Hi Michael,
Now you're getting into a deeper practical question. What you're talking about is a real-life countinuous power rating. The motor manufacturers will test their motors in a lab and may have added forced air or even chilled air during their rating runs. So a motor that the manufacturer rates for 180A continuous may not be able to sustain that power level when tucked away in the bilge of your boat.
For example, I believe that Motenergy (previously Mars motors) considers their maximum motor temp to be 150C. That measurement comes froma sensor buried in the windings and some of their motors include that sensor as contoller input. Personally, 150C seems a bit high, the motor will definately smell hot from a distance at those temps. With the temp sensor installed, a controller like the Sevcon Gen4 can set a temp limit to start reducing power to the motor to prevent damage. Since the temp can climb after you start to reduce power, that limit should start early. I believe that some vendors set their temp limit at around 125C, and that seem reasonable to me.
If you look at the Motenergy ME0913 motor, the manufacturer rates it at 180A continuous input. That 180 number probably sounds familiar, as far as I know, it is the motor that is used for the EY 180ibl with two of them in the EY 360ibl. Looking at the photos on their website, they drive those motors with Gen4 controllers. The 180A @ 48V gives you a continuous rating of 8.6kW.
Propulsion Marine uses the same motor and controller in the 5.5kW system. I know that this is a conservative rating, but James did this to avoid ever having a customer that overheated their motor. Sevcon has a display unit for the Gen4 called the Clearview that can display a variety of data including the winding temps in the motor. Propulsion Marine includes this display with their systems. In my boat, I've monitored the motor temps and while pushing my system at full throttle (1900 rpm), the winding temps seem to stabilize around 80C after about 30 minutes. That's with a 2.55:1 gear reduction and a 13x15.5 4 blade prop (the biggest that I could fit, even after enlarging the aperture a bit). Given that info, I could probably drop my gearing to 2.2:1, which would spin the prop a little faster which in turn increases the load on the motor which would increase my drive's output a bit. Then my motor would run a little hotter and my batteries would drain a little faster and the top speed of my boat while motoring would probably not increase very much. Since my boat motors 0.5kts faster with my current setup than it did with the Yanmar SVE12, I don't feel the need to run through that exercise. In reality, my typical motoring speed is between 4 to 5kts (1200W-2500W) which is well under the systems operating specs. Because I'm not running my system on the ragged edge (even at full throttle), then I don't expect any failures. I know that I won't experience a thermal shutdown because my drive is configured conservatively for my boat. Even if I went offshore, buying a spare motor and controller would cost less than the spare parts that many cruisers carry for their diesels. And replacing either of these components would be quicker than swapping an injector and bleeding an injection system.
I believe that if I loaded up the prop enough to run 180A continuous through my motor then I would start to run into heat issues. You can see that Propulsion Marine has a water cooling adapter to extend his conservative ratings to 7kW, around 150A. The rest of the components are the same.
The bottom line is that I believe that we need to consider our installations when looking at manufacturer ratings and I think that being more conservative when spec'ing your system components results in a more reliable system. Being able to collect real-world data like my temp readings under power carries a lot more weight with me than a spec sheet. I would be curious to hear about temp readings on some of the other vendor's systems when operating near their continuous power ratings, but that's really more their business than mine. I'm just a hobbyist that likes to get into the geeky aspects of these systems. On the other hand, I'm not trying to sell you anything...
Fair winds,
Eric
Marina del Rey, CA
PS. I personally think that gear reductions are more efficient and durable than most belt drive systems, but again, that's just my opinion.
--- In electricboats@yahoogroups.com, Michael Mccomb <mccomb.michael@...> wrote:
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> WELL, I am certain that everyone who has attempted to clue me in will be pleased to know that finally an explanation stuck and I now believe I understand why belt drives are not only quite acceptable but actually an advantage for the motors in a normal environment. Perhaps the new morning has refreshed my brain a bit.
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> Yet another question.
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> There are times when I must take my boat through a tidal flow. I try to avoid them but very occasionally I hit a destination at precisely the wrong time. In this circumstance I need to have maximum hull speed available and would therefore have to press the motors fairly hard. Motors cutting out in those moments in order to save themselves from heat overload would be really bad.
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> I am wondering if a genset powering the motors to their full extent combined with a bit of extreme additional cooling would be a way to ensure power when it is really needed. Perhaps duct the air conditioning direct to the motors or arrange for some sort of water jacket or perhaps even more unrealistically pack the motors in dry ice. :-)
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> Certainly it would help some but I am wondering about the heat transfer through the windings and if that would be sufficient to allow a hyper cooled motor exterior and or air circulation to be effective. Perhaps the cooling would only thwart the sensors and by doing so allow the motors to harm itself at the level of the deepest windings?
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> Perhaps a third motor if the situation is marginal? I'd definitely have to run the motors through some shutdown testing in order to understand when such an event would be likely to occur.
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> Would anyone have any thoughts on any of this?
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