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.

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.  

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.

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

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.