Electric outboard drive for small cruisers

  • 30 May 2020 07:55
    Reply # 9002534 on 8809939

    While surfing the web looking for website "looks" that might suit the JRA, I happened upon this:

    https://www.temofrance.com/en_GB/

    Maybe not applicable to small cruisers, but I've been thinking for some time that this is what electric propulsion for a small dinghy ought to look like - more like the "longtail" motors. Having tried unsuccessfully to get a small inflatable off a beach with a regular electric outboard, I was thinking of converting it to the longtail format.

  • 28 May 2020 21:08
    Reply # 8998853 on 8809939
    Anonymous member (Administrator)

    Darren, I am really impressed by that 2kW motor. It just reminds me og that technology has left me behind.  Anyway, I was never a whale on motors.

    Cheers,
    Arne

    Last modified: 30 May 2020 08:33 | Anonymous member (Administrator)
  • 28 May 2020 20:07
    Reply # 8998731 on 8809939

    David, 

    The big prop makes your new motor very much like a modern British Seagull.  They were one of the few manufacturers that got outboards right for displacement boats, with a large slow turning prop.  Of course, you'll now do it without the trailing cloud of smoke, crazy exposed flywheel, or incredible racket.

    Glad to hear it is working out well.

  • 28 May 2020 19:59
    Reply # 8998708 on 8809939

    Arne, 

    Given that we may have some difficulty convincing David to dissect his new motor, I'll take my best guess.  100A actually seems pretty tame from what I was used to with model airplanes, where it was not unusual to pump 100A through a very small controller and a much smaller motor than what David has.  Maybe an electrical engineer can step in with the details on mosfets, but my experience is that power mosfets are certainly capable of handling those currents.  Also, remember, even at full RPM the mosfets aren't continuously on.  I'm working from memory (thus there is good reason not to trust what I'm saying), but for example if you had a twelve pole motor you would switch the FET's 36 times in one full rotation.  At part throttle the Mosfets are on even less of the time.

    As for the motor, this could be done a couple of ways.  First the wire can be smaller than you would use for continuous rating, because it is only energized part of the time.  Also, you can do things like use two parrallel wires when a single large conductor would be unwieldy.  I've seen motors for models where there were only two or three wraps of wire on a stator.  It looks comical, but yeilds a very high KV motor (one that spins very fast at lower voltage).

    So, my guess is that there is no voltage boosting taking place in the controller.  Here's an example of a much tinier model airplane motor that could handle 2400 Watts @24V.  This motor would have much better cooling, but it is tiny compared to what is likely inside the Haswing.  The Haswing, really is just at the upper end of what would be used in large radio control models.

    Last modified: 28 May 2020 20:09 | Anonymous member
  • 28 May 2020 12:43
    Reply # 8997560 on 8809939

    Whatever is going on inside the motor and its controller, it's clear that the behaviour of both of them together is exactly what's wanted for motor-sailing. Full throttle = full RPM, but with current draw proportional to the load on the prop.

    The big prop certainly moves a lot of water around. Yesterday, during static testing with the boat not quite floating, full throttle resulted in a mighty whirlpool being formed, with occasional cavitation. There is an argument for always setting the motor at maximum depth when in use. Maybe there is also an argument for a home made anti-cavitation plate? Today, at LW, I see that there is a big scour in the seabed where the water flow has moved a lot of sand away.

  • 28 May 2020 09:46
    Reply # 8997313 on 8809939
    Anonymous member (Administrator)

    Darren,
    now I have slept on it. I certainly am no expert on electric motors, but I begin to doubt if they produce a 5hp motor running directly on 24V. That would result in very thick windings, and there would not be room for many of them. They may have chosen to do it that way, but I doubt it. Then the transistors turning on and off the coils, would have to handle 100A each.

    It would therefore not surprise me if the 24VDC first has been either stepped up to a hi-voltage DC, or even inverted to AC, and thus driving an AC motor.

    Arne


  • 27 May 2020 20:40
    Reply # 8996177 on 8995940
    Anonymous wrote:

     To reduce the torque and thus power of the motor, I guess the coils are fed with shorter and shorter pulses as the user of the motor reduce throttle.
    Neat.

