T-Pain Is On A Boat: Re: [Electric Boats] Re: Pros and Cons of Lower Voltage

Monday, April 11, 2011

Re: [Electric Boats] Re: Pros and Cons of Lower Voltage

Hi there Sailors,

I would just like to jump in here as a cruiser who cicumnavigated the globe in a Catamaran late 80's early 90's which was my third blue water boat and last year on the 24th of September I launched my 32.5ft 13000Lb yacht number 5 in the Azores with a 36V system I designed and put toghether in the Azores where materials is not easily obtainable and took off on the 26th for Morocco at possibly the worst time of the year without any sea or other trials.

The conditions was varied to say the least, becalmed to start with, into a gail and then a tropical storm. The only problem I had was to replace one brush holder out at sea due to the fact that the motor I was supplied as new new was not,luckily I had a spare brush holder and that saved the day. On my return from Morocco across the Gibb straits and my voyage around Morocco three harbours and west along the Spannish coast to the Portuguese Allgarve where I am now my, system took a beating and was tested to stress and stood up to anything the ocean could throw at it. I am sold on DC propulsion allthough I deviated from the norm all that remains now is a bit of refining and tweaking the system.

Regards and good sailing,

Paul.

----- ariginal Message -----

From: Myles Twete

To: electricboats@yahoogroups.com

Sent: Monday, April 11, 2011 7:28 PM

Subject: RE: [Electric Boats] Re: Pros and Cons of Lower Voltage

Pat-

Sure. But as another electrical engineer, it's not quite as you state. Motors are not simple resistors.

For a PM DC motor, 2 equations suffice:

Vm = Kb * w + Im * Rm

Tq = Kt * Im

Where Kt (newton-meter/amp) = Kb (volts/rad/sec)

Indeed, as you suggest, running a 48v motor at 24v and driving a boat with it will result in less current. However, the reasons for this are not quite as you suggest. Given the motor no-load speed at 24v is approx. half that of 48v, and given the exponential nature of drag in water, at 24v we expect just over half the speed attainable at 48v in water. Given the propeller torque needs drop nominally by half for each knot of speed reduction, the effect may not be 50% as you estimate at 24v. Say that 48v gave you 4kts---with 24v and the same motor we'd expect something greater than 2kts of speed. At 2kts, we'd expect 4x less torque than at 48v and 4kts. Since torque ~ current, we'd expect 4x (25%) less current, not 50% less.

In case this helps-

-Myles

From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of greenpjs04
Sent: Monday, April 11, 2011 10:49 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: Pros and Cons of Lower Voltage

Hi Eric,
Everything you say is true, but there is an implication that a boat user can easily adjust the current to achieve the desired result. For a given voltage, the main way to increase the current drawn by a motor is to increase the mechanical load. If you travelled from a normal lake to one filled with molasses, the current would certainly increase! To use examples similar to yours, if you have a motor that is designed to use 2400 watts of power at 48 volts, it will draw 50 amps. As you stated, a similar motor designed to 2400 watts of power at 24 volts will need to draw twice the amps, or 100. However, if you take that first motor and just lower the voltage from 48 to 24 volts, the current will drop in half (to 25 amps) - NOT automatically double to 100 amps. I know you already know that, but the original poster seems to be confused by that concept. (Of course, when using a PWM controller, you need to consider the average power, voltage, and current, but the theory is the same).

I hope I am helping here. I am not trying to be difficult, but as an electrical engineer, I always get the urge to correct misconceptions.

Pat

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