Motor Quiz

[Ether]

Originally Posted by EricH

It's also at least touched on at the college level. I'm curious why you say it's flawed--besides the fact it confused me every time I tried to use it! (I typically hold about a 2-3 decimal digit precision in the final answer, but I'll often go to 4-5 decimal digits or more during calculations.)

Several parallel thoughts come to mind:

1) for a detailed (and humorous) treatment, see this link.

2) rounding the answer to 11 volts as suggested here gives a profoundly wrong answer*. At 11 volts, the torque at 3700 rpm is 75.5 oz-in

3) with modern computers and calculators, there is no reason to drop any digits of precision during intermediate calculations. carry all the precision through, then round the final answer. the number of digits to keep is not a simple matter of looking number of digits in the datum with the least digits. in the specific case of this problem, tenths of a volt is a reasonable (and necessary) precision to get a reasonably correct answer:

61 oz-in @ 3710 rpm requires 10.52 volts

59 oz-in @ 3690 rpm requires 10.40 volts

... so if you want to round to 10.5 volts that would be reasonable, but not 11 volts*


*we are talking about correct answers to the problem as stated. I know that manufacturing tolerances of the motor could cause wide variation.