Kirk asked: “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?”
Maybe, but probably only indirectly---if the controller were responding to potentiometer position with a CURRENT output (i.e. a transconductance power amp) as you suggest, then you’d have no problem---FT: 49a; ½ throttle: 29 ½ amps; ¼ throttle: 14 ¾ amps, etc., subject only to the limitations that:
(1) the load can actually draw that much, and
(2) the low motor resistance and inductance won’t cause the controller to stay in current limit
(3) the controller’s current limit itself is weak
Why their controller is not a transconductance power amp:
(1) It’s a more complicated controller---not a lot, but see point (2)
(2) It doesn’t provide the desirable throttle behavior
a. Speed would not be proportional to throttle position
b. If prop pulls out of the water, the motor/prop will accelerate to MAX for any significant torque setting
Though there would be one desirable feature: Torque drive would help ensure motor or amp doesn’t fry when you hit rocks or get stuck in weeds
Look back at the TECH RESPONSE:
> > 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.
This clearly indicates that their amplifier is not a transconductance power amp. It also indicates that the TECH GUY is misleading you somewhat.
He wrongly implies that the motor will arbitrarily draw 140amps through any controller, regardless of the battery voltage (in this case he says 36v) and regardless of load and regardless of throttle position. This is absolutely wrong information.
Another TECH RESPONSE to “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.
This is misleading at best. For sure he is telling you only what the MK101 motor will do and not what the controller will deliver or limit. Worse, he is not mentioning the assumption: that the motor/prop are IN THE WATER! If you pull that motor/prop out of the water, it’s highly doubtful that hooking it directly up to 36v would cause 49amps of current to flow (over 1500watts!) to the motor---that would be bad engineering design.
I strongly suggest you educate yourself more on motors and controllers before doing something risky. Indeed, that’s what you’re doing here and sharing this information with us all and that’s great. A matched or demonstrated motor/controller combination is least risky, but frankly, I do not yet see why your original proposal will not work----ETEK permanent magnet motor with Minn Kota 49amp controller.
Caveats include:
(1) Ensure your throttle ramp rate is slow enough to ensure that the ETEK won’t try to draw high current on acceleration
(2) Provide a fast blow protection fuse to protect your controller in case the motor seizes or prop hits obstructions
(3) You know your boat’s motor loading and limit your throttle such that your boat never draws more than say, 80% of max MK rated current steady
(4) The low inductance and resistance of the motor will allow VERY HIGH currents to flow if motor loading jumps high enough---easily exceeding 49amps and can even reach 300amps (see Etek specs---300amps is allowed for short bursts)!
(5) The low inductance may cause excess switching stress on the controller
Short of building in a high speed current limit, the fuse (fast blow) would be essential.
About the ETEK and that “140amps”---where does that come from?
It’s the nominal rated amount of steady current that the motor can absorb without exceeding temperature limits assuming adequate cooling provided.
But that number is itself arbitrary as applies to what the controller sees. Life can actually be hell for the controller that drives these motors.
The effective internal resistance of the motor is nominally 25milliohms: 0.025 ohms.
What does this mean? Lots of things…for one, if you had a locked rotor, the motor and ramped up the terminal voltage, you would see:
· 40amps at 1v
· 80amps at 2v
· 160amps at 4v
· 320amps at 8v
· 1440amps at 36v!
So when you stated to the MK Tech Guy that this was a 140amp 36v motor, that’s not even quite correct.
Short Circuit Current for the ETEK is over 1400amps at 36v!
If the ETEK had not inductance, even many of the highest current rated motor controllers would fry if they didn’t have FAST current limits built-in.
And though the motor does have inductance to reduce the stress on a controller, some controllers will fry due to the ETEK having very low inductance.
While that can be remedied by adding a honkin’ inductor in series, it’s not pretty.
Someone else said it best when they stated that while you probably could drive an ETEK with a Minn Kota controller, why pay the money for the 8HP ETEK if all you are going to be able to deliver is a max of 2HP with the MK controller?
In case this helps-
-Myles Twete, Portland, Or.
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