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Geared motor vs brushed

Posted: Jan 01, 2018 7:05 pm
by Alex M
Forgive my ignorance - and mistrust ;)...

Without going into fundamentals of mechanics and hydrodynamics, would like to hear semi-educated opinion.

Here are 2 trolling/kayaking motors:
400W Torqeedo Ultralight 403, geared. (I understand they use PWM controller for variable speed).
360W Minn Kota Endura 30, brushed (either Max version with PWM, or an add-on 3-rd party PWM).
Or 500W Minn Kota 40, brushed, PWM.

According to Torqeedo: overall efficiency of their motor (not clear whether it's all Torqee models, or only tiny 403, or huge Cruise models), i.e. efficiency on the propeller, is 44-56% of input wattage, VS unspecified "conventional electric outboard" 30-35%, VS unspecified "trolling motor" 18-22%.

This is 2-3 times difference for "trolling motor".
I understand marketing reasons, but let's cut the noise. Will 403 really run 2-3 times longer than something like Minn 30 or 40, given same size N *wh battery and roughly same speed (less than hull speed), on the same 15ft kayak or canoe?

I understand that Torqee has designed a better propeller. Also, Minn is 6-9 lbs heavier (partly due to metal lower unit, VS plastic in Torqee). But - 3 times better?... Did they consider that modern "trolling motors" often have PWM? Or, that a trolling motor on a canoe or kayak is not immersed several feet into water (there are many kayak outfitters that custom-cut the shaft shorter).

Re: Geared motor vs brushed

Posted: Jan 01, 2018 8:50 pm
by amberwolf
Well, geared and brushed are two completely different categories of things, so you can't compare based on that.

Geared vs direct drive (and possibly vs belt or chain) you can compare.

Brushed vs Brushless (and other motor types) you can compare.

You can also compare one model to another, if you have the directly-comparable information, like efficiency--but remember that any of that information that comes from sellers of the items is suspect as marketing-blah-blah-blah. ;) However, they all usually inflate about the same amount, so you could probably compare whatever efficiency one brand claims vs another brand.

The best way to compare is to test the different systems on the same boat under the same conditions and speed, and get their "wh/mile" (how many watt-hours they consume over the same distance)--not a test most people can do without buying more systems than they actually need. :/

WIthout doing such a test, the only option you really have is to take the known tested values for any directly-comparable units of measure that are provided by the factories. If they don't give those, you can only guess.

The propeller design also makes a difference, so if they ahve different props that may change their efficiencies, though that might not be significant compared to the rest of the system efficiencies.

Pretty much any motor system that has a speed control is probably using PWM in the controller.

LIke for like, a brushless system will be more efficient than a brushed.

Whether a direct drive system is more or less efficient than a geared system will depend on the conditions / speeds / loads under which they are used vs the conditions / speeds / loads they were designed for (which are not usually specified).

Re: Geared motor vs brushed

Posted: Jan 01, 2018 10:15 pm
by Alex M
I should've mentioned that Torqeedo is brushless geared - not just a geared. Makes a typical whirring noise of a geared motor.

Minn Kota is brushed and (I think) direct drive. There is zero info on the latter feature at the Minn Kota website, but it "sounds" literally like a direct drive. Inside there are permanent magnets lining the inner wall of the hub - this must be the"stator", and the rotor is the axle with windings on it:

So the comparison I'm looking for, is geared brushless Torqee VS direct-drive brushed Minn Kota roughly of the same ~400W max input power, under the conditions when input watts are exactly the same, ex. 200W or 250W either motor. With the same battery, same low speed and same light kayak.
Both motors with various-speed PWM control (this is not a permanent feature of any trolling motor, you'll be surprised how many still use 5-speed knob).
LIke for like, a brushless system will be more efficient than a brushed.
More efficient, alright. Say, 20% more, 30%?
They claim 100-200% more (2-3 times more) - though this includes a reportedly better propeller. I realize that the claim was based on certain undisclosed assumptions, not to mention very wide range of numbers - 2* or 3* is a lot of margin.

Re: Geared motor vs brushed

Posted: Jan 01, 2018 11:29 pm
by amberwolf
There's no straight easy answer, without knowing all the details of the compared systems and the testing done to compare them, including test conditions.

If the drive they compare to is really not very efficient, then their drive could be that much more efficient.

Being brushless vs brushed is more efficient, so ther'es that.

