There is also friction and windage losses, not to mention magnetic losses. And then there is the fact that the motors resistance changes as it heats up and that the resistance through the brushes on a moving comutator is different than on a stationary one.
All you can be sure of with the below method is that the losses will be MORE than you calculated. You can get a bit closer by measuring the amount of power the motor takes to spin at the same RPM without a load and factor out the resistance losses. The remainder will be 'some' of the unaccounted for losses (friction and windage) but wont account for the extra losses due to high current (magnetic, etc.) Joe Smalley wrote: >1. Measure the resistance of the motor at zero RPM (zero backEMF). This can >be with a variable current power supply by applying a fixed current and >measuring the voltage across the motor. Resistance is volts/amps. This >wiring resistance is the fixed resistance of the motor. All losses in the >wiring resistance show up as thermal losses in the motor. > >2. Measure the current <through the motor> on a second by second basis >during a test drive. This can be done with an emeter connected to a laptop >in the car. > >3. Square each amp reading and multiply by the motor wiring resistance to >get the number of watts being dissipated in the wiring resistance each >second during the test. > >Note: This is a second by second current average instead of a cycle by cycle >power average so the real losses are actually larger than the indicated >losses if the current ripple is a large percentage of the average current >value. If the duty cycle is large and the frequency is relatively high, the >ripple is small compared to the average value and this method gives >reasonable results. > >4. Sum up the watts being wasted each second of the test and divide by 3600 >(seconds in an hour) to get the number of watt hours being wasted. > >5. Divide this number by the number of miles driven to get the number of >watt hours per mile being wasted in the motor. > >6. Read the <battery kilowatt hours> consumed by the car during the test >drive off an emeter reading <battery> current. > >7. Divide the kilowatt hours consumed by the car during the test drive by >the number of miles driven to get the number of kilowatt hours consumed by >the car during the test drive. > >8. Divide the resulting number in step 5 by the resulting number in step 7 >and multiply by 100 to get the percent power being wasted in the motor. > >If you do this with different driving styles, you will get different >percentages. If you drive with lots of motor current (low RPM), the >percentage will be larger that if you use lots of RPM and very little motor >current. > >Joe Smalley >Rural Kitsap County WA >Fiesta 48 volts >NEDRA 48 volt street conversion record holder >[EMAIL PROTECTED] > > >----- Original Message ----- >From: "Mark Dodrill" <[EMAIL PROTECTED]> >To: "EV List" <[EMAIL PROTECTED]> >Sent: Friday, September 27, 2002 4:59 PM >Subject: How much power am I losing? > > >>Since my Optima YT's are rolling along just fine (about 1k miles on them >>now), I want to turn my attention to increasing overall efficiency, to >> >reduce > >>my Amp/Hrs per mile. I'll be looking into front end alignment, LRR tires, >>etc, but I suspect my motor is where I'm losing the most. >> >>Since it is a 6.7" ADC (a bit small for my car), I suspect that I'm having >>a lot of electricity turned into heating up the motor instead of having >>the motor run faster (since I'm at the upper end of the power/torque it >>will do). I'm wondering how I can measure this loss. If I were to >> >measure > >>the air temp going into the motor and the temp leaving the motor, with a >>fan of a fixed CFM blowing air through the motor (in a sealed manner), can >>I use that information to figure out how many BTUs or Watts of heat are >>being generated? >> >>Any ideas on how to proceed on quantifying this would be appreciated. >> >>Mark Dodrill >> > > >
