In this article, we will show you different ways to program motor start/stop functions.
As most of you know, we are against giving you clear rules and practices when it comes to programming, because one of the benefits of programming is that the possibilities are nearly limitless. Users say it would be nice if there were at least some guidelines to help them choose a programming method, like a simple motor start/stop configuration.
It should be that simple, right? You have a start button, you have a stop button, you have a motor...that's how you program it. It's not easy. Each situation should be evaluated to determine the best way to write code.
However, here are four examples of start/stop motor programs. Two of them we use, one we don't use, and one we think is wrong.
1. "Sealed" circuit with normally closed stop button.
Right off the bat, many will correct me and say that we accidentally included "closer" instructions in our program. There are no errors in the program, yes we have a normally closed stop button connected to I:0/2. We use the XIC instruction to look at I:0/2. This is the most basic and basic instruction in RsLogix, but totally misunderstood. The normally open electrical symbol -||- is different from the -||- XIC programming instruction. In fact, Allen Bradley's documentation is misleading. "The instruction (also known as 'check' or 'normally open') acts as an input or storage bit." The problem is that they are trying to help the user connect with something they understand, which in this case is electricity, which it doesn't. This is ladder programming.
Now that you've got that out of the way, back to the Motor A sample. In fact, here is the above image again just so you don't have to scroll up to see it
Pressing your normally open Start Push Button will put a 1 in the I:0/0 data box which will in turn allow the XIC I:0/0 to be evaluated as true, making the OTE O:0/0 true, putting a 1 in O:0/0's data box which will turn on Motor A. Now when you let off of your Start Button, the top branch will be false but the bottom branch will "seal in" Motor A as long as your Stop Button isn't pressed.
Motor A not running had kept the bottom branch from being true till the motor started running. Note when comparing to Motor B. I'm trying to keep these explanations brief, but the advantage of the normally closed Stop Push Button compared to the normally open Stop Button is that if power fails or a wire is broken on the normally closed circuit, the XIC I:0/2 in Motor A's bottom branch will no longer be true, turning off the Motor A output.
2. "Sealed" circuit with normally open stop button. This code works the same as the code for engine A, except that the normally closed stop button XIC has been replaced by the normally open stop button XIO. We're seeing this in new programming code and textbooks at an astonishing rate. Once again, attempts have been made to relate circuits for start/stop control to ladder programming. While this approach is functionally fine, it has serious safety concerns because if you have a power failure on your stop button, your stop button, or a broken wire in your PLC input (all common on real devices), the branch below will Keep it true and the motor will continue to run. A PLC should not be relied on for security, but it doesn't hurt when designing circuits and programming for the highest level of security as an extra layer of protection.
3. The circuit for motor A containing the stop button has been moved out of the branch.
Here I go, already contradicting myself. I just said that the circuit and code should be designed with the highest level of safety in mind. This may be safer than Motor A's in theory and functions very similarly. Yet it lacks a key maintenance feature that many motors need.
The ability to "bump" a motor for testing, lining up tooling, or maintenance. Right or wrong, maintenance personnel will intentionally hold the Stop Button so that the Start Button acts as a way to momentarily start the motor, with it stopping the moment they take their finger off of the Start Button. By moving the Stop Button out of the branch and disabling this feature, it requires pressing the Start Button then the Stop Button to "bump" the motor.
This could cause more of a safety issue than it solves depending on the application but it will also lead to maintenance personnel bypassing all safety features to get the job done if they are not able to bump the motor with normal control. Usually this is done by pressing a screwdriver into the front of the contactor, a clear safety violation and very dangerous.
