3 phase motor reversing with delay and limit switches

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Old 12-05-13, 08:54 AM
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Lightbulb 3 phase motor reversing with delay and limit switches

Hi all,
I need help in designing a circuit to reverse a 3 phase ac motor using Fwd/Rev/Stop buttons. The motor is used to raise and lower a large strainer to lift out cooked items out of a large fryer and dumping them in a de-oiler.
The following logic needs to be incorporated:
a. Fwd button - starts motor in forward direction
b. Strainer lifts a few degrees and holds for 3 to 4 secs (to let the cooking oil to drain)
c. Continues to lift until strainer is almost vertical and items are tranferred to de-oiler.
d. lift limit is stopped by using limit switch.
e. Press 'Rev' button to start the motor in reverse
f. Limit switch stops the motor when strainer reaches into its seating position into the fryer.
g. The 'Stop' switch shuts off the forward or reverse operation at any time holding the strainer in place.
h. Process restart

Pressing the 'Rev' or 'Fwd' button while the motor is rotating in the opposite direction should either stop the motor until it comes to a halt and then the reversal process should proceed or there should be a slight delay (again until the motor comes to a complete stop) and then the motor rotation should start.

Hope my description is sufficient to help you guys understand my requirement. Any help, specially with a schematic, will be most appreciated.
 
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  #2  
Old 12-05-13, 09:26 AM
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Welcome to the forums from Pakistan.

This is a do it yourself site and that project would be stretching the DIY idea.

We can try to help you but we'll need pictures of what you are working with.
Do you have the three phase motor/hoist in place ?
Physical limit switches shouldn't be a problem.
Adding a timing circuit will complicate things somewhat.

I'm seeing two different three phase systems: 230/400 and 220/380
 
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Old 12-05-13, 09:37 AM
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I'd get one of these or a reasonable facsimile thereof. This will be able to perform all the functions you'll need.

Also, it will enable future add ons or adjustments.

[ATTACH=CONFIG]22472[/ATTACH]
 
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Old 12-05-13, 11:22 AM
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Motor control

The PLC shown will give you the ability to program a circuit with the functions you need. One question would be, is the motor speed already addressed through gearing or do you need to have control over the speed at which the motor accelerates. Is there an existing reversing mag or, are you designing this from scratch. A VFD can give you the ability to control the motor both forward and reverse as well as the speed at which the changes occur.
 
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Old 12-05-13, 01:37 PM
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Forward Reverse

Here is a diagram for a forward reverse control layout with your limit switches. You could add time-delay relays. I still like the suggestion using the Siemens PLC, it is a cost effective unit and fairly easy to program from the key board. Inserting the PLC into the control system will give you the ability to easily change your delay settings to get good control results.


Name:  Forward Rev with limits.jpg
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Old 12-06-13, 09:30 AM
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Hi PJmax, the idea of joining this forum is to do things ourselves... DIY.
I have all components in place including the motor which is operating on 400 Volts. The basic forward reverse circuit is not a problem and I had done it initially. But I realized, what if someone accidently pressed the reverse button while the motor was raising the strainer. The motor is rated at 1 Hp, 2800 rpm (no load), and geared down to 70 rpm through a gear box. This in turn drives a chain through another gear system connecting the strainer at its pivoting rod to either lift it or lower it. A sudden reversing of the motor while operating on full load will blow or break something. So, I have come to this forum for guidance and sharing the information for everyones benefit. I would post pictures if someone guides me how to do so. Thankyou.
 
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Old 12-06-13, 09:52 AM
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Hi bahtah,
Thankx for the circuit. What I failed to mention in my first post was that the Forward/Reverse contactors need to be latched. The pushbuttons are momentary press type (and are not interlocked) and return to their initial state when released. Also I need to add a few seconds delay once the forward cycle is started; to hold the strainer over the oil so that excess may drip off, and then it proceeds further till the end of its cycle, at which point the limit switch stops it from being driven further. The 'Reverse' can now be initiated to bring the strainer back to its lying position inside the heated oil.
 
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Old 12-06-13, 11:50 AM
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Forward Reverse motor control

The drawing I posted are momentary push buttons and show the HC contacts in your mag that will hold the circuit closed when you release the forward button. The dashed line shows that when you push forward you also open the reverse at the same time. To get a delay on the forward you could install a power-on delay (cube relay)between the forward and the forward coil so you have to hold the forward button in until the delay closed the circuit and then the HC will maintain the closure until the limit switch opens the circuit after you release the momentary forward button.
 
