Help me design the "Poorqeedo", another efficient electric boat

Discussion in 'Boat Design' started by mental_boy, Jun 2, 2010.

  1. mental_boy
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    mental_boy Junior Member

    Good to know.

    Rick, you mentioned earlier that a 7m monohull would require just 200w or so on the hull, do you have a drawing of this hull? I'm also curious to know the beam, and the power requirements above and below 6kts.

    The more I think about the weight of the structure of the catamaran deck and the windage, the more a stabilized monohull seems to make sense.

    I looked at the rules more carefully and it appears I can carry a hull up to 7.1m or so legally on my vehicle. Getting it on and off the vehicle it is a different story.

     
  2. Jeremy Harris
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    Jeremy Harris Senior Member

    Motors only really care about two things, maximum rpm and maximum current. There is no voltage limit, as such, only in as much as voltage determines rpm. A PM electric motor doesn't have a "power" rating as such, the only power that the motor has to worry about is that from losses within the motor itself, and these losses will mainly come from the current flowing through it. Manufacturers often print a voltage rating on the motor, but, unlike the current rating, this is rarely a "do not exceed" rating.

    As your motors have a very low Kv, and as the maximum rpm will be limited by mechanical factors (armature strength and brush/bearing speed ratings) I would think you could run them at much higher voltages without a problem. The advantage of doing this is that you'll be able to get much more power from the motors without getting them hot by running them at high current.

    Having said that, those motors you have do seem to have a high winding resistance, which means they are never going to be very efficient. I know that this is budget project, but you might be better off selling them on eBay and buying a couple of Chinese outrunners at around $30 to $40 apiece. The outrunners will deliver more power, be lighter in weight, be far more efficient and, having no brushes to wear out, will tend to be more reliable. They'll also work well at lower voltages (around 24V would be the most they'd need), giving you more battery options. The downside is that you'd need a higher reduction ratio to drive the props, but this isn't hard to do with standard industrial toothed belts.

    Jeremy
     
  3. portacruise
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    portacruise Senior Member

    brushless

    Jeremy, does this apply to the outrunner motors you mention below? I have been under the impression (mistaken?) that brushless motors in general could not be run over rated voltage or even significantly (50%?) below rated voltage. Maybe they have a protection circuit that kicks in?

    Thanks,

    Porta

     
  4. Jeremy Harris
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    Jeremy Harris Senior Member

    Yes, it does, Porta. Outrunner motors themselves don't have anything intelligent built in, they are just a bunch of magnets in a can with some windings in the stator. They will run at very high rpm, with the rpm limit being that of the bearings (and perhaps the strength of the rotor if you went to really high rpm). Medium sized ones, without skirt bearings, will easily run at 20,000rpm + with no trouble.

    The brushless controllers needed to run them can have rpm limits, set by the maximum commutation frequency they will tolerate. For low voltage (up to about 50V) sensorless operation, using RC controllers, this limit is quite high. Most RC type controllers will work at 100,000 to 150,000 electrical rpm (divide this by the number of pole-pairs in the rotor to get true rpm) and even the cheap (~$44 inc shipping) brushless electric bike controllers I've been using will run at over 70,000 electrical rpm. Many of these medium to large size outrunners are 7 pole pairs (14 magnets, 12 stator slots) so will run at 10,000 rpm on even a cheap electric bike controller. The cheap controllers I'm using need the motor to be modified to run with Hall position sensors, but this is cheap and easy to do. The RC controllers are just plug and play, but are only available cheaply for low voltage operation, up to maybe 30V maximum.

    The motors I'm using are around $40 plus shipping each, and need a controller that costs about another $30 to run from 24V.

    Here's an example of a reasonable set up for around 500 watts continuous:

    http://www.hobbycity.com/hobbyking/..._AerodriveXp_90_SK_Series_50-65_380Kv_/_1820w This $40 motor is rated at 1800 watts, so will run all day at 500 without getting warm. It has a Kv of 380 and is OK for up to about 12,000 rpm.

    http://www.hobbycity.com/hobbyking/...me=Hobbyking_SS_Heli_Series_70-80A_ESC_w/UBEC This $25 controller is OK for up to 80A on 24V, so more than enough for the motor above. It doesn't have a reverse built in, but a simple changeover switch wired to two of the motor phase wires will reverse the motor easily enough.

