Kirk,
Close, but not quite. It's kind of a chicken or egg argument, but my (admittedly simplistic) way of looking at it that the controller sets the VOLTAGE that the motor is getting, and the motor responds by spinning up to it's rated RPM for that voltage. The speed vs. voltage response is designed into the motor, and is generally known at the "Voltage Constant". For the E-tek the number is 72 RPMs per volt. So at full throttle (36V), you'd get 72x36=2592RPM. Half throttle (18V) will get you half that RPM. The amount of torque the motor needs to produce in order to maintain the selected RPM is what determines how much current it will draw. With no load (shaft spinning free) the motor will draw just a few amps, but adding any load to the shaft will result in a proportionate increase in amp draw. There are additional factors that Myles referred to such as motor inductance and inertial effects that conspire to make life difficult for the controller.
Another consideration... once you have robust enough controller so it it no longer the weak link, you'll want to start thinking about what the next-weakest link will be. Most likely it will be the batteries, (assuming you've done the wiring right). You'll want to set your reduction ratio so the motor doesn't toast your batteries at full throttle. I believe you said the Optimas were rated 55 amp-hours? You probably want to keep the W.O.T. (wide open throttle) amp draw under say 50A, and don't run more than 20-25A for any length of time.
With a bigger battery bank you could go to a lower reduction ratio to increase prop speed and draw more amps.
Hope some of this helps.
Jim
--- In electricboats@yahoogroups.com, Kirk Mason <gentlemancaller2009@...> wrote:
>
> Jim,
>
> Yes, that helps me understand the situation better. Thanks!
>
> I'm guessing when we have a complete 36V Minn Kota 101 RT system, what's happening when we advance the potentiometer is we are changing the amount of amps the motor is getting, and thus producing different speed / thrust levels? Does that sound about right?
>
> If that's the case, I can see what you mean about staying in the lower end of the power curve and maybe getting away with it. But that's a bit more of a restriction than I'd care to deal with on a regular basis, and the possibility of a failure would be a nagging concern all the time.
>
> If compatible, I would have liked to use the MK controller because I already have one. I was hoping it would drive the Etek to 70% power, but clearly it won't. So, it's no longer an option.
>
> I don't mind spending the money to get the right stuff; just thought I'd try to save a buck if I could use something already sitting in the shop. But no problem. I'm looking at the controllers most people use for Etek type motors, and at complete systems, too.
>
> I'll go ahead and install one more battery to bring the system up to 48 volts; get the right controller rig; and be able to operate the motor in the full power ranges it was designed for. That additional performance will make the additional expenditure less painful, I'm sure.
>
> Thanks to everyone who participated in this thread. It's been a big help, and kept me from making a big mistake.
>
> Best Regards,
>
> Kirk
>
> --- On Wed, 10/13/10, luv2bsailin <luv2bsailin@...> wrote:
>
>
> From: luv2bsailin <luv2bsailin@...>
> Subject: Re: [Electric Boats] Controller question: Minn Kota Says....
> To: electricboats@yahoogroups.com
> Date: Wednesday, October 13, 2010, 4:00 AM
>
>
> 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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