Is your 7x lathe really off?

[SwarfnStuff]

Hi All,
       The other day while threading by pushing the die holder with the tail-stock and cranking by hand I noticed that the lathe wanted to run forward as I reversed direction to remove the die.
       
       Anyway, this prompted me to do some checking. 

1) With the chuck jaws screwed in so I had a smooth surface, Speed controller to the off position, try turning the chuck in both directions. Result, more resistance going against the direction of the fwd/rev toggle.
2) Repeat with the machine set to reverse, result, same as 1).
3) Repeat 1&2 except put the Fwd/Rev Switch in neutral. Result, Free either way.
4) Repeat all of the above but with the emergency stop button engaged, Result, Free either way.

This leads me to conclude that there is definitely power to the motor and the Speed controller off is not actually off off, just not enough power to run the motor. (I thought there was a definite off position as there is a definite click at the zero position, evidently not on my  machine.)   

Were I fiddling about within the control box or any of the electrics I would automatically pull the plug from the power outlet. We are on 240V ac here in OZ.

   Anyone else have this problem, or is it just my Clone?

OK, perhaps I should have turned the machine off at the power point rather than just turn the speed dial to off when hand threading. In future the emergency stop will be pressed as it locks all power so no .
John B
   

[mfletch]

Is it permanent magnet motor as you turn the chuck you've turned the motor into a generator

[hopefuldave]

The speed controller has "IR Compensation" which balances the back-EMF generated by the motor windings and current required for the load - rotating the spindle and motor while speed's set to zero will generate an EMF which the controller misinterprets as back-EMF due to a load, leading it to try to compensate by putting current into the motor coils...

If moving in the right direction for the F/R switch setting the EMF makes it think that it's running into a lower load, so it reduces the current (to Zero, where it already is), in the wrong direction it sees it as an increase in load, so increases it, resisting the external speed change / motion and giving the resistance you're observing.

The KBIC DC motor controllers have an accessible "IR Comp" preset (maybe labelled "IC" - I disremember), this is usually the final step in matching controller to lathe and motor - too little and the lathe will slow excessively under load, too much and the RPM will "hunt" up and down in pulses, even under no load - the 7x12 controllers may have this preset too, but it's not something to mess with unless the controller needs setting up because of a hardware change!

In summary, yes, power off by the E-Stop switch or put the F/R switch in "neutral" (which disconnects the motor from the controller). Particularly for anything involving adjustments to chuck or rotating (potentially rotating?) work...

Hope that helps,
Dave H. (the other one)

[John Rudd]

Bottom line here....
If you want to fiddle with your machine......make sure its unplugged from the supply.....its the only way!

Dave, you are correct, it is IR compensation....labelled IR on the board

[SwarfnStuff]

Thanks for the responses chaps,
  Fletch, yes it is a permanent magnet but I am turning very slowly as I do not as yet have a hand crank but low level power generation is a definite possibility,
  Dave H, thanks for your input I knew if I posted here I would learn useful stuff. Never gave back emf a thought as electrickery is not my strong point and I stay away from the controller circuit. Except when it blew a couple of diodes, then it was out with the gear and replace the obviously blown bits. (I have enough spares to fix this same fault 5 more times as it was cheaper to buy 7 diodes than two).
  John R, yep, Good safety practice anyway so well pointed out. The E-Stop is easy to press plus closer than the power point and will be done whenever I need to, "fiddle" as you put it.
   I still reckon that the speed control switch should power off with the "click".
I just might check that out if for some reason I have the controller box off (IF I remember).
Thanks again,
John B

[steampunkpete]

I can confirm what has been said:
The "Forward-Off-Reverse" toggle switch disconnects the motor from power.
The speed control knob has no "Off" position (what would be the point when the "Forward-Off-Reverse" toggle switch has an "Off" position?).

Leaving the "Forward-Off-Reverse" toggle switch in either "Forward" or "Reverse" whilst working on the chuck should be avoided; I've seen my chuck suddenly make a few turns when I've left the toggle switch in "Forward" and the speed controller at minimum. I suspect the cause is a fluctuation in mains voltage causing a transient in the controller.

The toggle switch has an "Off" position for a reason; it would be wise to get into the habit of using it.

If your lathe is second-hand the speed control pot may have been replaced with a pot with an integral switch, or you may just have an idiosyncratic pot.

[modeldozer]

The switch incorporated with the speed control pot is a safety mechanism, forcing one to turn the pot to minimum before switch on.  It does not disconnect power from the controller. The NVR on/off, e-stop should be used for switching on and off.

Cheers
Abraham

[steampunkpete]

The switch incorporated with the speed control pot is a safety mechanism, forcing one to turn the pot to minimum before switch on.

I haven't got that on mine - yet another variation on the theme.

[SwarfnStuff]

Hi and thanks again all,
      As I posted this on the Yahoo 7x12 site too I have got many responses and good input. Of particular interest to me was from John Lindo that by setting the gear lever to neutral it gives better feel when threading by hand in the lathe plus avoids the possibility of getting caught by suddenly moving bits. All agree that cutting the power by some positive means, preferably unplug power or press emergency stop when hand cranking or adjusting the chuck etc is just plain good safety practice.
John B