Andrew Mawson wrote:
"Andy Wade" wrote in message ...

On 28/01/2012 15:20, Andrew Mawson wrote:

I think Andy that the shunt diode would need to be on the output side of
the contactor rather than the input terminals, ( ie across the
inductance of the motor coil) to be effective, and of course next time
the motor starts up the diode would vaporise as it would have 415v AC
across it. I'd like to be shown that I'm wrong, but I don't think so.

The diode will still be effective if the DC contactor is used to
isolate the DC PSU under AC motor-run conditions, but not for DC load
switching, as Bob suggested.

The braking sequence would be:

- AC off (open AC contactor)
- pause
- close DC contactor (with DC PSU off)
- energise DC PSU, by switching its mains supply, or otherwise
- hold for set braking period
- shut down DC PSU (DC contactor remains closed, so diode is in-circuit)
- pause while winding inductance discharges via the diode
- open DC contactor
- pause
- ready to re-apply AC.



Nice one. I hadn't thought of switching the DC psu at it's mains. Ok it
adds a couple more delay circuits but it's neat.

AWEM
but that was the basis of my earlier reply.......