T-Pain Is On A Boat: Re: [Electric Boats] Propulsion Marine 5 KW Electric Drive

Dave,

A small PMAC electric motor is 85 to 90 percent efficient.

Now lets drive 5 KW through that motor. That means that 500 to 750 watts is being dissipated as heat inside the motor.

Imagine running a hair dryer on the low setting inside a small aluminum enclosure with very little air circulation. Its going to get hot quick.

Most small electric motors used for sailboat conversions do not have adequate cooling fans to dissipate that heat on a continuous basis so we need to do everything we can to get rid of the heat.

Controllers, especially inefficient ones, also create enormous amounts of heat so should be mounted in a different heat space as the motor and on a heat sink.

The introduction of forced air cooling helps cool things down but even better is the removal of heat from the engine space. This can be done by enclosing the electric drive in a sealed area and then drawing the heat away from the motor.

The Sevcon Gen 4 controller has a temperature sensitive output that will turn on a fan at a certain motor temperature - we choose 70 degrees C winding temperature.

Another advantage of the Gen 4 controller, is it creates very little heat. It is rated at 180 amps continuous so this application is a walk in the park for it. But we still mount the controller to a large heat sink, offset from the wall with spacers.

Current at rpms is a function of the gear ratio chosen. It is vitally important that a motor does not have excessive AC current going to it. You can have low DC current and think everything is fine, but when you measure the AC current it can be right of the map. We like to choose a gear ratio that balances the DC and AC currents. 100 amps DC from the batteries - 100 amps AC to the motor.

I don't like to exceed 100 amps continuous on any 48 volt air cooled system. We are developing a water cooled system that will be capable of 200 amps but we will be using two battery packs and two stators and the most any one battery pack will see is 100 amps. The only place we will see 200 amps is the battery cable between the battery bus and the controller.

On Kapowai, we have two battery packs, a string of 4 D's and a string of 8 D's. Each battery pack sees approximately half of the load or 50 amps continuous. This way we avoid heat issues in the batteries, the wires, the fuses and the connections.

Boats are far different from cars because the motors are used continuously. Cars, golf carts, motorcycles all have intermittent loads on the motors. They accelerate then coast, go up a hill, then down, stop and wait at a red light or golf tee. These applications always have time to cool down and can consequently be used for short burst of power at high current levels. Boat applications running continuously do not have the opportunity to cool down and the heat builds up so must be managed.

We don't have any heat problem at all with Kapowai, largely due to the low currents throughout the system. Twin batteries and double stators! Low currents mean low heat.

We like to run the motor at no more than 1900 rpm, mostly because at these low rpms the system is very quiet and noise is another pollution we don't like. We could make more power by running faster, but we don't need it. With respect to sweet spots, the motor and controller run efficiently at all speeds. We only use what we need and motorsail all the time.

With respect to flooded cells: this is technology that hasn't advanced much in 100 years. Though many people would argue with me, I don't think that flooded cells belong on a boat in any situation. They are dangerous, vent explosive gases and lose there capacity in a straight line until they are done. Flooded cells especially should not be used on an electric boat because of the extended run times at high current levels.

AGM batteries are the best way to go for now and Lithium is very close to taking over once supply issues are resolved.

thanks for your interest.

Good luck on getting an e-boat soon.

James

On Oct 19, 2010, at 5:58 AM, dave cover wrote:

Jim

Excellent writeup. It's very helpful to see real numbers showing whats
going on. It looks like you've put a lot of effort into this.

First, a little introduction. I've been watching this list for a while
but do not have a boat of my own yet. I'm still in the planning stage.
I live in New England and hope to start out by sailing in Long Island
Sound. I commend you guys for the work you've done and look forward to
"joining the fleet." I learn something new with everything I read
here.

I own and drive and electric car, so I have a slightly different slant
on electric propulsion. There's a link to my car at the bottom of this
email. While many of the parts, concepts and terms are the same,
moving a boat through water is a lot different than rolling a car on
pavement. Driving a car, it's not unusual to pull 700 amps from my
pack taking off from a stop. Cruising the hills where I live I will
draw between 75 to 300 amps on a regular basis. Not what you guys see
sailing. I'm not here to tell anyone how to sail, I'm the newbie.

Ok, enough of the introduction.

You bring up a very good point that I haven't seen mentioned before,
heat. Heat is usually an indicator of inefficiency or problems with
electric drives. But even when it's a product of working hard, you
need to manage it. Your comments on forced air cooling are very
important and I hope more people keep this in mind when repowering
with electric. With a motor, heat can be most damaging when you try
and push a lot of current through a motor at low RPMs. Cars often see
this when starting from a dead stop, and I imagine a boat would almost
never have this. So cooling over a long cruise is probably more the
issue. Your comments on heat are spot on.

But heat can also rear it's ugly head in other ways. With your main
wiring (from pack to controller and controller to motor) you should be
aware of heat issues. If you have a loose battery connection it can
heat up enough to melt a battery post right off. It doesn't hurt to
feel (safely, don't get shocked) around after running electric to see
if any wires or connections are warming up. If you find a hot spot,
you need to address it. This applies to all the components in your
electric drive. I even have liquid cooling for my controller.
Controllers are probably more prone to heat failures than motors. If
any of you are running Curtis controllers, or any controllers that are
designed to dissipate heat through the case, be very careful how you
mount it. Mounting your controller on a vertical aluminum plate and
adding a simple 12v fan can save you hundreds or thousands of dollars.

Also, batteries can generate heat during charge and discharge. If
you're in the tropics and running hard, you should provide some
cooling to the pack. Most good chargers will have a way to monitor
pack temperature and adjust the charge to compensate. I have flooded
NiCad cells in my pack and I ventilate them during every charge. I
usually charge right after a drive, so they are a little warm to start
with. No sense in letting them overheat. This is mainly for the
hydrogen, but also to cool them. Don't forget, anyone with flooded
batteries must control the hydrogen! Floodies have great capacity but
require more care. Not sure if it's worth it in a boat. I'd avoid
them.

Jim, you also put a lot of effort into understanding the efficiency of
the propeller, but I didn't see much mentioned about the efficiency of
the motor. Electric motors have an RPM sweet spot for efficiency and
this should be incorporated in the calculations. While you will vary
the speed for maneuvering around the harbor, you should design your
drive to use the most efficient RPM range for long cruises. In normal
driving my motor runs from 0 to 4000 RPMs, but it likes to be around
2500 to 3000. So I plan my driving accordingly. I won't always shift
into a higher gear as you might with a gas powered car. I know,
sailing is different, but your should be aware of your motor speed and
use it most efficiently. The biggest mistake is to run your motor too
slow with too much current. This generates heat and heat is a problem.

Anyway, the ElectricBoats list has been great and I'm learning a lot.
Didn't mean to run off at the mouth like this. As I said in the
beginning, I know a lot about electric cars, but very little about
electric boats. I don't know when I'll have my own to sail, but I'm
working on it. If anyone in Connecticut want's some help with their
electric drive, I'd love to lend a hand.

Dave Cover, learning all the time
--
http://www.evalbum.com/2149