# more possibly dumb questions about BLDC motors



## kennybobby (Aug 10, 2012)

Are you able to specify how much torque and speed, power and range you need for this sailboat. Maybe pick a prop and use those specs to determine the requirements. From there you can narrow down some of the motor parameters.

Concerning the winding voltage and turns, you can use wire gauge and number of turns to rewind a motor from one voltage range to another. Smaller gauge and more turns for a higher voltage, and larger gauge with fewer turns for a lower voltage.

The speed of a BLDC or an AC induction motor depends upon the frequency of the current waveform, usually created using some sort of Pulse Width Modulation (high frequency chopping of the DC Buss voltage).
The torque is proportional to the amplitude or amplitude squared of the current waveform.


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## major (Apr 4, 2008)

With regards to the steel, you can get fully processed electrical grade silicon steel. Sometimes the semiprocessed is used and annealed after shaping to relieve stresses caused by the stamping or cutting process. Annealing can also reduce carbon in the material. Carbon is unwanted.


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## GrowleyMonster (Apr 7, 2014)

major said:


> With regards to the steel, you can get fully processed electrical grade silicon steel. Sometimes the semiprocessed is used and annealed after shaping to relieve stresses caused by the stamping or cutting process. Annealing can also reduce carbon in the material. Carbon is unwanted.


Thanks Major, that's good to know.


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## GrowleyMonster (Apr 7, 2014)

kennybobby said:


> Are you able to specify how much torque and speed, power and range you need for this sailboat. Maybe pick a prop and use those specs to determine the requirements. From there you can narrow down some of the motor parameters.
> 
> Concerning the winding voltage and turns, you can use wire gauge and number of turns to rewind a motor from one voltage range to another. Smaller gauge and more turns for a higher voltage, and larger gauge with fewer turns for a lower voltage.
> 
> ...


I don't have a torque figure. Electric sailors usually keep boat speed down around 3 or 4kts for greater range. Hull speed of my boat is about 6.3kts and at that point increased power has very little effect on boat speed. Displacement hulls push much easier at some reasonable fraction of hull speed. To put the power in perspective, when I bought the boat the original Atomic 4 engine had been mostly under water for at least several months. The day after I took it over, I added an outboard mount and bought a Nissan 4 stroke 6hp outboard. I made a bit over 6kts at about 3/4 throttle on an average day on Lake Pontchartrain. Advancing the throttle all the way increased rpm quite a bit but had barely any effect on boat speed. So a roughly 5kw motor could be quite adequate. Many guys are using the Motenergy 0907 for boats in my size range, usually with a 2:1 belt reduction, with adequate performance. A typical max range for 50% dod is 80 miles, at 3kts or a bit more. This with 200 to 220 amp hours of flooded cells, 48v, which is the most popular voltage for 25 to 32 foot keel boats with electric propulsion. I am fine with that performance level.

My initial repower will be the Motenergy 1010204201 motor with a Kelly controller, drawing from a bank of 8 GC2 6v golf cart batteries, about 210ah. Optimum loaded speed is lower than the older 0907 model, and I think if I cut my existing prop down I can run it with no reduction gear. That's all figured out. I am sticking with off the shelf components for now. All the technical questions are related to what I hope to do later, maybe next year, which is homebrewing a better motor to run with a bigger prop at lower speeds, like 1200 to 1500 rpm top end, loaded with a larger fixed prop, maybe 3 blade 12 X 12. Current prop is a folder, about 13" but I estimate only about 7" pitch. Blades, like most folding props, are fairly narrow. I want a fixed prop anyway for the regen, and high pitch will start turning at lower speed through the water, and larger swept area will give me a little more current from the motor. 

I pretty much have a handle on how a BLDC controller works. There is actually one fundamental difference from an induction motor controller... The phases are switched high, low, or off as needed and as determined by the rotor position via feedback from 3 hall effect sensors or optical encoding.

Most induction motors are going to run best at speeds much too high for direct drive to the prop. I actually was thinking about building an electric boat even before I started reading about it and messing with PM motors and alternators, and my idea was to use an induction motor and a variable frequency sine wave inverter for the controller, but I was told at the time that low frequency operation would burn out the motor quickly. Well, now I know that is sorta not true but I am going with BLDC anyway because that technology is well proven with a wide user base in my type of boat. And some PM motors are being used in direct drive already. I have never heard of a induction AC motor used successfully in direct drive.

Wow this got too long and I am tired so thanks for the reply and I'll check back in tomorrow.


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## major (Apr 4, 2008)

Hey G-Monster,

Some guys over on the ES forum design and build their own motors. A dude who calls himself Miles has some particularly interesting threads like this one: http://endless-sphere.com/forums/viewtopic.php?f=30&t=36368 And find some good references in the Resources: http://endless-sphere.com/forums/viewtopic.php?f=30&t=16056

Maybe lurk around over there a bit.

major


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