Mechanical Relay question

Hello People.

I am about to finish my automatics education.

For my final "exam" i got a hard, if not impossible task.

The task is as follows:
One push-button starts and stops a motor.
I can ONLY use basic mechanical relays. No time delay, counters or pulse relay of any kind.

I want to know if its possible to this with just basic relays?

Kind regards "SilverHelmet" aka Marius Boro from Norway

Good to hear that

Here's what i have come up with so far.

I hope I haven't misled you...... To me a "simple relay" is an eight pin, dual form c.

Think "toggle" circuit.

Stationmaster

This is a basic/simple relay for me

Your print shows 3 normally open contacts for K3. Not possible with a dual form c like the one in your picture.


Stationmaster

I think i got it now

Any physical toggle circuit needs to be completely independent of element timing. Stated another way, what happens if S1 goes to one state or the other and never releases? The circuit should achieve a stable state. I don't thnk yours will unless you specifically use overlapping contacts to guarantee your desired states are maintained.

Keith

You still don't seem to be using the 8-pin relays as they are designed. K1 for example has 2 normally open and 1 normally closed contact in your design, but neither of the n.o. contacts share a "common" with the n.c.contact as they would in actual relays. No such limitation exists when programming ladder in a PLC, but when using discrete relays you HAVE to take into account the INTERNAL format of the relay. Hint: It's printed right on the side of the relay.

Stationmaster

Looks like you got it to me....

Here was my solution, just slightly different... And in JIC format.

I used 8 pin relays, and one aux contact from the Motor Start. You could also use 11 pin mechanical relays, as you did.

View attachment RELAY TOGGLE-Model.pdf

-MUR

It's possible to do with 8 pin relays, although I don't think that the description requires it... 11 pin's are still basic mechanical relays, 8 pin's are more common, but still the same beast.

-MUR

I would say this will work 99% of the time. But this design counts on the relative speed of fairly loosely linked contacts. I'm betting 1 out of 100 times it won't toggle quite right.

Keith

kamenges said:

I would say this will work 99% of the time. But this design counts on the relative speed of fairly loosely linked contacts. I'm betting 1 out of 100 times it won't toggle quite right.

Keith

Click to expand...

The only problem I see, is if the button is not held down long enough, for K1 or K2 to energize, and not K3 to toggle. I don't know how to get around this with standard relays What specifically are you seeing?

-MUR

With standard relays you have multiple arms moving from NC contacts to NO contacts on energization. Most though do not GUARANTEE that ALL NC contacts will be broken before ANY NO contacts close. This is where errors can creep in especially after wear time.

Otherwise I'm not seeing a possible race problem as implied by kamenges.

There's nothing logically wrong with it. But with a typical form C relay the contacts are somewhat independent of each other. Your circuit assumes that the state of all contacts for a given relay will fully change at a given instant. But, for example, on the button press that turns the toggle off, you could lose CR3 before the NO contact of CR2 can complete the latch. The NC CR2 contact that releases the M1 latch and the NO CR3 contact in the CR2 activation branch with it can open before the NO CR2 contact can complete the latch. You are counting on the contact carrier momentum to be enough to complete the latch motion.

While this is unlikely it is possible. You can get around this by judicious use of early make and late break contacts. Or you can set up a latch for each state of the button press sequence so you hold a record of the button presses that are more independent of each other. This would take a couple more relays but it would get you the better part of that 1% error possibility.

Keith