Thread: CHIPS
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Unread 20-05-2004, 23:31
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Post Re: CHIPS

Quote:
Originally Posted by Paul Copioli
Next, Matt Adams I have one small correction to your motor power calcs for the drill with and without gearboxes. The available mechanical power when you have a gearbox must be lower than the motor by itself due to the efficiency losses. I know you know this because you accounted for them (the reason the drill/drill high/drill low all have different power numbers at the 40 amp limit), but you show that the drill in low has more power than the drill in high and the drill by itself ... no way is that possible. I think you may have transposed the data, but the drill by itself has to have more available mechanical power than with a gearbox ... Efficiency losses.
Paul-

I completely agree. I would like to explain how I came about the number for the horsepower.

I found in a post somewhere, that the ratio for low out of the planetary gearset was a ratio of 42.62 : 1.

My calcuation was as follows:

The stall torque of just the drill motor is 7.70 in-lbs, with a free speed of 19,670 RPM.

The expected speed with the gearbox with that gear ratio is 461.52 RPM. Hence, an efficiency of about 97.5%. This is honestly not realistic. However, I kept it uniform and assumed that this same loss would occur in the torque. So, to find the stall torque at 40 amps, I divided the stall torque of the motor times the ratio of the estimated free speed in low (450 RPM) to the ratio of the free speed of just the motor (19,670), then I multiplied that times the ratio of 40 amps / stall current (127 amps) and finally multiplied that by the efficiency of 97.5%.

The mistake of course, isn't TOO obvious, but here it is: the ratio is fixed and known, and I should have multiplied the stall torque by the true gear ratio, not the after-efficiency loss speed ratio. This would lower the overall output to somewhere around .493 HP in low, and .452 HP in high. This again, is not possible, but the benchmark for the actual motor without the gear box is based on one set of experimental data, the data I used in high gear is from another data set, and the low gear is purely theoretical.

However, I'll still say that I found these three pieces of data (though from independent sources) to be the most reliable pieces of information available to the general FIRST community at this time on this motor.

I hope this clears up some confusion.

Matt
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Matt Adams - Engineer at Danaher Motion
Team 1525 - Warbots - Deerfield High School