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Old 12-06-13, 01:40 PM
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Forward Reverse motor control

Here is a diagram with the time delay relay for the forward operation. You can go to the Marktime site and look at the relays available and all the information you need to install and set the time desired. When you press the forward button you hold it for the delay time and when the motor starts you release the button and the HC contact holds the circuit closed until the limit switch opens the circuit.The relay is KNOB adjustable not know....love auto spell check.Name:  Forward Rev with limits and delay.jpg
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Old 12-06-13, 06:14 PM
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Thanks for the circuit. Let me wire it up and see how it goes.
I think I might need two delay timers. One to hold the strainer after it is raised to about 10 to 20 degrees above its horizontal placement and the other to restart the motor from thereon so it may reach its vertical position and is stopped by the limit switch.
 
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Old 12-06-13, 06:45 PM
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Forward Reverse motor control

If you need a delay on the reverse, which I would suggest because if someone pushes the reverse while the motor is running forward, it will reverse before it coasts to a stop. To add the reverse delay, just duplicate the wiring shown for the forward delay on the reverse circuit.
 
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Old 12-06-13, 06:47 PM
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Hi bahtah, which software do you use to draw your schematics?
 
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Old 12-06-13, 09:06 PM
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Forward Reverse motor control

I draw them using "Power Point". When I was teaching apprentices that worked out well since I could draw multiple slides and then run through the operation of the drawing by making the changes to the drawings and showing them as a presentation.
 
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Old 12-13-13, 04:16 PM
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Hi again bahtah,
The circuit didnt work the way I expected. when we press the FWD button, the motor rotates to lift the strainer, but if during this period the REV button is pressed, the motor immediately reverses. This is because the power to the forward contactor is going through the reverse push-button. When the REV push-button is pressed, it momentary cuts off power to the forward contactor and activates the reverse contactor. What if we reposition the 'R' and 'F' NC contacts to before the FWD and REV push-buttons (after the STP push-button)?
Please offer your advice on this change.
Thanks

Name:  3Ph_Forward_Reverse.jpg
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Last edited by asami; 12-13-13 at 06:35 PM. Reason: Adding Diagram
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Old 12-13-13, 07:57 PM
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Reversing Mag

The R and F contacts are usually built into the mag. Did you install the time-delay relays. If you did not, then when the forward or reverse is pressed the mag will change direction. The R and F relays keep power from reaching both coils at the same time. You either go forward or you go in reverse, but without the time delay relays there will be no pause in this action other than the time for one coil to drop out and the other to close. With the time-delay relays you don't allow the circuits to be completed until the time delay you have selected allows the contacts through the delay relay to close. With the R and F contacts wired before the push buttons if you are running forward and push reverse, the forward coil drop out and the R and F contacts change state just like they do now and the motor will reverse. The time-delay would allow you adjust for the cooling oil to drain before allowing the motor to start again.
 

Last edited by bahtah; 12-13-13 at 08:27 PM.
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Old 12-13-13, 07:58 PM
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Give it a try this way.






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Old 12-13-13, 08:05 PM
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Rev Mag

Unless I am missing something it looks like you could push the forward and reverse at the same time and the circuit would be completed to both coils at the same time.
 
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Old 12-13-13, 08:16 PM
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You're right bahtah. This should fix that.



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Last edited by fubar2578; 12-13-13 at 08:16 PM. Reason: name correction
  #19  
Old 12-13-13, 08:33 PM
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That looks good but still has the same issue of being able to reverse the motor without a time-delay. He is trying to have a pause in the mags reversing and that is not possible without a relay to set the time. When the forward or reverse mag opens the AUX contacts change state and allow you to operate in the opposite direction (forward or reverse depending which way you were running). If you look at my earlier reply I put in a time-delay between the AUX contacts that change when the mag drops out or closes and the coil. If he installs two relays he can adjust how long you have to wait for the motor be able to be started.
 
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Old 12-13-13, 09:38 PM
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I agree two timers address the issue of delay between forward/reverse. However, it doesn't address lifting the basket so far and allowing the oil to drain before lifting it the rest of the way. To do everything I'm with Wirenut1110 on using a micro plc (smart relay). They are competitively priced enough to strongly consider.
 
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Old 12-13-13, 09:53 PM
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Thanks guys,
I'm off to the control board and will definitely use time delay relays for both the forward and reverse contactors. Putting in a time delay of approx 1 to 2 secs will do the job, because of the load on the geared motor, it will stop within this time. The only thing remaining will be the raising of the strainer to a few degrees and giving it a pause of 10 secs before continuing to its upright position until the limit switch stops it. On the reverse, i dont need the pause and can continue till it drops back into the oil. If I post a few pics of the strainer and arrangement, will it help?
Thanks again and I will get back again after incorporating the new circuit with the time delay relays.
 
