Assuming the driveshaft of the rear wheeled drive RC car follows the linear DC motor theory for stall torque and no-load RPM, also voltage supplied is constant and no inductance as well. Note: the motor is running above its operating voltage 3V, the motor voltage supplied was measured 5V.

Motor used: product.mabuchi-motor.com/detail.html?id=9

Forming hypothesized result: speed are directly proportional to voltage, I directly multiply the voltage ratio with the expected speed then multiply again by the gear ratio. Gear ratio was verified by calculating its' teeth. Actual applied voltage was measured using a multimeter and measure the voltage across the motor. The hypothesized stall torque were also multiplied by the same voltage ratio and the gear ratio.

The base of this experiment uses the formula


where the ω is the no-load speed, v is the voltage, k is the electrical constant or the torque constant assuming they are equal to each other.

Hypothesized results of a 7.3 gear ratio
Stall torque=0.065

No load rpm=4200

Measured result Stall torque=0.02

No-load rpm=6500

The stall torque is measured using an arm and applies the force on the weight scale on both sides. The no-load speed was measured using a non-contacting tachometer. It is found out that the no-load hypothesized speed is less than the measured no-load speed, the stall torque of the hypothesized result is higher than the measured results which is expected, because of power loss.

So, the question is why the measured no-load speed is higher than the hypothesized speed? Is it because of inaccuracy or some unaccounted factors?

  • $\begingroup$ "Yes." That is, both of those will contribute. You did start out by assuming no inductance, for example. $\endgroup$ Oct 26 '18 at 17:06

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