Power is made by torque multiplied by rpm. Torque to an electric motor is current. All electrical equipment is current limited, so the only way to get power out of a motor is with rpm and torque. If you double the rpm of a motor, you double its torque. Rpm is also limited, but to get the maximum output of a motor, you need to obtain maximum rpm and maximum torque. So a motor needs to spin fast in order to make its rated output power.
Conversely, a propeller is more efficient turning slowly. The slower a propeller turns the more efficient the propeller is.
Batteries are limited in the amount of energy they can store, so the more efficient the system is, the longer the range will be. Since range is probably the biggest determinant of whether someone converts to electric, it is essential to obtain an efficient driveline.
The only way you can have the motor turning fast, and the propeller turning slowly, is with a speed reduction system. Gearboxes provide a very efficient method of speed reduction, while being maintenance free for 10 years or more.
Without a gearbox or gear reduction method, the torque requirement of a large propeller, will be larger than the torque capability of the motor, and the motor will overheat, and may overheat far before it can reach its maximum power output because it can go over current before reaching maximum rpm.
Choose a reduction ratio that balances the DC currents going to the controller, with the AC currents going to the motor. ( 100 amps DC to the controller should be roughly equal to 100 amps AC going to the motor. )
James Lambden
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
The Electric Propeller Company
625C East Haley Street,
Santa Barbara, CA
93103
805 455 8444
james@electroprop.com
www.electroprop.com
On Aug 18, 2015, at 10:12 AM, artzboyz@yahoo.com [electricboats] wrote:
i'm a little confused by the reduction gearboxes. i've read that they are necessary to prevent motor overheating but, if that's true, why not just spin a smaller prop?
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Posted by: James Lambden <james@electroprop.com>
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