A.C. Versus D.C.

I think a 3ph AC motor will bee the best solution. It can bee tight and with a freq converter it is easy to control.

I've used 3phase AC motors with frequency drives for weighing conveyors all my life. This is not to say that DC motors/controllers are not good, just that I've never seen them used for this application. Oh, no wait, I have. They were used in small check weighers for checking the weight of products like milk, cereals, etc. For larger applications, like weighing coal, sugar, sand, etc. AC motors were utilized.

You can buy AC motors that are hermetically sealed and with corrosion resistant casings and shafts. For that, I'll have to suggest you talk to your salesman.

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One more vote for AC motors with proper construction: no needs for brushes (if DC motor must have them because of its rated power).

VFD maybe installed in the control room (or other space without a harsh environment).

The AC motor with a frequency drive will work the best for your application. The frequency drive can accept a variety of input signals from your devices and can be programmed to accommodate any type of delivery rate that you are seeking. If feasible, you will want to mount the drive in an area away from the "wash down" location. However, the motor should be a TEFC or "Totally Enclosed Fan Cooled" which works in just about most environments. If you are using a very aggressive or flammable wash down chemical, then most motor manufacturers have upgrades for those as well.

Allen / Bradley makes a great frequency drive for any type of use. Simply contact your local Rockwell Automation Allen / Bradley sales person and they will be glad to spec one out for you.

Good Luck!

Jeff

sixpac,

I too cast my vote for an AC motor and VFD! No contest over DC.

You can check out my favorite supplier for VFDs and inverter duty motors up to 100 HP: Automation Direct (Formerly PLC Direct):

http://web1.automationdirect.com/adc/Home/Home

Greg

Geez, this has turned into a sales platform!

AC drive and motor is best for wet applications, DC would be OK if it were Permanent Magnet type, but they cost way more.

Any decent VFD will work, but be aware of the environment in which you will mount the VFD. If outdoors exposed to the wetness, make sure you have a NEMA 4 or 4X VFD (IP65 or better if you are in an IEC country). You also should not allow the VFD to be exposed to direct sun or freezing temperatures (without adequate protection).

Also be aware however that you cannot just feed the weight signal into the VFD to modulate speed, you need a control system such as a PI or PID loop to keep it from "hunting". Many VFDs have that built-in now, but make sure you understand what that is or that the VFD supplier has really good and accessible tech support for you if you do not, otherwise you will never be satisfied with what you want it to do.

One other thing. In case you decide to mount the VFD some distance from the motor, be sure to read the manual as to the allowable distance. At certain frequencies, a standing wave may develop in the wires and jack up your voltage at the motor. At our installation, the project engineer placed a VFD in the MCC at the ground floor and the motor at the sixth floor. I voiced concern over that and, when we checked, the voltage at the motor terminals was 600V (for a 440V motor)!