Shorting 3 phase

[Jimmy D]

Hi Folks,

Soon I'll be be doing some rewiring and need your advice. My turbine is 8', 3 phase and 48v. I'm really just moving my shorting switch from the scary DC side over to the AC side. My question is, because there seems to be an awful lot of torque on the whole contraption when the switch is thrown, in the 3 phase situation could we not short 2 phases first then following a few seconds close in the third. Wouldn't this just soften the operation? I think that I unbalanced a blade set last year with an unavoidable fast stop.

Thanks for your time,

Jim  

[Flux]

Yes you can do a single phase short first then a 3 phase short. You can't do a 2 phase short unless you have access to the neutral.

A single phase short will reduce the braking torque. If it is a stiff machine it may stop it. If it doesn't stop in a few seconds then either short it completely or let it go.

Flux

[Ungrounded Lightning Rod]

A single-phase short will also dump the energy from the mill's inertia into the coils asymmetrically, heating one (delta) or two (y) of them more than they would be heated with a three-phase short.

If you want to stop in two stages I suggest, instead, using a heavy three-phase dump load for the first step.

Be aware that letting the mill spin longer before it stalls means you're collecting more energy to dump in the coils.  So you have to be careful to keep the first step short or even dumping some of that energy into a load will still let the coils end up with more energy and more heating than if you'd just gone straight to full braking.