plc control circuit

A PLC motor control circuit is designed to start and stop using only a single button. What could be wrong with the circuit if the motor fails to stop at the second push of the button?

Thank you

Start by dividing your problem in half.
Is the PLC telling the motor to run? (Is the output on?)
If yes, then the problem is within PLC or the code.
If no, then the problem is within the hardwire or motor area.

Within the PLC:
Is the output shorted?
Is the output forced?
Is the motor getting a start signal from elsewhere in the code?
Does the push button return an input when pressed?
Have you made a mistake in the code?
Does push button "Bounce" cause the motor to stop and restart a random number of times, too quick for you to notice?

Outside:
Have tou got an interposing relay, and does it release when the output switches off?
Does your output card have a leakage current higher than what will allow your relay/contactor to release?
Is your contactor welded in?
Do you have a hard wired override circuit, and is it activated?
Is the motor being driven by the load?

Suggest you search this site for "flip flop". What brand of PLC? Is this a student assignment? Show us what you have done and I am sure you will get further help.

rodolfo said:

A PLC motor control circuit is designed to start and stop using only a single button.

Click to expand...

Can anyone give me a practical application where you would do this with a motor?

While on many uses, safety would not allow this. I can see this being acceptable for some pumping appplications and maybe other uses.

I suspect that there might be some for a light in an automation application of a plc that I am just not aware of.

regards.....kc

CaseyK said:

Can anyone give me a practical application where you would do this with a motor?

Click to expand...

This is very common in applications controlling a large number of motors. Consider if you have 20 motors. You would have either 20 HOA switches or 20 Start and 20 Stop buttons. That’s a lot of switches and even more buttons. Plus consider the number of inputs and conductors. So in the interest of conserving space and money, companies have gone to the single button approach. You use one button to start and stop all 20 motors.

Here’s how it works – push the button once means you want to work with motor 1, push it again and it turns the motor on, push it again it turns the motor off. Now if you want to work with motor 12 you push the button 12 times and then push it once to turn the motor on or push it again to turn the motor off. Now here’s the cool part, say you are working with motor 12 and you want to work with motor 10, you might think that you need to push the button 20 times to get to motor 10, but not so, first you push the button once to tell the system you want to count backwards, then push the button twice to go from 12 to 10, then push the button once to turn on the motor 10 or push it again to turn off the motor 10.

The single button approach works for alarms as well. In the interest of keeping the system simple the alarms correspond to the motor numbers. So if you want to work with the alarms for motor 1 you press the button 101 times to get to motor 1’s alarms, then press the button once to silence the alarm or press the button again to clear the alarm.

It’s a great system, I use it all the time. It’s real easy for the operators to understand. You only have to teach the operators how to use one button as appose to 20 switches or 20 Start and 20 Stop buttons.



Of course I'm just joking.

Actually, I do that all the time with HMI's. For Independent modes (local, manual start/stop/speed), I just use a single button on the HMI. Most operators find it easier to deal with, as almost all of the discrete-operators (Auto/Manual, Indep/Coord, etc) all use the single-button toggle approach. This applies mostly to VSD's, though I also use it for pumps and such.

The face of the button (color and text) always indicate exactly what state the controlled device is in right now.

I wouldn't use that method for motors probably if I were using discrete pushbuttons, but for web-clamp's and such, I will, as long as the PB is illuminated.

CaseyK said:

Can anyone give me a practical application where you would do this with a motor?

While on many uses, safety would not allow this. I can see this being acceptable for some pumping appplications and maybe other uses.

I suspect that there might be some for a light in an automation application of a plc that I am just not aware of.

regards.....kc

Click to expand...

Tark said:

Of course I'm just joking.

Click to expand...

Good One Tark!!!

You had me right down to the end!

Best chuckle I've had all month!

Casey,

One time at a sawmill, the owner asked me to add Start/Stop Panelview controls for an existing chip conveyor in a remote part of the plant. There was no practical way to get any additional wires routed to the area. Working with the plant electrician, we found that there was ONE spare wire coming into the PLC from a control panel already at the remote location.

We tied the single wire to a spare 120 volt Input, and I added an "alternator" button on the Panelview screen, and that is the way that conveyor still works today.