Treated myself to a new toy, which is in fact an old 3 phase ex school Wadkin 10" AGS saw table.
The ad for it on eBay said the blade stops in 10 seconds and therefore no need for a DC brake.
I haven't looked for a mechanical brake yet, but wonder how it can stop so quick if it doesn't have anything mechanical.
I ask as although we have part sorted the lack of a neutral in the workshop, I wonder if this will need it, as did our new compressor with built in dryer.
Over to you Andrew !!!!
Bob
Hi Bob,
When I bought my Dominion Super Elliot combination wood worker it had been 'retired' as the place it was used (turning oak beams into floor boards) was caught by HSE for not having braking mechanisms incorporated. Hence it went on eBay. Not essential for my hobby use but desirable to have fast stopping. This beast has two 7.5 hp motors (20" Circular saw and 17" planer) and a
5.5 hp motor (cross cut saw) in all about 2 tons of heavy machine :)possible / probable that your unit has some such device incorporated.
A DC Injection Brake essentially is simple in concept. When the three phase power is removed, a source of DC current is 'injected' into one winding making it magnetic. This magnet acts on the iron of the motor as a brake. Very simple, but as usual the devil is in the detail. It is absolutely essential to design things so under no circumstances can DC be injected when mains is on the windings, or AC applied when DC is being injected.
My design has a pair of contactors that are not only mechanically interlocked (so only one can make at a time) but are also electrically interlocked. As well as this there are 'dead band timers' giving time for one contactor to have definitely dropped out before the other can be energised.
Andrew
Thanks for that Andrew, I knew you would have an answer. I'll get the saw wired in and see what happens. As its ex school, has a collection of safety things on it, but don't really see where it could get DC from. Bob
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