Once the magic smoke comes out, things don't work any more.
John Kasunich
jmkasunich@fastmail.fm
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Fri, 22 Dec 2006
Transistor loses its magic smoke
A while back I snagged a DB (Dynamic Braking) unit from the dumpster at work. I knew that once I had a VFD for the spindle, I would want to be able to stop it quickly, and to do that you need to dissipate the energy stored in the motor's inertia. A VFD doesn't dissipate the energy when it deccelerates the motor, it simply extracts it from the motor and moves it to the DC bus. If the bus voltage gets too high, the VFD will shut down (the alternative is to let it get even higher and then the VFD blows up). A DB unit solves this problem by throwing a beefy resistor or two across the bus to burn up the energy.
When I grabbed the DB unit from the trash, I gave it a cursory glance, and it seemed in good shape. But today I looked closer and found out why it was thrown away. One of the DC bus wires touched a sharp edge on one of the resistors. The resistor got hot, and the wire insulation failed. The burn mark on the gray wire below shows where it happened.
I haven't traced the circuit out to see exactly where the current went after the short, but the end result was that a TO-220 SCR on the control board got a lot more current than it could handle, and lost its magic smoke.
Fortunately for me, the main power transistor (a 600V, 30A darlington) is still intact. So are the two 100 ohm 200 watt power resistors and the 5K power resistor that provides base current for the darlington. A little 100mA 600V FET and some drive circuitry and it should work fine for my application. If I leave the resistors in parallel like they are now, the unit can disspate 2800 watts for a short time before the resistors heat up. There are thermal switches mounted to the resistors to prevent fire in the event of sustained braking.
(posted: 22 Dec 2006 18:22) (permalink)