T-Pain Is On A Boat: Re: [Electric Boats] RE: 48 volt solar panel array

Tuesday, January 28, 2014

Re: [Electric Boats] RE: 48 volt solar panel array

nicely put, Pat.

What I got from kcr's posting is something so basic I'm reminding myself not to forget it..... might even post it in some form right on my workshop wall.

Kcr points out that it's reasonable to control an electric motor's current draw - and therefore its power output - by simply selecting to feed it a few different discrete battery voltages. No more fancy controller is required. On a multiple battery array, he can vary the motor's drive with nothing more complex than a handful of knife switches.

Those knife switches make for the simplest, most inexpensive & dependable control with the smallest possible energy lost to the controller. Of course what is lost that way is continuous variability, but for a heavy boat that stores so much energy in it's own motion, I'd imagine that just turning the motor on/off while switching between speeds becomes a reasonable way to make the small speed changes necessary for docking.

Re: Ohm's Law calcs....I saw kcr's Ohm's Law example as more an illustration of voltage switching than an actual power calculation. For real calculation he wouldn't be using his battery voltage as the input voltage to the motor, it would be the battery voltage minus the small decrease due to voltage drop as well as a much larger input loss due to the back emf generated (Bemf) as the motor turns. Since kcr also uses the motor as a generator, he's aware of all that - particularly the Bemf part since that is where his generated power comes from when the diesel is doing the pushing.

Depending on the efficiency of the prop, Bemf can also be what provides a speed limit on the motor revs when the electric motor is doing the pushing.

just sitting here with coffee & doing some thinking this mornin'.....

Roger L.

----- Original Message -----

From: Forums

To: electricboats@yahoogroups.com

Sent: Tuesday, January 28, 2014 5:24 AM

Subject: RE: [Electric Boats] RE: 48 volt solar panel array

No. You are calculating two different things. Your math is correct for how much current is needed to still get 1000 watts at those voltages. His was how much current will be drawn if you feed a 48 volt motor lower voltages (24 or 12 volts).

Pat

From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of Steve Dolan
Sent: Tuesday, January 28, 2014 7:01 AM
To: electricboats@yahoogroups.com
Subject: RE: [Electric Boats] RE: 48 volt solar panel array

KCR,

I think you multiplied when you should have divided.

1000/48 = 20.8A

1000/24 = 41.6A

1000/12 = 83.3A

Steve in Solomons MD

From: kcr@kcrproducts.com

Sent: Saturday, January 25, 2014 6:38 PM

To: electricboats@yahoogroups.com

Subject: Re: [Electric Boats] RE: 48 volt solar panel array

The elec motor works pretty simply... V = I x R that is the rule --> Volts = Current(amps) x Motor Resistance(Ohms). Since (Watts=Amps*Volts) at 1000w and 48v, current(I) = 20.8 Amps; therefore the motor resistance is 48v / 20.8A = 2.3 Ohms.

The motor, at 48v uses 21 Amps, at 24v it uses 10.5 Amps, and at 12v it uses 5.25 Amps. If your motor is designed to take 48v then it will definitely run cool at 24v or 12v (much less power). The problem comes in when you take 48v and try to reduce the speed through resistors (old golf carts) or use pulse controllers(which cost money, overheat under high loads, and do not like salt water). However switching the battery voltage from 48v to 24v eliminates those problems.

I presently have a small 1000w 48v motor just to move in and out of dock and of course to charge my batteries when the diesel is running. Actual motor cost was $160 but I spent another $200 for couplings, brackets, etc. In the future I may replace the 1Kw motor with a 5Kw or 10 Kw as the price of lithium batteries (or sugar fuel cells) come down.