Stop an AC motor an a specific position?

[Barbqranch]

If you did install a VFD, you could arrange it so after the 15 minutes of run time, it went to a slow creep speed till it hit the switch that told it to stop in that position. That way it wouldn't coast past the turn-off spot.

 

[Danny89]

Interesting. Not a bad idea.


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[drcampbell]

Assuming this machine is geared down and rotates at ~60 rev/minute or less, this will actually be pretty easy & economical. No fancy servos. encoders, PID loops or PLC required.

Mount a big slug of iron on the rotating frame.
Mount a magnetic proximity sensor on the base of the machine, where the slug will pass nearby with each rotation.
(using a cam and a switch will require much-more-careful alignment, and will wear out the switch relatively quickly)
(before making your permanent mounting bracket, make a temporary bracket so you can position it in the right place -- the place where the machine begins stopping so that it coasts to a stop where you want it to stop)
Connect the normally-closed (N.C.) contacts of the proximity sensor in parallel with the stop button. (it may be necessary to add a relay to invert the logic)
Now when you press & hold STOP, it will keep turning until it's in the desired position.



If you want it to respond to a momentary press of the STOP button, add a sealing relay which unseals immediately and turns off the green light to let you know that the machine has responded to the STOP request.
You could also add a time-delay relay so it automatically stops (in the right position) after an appropriate amount of tumbling.

 

[Besoeker]

If you did install a VFD, you could arrange it so after the 15 minutes of run time, it went to a slow creep speed till it hit the switch that told it to stop in that position. That way it wouldn't coast past the turn-off spot.

I think you might need a brake to hold it in position.

 

[Gary11734]

Right now I have it programmed to mix for 15 minute once the maintained on off switch is turned to the on position. After the the 15 minutes the motor contactor opens and the motor stops turning.
I don't have motor nameplate data at the moment.

Just trying to find a simple solution to establish a home position so that I don't have to constantly jog the machine to get it in the correct position for loading and unloading the barrels.

Thought a rotary encoder could possibly work.

It's a polyphase ac motor.
Is it ok to put a drive on a motor if it is not a inverter duty motor?

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How about a jog switch tied to a limit switch at a predetermined position or Beam that breaks the circuit. I can always Frankenstein any setup if you give me enough time to think!

 

[Danny89]

I like the idea. Thanks a lot!

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[petersonra]

Depending on how perfect it needs to be you could probably do it with a couple limit switches or an encoder. There are also drives available that essentially turn a vfd and a regular induction motor into a system with Servo motor type accuracy. I have done it all three ways.

 

[drcampbell]

I like the idea. Thanks a lot!

Happy to help. Be sure to let us know how it turns out, with video if at all possible.

 

[MAC702]

Can you just put in a clutch that removed the load from the motor? Then disengage the clutch when it stops, hang a weight at what should be the bottom, and it will just go there for you.

Actually, looking again at the picture, that seems rather difficult, but I'm going to leave that there as part of the brainstorming.

Are you able to spin the shaft by hand at all (when the motor is off)?

 

[Jraef]

Right now I have it programmed to mix for 15 minute once the maintained on off switch is turned to the on position. After the the 15 minutes the motor contactor opens and the motor stops turning.
I don't have motor nameplate data at the moment.

Just trying to find a simple solution to establish a home position so that I don't have to constantly jog the machine to get it in the correct position for loading and unloading the barrels.

Thought a rotary encoder could possibly work.

It's a polyphase ac motor.
Is it ok to put a drive on a motor if it is not a inverter duty motor?

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Yes, if you take precautions, but read on.

Tumbler is loaded with barrels that are filled with powder.

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Standard simple VFDs don’t do well with eccentric loads, such as barrels with powder in them that will rise and fall inside as it rotates.

Environment is dusty

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That to me is the last straw against using a VFD unless the motor is small enough to be able to use one that is completely enclosed and rated for outdoor use, ie no cooling fans or air flow that will be blocked by dust.

If you did install a VFD, you could arrange it so after the 15 minutes of run time, it went to a slow creep speed till it hit the switch that told it to stop in that position. That way it wouldn't coast past the turn-off spot.

That’s a great approach and what I would do, but given the circumstances, not with a VFD. You don’t really need the variable speed and you can accomplish a similar thing with some brands of Solid State Soft Starters, which are more amenable to being put into sealed outdoor enclosures because they can have a bypass contactor to prevent heat buildup. Look for ones that offer a “slow speed move” option, this is EXACTLY what it is for. It gives you the ability to rotate the motor at 7% or 15% of rated speed for short periods of time (ie 30 seconds). (For those curious, it turns the motor into something akin to a “cyclo-converter” drive using rotating AC pulses but because it isn’t really a cyclo-converter the motor can’t do it for very long).

 

[Besoeker]

Jraef

Good points as usual.
A few comments in no particular order.

There are several unknowns.

We don't know how eccentric the loads are or how fast the driven machine rotates. If it is much slower than the drive motor the reflected inertia at the motor shaft may be much lower than that of the motor rotor. And thus not a big issue with the VFD.

Most of the motors I've used with a VFD, in fact most of the AC motors I've dealt with have been TEFC. Up to quite high powers. I would mount the VFD enclosure outside the dusty environment. There are ways around the issue if it can't be but at a cost.

My experience with Soft Starts is limited to those that run up to fixed speed and bypassed. Configuring them to emulate a cycloconverter sounds complex.

 

[gar]

181228-1300 EST

Danny89:

I doubt you require much accuracy.

First, you might simply put a pointer on the output shaft. After a signal is received the human response time to actuate a switch is some constant time plus a variable time with a standard deviation of about 30 mS. When the pointer is vertical you push the off button. There will be a stopping time from when the switch is operated to when the motor stops. Also this may include some variance. You adjust the pointer position so that the mechanism stops were expected.

Second, if the first method produces too much variance in the stop point, than add some electric motor braking. One way is to simply short the motor windings after power is removed. Thus, SPDT contacts. Or you can use DC injection braking like
http://www.rdmengineering.co.uk/dri...MIjviHo43D3wIVhUBpCh2ksgfmEAAYASAAEgIZRPD_BwE

Third, instead of using manual operation of the off button you add a prox switch to detect angular position, and from the prox information initiate stop using the coast to stop, or some other braking means.

If drift to stop, or braking of the motor itself, has too much variance, then look for a electro-mechanical brake whose coil connects in parallel with the motor leads, and is unbraked when power is applied to the motor.

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