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#1
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Re: Tetrix DC Drive Motor W739083 dynamometer data
Thanks for the information. However, it doesn't make sense in my mechanical mind...using a 2 to 1 gear reduction with 4" wheels. An unloaded motor runs @ 178 RPM, (2.96 revolutions second) multiplying this by the circumference of a 4" wheel. 2.96 x 12.56 = 37.17 inches per second. Based upon the math and considering the load will reduce RPM, +/- 3 seconds is plausible.
If the RPM is reduced by 1/2. +/- 6 seconds seems like the time to reach the top. Am I missing something? |
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#2
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Re: Tetrix DC Drive Motor W739083 dynamometer data
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I meant a 2:1 speed INCREASE from motor shaft to wheel. Effectively making a 4" wheel behave like an 8" wheel mounted directly on the motor shaft. Here's how I did the math: Climb height = 110 inch (2.79 meter) Energy = height X weight = height X mass X g Energy (Joule) = height (meter) X mass (kg) X 9.8 m/sec^2 Example: if mass is 5 lb (2.27 kg) then energy = 62.2 Joule Maximum motor output power (assuming a full Tetrix battery) is about 15 Watt at 90 RPM -- operating at different RPM will REDUCE the power. Time to climb = Energy / Climbing Power Climbing Power = Motor Power X drivetrain efficiency If drivetrain efficiency is 75% and two motors are used, then Climbing Power is 30 X .75 = 22.5 Watt. So, Time to Climb is 62.2 / 22.5 = 2.8 seconds, and climbing speed is 110 inch / 2.8 sec = 39.8 inch/sec. If 4 inch diameter wheels are used, then wheel speed is 39.8 / 4*pi = 3.2 rev/sec = 190 RPM. To match this wheel speed to the 90 RPM motor speed, the gear ratio is 190/90 or ~2. Last edited by Richard Wallace : 14-01-2011 at 05:23. |
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#3
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Re: Tetrix DC Drive Motor W739083 dynamometer data
Great information on motors/drivetrain for the minibot! Thanks!
Some rough analysis by FIRST Team 241 of the video at the Kickoff: The running bot seemed to have: 1) direct drive 2) 3" wheels 3) two motors It also seemed to climb the entire length of the tower in just under 5 seconds. That would seem to imply PI*3" per revolution = 9.4" or 12-13 revolutions in 5 seconds or about 144 RPM under load of the minibot. We don't have the ftc-020 First Choice kit yet and the abbreviated list of parts does not detail the wheels included. I see a grainy picture associated with the kit but I really could not make out 4" wheels. --- We plan to run experiments as soon as we get the kit. Hopefully we will get them soon. |
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#4
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Re: Tetrix DC Drive Motor W739083 dynamometer data
Thanks for the data Richard!
4"x1:1 versus 3"x1:2 (4"x1:1 is a 4" wheel with a 1:1 ratio, direct driven; 3"x1:2 is a 3" wheel with a 80T gear on the motor and a 40T gear on the wheel) Since mass will affect every race, is the added mass to get a 1:2 gearing on a 3" wheel worth the complexity versus a direct drive 4" wheel? I don't think so. My charts show that a 3"x1:2 is only 0.1 seconds faster than a 4"x1:1 since adding gears adds weight (gear mounting, extra axle primarily). I'd suggest trying it with both and seeing what you gain from the flexibility in the design from 4"x1:1. Edit -- Whoops -- the calculator didn't factor in another efficiency drop due to the extra gears -- so now 3"x2:1 is just on par with 4"x1:1. Last edited by JesseK : 14-01-2011 at 10:16. |
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#5
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Re: Tetrix DC Drive Motor W739083 dynamometer data
so from what I see everyone is planning on a tall gear ratio (to theoretically increase wheel diameter)
I am very uneducated on electrical motor performance but so far I thought Max performance was somewhere near the motors free speed (although I do understand you get full torque at any RPM assuming your motor does not burn up) if this was the case wouldn't you want a shorter ratio than 1:1 essentially creating a slower and higher torque drivetrain to ensure the motor was operating at peak RPM. I'm pretty sure I'm wrong based on Richard statements |
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#6
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Re: Tetrix DC Drive Motor W739083 dynamometer data
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You are likely thinking of a different type of motor. Maybe an internal combustion engine?* DC motors behave very very differently. Take a look at the spec Greg posted, it will clear things up. For a given voltage, a DC motor has the most Power output at about half of its free speed. Torque is highest at stall, and decreases linearly to zero torque at free speed. * My knowledge of combustion engines is pretty close to zero, but I think wenkels have performance like you suggest: http://4wings.com.phtemp.com/crafts/a2con/torque.jpg |
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#7
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Re: Tetrix DC Drive Motor W739083 dynamometer data
after looking at the graphs for the thousand time I sort of understand how to read them I see that maximum efficiency at 88 RPMs
I had the idea about Max torque at zero RPMs from electric dragsters But I guess that doesn't translate maybe brushless motors? Anyhow I'll gear it to hopefully bog to 88 RPMs Thanks for all help folks just goes to show 20 years of school rarely matches up to simply asking a question and accepting that you are clueless in the subject. ps Wankel's are awesome i Have a snowmobile with one |
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#8
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Re: Tetrix DC Drive Motor W739083 dynamometer data
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You are confusing power and torque. Your statement about maximum torque at zero speed is absolutely correct. However, power is equal to torque multiplied by angular speed. Since the speed is zero at max torque, the power is zero hence the efficiency is zero. This is why you will read about these motors burning out is you stall them at full voltage. At the other end of the curve, the torque is zero at "free speed" and, once again, the power is zero. For DC brushed motors, the max mechanical power is always very near 50% torque and speed (exactly 50% in a perfect, theoretical world). The max efficiency will always be at a lower torque (higher speed). Exactly how mush lower is the question... 20 years was not wasted. The laws of physics still hold and all is good... Regards, Mike |
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