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.