Lathe speed controller - Page 1
Forum section: electronics / repair

author: Mjolinor
author_rank: Frequent Contributor
timestamp: June 12, 2020, 09:52:24 am
content: The thyristors have gone on my lathe speed controller and I do not have any. I have ordered some but need the lathe.I have lots of triacs and figured I could use them as shown. Lower diagram is as it should be, upper is what I intend to do.Normally I would just suck it and see but doing it with mains on a PCB that I do not want to damage just makes me want to use a bit more caution and ask.The gates are connected to the same trigger signal through a resistor.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 12, 2020, 11:19:02 am
content: My experience with such controls for controlling a lathe, drill press, mill, shaper, etc. has always ended with dissatisfaction.Today, I use one of two approaches (assuming there is not a mechanical variable speed drive):1) Switch to 3-phase and VFD; or2) Switch to DC and PWM.I have never regretted making the change; although, getting it done takes a bit of effort and money. The latter has come down quite a bit from when I first did it years ago.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: June 12, 2020, 04:30:12 pm
content: It is PWM DC. I too would stick a VFD in there but it would mean putting the motor external as there isn't room inside to fit it where the DC motor is.I can't think of anything with variable speed that doesn't already use either VFD or PWM unless it is a universal motor.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 12, 2020, 05:16:34 pm
content: I was thinking of the cheap "light dimmer" controllers used for some induction and universal motors -- like some the small fractional HP lathes. Specifically, my large lathe (2 HP) is VFD/3-phase. My small Prazi (1/3 HP) was converted to DC/PWM.Sorry, I have no experience with swapping triacs for thyristors. Many years ago, I got a non-working thyristor controller and was able to resurrect it, but I used exact replacement parts.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: June 12, 2020, 05:30:10 pm
content: The absolute best way to convert now is to fit a BLDC kit. They cost about £150 all in and the motors pack a real punch, 1 hp into a 70mm diameter, 100mm long motor. They also have a huge advantage in that with the feedback the controller ups the power if you slow the motor down.

author: Yansi
author_rank: Super Contributor
timestamp: June 12, 2020, 05:38:36 pm
content: Quote from: Mjolinor on June 12, 2020, 05:30:10 pmExactly same you can do with the SCR control, as is often already done in these products.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: June 13, 2020, 06:48:29 pm
content: Well in the absence of any opinions I had to suck it and see. It doesn't work. No pops or bangs or other bad goings on, no blown components jsut no output.I guess I am waiting for the Chinese to deliver.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 13, 2020, 09:09:14 pm
content: SCR's are cheap. Why go China when you can go closer to home and get real stuff?

author: Mjolinor
author_rank: Frequent Contributor
timestamp: June 18, 2020, 11:08:55 am
content: Quote from: jpanhalt on June 13, 2020, 09:09:14 pmWell mostly because the stuff you buy here came from China anyway, costs five times as much and has nowhere near as good a warranty if anything goes wrong.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: June 24, 2020, 10:49:42 pm
content: Well, got the bits, put them in, still no worky.Op-Amp gone, replaced and all working now but no proof that my first idea wouldn't work.