Plastic Shredder Motor / Transmission?

Hello,

I’m in the process of building a Precious Plastics Shredder in order to regrind plastic molding flash for compression molding.

I came across a 5 HP single phase 220 motor and was curious if anyone had any advice for controlling speed. It seems like I would be better off getting a 5HP three phase motor and a 5HP VFD for more precise control of speed and direction as well as auto shutoff and reverse for safety purposes. It seems as if there are not really VFD’s for single phase motors, but I’m learning most of this as I go so i could easily be confused. Would anyone have any advice or thoughts about this.

Similarly, my thinking is to weld a frame and use an industrial chain and sprocket to transmit power from grinder box to motor. Does that seem reasonable?

I’m also looking at pregeared motors such as these https://www.grainger.com/category/power-transmission/gearmotors/ac-gearmotors

I definitely need more torque than speed and would love the run the unit on 110. Does that seem possible or would going to three phase 220 running through a VFD be my only real choice?

Any advice or thoughts would be greatly appreciated!

Thanks!
Destin

You’re at most going to be able to get ~1.75hp from a 120V motor without doing anything exotic. How much power does this application need?

Hey Destin,

Looks like an interesting project! Here are some thoughts from my end —

Agree with Jon, you need to find out how much power is needed or recommended. If you don’t know, you may have to experiment. It’s small, but I have an extra 120V single phase 1/3 HP motor that you can use if you want to try things before making a larger purchase. you could step down the gearing and see how much torque is needed to shred your plastic well. Is there a reason you are looking for speed control?

Many of these motors can be reversed by switching the polarity of one of the phases. So, possibly you could do something like that with a switch.

For framing, I would recommend 80-20 style framing which bolts together in order to avoid welding. For an example, similar to what the custom CNC router frame is made from.

Hope this helps, good luck!

-Nick P.

Thanks Jon and Nick,

The specifications call for 1/4 hp+ at 70 rpm

The torque required seems to be in the 50-100Nm range.

That is a great idea about the 80-20 framing, I’m going to look into local suppliers.

If you wouldn’t mind I’d take you up on your motor offer! There is no real reason I need speed control, I just thought it might be helpful when shredding different parts but if it adds very much cost to the project I’m happy to abandon it.

Thanks for the replies this is all very helpful!

-Destin

Destin,

Happy to help!

A vendor to consider is mcmaster.com for framing (or anything mechanical parts-related).

Feel free to reach out and we can coordinate a time to get you the motor.

-Nick

I can’t help with the motor, but I have a question about the project. Can this shredder be used to help recycle waste plastic from 3D printers?

[quote=“Destindouglas, post:1, topic:919”]
It seems like I would be better off getting a 5HP three phase motor and a 5HP VFD for more precise control of speed and direction as well as auto shutoff and reverse for safety purposes. It seems as if there are not really VFD’s for single phase motors, but I’m learning most of this as I go so i could easily be confused. Would anyone have any advice or thoughts about this.[/quote]

You are correct. Large single phase induction motors cannot be controlled via VFD, and there’s no good method for running them at anything other than their design speed. They cannot be reversed.

Your options are reduction gear/transmission or three phase motor and VFD. Two points there- one, some VFDs are ok to run on single-phase 240vac. But, 3 phase if you have it, single phase is hard on the VFD power caps.

Second, a three-phase motor does lose horsepower when run far from its design rpm. So, if you need a very different rpm, you may still need to do at least a fixed reduction gear.

would love the run the unit on 110. Does that seem possible or would going to three phase 220 running through a VFD be my only real choice?

A 240vac motor can’t run on 120vac without a transformer (that would be a pretty big transformer). 120vac motors pretty much only go up to 2hp, with rare exceptions.

I would suggest the best course of action would be to use the motor you have at its normal speed and employ a reduction stage if you need another RPM. What RPM are you targeting?

Hey Timur,

Yes! That is part of what we’re building it for. We’re regrinding flash from a record press here in Austin.

Thanks Danny,

I’m trying to hit around 70 rpm. I’ve found a 30:1 worm gear transmission that I think I’m going to try to see if its putting out enough torque to grind the material. Does that sound like a reasonable move?

Thanks,
Destin

Reduction to 70 rpm is definitely worm drive territory. Your single phase motor is going to be either 1745 rpm or 3450 rpm to begin with.

There is some kind of heavy duty grinder in the current UT surplus
auction. I have no idea what it was designed for or if it might work
for this project. It might be worth looking at, just in case.
https://swicoauctions.com/online/26/item/62643
A photo of the grinder is in the bottom of the cardboard box. For
perspective, that box is about the size of a forklift pallet.

I would be interested in some items of electronic equipment in that lot,
but it is likely to go for more than I would want to pay by myself.

I glanced at the grinder and thought it might be a food grinder, but to
me it looks too heavy to use for grinding food, unless it was designed
for grinding up bones. It is apparently listed as “Straub model for a
grinder” in the description, although their descriptions and photos are
not very reliable. The viewing instructions say that they are only open
in the morning Tuesday through Thursday, but when I went over there
Friday morning I had no problem getting in.

Bidding on this item closes Tuesday September-8, 1:05 PM

                         Lloyd
                         836-7893

I don’t recommend using 8020 for your build. The aluminum is great because it is light. The application here doesn’t need weight savings. It need strength. Also welds generally don’t loosen over time.