Thanks for the response.

We aren't allowed to use time delays, As far as I know. I should ask.

Edit : yes, we are only supposed to use control relays and the aux contacts on the motor starters.

 

It is not possible to my knowledge.

 

Do you know how much overlap there is on a slow break PB switch?
Haven't been able to find any info and I've never had the need.

Wondering if it would output a 20ms+ pulse if NO and NC were in series.
To replace TR1 contact.

 

You should be able to wire the relays to create latches and then use relay-logic to set and reset the latches as needed.

But this seems pretty advanced for the level of course that the OP appears to be taking.

 

You should be able to wire the relays to create latches and then use relay-logic to set and reset the latches as needed.

But this seems pretty advanced for the level of course that the OP appears to be taking.

The only way that I know of to make a latching design that doesn't immediately progress to the next latch and to the next and so on, is with a pulse to set the latch. This requires a timer.There is another way to create a pulse; that is to feed through a relay's NC contact to its coil, but this creates not just one pulse but a tattoo gun. In theory you could use the first pulse to latch another relay that interlocks the first, but I've never been able to make that work in practice, and even if it did work, the system would grow exponentially in numbers of relays as more latches are needed.

 

All of the professors tell me it's possible, nobody else is aware of how to do it. They said you need three control relays and a motor aux's.

 

All of the professors tell me it's possible, nobody else is aware of how to do it. They said you need three control relays and a motor aux's.

WOW, with only THREE relays? I'm throwing the B.S. flag (not on you, but on the professors). Have these professors built the circuit? Or any circuit? I will eat my shorts and post a picture of it here if it's true. Literally. Please, pledge to come back here and post the solution once it's given.

 

Pinky swear I will.

 

Still haven't figured it out.

It was strongly hinted today that it involved parallel connections, still pretty stomped though.

 

I did not get much time today, but it seems as a start that a logic index or relay state sequencer would be used for the four states.
i.e. starting with
000 start (all OFF)
100 Motor A on (B off)
110 Motor B on (A off)
111 Both Motor A & B on
000 reset counter/latch. Both OFF.
Or a variation of the above?
Max.

 

The only way that I know of to make a latching design that doesn't immediately progress to the next latch and to the next and so on, is with a pulse to set the latch. This requires a timer.There is another way to create a pulse; that is to feed through a relay's NC contact to its coil, but this creates not just one pulse but a tattoo gun. In theory you could use the first pulse to latch another relay that interlocks the first, but I've never been able to make that work in practice, and even if it did work, the system would grow exponentially in numbers of relays as more latches are needed.

I'm gonna think about this some when I get a chance. I've done quite a bit of asynchronous event-driven logic design on ICs using latches and I'm curious whether the approach I used translates to relay-based designs. It seems like it should, but there may well be a hidden gotcha that I haven't thought about.

I'm not too up on the terminology related to motor control components. When we talk about a "control relay", I'm envisioning a SPDT switch that is activated by a relay coil such that we have a common terminal and a NO and a NC terminal (referring to when the coil is deenergized). I'm also assuming that these come in multi-pole designs that would be fair game.

But I only have a vague notion of what a "motor starter" is, particularly within the context of this problem. Could you describe it (as it applies to this problem)? I'm again envisioning a device that, when energized, provides power to a motor, perhaps in a way that meets the start-up requirements of the motor. But does it have to remain energized to keep the motor running, or is that normally done via a second relay?

 

But I only have a vague notion of what a "motor starter" is, particularly within the context of this problem. Could you describe it (as it applies to this problem)?

A motor starter is usually a contactor which is a heavy duty (3ph) relay, which almost always incorporates N.O/N.C. auxiliary contacts for comunicating to other logic components such as control relays etc or for latching/retaining contacts etc..
Max.

 

There are simple ladder logic equivalents for AND, OR, NOT and so forth. There are also ways to use those basic functions to build memory and thus state machines. You obviously have a challenging task given your level of knowledge. Now a basic truth table has three columns:
1st column is Boolean variable A
2nd column is Boolean variable B
3rd column is the "value" of the Boolean Function

Let's do an example:

"Output is a 1 if A is a 1 AND B is a 1"

Truth Table looks like

 A  B | Y
---------
 0  0 | 0
 0  1 | 0
 1  0 | 0
 1  1 | 1

As you can see the four rows of the truth table contain all possible combinations of the 2 valued Boolean inputs. If we had 3 inputs the table would have 8 rows. Can you write the 8 combinations of inputs for such a table.

In ladder logic this table can be implemented by two normally open switches in SERIES with the coil of a control relay. Both switches must be closed to energize the coil.

The OR function can be implemented with two normally open switches in PARALLEL with each other, and in series with the coil.

There is a fundamental combinatorial circuit called a latch. A latch is like a single binary digit of memory. Can you find the Boolean equation for a latch? I'll bet you can.

Good Luck.

Edit: (after reading other posts)
Sounds like the "state" information is kept in the contacts on the motor starter. Does it have contacts for "Motor On" and "Motor Off". So you could write an expression like:

"START/STOP" .AND. "MOTOR A OFF" .AND. "MOTOR B OFF" --> Energize "MOTOR A ON"

 

Ladder Logic.
http://en.wikipedia.org/wiki/Ladder_logic

Electrical engineer students normally are not taught those...

Nonsense. We learned them where I went.

 

I did not get much time today, but it seems as a start that a logic index or relay state sequencer would be used for the four states.
i.e. starting with
000 start (all OFF)
100 Motor A on (B off)
110 Motor B on (A off)
111 Both Motor A & B on
000 reset counter/latch. Both OFF.
Or a variation of the above?
Max.

can this be done with 3 ordinary control relays and a couple of starter aux contacts?


