Only when the motor stalls (mechanical jam) which doesn't happen very
often. But when it does, it blows the controller in less than a
second.
Ok, its cause the maximum current occurs durring stall(at least with most
motors).
I assume when the motor stalls, it behaves more or less like a short
circuit placing quite a few amps down the MOSFET (IRL7843 with an rds
of 5 milliohms) which ends up with a crack across it. The MOSFET also
has a source to gnd series 0.01ohm 6W current sensing resistor, which
also blows up. (100 amps at 0.01r is 100W)
Well, it behaves as a resistor. Once the motor starts turning a back emf is
generated which reduces the current.
The system is designed to operate at 10-20A nominal, so the controller
is OK under these conditions, the jamming is an unexpected situation.
Ideally, i'd like a fast "intelligent" fuse that can trip during these
unexpected situations.
I don't have access to the controller's internal circuit, I know it
has some form of current sensing (it has tripped before) but somehow
it is not catching these fast high-current conditions.
Any ideas?
Well, it really depends on what you need. If the stall is only temporary and
the motor and "restart" somehow then there are many options.
If you have to manually remove the jam then there are a few other options.
But realize that the mosefets blowing is a good thing else your motor could
end up getting fried.
Some possible options:
1. Use a circuit breaker.
2. Use a thermal switch. (kinda an automatic circuit breaker but probably
not really a smart way to do it... but cheap and easy)
3. Use a smart current limiting technique like current folding. This may or
may not be easy to do for you depending on your electronic skills or ability
to find and integrate such a device into your system.
4. A fuse(if you like wasting fuses).
5. Current limiting resistor(like a lightbulb). Not the best method as it
wastes power and might cause the motor to run slower but might introduce
just enough resistance to limit the stall current so the mosfets don't blow.
(I don't recommend this one as its really just very dumb current limiting
but then again might work fine for your needs)
3 Is the best and can be accomplished relatively easy. What you do is have a
small very precise resistor in the path.
like a 0.010Ohm resistor, say. Since you know the voltage is 24V then the
current through the resistor will give a voltage on it of I*0.010. The power
dissipated in the resistor is very small and has little impact on the
circuit. (for about 30A it will dissipate 9W)
You then sense the voltage drop across it which you can then compute the
current. When the current passes a threshold(say using a comparator) it will
trigger a relay or mosfet to turn off the power. (or even switch in a high
resistance light that could be used as a malfunction indicator)
It can be accomplished with just a few components like are resistor,
comparator and transistor.
Better techniques would use current folding which actually reduce the
current but require a more components and knowledge.
I'm sure theres other techniques out there too but you really need to decide
what you want to do and what you need to do for your specific application.
