Thread: Building an electrical control box

My field is small electronics not this stuff so much. But I can answer that in 2 ways
In some quality electronics they fitted a very small mains switch but the unit had a small power supply which was always on. This drove a zero crossover switch which only allowed power up to the main unit when the mains phase was passing through zero. The failure rate of both switch and unit was very low.
So it is not how often you use the switch its what the phase of the mains is at the time. It may last for 20 years or 1 use if its under rated, toss your coin.

Originally Posted by .RC.

Not yet, I expect it to take a week at least.... Takes a long time for stuff to travel north..

isnt Rockhampton in Australia? .....they taught me at primary school that it was...the lying buggas.....

Well it did turn up in the mail

However there is a problem... It has a NC contact and I need a NO for the control circuit...

Originally Posted by rrobor

My field is small electronics not this stuff so much. But I can answer that in 2 ways
In some quality electronics they fitted a very small mains switch but the unit had a small power supply which was always on. This drove a zero crossover switch which only allowed power up to the main unit when the mains phase was passing through zero. The failure rate of both switch and unit was very low.
So it is not how often you use the switch its what the phase of the mains is at the time. It may last for 20 years or 1 use if its under rated, toss your coin.

Hi Rrobor,

What you say is partially correct, but this is 3 phase, it's pretty hard to find a point in time when the voltage is zero across all three phases....

AC4 contactors are rated for 6x on make, 6x on break, whereas AC3 contactors are rated for 6x on make 1x on break.

So the issue is not whether the AC3 contactor can handle breaking the circuit or not, (if it failed after one operation, it's a faulty contactor) it's more just a question of how long it will last.

Looking at the data on telemecanique, it goes from 1,0000,000 operations down to 200,000 operations. Just taking a guess, if these contactors are rated at 200,000 operations, then you might expect failure after 40,000 operations, probably unacceptable in an industrial control application. But might be quite acceptable lifetime in a home workshop with intermittent usage. Especially if the cost is zero.

Regards
Ray

I have an Auxillary somewhere..i will send it up

Originally Posted by RayG

Hi Rrobor,

What you say is partially correct, but this is 3 phase, it's pretty hard to find a point in time when the voltage is zero across all three phases....

AC4 contactors are rated for 6x on make, 6x on break, whereas AC3 contactors are rated for 6x on make 1x on break.

So the issue is not whether the AC3 contactor can handle breaking the circuit or not, (if it failed after one operation, it's a faulty contactor) it's more just a question of how long it will last.

Looking at the data on telemecanique, it goes from 1,0000,000 operations down to 200,000 operations. Just taking a guess, if these contactors are rated at 200,000 operations, then you might expect failure after 40,000 operations, probably unacceptable in an industrial control application. But might be quite acceptable lifetime in a home workshop with intermittent usage. Especially if the cost is zero.

Regards
Ray

What I said is correct whether it be single or 3 phase, If your contacts are not rated at the peak current expected then it is luck, no more or less, how long they will last.
There is no calculation you can do to find out, its purely toss a coin stuff

Originally Posted by rrobor

There is no calculation you can do to find out, its purely toss a coin stuff

Maybe you should tell that to the manufacturer's... here is the calculation that rrobor says can't be done...



Regards
Ray

Ray you shot yourself squarely on the foot in this post before, please consider what you post, what it says and what I wrote. In that way you may just save blowing off another toe.

Originally Posted by rrobor

Ray you shot yourself squarely on the foot in this post before, please consider what you post, what it says and what I wrote. In that way you may just save blowing off another toe.

Not true, you are failing to understand that AC3 and AC4 contactors are both rated for the
load required, the difference is in the duty cycle.

Just curious, have you ever admitted being wrong?

Regards
Ray
PS I have a few toes left...

Ok point !/ Not true, you are failing to understand that AC3 and AC4 contactors are both rated for the
load required, the difference is in the duty cycle.

Now at no point did I say this was my field, in fact I said it was not. What I did say was that if you use an under rated contact you are dealing in lucky chance.
I took the word of 2 Qualified tradesmen that this switch is not correct so please take that point up elsewhere. If I am incorrect Yes its not an issue to say oops but what I said is correct so please read exactly what I said before you go off at tangents for reasons best known to yourself.

Originally Posted by rrobor

Now at no point did I say this was my field, in fact I said it was not. What I did say was that if you use an under rated contact you are dealing in lucky chance.
I took the word of 2 Qualified tradesmen that this switch is not correct so please take that point up elsewhere. If I am incorrect Yes its not an issue to say oops but what I said is correct so please read exactly what I said before you go off at tangents for reasons best known to yourself.

Funnily enough, I understand perfectly what you said, there's no reason for you to get upset about it.

Your point about zero voltage switching is not applicable for three phase circuits.

Your point about contactors failing because they can't handle the current is not applicable in this instance, for reasons i've been trying gently to explain.
I'm unclear as to why you aren't accepting that correction.

I'll expand a little on the zero voltage switching, (remember, we are talking single phase)
I have used a type of crydom SSR's in the past which are designed for zero voltage turn on, and zero current turn off, the problem with zero voltage turn off is that is often when the current is at a maximum for inductive loads.
So a solution which switches off at zero voltage crossing points on the mains is exactly what you don't want in a lot of circumstances.

Regards
Ray

I made the point as to zero cross over to explain a point, not as some form of new hope. At no time ever did I suggest your contact was too small, others did. I made the point that if you have current over the rating of the contact its lifespan will depend on luck. Where oh please tell where that is incorrect.

Originally Posted by rrobor

I made the point as to zero cross over to explain a point, not as some form of new hope. At no time ever did I suggest your contact was too small, others did. I made the point that if you have current over the rating of the contact its lifespan will depend on luck. Where oh please tell where that is incorrect.

Ok, I guess I've tried and failed, and I'm sorry I can't think of another way to explain it.

Regards
Ray

PS: Just out of interest, when I was talking before about inrush currents on contactor and solenoid coils, had you ever come across the problem before? Or was it all new information for you.

Please do not be condesending Ray. That you do not understand what people write, and then explain concepts that were not the issue is your concern.
I made the point about current flow through contacts and you post specs on a contactor. Can you really not understand that that was not the issue I wrote about.