220v AC to 5v DC

[dano]

Let me start by giving a little background info. I repair CNC machinery for a living. The other day I had a problem that required I know whether the 3 phase coming in to the machines main relay was dropping out or if the relay itself was dropping out first. It was a situation of which came first the chicken or the egg. I run into this type of problem on occasion, not enough to justify expensive test equipment so I'm looking for an inexpensive way of monitoring the voltages. I was thinking along the lines of a PIC to compare two voltages and let it determine which one went low first. My problem is how to convert the 3 phase to something the PIC can handle. Any advise from the experts here?

thanks

dan

 

[zachtheterrible]

Very simply: a transformer. Maybe use a clamp diode to clip the wave if it goes above what the pic's analog input can handle.

 

[dano]

I've thought of a transformer and a bridge circuit but my concern is will the signal be delayed by the magnetic field of the transformer decaying when the voltage is removed. This could give me false info.

Dan

 

[john1]

Hi Dano,

I am intrigued by your post - '220v AC to 5v DC'
The question in your post seems to bear little relation to the title.

Not knowing what CNC machine is, i tried 'Google', which was
ovewhelming in its wide expanse of information about CNC machines.

Except of course, that it did not give the information required,
namely - what is a CNC machine ... ?

Then i hit upon the simple answer of using Google Images.
Judging from the pictures thus obtained, i now think that they are
the latest type of computer controlled metalworking machines like
lathes, routers, milling machines, grinders, and suchlike.
Most of which run on a three phase supply.

Deciding what the main relay is was not so difficult, i have decided
that this is the relay that feeds the three phase supply to the
machine, and can cut the supply if the system decides to do so.
Here in England these are called 'Contactors'

The operator can usually press a button to engage the contactor to
feed the supply to the machine, very often the machine itself can
disengage the contactor, or the operator can press another button
to stop the electrical feed to the machine.

Now it seems to me that your problem is that the contactor is dis-
engaging, either as soon as its engaged, or shortly afterwards.
Although why this should bring into question the three phase supply
to the machine i cannot quite see.
Unless you are thinking that there may be a brief interuption on one
of the phases, causing the contactor coil to be interrupted and drop
out, leaving the machine dead.

Anyway, whatever the reason, you want to decide if the three phase
supply to the contactor is dropping out,
or if the contactor is disengaging.

Qu:
****
The other day I had a problem that required I know whether the 3 phase
coming in to the machines main relay was dropping out or if the relay
itself was dropping out first.
****

That doesn't sound too difficult.
Using a micro-controller might be a little excessive.

But first, have i got the basic question correct ... ?

And could you explain what you mean by 'first'
Do you mean the contactor is dropping out and then the supply to it
is removed?
Do you mean the contactor is dropping out at apparently the same
time as the supply to it is being removed ?
What is next, after the contactor is first ?

John

 

[dano]

John, sorry for not being clear enough. Yes you did get the initial question right .
The CNC machines that I work on are indeed computer controlled. Some are lathes others are milling machines. Some would fit on a table top, others are as large as a small house. The machine I happened to be working on was faulting giving a phase loss error. The machine has a contactor which controls the 3 phase power to the machine. This contactor was dropping out, but I couldn't tell if it was because the machine sensed the phase loss and shut the power off to the coil of this contactor or the contactor was dropping out causing the phase loss error. So I could'nt tell which happened first,A) coil voltage turned off or B) 3 Phase interrupted. The problem ended up being the contactor (it would drop out whenever it felt like it). So I don't need a solution to the problem on this machine but I do see similar problems and would like to be able to "see" whats happening. I want another "tool" in my toolbox for testing problems of this sort.

thanks
dan

 

[dano]

I should add that the 3 phase is 220 volts AC, I was thinking of converting it to 5 volts DC but not smoothing the rectified voltage and then use a missing pulse technique to see if I lost a phase. I don't know if this would work or if there is a better way.

dan

 

[gerty]

We had a simillar problem where I used to work. We wired two 220v relays across the incoming line, phase a to b and phase b to c. We then wired the contacts to the PLC as inputs. Anytime there was a problem we could do a
'contact histogram' which would show the last state.You cold also do a histogram on the stop button to see if it's the operator stopping the machine, which was the problem in our case.We had Allen Bradley PLCs , other brands may not has the contact histogram option..

 

[john1]

Hi Dano,

Thanks for the more detailed explanation.
Yes, i can understand wanting an indicator of some sort which will show the order
in which the voltages disappear.
The contactor coil is probably AC operating, although it doesn't have to be, as
i'm sure you know. It is probably fed through a small chain of other contacts on
smaller relays, any of which could cause it to dis-engage.
These smaller relays in turn may be arranged with optical sensors here and there,
or temp sensors, or whatever the manufacturers designers think is relevant to
shutting off the supply.
Sometimes on spinning parts the motors are deliberately shorted to provide a
quicker emergency stop time, i'm sure you know about this.

In your position, faced with this sort of problem i would probably ask for an
eight track memory oscilloscope to be left running as a monitor on the problem
machine, with a delay on a one or two second sweep, connected to the various
points that would give some information on the source of the problem.
The delay arranged to give the last one or two seconds before an unexpected
shutdown occurred, up to the point of shutdown.

But you are looking for a simple device that would merely indicate which of two
points was the last to remain 'live'. This is awkward because you may wish to
compare DC circuitry with AC circuitry, i don't think you can confidently make
those comparisons because the DC circuitry will (almost certainly) be backed up
by capacitors.

A small and simple to make device that will do this job for DC points, could i
feel be easily designed, and you could build such a device for a negligible cost.

However, for AC the situation is different, and your suggestion of comparing the
pulses or waves is certainly worth thinking about.

I would be interested to read any thoughts you have on this.

Regards, John

 

[Russlk]

If you use a three phase transformer and full wave rectifier, a missing phase is easily detected by the increase in ripple. A level detector, set just below the ripple will give instantanious (or as close as you can get to it) detection.

 

[dano]

Thanks for the replies. I think I'll order a transformer and see what kind of results I get.

Dan

 

[ifeji john]

request

pls i need information on light dependent switcher