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Old 12-13-13, 10:19 PM
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Asami, A picture might help. A repeat cycle timer with an ice cube relay MIGHT be an option depending on how far it is lifting.
 
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Old 12-14-13, 01:13 AM
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Forward Reverse motor control

The 330 timers provide the delay on start for the forward and reverse. The 338 timer is a recycle time that can have on/off delays of unequal time. The idea is you push the forward button and power is applied to the 330 timer which will not close until the delay period has passed and will stay closed until all power is removed. The contacts in the 330 close to complete the control circuit for forward as well as provide a by-pass of the HC contact on the forward button so when the 338 timer cycles off the forward control circuit remains energized. The 338 timer provides the ten second off and then closes until the limit switch opens the forward control circuit. When you push the reverse button the other 330 timer provides the delay then closes and the motor runs in reverse until the limit switch opens the circuit. I agree with wirenut1110 and fubar2578 that the little Siemens PLC would be a better fit. They have load contacts on the standard unit, more than enough for what you are doing.


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Old 12-14-13, 10:24 AM
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Hi all,
I just came back from my setup and guess what? I finally managed the logic minus the pause relay for the strainer. I will put up the circuit as soon as I draw it out. Now here is what I did, and without the time delay relays. Going back to the circuit I presented in my previous post (which, by the way, gave the same problem of rotation reversal whenever the opposite button was pressed), I added two NO switches in parallel to the FWD/REV buttons operated off the same contactor. These I managed through auxiliary switches attached to the contactors. Now when the FWD or REV button is pressed and the motor starts rotating in one direction, the other button is bypassed and no effect takes place if it is pressed. The operation can only be stopped by pressing the STP button. Problem solved.
I will be putting up the new circuit diagram in a little while, which will make things quite clear.
Best Regards
 
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Old 12-14-13, 10:56 AM
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Hi again
As promised, here is the circuit diagram.

Name:  3Ph_Forward_Reverse.jpg
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Old 12-14-13, 11:31 AM
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Forward Reverse motor control

Looks good. now all you need is your time-delays.
 
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Old 12-14-13, 04:51 PM
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Thanks for all your help. Do you think I still need the delay for the FWD and the REV operation or will it do?
 
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Old 12-14-13, 11:29 PM
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Forward Reverse motor control

Not knowing how quickly your motor stops I can't say for sure. If when you push stop the motor is still rotating when you push reverse or forward the motor must overcome the counter rotation before it can go in the direction you selected, that can be hard on the motor over time. If your motor comes to a stop before you change direction then you should not need any delay. Are you still going to have the motor go forward for a few seconds and then pause for ten seconds then start moving again automatically, or are you going to have the operator do that?
 
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Old 12-15-13, 11:33 AM
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Hi again.

I tried out the whole system today and the motor stops pretty quick because of the load but to be on the safe side I am also going to install the time delay relays between the fwd and rev functions. I was also searching for electronic braking of 3 phase ac induction motors and found a couple of sites giving some insight into designing a circuit for this purpose. I might try that approach also to bring the motor to a quicker stop.

As far as the 10 sec pause is concerned, although the operator has the control, but I would rather do it automatically so that the operator is free to do other functions and concentrate on his primary work of snacks frying.

What I forgot to mention uptill now is that the motor is 1 Hp, 1400 rpm (no load), operating a pulley using belts for rpm reduction and it drives a reduction gear of 40:1 ratio which again is connected to the strainer lifting mechanism through a chain driven gear. I would post pictures of the whole system, but the pics I took are of a higher resolution and the file size is much more than the allowed limit on this site.
 
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Old 12-16-13, 12:49 AM
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Forward Reverse

Your description makes me think a VFD would be a good application for your machine. You could get rid of the belts, have ramping ability and speed control and dc braking. Although the motor would need to be inverter duty rated and may need a cooling fan if your speeds are low. Just a thought incase you are building these machines.
 
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Old 12-16-13, 03:58 PM
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Thanks for the advice. I bought an old setup of the snacks frying system and have overhauled the entire mechanical lineup. Had to build a new control panel to start/stop various motors (there are 8 of them, all rated at 1 hp. Some are geared and some are belt/pulley driven. Two drives were difficult, one of which this forum helped me build. The second also has a sequenced pair of motors, which I am working on and will post a circuit for guidance from you all. The setup came with some spare parts including some contactors (mostly telemecanique), some 'UP' timer relays from Omron, normal DPDT relays, voltmeters, switches, etc. I am trying to use the good stuff I already have to make the machinery to operate instead of buying new from the market until I have to absolutely.
 
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