Wbahn,
The "control relay" refers to ordinary relays, usually 4PDT "ice cube" relays in my experience, but can also be SPDT, DPDT. Timer relays, dual coil latching relays, pneumatic time delays, and misc specialty relays are usually not referred to as "control relays."

 

Nonsense. We learned them where I went.

I'm not aware of any current EE curriculum that includes them -- though I am sure that there are going to be some, particularly in programs that lean more toward power and industrial control.

I've never had any formal education in ladder logic. My only significant exposure was the wiring diagrams for the hydraulic test stand I worked on in the Air Force and I had to figure out what everything meant on my own. Doing so was actually rather fun, but of course if the test stand didn't use something, then I got no exposure to it at all.

 

Have a question that has my entire class stumped ;

We have two motor/motor starters, with plenty of aux's (Both normally opened and normally closed), and control relays.

We have to wire a single push button start/stop to do the following ;

On the first start press, Motor A turns on. When you hit stop, it stops.

On the second start press, Motor B turns on. When you hit stop, it stops.

On the third start press, Motors A and B both turn on. When you hit stop, both stops.

After that, it has to repeat.

We're all drawing a blank and I've been pondering it all day, and I have about 60 sketches of "****, that's not rights". Any help?

I need some clarification. "It says that there is a single push button start/stop", but then it talks as though there are two buttons - one start button and one stop button.

Which is it?

 

There are simple ladder logic equivalents for AND, OR, NOT and so forth. There are also ways to use those basic functions to build memory and thus state machines. You obviously have a challenging task given your level of knowledge. Now a basic truth table has three columns:
1st column is Boolean variable A
2nd column is Boolean variable B
3rd column is the "value" of the Boolean Function

Let's do an example:

"Output is a 1 if A is a 1 AND B is a 1"

Truth Table looks like

 A  B | Y
---------
 0  0 | 0
 0  1 | 0
 1  0 | 0
 1  1 | 1

As you can see the four rows of the truth table contain all possible combinations of the 2 valued Boolean inputs. If we had 3 inputs the table would have 8 rows. Can you write the 8 combinations of inputs for such a table.

In ladder logic this table can be implemented by two normally open switches in SERIES with the coil of a control relay. Both switches must be closed to energize the coil.

The OR function can be implemented with two normally open switches in PARALLEL with each other, and in series with the coil.

There is a fundamental combinatorial circuit called a latch. A latch is like a single binary digit of memory. Can you find the Boolean equation for a latch? I'll bet you can.

Good Luck.

Edit: (after reading other posts)
Sounds like the "state" information is kept in the contacts on the motor starter. Does it have contacts for "Motor On" and "Motor Off". So you could write an expression like:

"START/STOP" .AND. "MOTOR A OFF" .AND. "MOTOR B OFF" --> Energize "MOTOR A ON"

OK, I'll bite. Here's the logic table:

|    Inputs   |            Stored bits             |      Outputs    |
|             |  Run mode | Last motor running was:|                 |
|Start | Stop |           |    motorA   |  MotorB  | MotorA | MotorB |
|  0   |   0  |     0     |       0     |     0    |    0   |    0   |
|  1   |   0  |     0     |       0     |     0    |    0   |    0   |
|  0   |   1  |     0     |       0     |     0    |    0   |    0   |

|  1   |   0  |     0     |       0     |     0    |    0   |    0   |*
|  1   |   1  |     1     |       0     |     0    |    1   |    0   |
|  0   |   1  |     1     |       0     |     0    |    1   |    0   |
|  0   |   0  |     0     |       0     |     0    |    0   |    0   |

|  1   |   0  |     0     |       1     |     0    |    0   |    0   |
|  1   |   1  |     1     |       1     |     0    |    0   |    1   |
|  0   |   1  |     1     |       1     |     0    |    0   |    1   |
|  0   |   0  |     0     |       1     |     0    |    0   |    0   |

|  1   |   0  |     0     |       0     |     1    |    0   |    0   |
|  1   |   1  |     1     |       0     |     1    |    1   |    1   |
|  0   |   1  |     1     |       0     |     1    |    1   |    1   |
|  0   |   0  |     0     |       0     |     1    |    0   |    0   |

|  1   |   0  |     0     |       1     |     1    |    0   |    0   |
|  1   |   1  |     1     |       1     |     1    |    1   |    0   |
|  0   |   1  |     1     |       1     |     1    |    1   |    0   |
|  0   |   0  |     0     |       1     |     1    |    0   |    0   |
*We need to make a provision to start with motor A, when there is no stored bits.

So, how do we store 3 bits of information and set 4 unique output states, using 3 ordinary relays, and a couple of aux contacts?

 

So, thinking about it just a bit, here is what I've got so far.

Imagine that you have two relays each of which stores whether or not the motor it is associated with was running at the time the stop button is pressed.

With those two relays, you can then route the signal from the start button to the motor starter coils in a very simple way.

This leaves you with one control relay available to implement the "write" to the other two relays when the stop button is pressed.

 

So, thinking about it just a bit, here is what I've got so far.

Imagine that you have two relays each of which stores whether or not the motor it is associated with was running at the time the stop button is pressed.

With those two relays, you can then route the signal from the start button to the motor starter coils in a very simple way.

This leaves you with one control relay available to implement the "write" to the other two relays when the stop button is pressed.

Do you have time to draw that out sir? Doesn't have to be in ladder if that's foreign to you; I will translate.