motor control
motor control
(OP)
Hi,
I have a dc motor that normally rotates in one direction for steel coil winding. I have noticed that after threading the coil onto the mandrel (beginning to wind the coil), the shaft first rotates in the opposite direction (slightly) and then begins to rotate correctly. I use an Avtron M785 tach (1200 PPR). Is there anything in particular that I should look into to resolve the 'reversing' issue? I am thinking that it is a tach issue.
Thanks
I have a dc motor that normally rotates in one direction for steel coil winding. I have noticed that after threading the coil onto the mandrel (beginning to wind the coil), the shaft first rotates in the opposite direction (slightly) and then begins to rotate correctly. I use an Avtron M785 tach (1200 PPR). Is there anything in particular that I should look into to resolve the 'reversing' issue? I am thinking that it is a tach issue.
Thanks





RE: motor control
RE: motor control
RE: motor control
RE: motor control
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RE: motor control
Food for thought:
What does slightly mean - half a rotation - 3 or 4 rotations?
I would think the amount of reverse rotation could help you zero in on the problem.
Is "threading the mandrel" an auto or manual process?
Is there any kind of tensioning system (mechanical or otherwise) that might be "fighting" you? (This is my suspicion.)
Why do suspect the tach - is your application really positioning the winder? Is the tach being used as a data source for a counter? Is the tach for velocity feedback (speed control)?
Finally, no offense intended, could the reversal be normal? Is the reversal detrimental to the operation. If not, the problem could be "not a problem". I have been burned seeking solutions to a problem that wasn't. You know the old saying - If it ain't broke, don't fix it!
RE: motor control
I don't believe it to be the tensioning system bc when the mandrel reverses, it creates slack, which means that the bridal system beforehand is not pulling back. The tach is used as a speed fdbk that goes into a speed amp.
The drive I'm using is the GF2000.
RE: motor control
Just a thought ,but is the mandrel seeking a be balance position on start?
RE: motor control
RE: motor control
If your dc motor has a differentially compunded series field winding and if the starting inrush current is high enough, then the motor could start in the wrong direction. Check if your motor is differentially compounded and change to cumulative compounding.
Differential compound - series opposes shunt (main) field.
Cumulative compund - series field aids shunt (main) field.
RE: motor control
RE: motor control
HP- 400
Arm. V- 250
Arm. I- 1300
Base Speed- 300 RPM
Top Speed- 1340 RPM
Field Excitation- 29.5A
Field R- 2.85Ohms
Generator:
Power- 320 KW
Volt- 250
Load I- 1280A
Field Excitation- 18.8A
Field R- 16.5Ohms
Gear Ratio:
1.55
Inertia:
25.717 lb-ft^2 (full load)
3.117 lb-ft^2 (no load)
Speed Ref and Speed Fdbk (from tach) goes into a Speed Amplifier. An error is calculated and goes into a Current amplifier along with a tension ref and a current fdbk. another error is calculated and goes into a phase controller which then supplies the motor with voltage. (from how i understand it to be)
RE: motor control
So if my assumption is true, here's a scenario:
Consider what happens if the measured speed has a calibration zero error of, say, +0.1%. If the reference is true zero, then your controller sees a speed error of 0.1%.
How does the controller behave with such an error? If you have an integral controller, your error will cause the demand to your motor to build up in the negative direction - i.e. it will try to reduce the error. But the motor is turned off, the motor can't slow down and the error persists. When the motor is first turned on, it runs backward as the controller tries to drive the error to zero (i.e. drive the actual speed to -0.1%). As the speed reference ramps up, the error becomes negative, and the controller accelerates the motor up to running speed in the forward direction.
RE: motor control
Question :has the machine always been prone to do this or could it have slowly developed the tendancy as it grew older?
Wear creates mechanical play or backlash - could this have something to do with it ?
Some kind of simple ratcheting device could prevent unwanted backwards movement.
Difficult to say, having not seen the machine.
RE: motor control
RE: motor control
<nbucska@pcperipherals.com>
RE: motor control
RE: motor control
that's the problem, and i'm looking for a solution so i'll update this when i figure it out.
thanks again for everyone's help.
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RE: motor control
okay the problem resides when a reel in the beginning of the process switches over from being speed regulated to being voltage regulated. the switch causes tension to drop 65%, which means less control between the entry reel and the exit reel. the exit reel, during this state, backs up due to the movement of the entry reel.
///Please, clarify how this back-up works.\\\
we don't use brakes, during the transition, and the solution will be software based.
///Please, clarify if there happens to be any dynamic braking over the software solution.\\\
RE: motor control
Also you mentioned to Cabber that it always backs up when first started. Is this only when the web is attached or even when you are in speed mode with no web and starting from zero?
Besides all the other possiblilities mentioned, the problem could be a web handling/drive problem. If in fact you go from speed to torque, if the tension build up is high between the coiler and the bridle at stall, you could be switching to a torque value which is less than the existing force on the web. Thus the web pulls the coil back slightly at the moment of changeover.
You should pay particular attention to the current being supplied to the motor at the instant of changeover. Does it really go reverse or is it just dropping to a lower level.