Dryer Coming down By Itself

[dgunderson]

I've been working with an Istobal M22 with profiling dryers. Mysteriously, the dryer has lowered itself to the bottom stops twice, we have it on video. On one occasion the raise/lower motor continued to run even after the blower met the bottom proximity switch. This caused the belts to wrap around the pulleys backwards. Unraveling that mess was no small trick. I have some theories; someone butt dialed the machine on their cell phone and lowered the dryer or someone with remote access lowered it intentionally to piss off the owner, but that doesn't explain the belts unraveling, the prox should have stopped the motor. Next possibility is that the computer has a glitch either in the hardware or the software, but again there is the belt issue and the fact that it apparently did not acknowledge the bottom proximity switch. Finally, there is the possibility that the VFD did it, if it operated independently from the computer that could have explained the belts and the prox. Right now I have the computer inputs to the VFD disconnected and we are running without profiling. Istobal is stumped. Anybody else out the seen anything like this?

 

[JGinther]

Not familiar, but might be able to help with the logic... Based on the description of events, I would think you could eliminate anything having to do with software. I would expect that the dryer down output would be shunted in the software anytime the lower prox was made... regardless of remote operation or internal normal operation programming. So all that is really left is a faulty output from the PLC or a faulty input on the VFD, or some wiring problem or maybe wet terminals in between. I don't know that machine in particular... Is there any other relay/device between the PLC output and the VFD input?

 

[dgunderson]

There is nothing between the PLC output module (D07) and the VFD (T6) as you can see in the attached schematic. Currently I have wires 5 and 6 disconnected at T6. If the VFD caused it it can still theoretically do it again. To lower the blower there has to be a +24 VDC applied to terminal 4 on the VFD. I've lowered the blowers by placing a jumper between terminals 6 and 4 on occasions when the blower was caused to overshoot it's proximity switch.

 

[JGinther]

I would measure voltage between 3 and 4 when machine is resting, and 'mechanically agitate' the plc. I don't know if it has solid state (transistor) outputs or mechanical relays, but if you see voltage bounce when 'bumping' the plc, I would suspect the output is defective. If its resting voltage is more than 3 or 4 volts, I would also suspect that output. I have seen inputs on devices detect 'on' at 10V dc.