RPM control for medium size DC motor - Page 1
Forum section: electronics / projects

author: DmitryL
author_rank: Regular Contributor
timestamp: November 10, 2020, 01:02:57 pm
content: I have upgraded my EmcoUnimat mini-lathe with a much better motor - it is 200wt DC (not universal, polarity change affects rotation direction). It is rated for about 200-220 V DC.. For experiments I use a rectifier + VARIAC, but it is not an option for normal use...So, currently I see a couple of options:1. mains->rectifier->good old Triac dimmer + optional power filters2. mains->filters->rectifier-> simple 20KHz PWM with high voltage N-channel MOSFET.I don't need RPM feedback and clased control loop.Any ideas which option is better and why ?There is no need to save cost, it is just for myself..Thanks!

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 10, 2020, 03:02:24 pm
content: Hi, I have a 70 year old South Bend 9A ( your UK: Boxford 9 inch lathe)It still has the original 120 V 1 phase capacitor start motor, using the original start capacitor which is a worry!I am also thinking of a DC conversion similar to yours.I will use your option 2 @ 30 kHz using IXGH32N60C pwm push pull.First need to select a motor, looking at a surplus treadmill motor.

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2020, 04:03:04 pm
content: Either option will work, but what is wrong with using a variac and rectifier?

author: DmitryL
author_rank: Regular Contributor
timestamp: November 10, 2020, 04:07:45 pm
content: It is a tiny lathe, fits on a table. VARIAC takes valuable space on the tableThe regulator can be made in quite a small box nowadays.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 10, 2020, 04:45:46 pm
content: Personally, I would make a semi-controlled bridge with 2 TIC116M thyristors and 3 1N5408 diodes (1 diode for free wheeling)You can try option 1, but: it will have to be modified:mains-> good old Triac dimmer> rectifier + optional power filtersWhat about braking?Have you planned a braking resistor?

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 10, 2020, 05:01:06 pm
content: And reversing?I use reversing to run back on the leadscrew when threading, and when coil winding.Need to make sure the chuck is tight first!

author: Benta
author_rank: Super Contributor
timestamp: November 10, 2020, 05:03:50 pm
content: For that size and type of motor, 20...30 kHz is utopic. The iron losses will be tremendous.2...4 kHz will work, but be audible.

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2020, 05:13:54 pm
content: Quote from: DmitryL on November 10, 2020, 04:07:45 pmI would probably mount the variac to the bottom of the table facing down to get it out of the way, or do the same with any other speed control.

author: james_s
author_rank: Super Contributor
timestamp: November 10, 2020, 06:09:13 pm
content: Either one ought to be fine. I'd be tempted to design a fancy speed controller using a H-bridge of mosfets with PWM control from a microcontroller. Then you can have soft start, forward/reverse, and any other fancy features you think up. You can make it simple for now and then add new features in software later if you have a need for them. The whole thing could be made very cheaply with only a handful of parts.

author: Pawelr98
author_rank: Regular Contributor
timestamp: November 11, 2020, 03:26:32 am
content: How about a different approach ?Turning on/off full 50Hz cycles.One full sine wave goes through as mosfet or triac is condcuting.The next one does not pass through as the triac/mosfet is not conducting.It would be like a low frequency PWM.uC should do it easily.A test setup may be made with a CD4017 and a mosfet.Resistor + LED/zener to clamp the gate input voltage.Two diodes for OR function to get two half-waves through.Such setup will produce a 50% duty cycle that is synchronous with the sine wave.Will show how the motor reacts to such slow PWM.Otherwise I would just use standard PWM using 555 or 494.Though 494 can also do overcurrent protection and softstart.Simple triac dimmer circuit cannot do that.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 11, 2020, 04:17:35 am
content: There are some reports that motor torque impulses on small lathes can leave an imprint on the fine finishing cuts.I just searched "lathe motor speed fluctuation versus finish" and some hits came up.One company has option for operator selection of speed dither to reduce tool chatter.And some reports that 3 phase or DC is better than 1 phase motors, belt better than gears.And other opinions that the drive belt joiners clicking over the cones cause finish imperfections.I have not experienced any of the above here . My SB9A drive train hasmotor > belt > heavy pulley > cones > belt > spindle cones .

author: Pawelr98
author_rank: Regular Contributor
timestamp: November 11, 2020, 04:37:11 am
content: If torque impulses are a problem, then triac + rectifier + capacitor is a better option as you get roughly constant DC voltage.But then the magnetic field doesn't collapse instantly, it still holds for some time and any current goes through the parallel diode.With fast enough PWM I don't think there will be any issues.

author: Berni
author_rank: Super Contributor
timestamp: November 11, 2020, 05:48:08 am
content: Quote from: mag_therm on November 11, 2020, 04:17:35 amIts the single phase AC motors that have a pulsing torque problem. They only actually produce torque when the sine wave is high so the output torque pulses at 100 times a second, this is also the reason why single phase asynchronous motors are so loud while 3 phase motors are so quiet (most of what you hear is just bearing and fan noise)DC motors don't have this problem and even when run with 1KHz PWM the current can't die off fast enough in the motor, so it keeps producing torque even between PWM cycles.But yes intentional speed dithering is a thing on lathes. But the reason is not the motor. The actual reason is that when you have something long in your lathe chuck you actually create a tuning fork. As you cut along it you tune this long turning fork of a workpiece to different frequencies, at some point it finds a resonance with something and starts to oscillate, this can be heard as a screeching and it can leave a pattern in the cut finish. But resonance takes a bit of time to build up to full amplitude, so by constantly changing the motor speed up and down slightly you quickly pass trough those resonance points so that they don't have enough time to build up in amplitude.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 11, 2020, 07:33:35 pm
content: Good old Triac Dimmer would be the simplest to start with, but you've got the order wrong. Put the Dimmer on the AC side, so:Mains socket -> Filter -> Dimmer -> Bridge -> (LC filter) -> Motor.The LC filter part may make the motor run smoother, but the motor itself may have enough self induction.But High voltage FET's or IGBT's are also cheap and easy to come by these days, and the higher switching frequency reduces torque ripple for the motor. Higher frequencies also means smaller filtering components.I would not bother to make a full bridge. There is no need to switch direction fast, and a simple dual pole switch is good enough.