Kirk,
I like to use plumbing analogies too, but they have a way of breaking down if you carry them a bit too far.
Here's another way of looking at it...
For any given voltage, the motor will try and turn at a given RPM, and will draw whatever amount of current it needs to maintain that RPM unless there is a weak link, which in this case is the controller.
If you limit the torque demand and/or manage the throttle setting so the motor doesn't want more than 49A you might get by with it for a while if you take it easy and "sneak up on it". Might be worth a try if you don't mind a little fireworks!
Jim
--- In electricboats@yahoogroups.com, "gentlemancaller2009" <gentlemancaller2009@...> wrote:
>
> u
> u
> 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.
>
> 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.
>
>
> 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.)
>
> Thanks and Best Regards,
>
> Kirk
>
> --- In electricboats@yahoogroups.com, "Myles Twete" <matwete@> wrote:
> >
> > In other words, if MinnKota's controller will "fry itself" on overload, then
> > its design does not have current protection.
> >
> > But just so you're not misunderstanding anything here, an ETEK-type PM DC
> > motor rated at 48v and 140amps max steady will itself only draw the current
> > necessary to deliver the torque drawn from it. If you do not load the ETEK
> > motor, it will draw insanely low current. Conversely, to get it to draw
> > 140amps, you need to load it at its max continuous torque level.
> >
> > At half that torque (maybe that's your load?), you'd expect 70amps.
> >
> > At ¼, you'd expect 35amps.
> >
> >
> >
> > So bottom line is this: You shouldn't fry the 49-amp controller if your max
> > torque load is less than "Max Steady ETEK Torque" * 49/140.
> >
> >
> >
> > -mt
> >
> >
> >
> > From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
> > On Behalf Of Kirk Mason
> > Sent: Tuesday, October 12, 2010 1:03 PM
> > To: electricboats@yahoogroups.com
> > Subject: [Electric Boats] Controller question: Minn Kota Says....
> >
> >
> >
> >
> >
> >
> >
> > Group,
> >
> > Talked to Minn Kota Tech "Dave" today...I've got to admit what he told me
> > came as a surprise, and contradicts what I thought about how controllers and
> > motors work together.
> >
> >
> >
> > SYSTEM: three 12V 55A Optima batteries; one Minn Kota RT-101 controller
> > rated at 36 volts and 49 amps; one Etek-type motor rated for 12, 24, 36, or
> > 48 Volts (@36 volts / 140 amps = 3200 rpm.)
> >
> >
> >
> > QUESTION: Will it fry the controller, or will the controller simply not
> > provide enough amperate to drive the motor up to speed?
> >
> >
> >
> > ANSWER: The motor will seek to draw whatever amperage it's rated for. In
> > this case, at 36 volts, the motor will seek to draw 140 amps through a
> > controller rated at 49 amps maximum. Results: it will fry the controller.
> >
> >
> >
> > CLARIFICATION REQUEST: I thought the controller would simply not provide
> > enough power to drive the motor. You mean to tell me the motor will
> > actually pull whatever amperage it's rated for through the controller?
> >
> >
> >
> > REPLY: Yes. In fact, when we take our 36 volt / 49 amp 101 motors and
> > connect them directly to a battery pack without the controller, they pull 49
> > amps: no more, no less.
> >
> >
> >
> > (I've got to add here: that sounds crazy to me. I'd think if you hooked up
> > a 36 volt / 165 amp battery pack directly to a motor rated at 36 volts and
> > 49 amps maximum, that it would burn out the motor. But Dave says no. The
> > motor will only draw the maximum amperage it's designed for.)
> >
> >
> >
> > CLARIFICATION REQUEST: So if I have three 55 amp batteries in a pack
> > providing a maximum of 36 volts and 165 amps; and the motor is rated at
> > being able to produce 3200 rpm if supplied with 36 volts at 140 amps; and
> > the controller is only rated at 36 volts and 49 amps; then it's not a matter
> > of the controller not being able to bring the motor up to speed: that the
> > motor will seek to pull 140 amps, and that will fry the controller?
> >
> >
> >
> > REPLY: Yes, that's right.
> >
> >
> >
> > So, there we have it, right from the Tech. I've got to admit, that is
> > contrary to what I thought about how the systems work:
> >
> >
> >
> > I thought the controller simply would not provide enough amperage to bring
> > the motor up to speed at 36 volts.
> >
> >
> >
> > But instead, Minn Kota tells me the motor will seek to pull whatever amps
> > it's rated for through the controller, resulting in potentiometer toast.
> >
> >
> >
> > Well, no problem. It was an effort to use what I had, and save a few bucks.
> > But it's not gonna work, so I guess I'll get a suitable controller.
> >
> >
> >
> > Thanks for the input; and I hope this helps someone else as it did me.
> >
> >
> >
> > Best Regards,
> >
> >
> >
> > Kirk
> >
>
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