On 13/10/10 09:03, gentlemancaller2009 wrote:
>
> Myles, you are preaching to the choir. ;-)
>
> What you describe is EXACTLY what I thought would happen, but the tech told me otherwise. I'm still having trouble wrapping my head around what he said.
>
> 1. The battery pack has a maximum of 36 volts and 165 amps.
>
> 2. The controller has a maximum output of 36 volts and 49 amps (I would think to any motor it's governing: a Minn Kota or Etek).
>
> 3. To me, it seems the motor can only get whatever the controller will allow: in this case, 36 volts @ 49 amps: which means the motor will be running at something less than what it does at 140 amps. (And again, that would be fine with me.) But the tech says no.
Their controller obviously doesn't have effective current limiting.
Presumably their own motor has a 'stall current' that doesn't exceed 49
amps, so they didn't bother adding extra protection to the controller.
This sort of thing is typical of systems that are designed as whole,
especially so if they are trying to get the system price down.
I wonder if it's short circuit protected though. Damage to the wiring
could blow your controller which doesn't sound optimal.
A 'generic' controller, like my Alltrax has to be designed to be more
forgiving as the controller designers don't know what the users of their
controller might subject it to. So they build in things like software
settable current limiting.
> I see the controller like a faucet between a water pump and a spray nozzel: the pump supplies water at a certain output pressure; the faucet determines how much of that maximum pressure reaches the spray nozzel; and that determines how far it will shoot water. If I don't open the faucet all the way, the nozzel doesn't shoot as far. It's not a case of the nozzell sucking maximum water pressure past the faucet. But it sounds like that's exactly what the Tech is telling me will happen.
>
> I don't see how the motor can "draw" more than 49 amps through the controller: but that's what Dave said would happen.
If you don't load the motor it won't. But what if it stalls ?. Say your
prop gets snagged in weeds. The current will near instantly go to the
maximum the motor, the controller will get hit with 140amps (or more)
and it will burn out.
>
> What am I not understanding here? (That's not a wisecrack: I really want to know. If there's something fundamental I'm misunderstanding, I need to know. And I'd appreciate anyone explaining it to me.)
I think the Minn Kota tech has simplified the issue a little. But what
he said is essentially correct.
If you took your E-tek motor, put it in the boat so it has appropriate
prop drag, connect it via a peak-hold current meter and then connect it
straight across the 36 volt battery, I'm pretty sure for a short period
of time as the motor struggles against the drag of the prop (and the
boat) to get up to speed, you will see current at the maximum 140amps
the motor can draw. As the motor gets the prop spinning and the boat
begins to move the current will rapidly drop down to what ever power
level is required to move your boat. The peak current might only be for
a very short period of time (you probably wouldn't see it at all with a
digital ammeter that didn't have a 'peak hold'), but you are talking a 3
times overload for the Minn Kota controller. It's not going to like that
very much even if it's for less than a second.
On my boat two things limit the current - the controller has current
limiting, which I have set just below the 10 minute rating of my motor.
But it also 'ramps' up the power according to a settable 'ramp' rate, so
even if I twist the throttle round to full more or less instantly, the
controller will ramp up the voltage to the motor over several seconds,
not instantly switch to full power. This also helps to limit the max
current, being a 'soft' start.
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