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
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.
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