Measuring DC power of PWM motors <1000w - Page 1
Forum section: electronics / beginners

author: LCA078
author_rank: Contributor
timestamp: December 01, 2022, 01:24:42 pm
content: I'm building a test bed to characterize airflow vs power for a variety of DC fans. I'll be testing mainly larger electric ducted fans (EDF's) used in the RC hobby world along with various DC fans used in industry. Goal is to find the most efficient set up to provide 1,000 cfm using 11s LiPo backpack battery pack as the power source. For my test bed, I'll use DC power supplies in lieu of LiPo packs (I have a couple of ebay Sorensen's being delivered soon) but need advice on measuring the power/watts/amps/etc.The RC world uses inline watt meters like this:https://powerwerx.com/watt-meter-analyzer-inline-dc-powerpole?gclid=Cj0KCQiAvqGcBhCJARIsAFQ5ke7Ed60T8qsLBFjMgrWZlVE_BGg9O-NqeMrwT_0_4UixuaEKfnC6aL4aAsdbEALw_wcBbut I'm a bit skeptical of their accuracy because of the PWM nature for the DC motors. I'd love to use these meters because they offer a lot of battery-related info (watt-hrs, etc.) instead of just a plain clamp meter only showing amps.I'll more than likely stay in the ~40v range with 6-10amps ranges to get the needed airflow. Data logging is not required (at this time) for my initial set up. What do ya'll recommend as a power meter for my test bed?

author: Manul
author_rank: Super Contributor
timestamp: December 01, 2022, 04:24:47 pm
content: Most likely you don't need nothing at all, because you will see voltage and current readings on your lab power supply. Some better lab power supplies will do the math for you and show the power too. I expect any decent lab supply to be quite accurate even with PWM load. More significant inaccuracies might happen while measuring air flow (my guess).

author: Benta
author_rank: Super Contributor
timestamp: December 01, 2022, 06:58:15 pm
content: At those power levels we're probably talking sensorless BLDC motors for the ducted fans?It's of no great consequence.There's no reason to make big scientific measurements. Measuring DC current and voltage from the cell-pack or power supply will be enough. The measurement will be a bit skewed by losses in the controller, but that'll be in the range of a couple of percent, no more. Atmospheric pressure and humidity on the test day will probably have a higher impact.

author: LCA078
author_rank: Contributor
timestamp: December 02, 2022, 01:53:26 am
content: Yup, BLDC motors on the EDF's. Something like one of the 120mm versions here:https://hackermotorusa.com/product-category/streamfan-edf-power-systems/Of course I won't be running them anywhere near the high end...probably 10-20% of max power. (BTW- I can't believe the max power ratings on those small little things- gotta be measuring a spike load!)Maybe I'm too old school as I always treat the power supply strictly as a dumb supply. That's why I was stuck on needing additional external measuring devices. I'll see how good I can use the Sorensen V-A. readings when they're delivered next week. And for the price I paid for those power supplies, they better be accurate!I'll probably buy one of the watt meters for testing watt-hr capacities anyway.And yes, after thinking through the inputs above, I agree the error in my measurements will probably be more with the airflow and atmospheric nuances than anything electrical. Good feedback!