    To drive the RC controller, you need a source of pulse position modulation signals. The easy way to do this and get a hand throttle is to use a cheap $9 servo tester, like this one: http://www.hobbycity.com/hobbyking/...roduct=6554&Product_Name=TURNIGY_Servo_Driver (you can get these for around $5 if you shop around).

    The above parts, coupled to a belt reduction drive, made using toothed belts, will probably give you up to about 1kW reasonably reliably. For maybe $50 to $80 more you could get maybe double that power reliably and maybe 3 or 4kW peak.

    I'm using rewound $40 motors in my project and have found them to be pretty simple things to play about with - there's essentially little in the motor to give any problems.

    Jeremy
     
  5. portacruise
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    portacruise Senior Member

    Thanks, Jeremy-valuable information. If you buy a 24v controller to run with a particular outrunner and set up your drive system, can this SAME one be used without alterations, if you want to change to 48v input? I am trying to get a feel for the range of voltage input that a particular controller setup will tolerate. Wonder if there are controllers available with a broad generic range of input voltage.

    Thanks!

    Porta

     
  6. Jeremy Harris
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    Jeremy Harris Senior Member

    Glad to be of help.

    At the budget end of the brushless motor market there are really only two options for controllers; either the sensorless RC model types (like the one from HobbyCity that I linked to earlier) or electric bike controllers (most of which need the motor to be fitted with Hall position sensors).

    Here are the pros and cons of each:

    RC controllers:

    Pros: Small, light, only three wires to the motor, relatively cheap, at least for low voltage, high current models.

    Cons: Needs a source of pulse position modulated pulses as a throttle signal, limited range of high voltage controllers, no current limiting (so can overload if the motor gets bogged down), very expensive for anything over about 35V.

    Electric bike controllers:

    Pros: More robust than cheap RC controllers (built in current limiting), easy throttle control (a variable voltage or pot can be directly connected), many are easily programmed with a PC, cheaply available at voltages up to 48V and relatively easily modified to work at up to 100V, easy to reverse, as most have a direct reverse connection.

    Cons: Slightly more expensive than RC controllers, not as readily available in very high (>40 A) current ratings, generally a fair bit bigger and heavier than RC controllers, usually need Hall sensors to be fitted to the motor, some knowledge of working with poorly supported Chinese made electronics needed to get the best from them!

    Generally, the RC controllers may have some form of low voltage cut-off or may not. On some models it's programmable, often by an arcane sequence of power-on throttle movements, sometimes via some sort of plug-in programming card. Ebike controllers are more flexible, in that many are programmable using a PC, which allows low voltage cut off settings to be changed, as well as current limit settings. The small controllers I'm using, as standard, have a programmable low voltage cut off setting from about 20V (OK for 24V operation) to about 50V (OK for 60V operation). They come in versions ranging from a small 6 FET (cigarette packet size) 25A controller (which can be modified for up to 50A) that costs about $44 inc shipping, to a 24 FET monster capable of well over 100A. In between there are 12 FET and 18 FET versions. They all come direct from China - I get mine from a chap called Keywin Ge in Shenzen, who is very helpful, but in line with most Chinese vendors doesn't really offer Western-style standards of support.

    It's perfectly possible to have a controller that will work equally well on either 24V or 48V, using either an expensive high voltage RC type controller and an unmodified RC outrunner motor, or using an ebike controller and an outrunner fitted with Hall sensors.

    Hope this helps.

    Jeremy
     
  7. portacruise
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    portacruise Senior Member

    More useful information, thanks Jeremy! I see the advantages of building with a component system approach.

    Several years ago when I was looking at off the shelf brushless, at least some motors came with built in controllers and did not have much latitude in what inputs were tolerated. The turnkey approach was fast, but I see how it limited performance to what the motor was originally designed for.

