Speed 300 Motor Tests

My old Graupner Speed 300 motor gave it's life to science :) Speed 300 internals

Graupner's 6V Speed 300 motor is a re-badged Mabuchi RK-370. It's can spin faster than 30,000 rpm, and is much more rugged than the similary sized Speed 280. What many people don't appreciate (including some manufacturers apparently) is that this motor is factory timed for forward (counter-clockwise) rotation. It has to be geared when spinning a large prop, which usually requires running the motor backwards. I wanted to see how this would affect its performance.

Speed 300 timingI hooked the motor up to a 5V power supply, and measured the current and rpm with factory timing (CCW). Then I loosened the endbell and rotated it clockwise until the current was the same, but with clockwise timing. The total distance moved was 7mm, corresponding to an angle of 33° (±16.5° from neutral). The point of lowest current was appoximately in the middle of this range (current is a little higher than no-load, due to drag caused by the tachometer disk). Note the rpm increase with advanced timing.

Timing Volts Amps rpm
CW 5.0 1.16 24210
CCW 5.0 1.16 26130

Now to see how timing affects performance. For both advanced (CCW) and retarded (CW) timing, I measured the current and rpm with the motor driving a GWS 4x2.5 inch prop. With this load the motor is working close to maximum power output. Here are the results.

Timing Volts Amps rpm Power In Power Out Efficiency
CW 5.0 7.5 17010 37.5W 20.9W 55.7%
CCW 5.0 7.6 18060 38.0W 25.2W 66.3%

Advanced timing gave over 1,000 more rpm, with minimal current increase. How does this relate to power and efficiency? Input power is simply Volts multiplied by Amps. Output power is proportional to rpm³ (the actual calculation is quite complex and requires a 'prop constant'. Suffice to say that I have already calibrated the prop). Efficiency is output power divided by input power. With advanced timing the motor produces 19% more power out for the same power in! You can also expect the brushes and commutator to last longer, due to reduced brush arcing and lower temperature.

Mabuchi only provide specs for the 7.2V version of their motor, but they will custom-wind a motor to their client's specifications. GWS sell three models, the EM300, EM300H, and EM350. To find out how Graupner's motor compares to them, I measured my motor's wire diameter and number of turns per pole. This required cutting one winding off the armature (sniff!). The results were interesting.

Motor Volts Turns Diameter




















So it would appear that GWS's EM350 is exactly the same as Graupner's 6V Speed 300. But GWS rate all their motors at 7.2V, even though they are a lot 'hotter' than Mabuchi's 7.2V motor. Just to add to the confusion, Graupner now offers a 7.2V Speed 300, which is similar (identical?) to the Mabuchi 7.2V motor.

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