally controls a motor at the motor's full voltage rating. These are usually called a manual starter. However the ampacity or HP is limited to smaller motors.
With the momentary start stop PB stations it actually controls the motor contactor (a relay of sorts). The start PB is momentary, pushing it energized the contactor coil pulling in the contactor, once pulled in you can let go because the contactor has its own contact which itself closes (holding circuit) and holds the contactor in.
The stop PB is a normally closed contact and when you push it the contact opens. This N/C contact in placed in series with the coils holding circuit. Pushing the stop PB breaks that circuit and the coil is de-energized and drops out. This commonly is called 3-wire control.
The control circuit as described is commonly 120vac and can be other voltages such as 24vac. The coil voltage of the contactor is the same as the control voltage in this example. When you are controling a 480v motor using 120vac a 120vac control voltage is much safer that 480v.
As one of the other posters pointed out when power is lost with this type of control the contactor drops out and will not pull back in until the start PB is pushed where a manual starter would simply restart the motor if left in the ON position.
Also, with 3-wire control is is posible to have multiple PB stations that will control the same motor.
 
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