Re: [CAD_CAM_EDM_DRO] Re: E-stop relay welds contacts for load dump resistor

In my experience a dump resistor works well on a servo system. I usually
switch the mains to the power supply transformer and switch the dump
resistor with a reasonably beefy contactor. Contactors are designed to
switch high voltages and high currents reliably and safely. They are
also available with a mix of normally open and normally closed contacts
so you can use one contactor to switch both the mains and the resistor.
Most contactors have an option to attach extra contact blocks which can
be used for the latching circuit etc.

With a dump resistor the cap still takes a finite amount of time to
discharge. This means that the drives will remain powered up for a
fraction of a second after you hit estop. If you are moving fast and hit
estop the drives try hard to stop, dumping a lot of power back into the
reservoir cap. This dumped power will be enough to keep the cap charged
even though you have the resistor in circuit. On my converted Colchester
Triumph lathe for instance, if I hit estop at full speed it stops like
it has hit a brick wall.

Les

> I would maintain that using a dump circuit as suggested is not the best
> approach, as it leaves the motor freewheeling and uncontrolled every
> time the E-stop is engaged. It's like turning off the ignition key of a
> runaway car on the highway at high speed. Yes, it stops the engine, but
> you are still going 65 and now you have lost your power steering and
> power brakes. I do submit that it would be helpful in case of a driver
> runaway condition, where the driver is running the motor independently
> and in contradiction to the input signal. If the driver is well
> designed, then this should be a rare occurrence.