Programming Challenge: Timed Delay Reversing Motor Starter

Gents:

I teach technical training for offshore electricians, and the advanced classes dip into some ladder logic/PLC troubleshooting. As a consequence, I find it a good idea to give them some simple programming challenges and have them build them on our motor starter panels (with actual components).

One circuit that gives them some trouble is to build a reversing starter with a forced cooldown delay (5-10 seconds). In other words, build a forward reversing starter. Add a forced ten second delay between reversing directions. For example, once the motor is stopped by the stop PB, require a ten second delay before starting in the opposite direction, but allow an immediate restart in the same direction.

I have a few solutions for this, one using a TON relay (or two) and one using TOFF (or two), but I wondered if there are any particularly elegant or simple solutions I'm missing.

Care to help?

This is the simplest solution I could come up with...

That is a simple one, and it's very similar to the solution I'd do with TOFF timers. Although I was taught not to put them in series with a coil, so my ladder logic to prevent them from energizing until I needed them to was a little more complicated. But if it works (having not build logic with timers in line with coils, I'm not entirely sure how/when they're energized) that's definitely a bit simpler than what I have now.

Thanks!

The "inline" timers and coils is applicable to Allen-Bradley Logix5000 systems - sorry to confuse you with this, I am just so used to doing it.

Here I have just put the timers on branches, so should work in most other systems that don't support "inline outputs".

_JT said:

That is a simple one, and it's very similar to the solution I'd do with TOFF timers. Although I was taught not to put them in series with a coil, so my ladder logic to prevent them from energizing until I needed them to was a little more complicated. But if it works (having not build logic with timers in line with coils, I'm not entirely sure how/when they're energized) that's definitely a bit simpler than what I have now.

Thanks!

Click to expand...

A simple "rule-of-thumb" - "output" type instructions, such as -( )- or TON/TOF etc., do not change the current rung logic state, true or false. They simply ask the processor to "do something". They are classed as "non-conditional" instructions.

Compare that to "input" type instructions, such as -| |- or EQU etc., these do affect the current rung logic state. These are classed a "conditional" instructions.

Here is a revised version, which guards against a stuck start p/b... in the previous code, a motor would restart when the Stop p/b is released. The ONS instructions take care of this situation nicely.

Here is my solution using a single TOF:

what plc are you using?

Anirban Hazra said:

Here is my solution using a single TOF:

Click to expand...

You may have saved a timer, but the extra logic needed to drive it for both cases sort of defeats the object in terms of memory usage, execution speed, and visual complexity.

Also, the _JT wants a solution that doesn't use mid-line outputs.

daba said:

You may have saved a timer, but the extra logic needed to drive it for both cases sort of defeats the object in terms of memory usage, execution speed, and visual complexity.

Click to expand...

I never claimed that my Logic achieved any of those.I just posted my alternative solution using a single TOF.

I did not have memory usage on my mind while building this, but I guess a Timer uses 3 words(48 bits) and I have had to use 6 more instructions in place of another TOF. I doubt that they put together would use more memory than 3 words.As for visual complexity, I don't think a "seal-in circuit" within a "seal-in circuit" is that complex.But then again, that's just my opinion and I may be calculating the memory usage wrong.

daba said:

Also, the _JT wants a solution that doesn't use mid-line outputs.

Click to expand...

In that case, he will just have to build the inner "seal-ins" in 2 separate rungs.