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Multiple pallet are conveyed on a roller conveyor. The conveyor motor is "across the line" started and friction is the only stopping force when the motor is turned off. The stopping distance for the maximum load is about 30-40 inches. Is there a simple and low cost solution to stop the conveyor faster? A distance of 18 inches or less would be great. A brake motor (integral brake mounted to the motor) is too abrupt and cause high stress in the drive system. VFD drives and other DC injection controllers will do the job but add significant cost - (there are about 150 drive motors).

Total load weight varies is about 12,000 # maximum and the conveyor drive is a 2 HP 1750 RPM motor driving though a gear reducer. The inertia of the weight of the pallets is about 85% of the total system inertia. An average motor braking torque of about 1.5 FT-# is all that is needed to stop the load in this distance.

Sounds like real dilemma. VFDs would do the job, but price is an issue.

If you may, try to experiment with a reversible starter. What if you reverse motors for a very short time with reversible contactor? Let say 1 sec. I don't think for such short time your overloads will trip.

You could experiment with a crude form of DC injection using just a diode and resistor into one winding.

Suggestion, add a second contactor in parrallel with the motor contactor. This one would be small and only have 2 phases with a diode & resistor in series with one phase. To stop the main contactor would drop out so now the motor is only getting half wave across 2 phases then a couple of seconds later the second contactor would drop out.
Another way of course is to have a completly seperate DC source but that involves dropping the power right off the motor before connecting the DC.

You could add the DC to a group of motors by interupting the main supply or for that matter adding a large VFD to supply the whole conveyor
I once worked on a tripper car that had similar overshoot, we added an electric disk brake with an adjustable DC supply but that is quite complex for the number of drives you have.
Plugging (reversing for a second) also would work but that can be quite harsh on the mecanical equipment and you have to be carefull not to start going in reverse

Roy

Search “dc injection braking” to find complete systems or simply design your own.

There is an easy and cheap way to stop an AC motor by injecting DC to the AC motor. The typical system includes a DC contactor, timer and low voltage source and 1, 2, 4, or 5 diodes. To stop a 480 volt 3-phase motor quickly about 120 volts of full-wave DC is applied for a second or two when the motor starter is turned off.

To stop the motor more smoothly you could use half-wave with 1 diode and use a lower voltage. The current will be high because it is limited by the DC resistance of the motor.

To protect the diode you could connect a varistor across the AC to the diode or connect a diode across the DC output side of the diode. This will make the diode last much longer.

I hope this helps,
William Hinton

Responding to Art Terpstra's 09-Nov (01:11) question... this subject has been covered in great detail in this forum. I suggest you search the Control List archives!

Regards, Phil Corso (cepsicon@aol.com)

Hello,
Simple, move the current stopping apperatus, further back down the line away from the required stopping point...or add a timer from entrance to the conveyor to stop. Is it possible that production increases have put a burden on the old conveyor systems?...I ask this cuz it may make sense just to turn down the speed of the conveyor (dare i say move something slower?) Not in our life times! (yeah right)

Speed changes can be accomplished with gear ratios and sheeves or sprokets as you know. )Photo eyes and or electromechanical cat wisker switches...you want cheap and reliable then go to the way back machine....keeping it simple (and cost effective)is always tough when going high tech.

Best Regards,
Mark