Hey all.
Just registered on the forum. I'm an electrician but have moved over ibto the security field for the ladt 7 years. Working mostly on cctv, intrusion and fire alarm.

I registered on the forum as I stumbled across it looking for motor control info online. I was asked to have a look at a set of electric cow shed scrapers yesterday that weren't working. The contactor was not engaging when coil was getting energised.
Unfortunately when I went to remove the contactor from the dinnrail in the enclosure to get contactor info it started working before I got to diagnose whether it was the overload, contactor or coil. Once contactor engaged I still had to adjust the oil pressure valve to get the unit working correctly but was fine when I was leaving. I'm waiting for it to stop again and the phone to ring! It got me thinking and I can't really understand the control circuit. Im hoping some of you guys that are more familiar with motor control can explain.

The scrapers are driven via a hydraulic pump. The pump is controlled by a hand off auto switch. On "hand", the pump engages and drives the scrapers for one complete scrape and it returns to its home position. Contactor disengages when limit switch is hit at home position. "Auto" engages an analogue timeclock which starts the same sequence. Again it stops once once scraping unit returns to home position.
The contactor is a 3ph 25amp unit with 24v coil. There is an adjustable overload in the 7-10amp range piggy backing the contactor.
Control circuit is 24v and has an oil level switch and "home" limit switch in ciruit.
My question is this- how can a one direction motor control circuit have a stop limit switch in circuit that will not prevent the motor from re engaging without the use of time delays?

I am preparing for the call back and want to havd my head around the circuitry!

Thanks in advance.
Dermot.

 

I'm having a hard time visualizing this, but the automotive windshield wiper might b close or say a rotating wheel that has 4 positions.

The wheel could have depressions along the edge where a roller will drop into. So, to start the mechanism one bypasses the limit switch until the motor has left that limit. It would then stop at the next one. Hall effect 2-bit sensors sense the positions 00 01 11 10 using a grey code where only one bit changes at a time.

Wipers can get wierd too. Some cars make the first stroke the same number of degrees, but the second stroke starts in another position. One car I had, had a park winding. Turning off the wiper switch would immediately park the wipers without a complete sweep.

 

Hi. Thanks for the reply.
Below is a pic of the scraper that im referring to... hopefully!

 

Ok ill just try insert a link maybe
http://www.dairymaster.ie/automatic-scrapers/automatic-scraper-system/

 

My question is this- how can a one direction motor control circuit have a stop limit switch in circuit that will not prevent the motor from re engaging without the use of time delays?

I am preparing for the call back and want to havd my head around the circuitry!
Dermot.

I'm not really sure of your question?
Does the motor go into reverse when it hits the end limit or is there a delay?
Max.

 

Lots of missing info. Still, our discussion of what we might find in a ladder diagram does no good. The problems will be confined to the pieces you have.

#1- is there any external, VISIBLE damage?

#2- I've seen way too many such control panels not properly closed, and the terminals and components damaged from moisture, dirt, insects, and people. If so, it is part of your job to repair that stuff too, and the people that do that damage never want to pay for it. If that becomes the situation, the wisest move is to politely decline to attempt service.

#3- if no components are visibly damaged (wiring included), carefully remove covers of limit switches, j-boxes, etc. to examine for moisture and corrosion in particular.

#4- hand-operated items like that H-O-A switch will "wear-out" and malfunction or fail without being externally, visibly damaged. Check it out carefully.

#5- as so much manufacturing has left the USA, increasing amounts of Electrical components are imported trash of IEC style. The system you have there is probably NEMA style, but may be mixed components, or IEC style of mixed and questionable quality. The harm done by dirt, moisture (including condensation of humidity) and insects cannot be overstated!

 

No... thats the thing. Theres no reverse. If it was 2 directional with two limits it would be more straightforward!

When pump runs it drives a ram. The ram pushes and pulls on the ground bar for the length of the stroke- approx 2 foot. The scraper unit sits on the ground bar. There are notches on the ground bar that catch a tooth on the scraper. When the scraper reaches the end of the ground bar a mechanism changes in the scraper and it starts to return to home on the moving ground bar. When the scraper gets back to its final position it hits a limit switch which stops the process.
I hope I'm making sense!

Thanks for your help.

 

Typically the "end-of-cycle limit switch on such systems are mechanically cleared: the machine strikes it and coasts past it, clearing the switch arm and allowing the contacts to return to the "start of cycle" position.

 

It's hydraulically operated!!! The motor operates a pump, the valves reverse and otherwise control the action.

 

To further clarify:. You must have clean, firm conductive connections and conductors throughout the system! All clean, tight terminals, all clean, dry contacts.

Find 'em, fix 'em, verify 'em.

 

Then I am assuming the reverse is done hydraulically which normally can be done 'instantaneously'.
For any real reasoning and diagnosis, really a electrical and hydraulic schematic is needed.
Worst case scenario, it will take some reverse engineering, for e.g. I assume there is a three way hydraulic valve and is it dual coil or single with spring return.
Max.