    For RC controllers, "Source of pulse position modulated pulses"? Is this usually a pot throttle for electric boats and how are the written directions for picking the range of values and other setup procedures? Do they come with heat sinks appropriate for e-boats. Would a circuit breaker give enough time to prevent damage on overload since they are not protected. I know I have some reading to do. I would be a complete novice at something like this.


    Thanks again.

    Porta


     
  8. mental_boy
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    mental_boy Junior Member

    Thanks Jeremy,

    Good to know motors don't really care what voltage they run at as long as amperage is kept in check. Someone at endless sphere mentioned that the number of segments in the commutator is one factor for max voltage, something like 10v per segment max. I believe that there will be some extra heat from the commutator and core losses, so cooling still seems like a good idea (and cant hurt)

    How did you decide the resistance of the windings in the Sanyo servos is high? I imagine it's high compared to an RC plane motor, but those have a low number of turns and a high Kv.

    The main thing I don't like about the RC plane motors (other than having to spend more money) is the high Kv and the need for a large gear reduction. You even rewound your motor to get a lower Kv, right?

    I think I'm just going to use these motors with 36v and direct drive, and if I feel the boat is really underpowered I'll repower it with something else. First I'll test the motors so Rick can calculate the efficiency for me (as he so kindly offered).


     
  9. Jeremy Harris
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    Jeremy Harris Senior Member

    The easiest way to convert a simple pot throttle to drive an RC controller is to buy a servo tester (they're only around $6 to $10). These have a pot that can either be coupled to directly, or replaced with another of the same value mounted to a suitable control.


    The RC controllers will usually benefit from a bigger heatsink, although I've been running my milling machine with a outrunner motor, standard RC controller and servo tester for a speed control for some time without one. My guess is that the loads that the milling machine motor sees are a lot greater than those from a prop, particularly at low speeds (high torque at low speed is the worst case for brushless controllers).


    There's no need for a circuit breaker if an ebike controller is used, as they have fairly comprehensive over-current protection. A circuit breaker wouldn't be fast enough to save the electronics in the event of an overload, plus, unlike a brush motor, controller failure will always result in the motor stopping (a brush motor controller will sometimes fail to a short circuit, running the motor at max rpm, this can't happen with a brushless controller).

    Your best bet would be to do some reading over at the Endless Sphere forum, perhaps in the "Technical" or "EBike non-hub motor drives" sections. Although focussed on electric bicycles and motorcycles, there is a great deal of directly applicable knowledge there, albeit scattered about a bit.

    I started out knowing little about this stuff a couple of years ago, but by a mix of experimenting and working with some of the good guys on the ES Forum I now know enough to probably be quite dangerous.........

    Jeremy
     
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    Winding resistance is a function of Kv and motor diameter. Your motors have a small diameter and a very low Kv, so they have to have a high winding resistance. In other words, I guessed!

    They aren't all high Kv and having a low Kv is a bit of a mixed blessing. As I mentioned above, to get a low Kv and low winding resistance (for good efficiency) you need a big diameter. This then limits the speed that the motor can spin. One advantage of a high Kv is that you can get a lot more power from a small motor this way. As motors (from a thermal perspective) only care about the current flowing through them, and because current is directly proportional to torque (for a PM motor), if you hold the current constant and double the rpm you double the power that the motor will make, all for no extra heat load.

    I rewound a motor as an experiment to try and get direct drive to work. It wasn't a success, as the motor losses were too high. I reverted to an unmodified motor and a toothed belt drive, as the mechanical losses from the belt drive were lower than the electrical losses from the motor modified to have a lower Kv.

    There are RC motors with a Kv of around 170 to 380 that are reasonably well suited to driving a prop with a single stage belt speed reduction. The motor I have driving my milling machine is a Turnigy Aerodrive one costing about $60 (this one: http://www.hobbycity.com/hobbyking/...TGY_AerodriveXp_SK_Series_63-74_170Kv_/_3250W ). This has a Kv of 170, so running on 24V it only runs at around 4000rpm. This can easily be reduced down to run a big prop at an efficient speed with a single stage belt drive (my milling machine uses a 2:1 drive using a 5M HTD belt). Derating this motor for use in a boat would still give a comfortable kW or so of useful power. I'm driving my milling machine motor at 24V and am using this $25 speed controller (with a servo tester as a variable control): http://cgi.ebay.com/RC-Model-100A-B...Control_Parts_Accessories&hash=item5ad825c9b9 . I had to add a small 5V power supply to drive the servo tester, as the controller didn't have an onboard 5V supply available (some do, I just didn't check before I ordered this one!).


