Michael,
The rating on an electric motor is in fact the 100% duty cycle for that motor. At 100% the motor will not overheat unless cooling is compromised by dirt or confinement in a space where air is not moved enough (ambient heat problem). This fact alone is a problem on a boat. Water cooling or ducts that bring the air from free space can fix this but we all want dry conditions for our electrical installation. The insulation has a heat rating. Consult the manufacturer for this magic number to avoid overheating based on ambient temperature. most AC motors are rated with a 60 deg difference from ambient. On a 110deg F. day you may have to reduce the power you draw to save a motor depending on insulation temperature ratings.
Electric motors draw current based on the torque load applied to the motor shaft. They are rated for 100% duty cycle. They can deliver more than the rated 100% duty cycle. They will do this at their own detriment as this is when they will overheat. Cooling schemes will allow for 100% duty cycle when operated at heaver loads, and if cooled enough can be run this way for the same life time as at 100% of rating. When you look at duty cycle of a product you may find evidence that the motor is smaller than the stated HP. This is a method employed by manufacturers to produce products that may not need to be run for long times, saving on motor costs and weight.
In designing your drive, you may consider running your motor at more than 100% output for short periods. Never design this way for any emergency conditions because you have no idea how long such conditions may last. You can however figure higher loads for docking needs and save on motor costs. Note the controller must be sized for the higher loads when employing this kind of tactic. You may ask when you might need such tactic? If the controller is sized for 200% conditions you would likely be safe to set your forward/reverse time to "0" and get faster response times.
Cooling fans on variable speed drives are the weak link. Slow turning fans fail to move air. Loading a motor at low speeds tend to overheat the motor. A cure for this problem is a thermostat controlled fan, large enough to move the same amount of air as the OEM fan on the end of the motor. It might be worth noting that squirel cage fans are not as efficient as a standard prop type fan for the same reason jet drives loose in efficiency wars.
I hope this answered some of your questions.
Kevin Pemberton
On 09/11/2011 07:38 PM, jrmdive wrote:
I'm not sure anyone can really answer your question about how "close to it's rated power" other than you and your comfort level. Ratings are very subjective, just look at how the different vendors even here rate their systems that use the same Motenergy motors. I look at the power rating as more the motors ability to handle heat, as by throwing James' water cooler on the 5.5KW system (his rating, see below) it can become a 7.5KW system with all the same parts (maybe a bit bigger controller).
I have a Propulsion Marine 5.5KW system, and the rating comes from the max power that it can continuously run and keep the winding temperature at 100C installed in the boat. So, I can run the motor at 100% of its "rated power" for eternity (or at least until the batteries give out). Other manufacturers rate the same motor at 7.5KW and up in the same configuration, depending on how hot their willing to run the windings. I'm no genius, but I do know that the more current you put in, the more heat you generate, which is why folks can play volt games and get more "power rating" with the same gear.
Josh
-- BTW, as shown again this afternoon, I think one of the best things about my electric boat is the ability to spin the prop at any speed desired without worrying about stalling an ICE. It makes coming back into the slip a dream, even in iffy conditions. My wife took the boat out, and put her back in the slip, entirely without any prompting from me today, and looked like a pro doing it. All the other stuff like quiet, less maintenance, electric sailing, etc. is just gravy :-)
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
>
> thanks Jeremy.... i have to get my head in gear to read your stuff but it is for responses like yours that i throw my thoughts out there... good replies to my suppositions
>
> so now I am coming around to thinking that the bigger motors are the way to go BUT that i should go ahead and stick with the smaller gen sets and battery banks... by doing so I will get only very slightly more from the motors but significantly increase their robustness by upscaling.... sometimes i feel like a ping pong ball but as i say, that is why i throw it out here :-)
>
> let me ask this, what would you consider to be running a motor at "close to its rated power"
>
> 60%, 80%, 95%???
>
-- AspireOne Netbook running Ubuntu 11.04 Safe, Secure, Easy to use, Free Open Source Linux OS. Leaving Virus worries to the Windows Users!
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