So I have this universal motor from an old washing machine. I'm trying to build a plastic shredder with it. I bought a cheap PWM dimmer to control the engine's rotation but it's still too fast for what I'm aiming for, plus it has problems starting with knob set to the lowest setting.

At this point I'm stuck; should I look for a different kind of engine? Or is there some kind of circuit that I could build that will let me control the rpm with decent troque?

>>2928624 (OP)
>plastic shredder
>universal motor from washing machine
>PWM dimmer
You're dying in the dunning-kruger hole. A 500W-ish motor isn't gonna shred plastic at 20RPM. You need a gearbox or a gear motor. On the very cheapo end, used car wiper motors are decently strong and come with their own gearboxes and can be controlled with 12V PWM. "decently strong" is still in the realm of shredding PET bottles slowly, not shredding 3D prints.

>>2928628
so basically I should pick a different engine?

>>2928631
What do you want to shred? If you want a meme shredder that you use to shred 2 PET bottles then put it away, you can do it with the wiper motor. If you want a plastic shredder that can do larger objects and will not crap out after working for 5 minutes, it's going to be a larger investment. You need something like a 1000W+ motor with a worm gear that takes it to about 30-60RPM and an all metal construction for the shredder itself. These are usually using induction motors, and those are speed controlled with VFDs, but you don't really need the speed control to begin with. There are a bunch of projects like this on youtube.

The washer motor is basically useless in either case because you can't get it geared down cheaply enough. Gearboxes for high torque are expensive.