Armature Feedback vs Tachometer
Armature Feedback vs Tachometer
(OP)
I'm working with a weighfeeder that uses a 3hp 180V 1750rpm DC motor controlled by a Bardac speed control. The Bardac always used tach feedback for regulation, and we have been instructed to remove the tach and switch the controller over to armature feedback. This weighfeeder regulated the amount of coal for the combustion process, and has to be very accurate. The weigh feeder uses a Merrick MC^2 to control the Bardac. Historically the tach's have proven very accurate, but are slowly being phased out as they fail on other weighfeeders.
Is armature feedback an acceptable form of regulation for a critical process like this? We performed three material tests today and hit 1t/hr on the money all three times. Tomorrow we will remove the tach and try again, but I would like some guidance on this.
Is armature feedback an acceptable form of regulation for a critical process like this? We performed three material tests today and hit 1t/hr on the money all three times. Tomorrow we will remove the tach and try again, but I would like some guidance on this.





RE: Armature Feedback vs Tachometer
The weighfeeder uses an encoder mounted on the tail pulley of the converor to determine the the beltspeed and then uses a 4-20 to adjust the DC speed controller.
RE: Armature Feedback vs Tachometer
Note that the speed error is related to load change. If your load doesn't change or only changes over a small range, all of the above speed errors are reduced in the same proportion.
A coal conveyor probably doesn't vary much in motor loading thus the armature feedback error is quite low.
RE: Armature Feedback vs Tachometer
It is very important to clearly define your sample size. It is far more difficult to hold +/-1% from second to second compared to hour to hour.
RE: Armature Feedback vs Tachometer
RE: Armature Feedback vs Tachometer
RE: Armature Feedback vs Tachometer
Do you have any way of:
1.) Measuring the armature current in a way that is useful for this purpose?
and
2.) Scaling this measurement into an voltage value equivalent to your armature voltage measurement, then subtracting this from the armature voltage measurement to isolate the velocity component?
Curt Wilson
Delta Tau Data Systems
RE: Armature Feedback vs Tachometer
RE: Armature Feedback vs Tachometer
It's crucial to get your weigh scale aligned properly, any idlers that are high or low will cause large errors but correctly aligned you can expect +/- 0.5% or better with a chain calibration. Training idlers are the worst in my opinnion.
Regards
Roy
RE: Armature Feedback vs Tachometer
RE: Armature Feedback vs Tachometer
In mining to verify a scale we would run the belt at a known speed, stop, then weigh the ore from several feet of belt on a lab scale. We had a couple of plates shaped to the belt profile fixed a set distance apart.
I can't think why your arangement should be any less accurate than tacho feedback. The weigh scale doesn't care.
Roy
RE: Armature Feedback vs Tachometer