G'day All
At 05:42 AM 14/02/2010, Bob Noble wrote:
>I think you make a good point here Kevin.
>However, I think most Mosfets would fail shorted first.
>
>However, if one of the legs failed shorted, <snip>
I haven't properly read all the posts on this thread, so appologies
if this is repeating what someone else has already said:
A DC motor controller, for typical DC motors, will usually fail short
(initially) then go open circuit (as the transistors vaporise). That,
however, is overly simplistic. I have pulled down Curtis controllers
that were down on amps - and found a number of blown mosfets, where
the rest were still working. The mighty Zilla (series DC motor-)
controller (for on-road electric vehicles, 96 volts and above) has
bucketloads of self-testing and will drop out the main contactor if
it finds a fault - and as far as I know, has only had a couple of
power stage failures, ever. Cheaper controllers, Curtis, Kelly, etc
lack self-testing so it is up to the system designer to add what is
needed to make them safe. On a boat, the prop and hull will limit the
maximum failure speed, the inertia of the vessel should prevent rapid
changes, all you need is an emergency stop system that is available and works.
A "brushless DC" motor (actually a permanent-magnet-
motor) controller and 3-phase AC induction motor controllers are by
their nature a more complex beast. It is possible to have the 6-leg
H-bridge fully monitored, so if one transistor fails short, the
control system notices and shuts down the other five transistors -
the 10-year guaranteed Siemens on-road electric vehicle controllers
do this (as I understand, never seen one though). Cheaper controllers
do not monitor the H-bridge, just the overall H-bridge current, so
the likely scenario is DC braking of the motor, but under control -
not exceeding the overall current limit. (this is the 'Achilles heel'
of wheel motors - if one side of a road car does this, the car will
likely spin out of control. Nice lawsuit there). On a boat, this
behaviour is unlikely to be a problem.
What this comes down to, is that you get what you pay for, regardless
of AC or DC. There have been DC systems in the past that cost as much
as the expensive AC systems of today - brushed DC has fallen out of
favour with the 'high end' crowd, and why not if you can get 10+
years with almost zero maintenance included for your money? The
better DC systems have in them a similar level of engineering as the
mid-level AC systems. The cheapest AC systems (and this includes
brushless DC) are worth what you pay for.
Already said, you get what you pay for, however... if you (the
builder/owner/
put to use, to put together a system that suits you (and the
operating parameters of the boat) then why not?
For example: remove electronics from the motor control system
completely. What? (I hear) how? If you are (or have a friend who is)
a machinist, take a page from the 1920's electric car scene, and
build a drum controller, which switches battery connections (and
motor taps if possible). With a 6V set of batteries you can have [6V]
- [12V] - [12V plus weakend field] - [24V] - [24V plus weakened
field] - [48V] - [48V plus weakened field]. 7 steps forward, 7 steps
backward. In the middle is a position that disconnects the motor and
series connects the batteries for charging at 48 volts. You need to
be careful in design, so that all batteries are loaded together.
Similarly you can use contactor controller, or a 'rectactor'
controller, that uses contactors and big diodes. If anyone is
intersted, I can provide more information (plus can be found in the
electric vehicle discussion list library at
http://www.evdl.
If you have a bit of electronics skills, there are plenty of surplus
power transistor modules on Ebay cheap - a simple PWM (pulse-width
modulating) transistor chopper can be built by anyone with basic
electronic skills - all the current limiting, etc needed in forklifts
(and other similar EVs that the controllers were built for) become
much less important on a boat (until something goes wrong). Dealing
with the failure mode is where things can get expensive if you don't
go about it the right way.
Regards
[Technik] James
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