Magnet problem, need help please.

Hello all. Another newbe...

Reworking on the old grinder. Installed new softstart, new contactors, sensors... Machine is ready to go exept for the magnet. Imagine big turntable where operator put big chunk of metal. That truntable start rotating and another tool from the top start grinding same chunk of the metal. Turntable needs to be magnetised in order for part NOT to flu from it.

Plc used is MicroLogic 1200, program that I am working with is RSlogic 500. Have DC drive that is suppling + - to the magnet. To supply supply power to the turntable (magnetize) it is simple, now my problem is how to de-magnetize. I am stuck there. Need help if someone can "push me" in the right direction.

Inputs are:
1. Magnet on - push button - start magnetising
2. Magnet off - push button - start de-magnetization.

Outputs are:
3. DC Drive - run
4. Contactor C1 - for delivering + - voltage
5. Contactor C2 - for delivering - + voltage
6. Resistor bank 1
7. Resistor bank 2
8. Resistor bank 3
9. Resistor bank 4
- resistor banks controling the speed of the dc drive

Basicly, when magnetising:
4 on
6, 7, 8, 9 on - to get max voltage reference to the drive
timer 1/2 sec
3 on

when de-mag:
3 off
timer 1 sec
9 off - decreasing the speed of the drive
4 off
5 on
timer 1/2 sec
3 on
timer 1 sec
3 off
timer 1 sec
8 off
5 off
4 on
.......... and continiue like that with contactors changing in order to change the polarity and resistors droping down in order to decrease the speed until 0. Last step will be drive off, 1/2 sec delay and then last contactor drop out.
Timers are used in order to protect drive from backfeed (in addition of power resistors and other rectifiers which are allready installed on the machine).

I understand that sequencer will be the best in this case, however, i can not get it to work properly. I guess, not enough experiance.

Thank you in advance and sorry about spelling...

p.s. Just to clarify something, fisical outputs are NOT one after another. There is different voltage that is used, plc is relay type. I guess that sequencer is out of the question...

Several years ago I did some work with Cone-Blanchard on their Blanchard grinders. In the PLC programs I have, control of the magnet was up to the operator, not automatic. It was interlocked so the operator couldn't turn the magnet off while the turntable was moving, but when a grind cycle was finished the operator had to turn off the magnet in order to remove the finished pieces.

Steve Bailey said:

Several years ago I did some work with Cone-Blanchard on their Blanchard grinders. In the PLC programs I have, control of the magnet was up to the operator, not automatic. It was interlocked so the operator couldn't turn the magnet off while the turntable was moving, but when a grind cycle was finished the operator had to turn off the magnet in order to remove the finished pieces.

Click to expand...

Thank you for your response. That is the case on this machine, too. I just did not write complete logic of the machine overhere, did not want to bother people.

- turntable must be still before magnet can be turned on and it can not be activated (exept jog function) unless magnet is on
- magnet can not be turned off for as long as turntable was moving (at any speed, - has slow and fast speed's).

In order for the operator to remove workpeace from the machine, it has to be de-magnetised. Not becouse of the machine. If it is not de-magnetised, will couse problems down the road (sticking to other metals, etc... )
Only way to remove magnetic force from it is to change polarity of the field while voltage supply is lowered to 0.
I hope that this help a bit. Again, thank you for reading.

Andy

Actually, a sequencer instruction would be a good choice to control the demag cycle. The key to using it is to get the correct pattern of which outputs are on and which outputs are off at each step of the sequence. If I understand correctly, your sequence is something like this:

Voltage level 1 forward
Stop
Voltage level 1 reverse
Stop
Voltage level 2 forward
Stop
Voltage level 2 reverse
Stop
Voltage level 3 forward
Stop
Voltage level 3 reverse
Stop
Voltage level 4 forward
Stop
Voltage level 4 reverse
Stop
...
additional voltage levels
...
End of sequence

Another possibility would be a reversing drive. I'm assuming you're feeding the drive's armature voltage output to the magnet. I've only worked with AC drives for the past few years, but aren't there regenerative DC drives available that are capable of contactorless reversing? With a +/- 10 VDC analog output from the PLC, you would simply need to reverse the polarity of the velocity reference periodically as you ramp the amplitude of the reference down to zero.

Steve Bailey said:

Actually, a sequencer instruction would be a good choice to control the demag cycle. The key to using it is to get the correct pattern of which outputs are on and which outputs are off at each step of the sequence. If I understand correctly, your sequence is something like this:

Voltage level 1 forward
Stop
Voltage level 1 reverse
Stop
Voltage level 2 forward
Stop
Voltage level 2 reverse
Stop
Voltage level 3 forward
Stop
Voltage level 3 reverse
Stop
Voltage level 4 forward
Stop
Voltage level 4 reverse
Stop
...
additional voltage levels
...
End of sequence

Another possibility would be a reversing drive. I'm assuming you're feeding the drive's armature voltage output to the magnet. I've only worked with AC drives for the past few years, but aren't there regenerative DC drives available that are capable of contactorless reversing? With a +/- 10 VDC analog output from the PLC, you would simply need to reverse the polarity of the velocity reference periodically as you ramp the amplitude of the reference down to zero.

Click to expand...

Yes, you are correct. There are dc drives that can do that. Problem is backfeed from the magnet. I must have contactors that will take punishment from backfeed together with power resistors. It is little off the topic, but, did you ever play with 9 volt battery and supply that voltage to magnet? If magnet is stuck to the metal, and you disconect battery, there is allways spark. If you are touching metal or do not have good boots, you will feel shock, about the same strenght like 110V outlet. It is from energy collected in the magnet. Just imagine what will happen with magnet (in my case) feed with 100V dc and 30A. And switching polarities fairly fast when de-magnetising...
Now, I am not engenier, just the dirty electrician (sparky) . Do not ask me for formulas and other definitions. My kwnolidge is primarly from the field work.

Your sequence is correct. Now, I belive that somebody told me that sequence will control all outputs. In my case, outputs are all over the place. Plc that I am using is relay type. Let's just say that Com1 and Out1 are my control for dc run (control voltage from dc drive), Com3 and Out5 and 6 are for contactors (general 110 control voltage) and Com8 with Out12, 13, 14, 15 are my resistors (control +10V reference from the DC drive).

If I use sequencer, what will hapend with other outputs? I still need independant control for other devices... motor runs, indicational lights etc. Or I am mistaking?

Thanks for your time.

The sequencer controls 16 outputs. You use the mask to eliminate specific bits that you don't want to sequencer to control. Your programming will be a lot easier if the outputs under control of the sequencer are consecutive.

Steve Bailey said:

The sequencer controls 16 outputs. You use the mask to eliminate specific bits that you don't want to sequencer to control. Your programming will be a lot easier if the outputs under control of the sequencer are consecutive.

Click to expand...

Thank you. I belive that was what was missing. Will pull help files from program to learn more. Thanks again.

Andy