I'm not an electric motor wizard. I really joined this group to get an idea of what it may take to make an electric boat with my students. However, I am quite knowledgeable of DC electrics in boats (Certified ABYC Electrician) I think I can clear this up.
There are three ways an item can be rated for amperage. What it can give what it can take and what it will draw.
The batteries are rated on what they can give.
The motor is rated on what it willl draw Usually under maximum load.
And everything in between, if I am not mistaken is rated on what it can handle.
This being said you want everything in between the battery and the motor, ie the controller to be rated higher than the motor.
The motor will only draw what it needs to do what it is asked to do unless something goes wrong. At that point overload protection devices come into play. Which by the way are rated at the point they will shut off power.
The batteries will only give off what is needed, so if you have a 120 amp motor and a 360 amp/hr pack you will only draw the 120 amps. And that is only if you are running under a full load. By the way you probably should get a larger motor if you find you are constatntly running against the maximum amps.
Furthermore, if you put a 49 amp controller or switch between the battery and the motor, once the motor is asked to draw more than 49 amps, you will damage the controller.
I hope I am clarifying things for you and For any of you that are more familiar with electric motors if I am incorrect feel free to correct me. I am basing this info on basic DC electric knowledge and experience.
--- In electricboats@yahoogroups.com, "luv2bsailin" <luv2bsailin@...> wrote:
>
> 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
> > >
> >
>
No comments:
Post a Comment