Also there are different magnetic designs of motors, and while you can just punch out a bunch of laminations, stack them on a shaft, and wire them up, that doesn't mean it works as well as it could if you optimized it's design. Could be the difference between 60-70% efficiency and 98%+ efficiency, at the right RPM/voltage/load.

A motor that runs closer to it's unloaded speed, when loaded, will be more efficient (less current to drive it) than the same motor running much less than it's unloaded speed, when loaded the same.

Since the motors arent' the same, they can't be directly compared, but it is possible that the DD brushed motor is running far enough below it's unloaded speed at the test loading that it is far from it's intended efficiency, and that the geared brushless motor is running close enough to it's unloaded speed at the test loading that it is close to it's intended efficiency, and the difference could be huge.

Gearing a motor down means you can run it at higher RPM under load and keep it closer to it's efficient zone, than with a directly-driving motor, all other conditions the same. So it's possible that being a geared motor makes a significant efficiency difference---even though the gearing itself will eat some of the efficiency (a few percent, typically).

It depends on the test conditions, and if they are biased toward one system or the other, or if the conditions are even the same between the two tests (some marketing departments provide the worst case test conditions for their competition, and best case conditions for their own stuff). If they provide the test condition data along with the test results, then you can judge for yourself.

Otherwise, there's no certain way to know without testing both yourself under the same conditions. :(

Re: Geared motor vs brushed

Posted: Jan 02, 2018 12:22 pm
by major
You are comparing very different products. $109 brushed direct drive vs a geared BLDC at $1700.

The prime objective of the costly system appears to be low mass whereas the other product is primarily low cost.

The high 90's efficiency of the high speed BLDC seems overstated and you'd also have to account for controller and gear loss. The brushed motor efficiency likely does suck. The motor manufacturer has been beat on for decades to cost reduce so materials and process suffer. It also runs at 12 V (vs 30 V for the BLDC). At reduced speed/partial load, the brushed motor alone could be as low as 60-70%.

Judging from the prices on the Torqeedo website for accessories, I'd say they get top dollar for their products. Minn Kota sells good quality stuff for reasonable prices from my experience (with battery chargers).

As for the claims, how this all relates to propulsion of light weight watercraft, I think it might be possible to double the distance. That would not be solely due to motor efficiency though. I guess it boils down to how you want to spend your money and expectations of performance. Compromise might be a prop change on the brushed motor and Lithium battery.



Re: Geared motor vs brushed

Posted: Jan 02, 2018 3:03 pm
by Alex M
Either model needs an adequate battery 900-1000 WH. If you add 900 WH Li battery to Minn, quality PWM remote, waterproof battery box (they don't exist, need to make something), then Minn becomes $1,300 and Torqee $2,400 (comes with a small battery of limited usefulness, you have to buy 900 WH anyway).
Right now Torqee sells from $1,400 by some outlets, not a hot item and besides, 2018 model 403A is about to come. But 900WH Torqee battery still costs from $800.

Cost is not everything. There are other things that could make one model more suitable than another.
For example, Torqee - poor availability and high cost of parts, slow 3-5 seconds response to throttle, overly sensitive protection system that pops error code up now and then (motor stops, it's not just an annoyance on display), brittle plastic fin. OTH Minn - heavy aftermarket Li battery that weighs 50% more than similar size Torqee battery, need to make a battery box and beefy #6 cables with waterproof connectors, need to install remote AND display or at least voltmeter (the latter doesn't come with remote). Either model has flaws in their "one-fit-all" mounting system, but let's not go there now.

Installing a better prop on Minn brushed direct drive - either from RC aircraft or original Torqee prop - is something that people talk about, but I haven't seen real reports.

Just wanted to get the bottom line on efficiency in watts per speed, particularly for Minn. For Torqee some users have run real-world tests, though on heavy Hobie cat, not a kayak. Ease of repairs with Minn (= trip to Walmart or instant $60 order of the lower unit from zillion of sellers) means a lot to me, in remote location.

There was no intent to bash either model, just trying to establish the facts.

Re: Geared motor vs brushed

Posted: Jan 02, 2018 3:41 pm
Don't forget that with a "better" motor, you won't get much benefit out of it trying to drive a displacement hull faster than its maximum theoretical hull speed...its all wasted energy above a certain horsepower...

Re: Geared motor vs brushed

Posted: Jan 02, 2018 3:49 pm
by Alex M
Chvidgov - yes, driving displacement hull faster than a hull speed - would be very inefficient. Resistance forces increase many times. That's why one of conditions that I set was "slower than a hull speed".