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Old Jan 06, 2005, 08:52 PM
Long live the SHP!
thomer's Avatar
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Joined Mar 2003
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Graupner Speed 400 brushed motors

I have a Graupner 4.8 Volt Race 400 brushed motor in my Microjet. Hobby Lobby's website says to only use it geared. Is the reason so you don't fry it with higher voltage? I have been keeping the voltage below 7.4 volts (using a 2s lipo) The salesman at HL wasn't able to answer my question so I am looking for help here. Thanks in advance!

Kevin
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Old Jan 06, 2005, 10:05 PM
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Ben Lanterman's Avatar
St. Charles, MO USA
Joined Jan 2001
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I have a MicroJet and use either a pack of 8 350mah or 500mah nimh cells for power. When I land the motor is very hot.

There is a test here

http://www.rcgroups.com/links/index.php?id=4342

I thought over your problem also when I started flying the MicroJet but since the manufacturer recommends 7 to 8 cells and the motors are cheap and the flying really fun at full power I decided to go with the higher voltage. The airplane is still flying.

I did discover that I couldn't throw it hard enough to get it flying - thank goodness for bungee launches.

Ben
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Old Jan 06, 2005, 10:57 PM
Long live the SHP!
thomer's Avatar
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Joined Mar 2003
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Hey Ben,

Thanks for the info. I think 8.4 or 9.6 volts would fry the 4.8 volt Race pretty quickly. I'm still trying to figure out why they recommend the lower voltage motor for geared use only.

Kevin
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Old Jan 07, 2005, 02:20 AM
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perttime's Avatar
Tampere, Finland
Joined Nov 2004
4,318 Posts
That is quite simple really. The "lower voltage motor" wants to run faster for a given voltage. That means that you have to let it run fast. A direct drive prop that lets a 4.8 Volt motor run fast enough would be too small to produce useful thrust.

If you put a big prop on the motor, it still tries to run fast but to run fast it needs a lot of both Volts and Amperes. The Amps will fry the motor.

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Old Jan 07, 2005, 05:22 AM
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East Anglia, UK
Joined Sep 2002
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Yep. As teh man says. teh 4.8v race motor is a great performer when geared, but wants to do upwards of 30K RPM to develop efficiency and power.

If you could gear it a bit, it would be OK, but the 6v motor is the best compromise for DD on 2s LIPO, and the 7.2v on 3s LIPO, both using Gunther of 5.5x4.5 props.

Oh. I did a quick motocalc, and the 4.8v motor will be OK on a 4.1x4.1 prop on 2s LIPO.

No more thrust than the 6v motor, and a lot more current, but faster definitely.
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