Re: Motor driver current snese protection


"Joerg" <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:5I%9j.32020$JD.20138@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Spehro Pefhany wrote:
On Tue, 18 Dec 2007 18:03:35 -0600, the renowned "Mook Johnson"
<mook@xxxxxxxx> wrote:

This is the classic triple tottem pole 3-phase inverter. There was a
failure of the motor wires getting shorted together and the bus
capacitor
(100uF charged to 500VDC) dumped through two on IGBTs and blew the 0.1
ohm
current sense resistor. This presented 500V on the input of the opamp
that
was measuring the voltage across that resistor. It blew and connected
500V to the +12V rail and blew everything on that rail including the TVS
(SMB package).
:(

The reliability guy suggested a SMC or DPAQ diode across the 0.1ohm
resistor
to catch the voltage if it were to blow again to hopefully save the
tool.

Sounds doob-yuss.
Would it make more since just to beef up the 0.1 ohm resistor to handle
the
pulse load of a 500V 100uF cap discharging into it? (thinking metal bar)
The
cap ESR is ~2ohms so the theoretical peak current would be 250A.

Sounds expensive, and might 25V with 0.1R source impedance not
potentially cause some major grief in the op-amp circuit?
Any other protection schemes? (HV resistor between the current sense
and
opamp with clamp)


An impedance (eg. a 10K 500V resistor rated at say 1W-- check the
pulse ratings) in series with the 0.1R and put a diode or tranzorb on
the other side of *that*.

Then I'd also do that for the other side. In case that side goes open for
some reason and the whole motor current would decide to flow through the
measuring box and then ground instead, melting a few thing along the way,
orange sparks flying and that sort of thing.

--
Regards, Joerg

http://www.analogconsultants.com/

I wonder if putting a small sarcificial .5 ohm resistor on the collector of
the top IGBTs would help the situation. If it blows open the rest of the
circuit would fall to ground. Since it would be 5 times the resistance of
the .1 ohm and physically smaller (say a 2010 vs Vishay WSR 2W) it "SHOULD"
blow first. Kind if a fuse of sorts. The peak phase currents we're talking
about here is 1.5A RMS. Duty cycle should be about 70%. Of course the 2010
resistors are typically limited to 250 max so at 500V would they just acrh
across and short instead of blowing open. What I really need is a fuse.
Are there SMT Fuse type devices around? Runs fine at say 3A continuous and
pops when > than say ~10 amps is applied?


Maybe a thin trace in the PCB?



.

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