Electric outboard drive for small cruisers

  • 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.

  • 27 May 2020 16:12
    Reply # 8995638 on 8995174
    Anonymous member (Administrator)
    Anonymous wrote:
    Anonymous wrote:

    Still, I ran the motor in air, and was surprised to find that at full throttle the current draw was only 3.2A. So that's without any water loading on the prop.

    I had been thinking about this and it doesn't surprise me at all. I was thinking about the scenario where the prop gets out of the water while at full throttle. If the motor were to spin unrestrictedly it would probably go up to 20.000 rpm or something (surely not good for its health). So there has to be an rpm restriction, meaning if the prop is allowed to turn freely it wouldn't use that many amps at all.

    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


  • 27 May 2020 15:16
    Reply # 8995461 on 8990425
    I wrote:


    I didn't actually check how much the motor can safely turn with the rudder centered, I just (wrongly) assumed I'd always keep the outboard centered. When maneuvering in harbours though this would be very welcome as Tua-Tua reverses like a dog. 

    Got the inboard engine lifted out today by Hiab for €50 (I wouldn't even have had to pay that but wanted to pay something) by a colleague of my brother's. While there I checked how much the outboard can move with the rudder centered and it goes clear 360 degrees by a very comfortable margin, so that's going to improve tight space maneuvering by a lot! A "stern thruster". :)

  • 27 May 2020 12:58
    Reply # 8995174 on 8995144
    Anonymous wrote:

    Still, I ran the motor in air, and was surprised to find that at full throttle the current draw was only 3.2A. So that's without any water loading on the prop.

    I had been thinking about this and it doesn't surprise me at all. I was thinking about the scenario where the prop gets out of the water while at full throttle. If the motor were to spin unrestrictedly it would probably go up to 20.000 rpm or something (surely not good for its health). So there has to be an rpm restriction, meaning if the prop is allowed to turn freely it wouldn't use that many amps at all.

  • 27 May 2020 12:41
    Reply # 8995144 on 8809939

    The shunt has two little screws for the meter cables, as well as M6 studs for the current carrying cables, so that's OK.

    It was LW this morning, so Weaverbird was dried out. Still, I ran the motor in air, and was surprised to find that at full throttle the current draw was only 3.2A. So that's without any water loading on the prop. The voltage across the shunt corresponded fairly closely with this - 0.00022V against the theoretical 0.00024V - so I'm gaining confidence in the ammeter.

    So this afternoon, I'll go back aboard at HW and move the mooring strop to the stern so that I can carry out a static pull against it. That should result in maximum prop load and maximum current draw.

  • 27 May 2020 09:24
    Reply # 8994883 on 8809939
    Anonymous member (Administrator)

    Yes, of course, I asked because there were so many numbers flying around. With a shunt resistance of only 750MicroOhm (the lowest I have seen), any small voltage drops in the connectors could give errors. Therefore, make sure that the current meter is connected directly over the shunt.
    Personally, I would have liked to have a big bypass switch over the shunt. One does not have to monitor the current constantly.

    Arne

     


  • 27 May 2020 08:26
    Reply # 8994831 on 8809939

    Surely if at 100A the voltage drop across the shunt is 75mV, the resistance has to be 0.00075 ?

  • 27 May 2020 08:17
    Reply # 8994829 on 8809939
    Anonymous member (Administrator)

    David,

    I wonder what the specs of the shunt resistance says, 0.001, 0.005 or 0.01 Ω ?  My guess is 0.001Ω....

    Arne

     


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