Emergency stop for runaway large DC motor

[geshka]

Just fit a normal manual E-Stop in reach of the operator,

You REALLY should have one of these anyway.

I'm going to assume you are not silly enough to leave the lathe spinning un-attended btw!

Yes, indeed.  I have one, though it is my reflexes against microprocessor reflexes. I believe, latter is faster 

[geshka]

So folks, in addition to mechanical solutions and E-STOP - is there any practical schematic to measure , let's say, Vds of the MOSFET and shut down the power ? 

[mag_therm]

Measure Vds into a  Hi Z input low pass filter. V < Vlo means stuck ON or shorted ; V> Vhi means OFF, In between = running OK

I have not tried that directly here but in my 300 Watt DC converters I use a floating snubber, where its DC level varies.

[Xena E]

Just some thoughts:

Pops has a DC motor driven lathe, humble Myford ML7. The controller is an old Fenner Speedranger, that is a three phase thyristor job but the principles are transferable.

We built a controller that sensed the DC supply to the motor and the control signal to the Speedranger, it was basically a scaled comparator that would drop out the no volt release if it 'saw' excess output, over controller input.

Purely analog.

The reason it was necessary was that one time the 3 Thyristors in the Fenner failed dead short, which is the kind of rare scenario that I understand the op is concerned about.

As for the emergency stop safety switch, an unfortunately named dead man foot bar must be the best choice IMHO for a lathe.

Regards,
Xena.

[Xena E]

As an idea if you want to sense Vds, you could perhaps use an optocoupler, the fun would be keeping the emitter current within specs.

[Johnny B Good]

Remember this very important safety item. If you sense overspeed the fault circuit must latch itself to prevent re-applying power when the RPM slows below the trip point during braking!! The circuit should be wired to self latch until manually reset!!

An SCR into a brake resistor would do the trick.

 I second that suggested solution and add that, given a suitable braking resistor, the SCR can crowbar the PSU into submission and swiftly blow the protective fuse or trip out the circuit breaker to shut off the power (or best of all, simply cause the ( assume) smpsu to trip out from the overload).

 The SCR being the one exception to that old definition:

"Transistor - a ten dollar three legged device designed to protect a ten cent fuse."

 Just make sure to use a suitably rated SCR to handle the resulting transient surge current.