    You may have better luck than I did with direct drive. The trolling motors are direct drive, with a max rpm of around 1200 I believe, and they can be made to work reasonably well with a decent propeller.

    Good luck, I've found that a few hours of experiments are worth days of speculation and calculation!

    Jeremy
     
  11. portacruise
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    portacruise Senior Member

    direct drive

    If you decide to go with troll motors to get direct drive, this blog gives some good info more or less along the lines of what I have done to increase the power by doubling voltage in. (12v X 30A = 360 watts can be doubled) The minn kotas described can be had for under $100 at sports and on line stores. They can probably be run out of water without overheating if you drop the current somewhat, but the seals will be ruined and the motor then cannot be submerged.

    http://www.cfnet.net/tm/

    USA external plug and play PWM speed controls at:

    http://tncscooters.com/partsdb.php?type=ALL

    Enjoy.



    Porta






     
  12. mental_boy
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    mental_boy Junior Member

    Thanks Porta,

    If I decide to use trolling motors I'll probably just modify them like mcdenny and hang them right off of the back. And I'll definitely check out the blog.

    As for speed controls I already have some 36V, 30A curtis controllers and some 24v ones as well.


     
  13. mental_boy
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    mental_boy Junior Member

    So it sounds like if you're using low voltage, small diameter motors, then high Kv and belt reduction is the way to go to maximize efficiency . Also seems like motors with low Kv and High winding resistance could be very efficient if run with high voltage.

    Big diameter would be a cool way to go, but there's just not much out there. There's that fisher paykel motor I mentioned in your thread, and the el cheapo bike hub motors, which probably won't win any efficiency contests.

    I'm amazed you can use a turnigy motor to drive a mill, the mill I use has something like a 3hp ac motor.


    Agreed.

    I suspect these motors I have will handle significantly more power once they are force cooled. I also suspect the amperage rating is somewhat conservative. I arrived at this conclusion after looking at the puny windings on one of the ~6 amp continuous electrocraft motors I have (60% of the sanyo's cross sectional area), and seeing the abuse others dish out to motors with similar sized windings.

    Anyway, that's just speculation. I have three Sanyo motors, so I'm thinking of force cooling and bench testing one at 36V, 10A to see how it holds. Even if the motor is not particularly efficient at that load it would be nice to know I have the extra power on hand.

    Again, I agree that upping the voltage would be better, but that means another controller, more batteries and higher RPM.
     
  14. Jeremy Harris
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    Jeremy Harris Senior Member

    Upping the voltage certainly will improve efficiency, based on my experience. I'm convinced that the mechanical losses from a reduction drive are generally lower than the electrical losses of running a small motor at high current/torque.

    The milling machine is only a small one. a Taig bench top model. Normally it has a motor that is this size:

    [​IMG]


    Here's a photo of mine with the outrunner:

    [​IMG]

    The power supply, motor controller and servo tester are fitted inside the black box on the right. The controller has a built in zero volt lockout, so the motor won't run if the power is turned on with the speed control at anything other than zero - a nice safety feature on a machine tool and one that saves me having a contactor. That baby outrunner is rated at 3.25kW, or around 4.3hp (for comparison, the big, standard AC motor is rated at 1/4hp...............). Mine is limited by the power supply capacity to 500watts, which is about 2/3hp, but still around 2 1/2 times more power than the big AC motor. It'll run all day taking big cuts with a flycutter without showing any signs of loading the motor up.


    As you have these motors to hand it seems daft not to at least try them and see what they'll take.

    Jeremy
     

  15. Guest625101138

    Guest625101138 Previous Member

    Attached shows the hard chine version of 7m hull for 6kts. It requires a little over 200W on the hull to do 6kts.

    Rick
     

    Attached Files:

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