Nice, detailed explanation as usual. Thanks Eric. The term "lugging" in my vocabulary refers to what you're doing to your engine when you try and pull too heavy of a load in too high a gear. The analogy sort of works, though the term probably wasn't the best choice. Anyhow, since most of our installations don't have provisions for shifting gears, we need to find a happy compromise when setting up our drives. Too low a ratio and you get overheating problems due to high amps and low airflow. Too high a ratio and the motor will be happy but you give up top end speed. I sort of understand the eddy current effect Arby is talking about but I think it's pretty small compared to the raw heat generated in the windings, which is proportional to amps SQUARED times the winding resistance.
My main point is that it finding the right gear ratio can go a long way towards an efficient and (relatively) cool running motor.
Jim
--- In electricboats@yahoogroups.com, "Eric" <ewdysar@...> wrote:
>
> It is my understanding that pretty much all electric motors have their power patings limited by how much heat that they can dissapate. Almost any electric motor can be run at more than its rating, but it will fail if allowed if overpowered for too long. Most electric motors have fans attached to their main shafts so that the fan turns at the same speed as the motor. Power sanders and drills have cooling fans built in and larger power equipment like bandsaws, lathes and drill presses often have TEFC motors (totally enclosed, fan cooled) that get rid of heat under normal operation.
>
> For our common motors, whether DC or PMAC, the motor has cooling vents and you can feel the air move through the motor while operating. The slower that the motors turns, the less air is pushed through the motor. Unfortunately, some motor manufacturers rate their motors under very controlled conditions, in air conditioned labs or with large amounts of forced cooling that may be difficult to replicate in a marine installation. So it is possible for many of our motors to overheat even when running at less than their "rated" capacity, because our installation may compromise the ability of the motor to cool itself. Running the motor slowly (less cooling from the internal fan) at high loads (more heat) can make the problem worse. Direct drives (1 to 1) and low reduction ratios have to be engineered to take this into account. Running the motor slowly at low loads is not a problem.
>
> So knowing the temperature of our motors while operating is a useful piece of information, but that can be difficult to determine in many installations. Mars Motors (now Motenergy) will install a temperature sensor in the windings of many of their motors, my Propulsion Marine drive came with a temp sensor in the PMAC motor and the temperature is displayed along with current information on the drive display. This is much more accurate than relying on smell or putting one's hand to the motor case, though if the motor feels cool, it's propbably not overheating. Some of the more sophisticated controllers, like the Sevcon Gen4, can use the temp sensor to reduce power if the motor starts to overheat, protecting the system from self-infliceted damage.
>
> It's my opinion that our installations benefit from simplicity, so any system to improve cooling should be kept as simple as possible. On my boat so far, the highest winding temperature that I've seen on the display is 70 deg C. So until I see temps around 100C or above, I don't think that my boat will need addditional cooling for the motor. Since James has already developed a water cooling option for these motors, with documented positive results, I would probably tend towards that solution rather than try to re-invent the wheel.
>
> Fair winds,
> Eric
> Marina del Rey, CA
>
>
> --- In electricboats@yahoogroups.com, GNHBus@ wrote:
> >
> > This is an interesting statement Jim,
> > Are you discussing a specific electric motor, or does this happen with all
> > electric motors?
> > I think "lugging" is understood, like "bogging down" ????
> > Under what conditions does this "lugging" occur ?
> >
> >
>
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