    Arne


    Given your background Arne, you could very quickly come to a better understanding of this than me.  One of the great parts of the JRA is that it is populated by a bunch of very intelligent and creative folks who are very generous in  sharing what they know.  Glad I could contribute this time.

    Yes, the motor speed is controlled by Pulse Width Modulation (PWM).  One thing that might make this discussion useful to David and Oscar, is that depending on the winding of the motor, the frequency of the PWM controller, as well as some other variables, the system is not equally efficient across the entire throttle range.  Very low throttle settings can be particularly inefficient.  If your motor makes a harmonic "singing" sound at low throttle this is particularly so.  When you also consider the exponentially increasing power demands as you approach hull speed, there is likely to be a sweet spot somewhere at an intermediate throttle position that would give the maximal range.  Some tests in non-tidal water using a gps, and keeping an eye on the amp-meter could be informative for helping maximise range.  

  • 27 May 2020 19:00
    Reply # 8995940 on 8809939
    Anonymous member (Administrator)

    Thanks for that, info, Darren,
    always something new to learn. Quite obvious when thinking of it: Instead of leading the power to the rotor coils, over commutators, a number of coils at the stator are switched on or off electronically, and with a permanent magnet for rotor. The key to this is the electronic rotation sensor. This ensures that the right stator coils are turned on and off at the right moment, and that the speed is not higher than asked for from the power control. To reduce the torque and thus power of the motor, I guess the coils are fed with shorter and shorter pulses as the user of the motor reduce throttle.
    Neat.

    That rotation sensor reminds me of the azimuth pulse generators (APG) which were a critical part in the ATC radars I used to maintain.

    Arne


  • 27 May 2020 17:16
    Reply # 8995760 on 8995638
    Anonymous wrote:

    Now I looked up David’s posting (9.3.) about that 5hp Haswing Protruar 24V outboard. I am quite impressed. Since it is said to be brushless, I am pretty sure it is an AC motor, either of a synchronous or asynchronous type. To make it run on 24V DC, there must be  a built-in DC-AC inverter. This both regulates the AC voltage and the frequency of it before going to the motor. Therefore, the motor cannot run away since the rotating field created by the stator coils will lock the rotor’s speed to the same speed (or a little lower if it is an asynchronous motor).

    The great things with these modern switch-mode supplies of later years (AC-DC or DC-AC), is that they have very little energy loss, and they are very compact.

    Arne



    It might be better to think of the motor as an electronically commutated DC motor. Here is a slightly cheesy video that makes an introduction to the idea. Nonetheless, the system does have the characteristic in common with switched-mode power supplies that there is little energy loss. I used to build and design electric radio control aircraft as a hobby, the leap from brushed DC motors to brushless DC was a major step forward in power density, reliability, efficiency, as well as reducing the amount of electrical noise (radio EMI).

    The limited current and RPM David saw from the motor dry is related to the KV of the motor. Based on the number of turns of wire per pole and the back electromotive force that the magnets generate in the coils, the motor will only turn so fast. Basically for a given voltage and a given number of turns of wire on the stator, the motor rotation will become self limiting at a given rpm, this is the KV for that motor (its no load rpm). The controller in the Haswing is probably using the back emf in the windings to determine how fast the motor is turning and uses that to determine when to energize the next set of coils (this is called a sensorless controller). It could also use Hall Effect sensors to measure rpm from the changing magnetic field (this is called a sensored contoller). Either way, the controller could also be used to limit the rpm even lower than the inherent KV of the motor.

    If this was a model airplane motor I wouldn't worry about running it at full rpm with no load. However, I suspect the Haswing may have some underwater seals that could be damaged by running the motor dry. This might be worth investigating before further dry runs of the motor.

  • 27 May 2020 16:36
    Reply # 8995669 on 8809939

    Now that the tides are taking off towards neaps, she didn't really float this afternoon, just bumped on the bottom, and I couldn't carry out my full plan of a static pull against the mooring. While still aground, I did manage a short test. I ran at just over 90A for few minutes (confirmed by a voltage of 70mV across the shunt), then turned the throttle the last few degrees to full on, the RPM didn't increase much - and the breaker tripped. So I'm assuming that breaker isn't any good.

    I started to install the new breaker, and found that the cable lugs didn't fit. Sigh. I'll have to dress them down tomorrow and try again.

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