Eric,
Thanks for your input. The points you have made are great and I am not disagreeing at all. To add to your explanations I introduce the following.
As an add-on to the below. In order to state the way a motor will handle heat conditions must stay consistent. For instance how many degrees will the motor run above ambient (room temp). We can find the top temp that our wire will survive by the manufacturers insulation temperature. If we know what temperature increase can be expected then we know what temperature ambient can be and have the motor hold up at rated HP.
I would not buy a motor tested at other than lab conditions, because the motor would have an unreliable rating. Lab conditions give us a standard we can translate to our individual operating conditions. In an industrial setting motors have heaters that control contractors to maintain a safe condition for the motor. small equipment have heat switches among the windings of the motor that interrupts the power if it gets to hot. I once had a surface grinder that was being run in a uncooled room. To use it I chose to increase the heaters ratings so the power would not be interrupted on hot days. This choice may have limited the useful life of the motor, but the compromise was acceptable to the owner.
Motors are designed for the conditions they operate in. How much water they can handle depends on how they are designed. Open case motors are not designed for any amount of water. Closed case motors can withstand some water but no motor is designed to be run in water. The housing the motor sits in may be designed to keep the water out. The best design to handle water has the motor coupled via magnets, the shaft never sees the outside of it's case. Most high quality submersible pumps use this kind of arrangement.
If you could find an explosion proof motor coupled by magnets it would be safe to say it would run for years under water, no matter what the water had in it. This however brings quite a price tag. Compromise is the key to affordable products. Our installs are no different.
Kevin Pemberton
On 03/23/2011 12:41 PM, Eric 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
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