Call for a hack idea!

[Lenp]

Here's the story.
I have a small, very common Chinese lathe, with a DC motor and controller.
To start the lathe, the power switch must be on, the chuck guard down, the forward-off-reverse switch must be in forward or reverse and the speed control must be rotated from off to the desired speed. Disarranging any of the above will stop the lathe.
If the speed control is left alone, it must be turned to the off position then on to a speed setting to restart the lathe.

Focus is on the speed control. It has a switch at the CCW off position, that closes at the off position. When it opens it resets some protection circuit in the controller and the motor will start. Now this works OK for most applications but if you are doing something, like a quick cut on many parts, it is a nuisance, increases wear on the switch plus returning the speed control to the same setting is touchy.

So, what I am looking for is a simple circuit to mimic the speed control and switch operation. The problem is that if the speed control is set to anything but a real low speed, and the switch is shorted then opened to reset the protection, the motor will start with a surge, sometimes blowing the 5A main fuse. The control system was apparently designed to force a slow ramp up to speed, and any design will have to duplicate this. The speed control is a common 5K potentiometer, wired as a rheostat with a NC switch at the CCW position. The resistance increases, to increase the speed

I am aware of digital potentiometer IC's but would like to avoid all that complexity.
I might also consider an LDR coupled with an incandescent lamp driven by a ramp-up circuit, but now, the jury's still out!

The ultimate hack would be a foot switch; press to start with a quick ramp up to a set value, and when released, it stops.
I guess I could get a pot, add the switch then cobble it up to some kind of footswitch, but I think there may be a better way!

There is really no safety issue, since when the chuck guard is lifted the motor is disconnected.

Ideas?

[ShopRat]

Would a "step start" be good enough? A small-value resistor switched in parallel to the rheostat could give a slow start speed, then open that switch to "jump" to the speed preset by the rheostat. That would still need the "on-off" switch, needing three actions to restart the lathe. If you need more than one speed step, complexity would exceed the more elegant approaches.

Any chance this is some implementation of an old servodrive type signal, such as 0-10V or 0-20mA? That might open up a number of possibilities, including a permanent soft-start feature.

Dale

[Lenp]

So, if I understand your idea....

1. Speed rheostat is preset at desired speed (it's switch is open)
2. Additional resistor (variable) is switched in to shunt speed rheostat to set a low safe speed
3. Momentarily close the external switch that is wired parallel to speed control switch
.
.some time delay?
4. Remove shunt resistor to go to preset speed.

New Glow....
Maybe a shunt capacitor would work instead of a resistor. It would be low resistance at start up, then becomes higher as it charges, giving an inherent 'soft start' effect plus, ...It should also work with the normal starting procedure. Humm.....

To comment on your post, the motor is a simple DC motor, but just maybe the control is really a current source since a rheostat is used. I would more likely expect to see a potentiometer, if it was a voltage signal.
Looks like it's time for the clip leads and meter to find 'was ist los'

Got work to do!

[ShopRat]

Hi, Len,

Yes, you followed my "quick fix" correctly. I'm guessing the Chinese didn't invent anything expensive. Also guessing there's a "black box" control behind the curtain. Who knows, maybe they got a deal on a shipload of the "black box" for my '80's JD lawn tractor that looked at the seat switch, neutral switch, PTO switch, etc., although I think that was mostly an interconnect circuit box without any "electronics."
The rheostat could be the bottom half of a voltage divider, or something completely different! Increasing resistance with increasing speed (not to mention the power rating) suggests the rheostat is NOT in series with the field winding, for example. I would expect the cheaper "universal" motor (common on portable tools) in your lathe. Another reason for their start method could be getting reasonable life out of the motor brushes.

I should have mentioned that I was referring to industrial servodrives, not hobby servos. I've heard of 4 "protocols" that emerged as de facto standards in industry, communicating speed/direction from a logic/control section to the drive power section. I know we're still using one of them for the "drive" signal from a robot to its adhesive/sealer dispenser. Anyway, that was what I wondered if the Chinese had "recycled" to use in your lathe.

Aha, there's your problem! Speaking German to a Chinese tool .

Good luck,

Dale

[Lenp]

Hi Dale,
The rheostat is a carbon control, and definitely not directly connected to the motor windings. The motor has that PWM whine sound so until I scope it, it's just a guess. The board has a few trim pots, so they may set min/max and maybe some drive waveforms. I suppose PWM is the best (read cheap) way to get big torque at low speed.

Since there is a gear train driven by the motor through a cogged belt, there probably isn't much slip on startup, so the slower speed would be desirable to minimize damage.
Some careless operator may start the lathe with the carriage too close to the chuck and then it gets a battle scar, or worse!

I've been distracted from that project for a short time because of some gotta-be-first jobs, but I'll get back and let you know what I find.
I am still mulling that cap idea!
Thanks for your input,