BLDC motor power measurement - Page 1
Forum section: electronics / projects

author: perucho
author_rank: Newbie
timestamp: April 11, 2017, 02:08:51 am
content: Hi, I have a BLDC motor installed in a small mill and want to perform some test to see how much power the motor consumes at different loads and operating speeds. The motor is a 3 phase / 8 pole motor, nominal current is 10 A and voltage is 48 Vdc. I am interested in getting only the motor's consumption so I want to take the measurements after the motor driver but I am confused on which voltage and current measurements I should take. Would it be ok to measure the voltage from one phase to ground and the current of only 1 phase and assume that the current in the other phases will be the same, as well as the voltages in the other phases to ground? Since the voltage is a PWM im planning on using an oscilloscope to get the both average values. Final doubt is how the power should be calculated with these values, simply V x I or should it be treated as a 3 phase AC system? I guess I am missing something here!thanks in advance for your comments!

author: max_torque
author_rank: Super Contributor
timestamp: April 11, 2017, 11:46:48 am
content: Is there any advantage to knowing what the motor itself consumes?You are i suspect always going to run it with the control inverter, so you may as well include that loss in your power budget too. And that means an easy DC power calc, just measure V & I on the DC link.Otherwise just google "3ph power measurement techniques" for an entry into a whole world of fun / pain (<< delete as approriate)

author: Siwastaja
author_rank: Super Contributor
timestamp: April 11, 2017, 03:53:31 pm
content: Just measure the DC input to the inverter. Anything else is complicated, requiring real understanding of the actual inverter circuit, and taking multiple synchronized high-speed measurements, while dealing with noise.Guesstimate the efficiency of the inverter as something like 97% and you are very close.

author: perucho
author_rank: Newbie
timestamp: April 11, 2017, 06:19:45 pm
content: Hi, thanks for your comments. So actually my first idea was to measure as you both suggest, directly on the DC bus, but since I am going to do the tests at different loads and speeds I don't know exactly how the efficiency curve of the driver is looking at partial loads. So I thought it might not be so complex to do it directly on the motor side, but the more I get into it the more confused I get, plus I haven't really find any good explanation on the web. As you said Siwastaja, I have tried looking at the drivers voltage output with the oscilloscope and yes I get a lot of noise coupling from somewhere!! I saw you are from Finland, I am actually using a driver from a finish company Electromen, perhaps you know them. The problem is that they have very few documentation and the driver behavior is almost an unknown blackbox.Anyways if you guys have some good literature you would recommend for this topic it would be great! I am actually planning on building a simple motor driver for this milling application so it might be useful in the near future.

author: sanwal209
author_rank: Regular Contributor
timestamp: April 11, 2017, 07:27:22 pm
content: Its easy. Your invertor must have 3 half bridges to drive the phases. Add shunt or hall effect based current monitor IC in series to each half bridge. After than you have three current to voltage converted outputs. add them by using summer(use op amp) and if you are using PWM to control current too then use RC filter after output of each sensor and feed into summer.

author: max_torque
author_rank: Super Contributor
timestamp: April 12, 2017, 06:22:56 pm
content: https://www.yokogawa.com/library/resources/media-publications/electric-motor-power-measurement-and-analysis/Remember, with a PWM inverter your phase voltage must be sampled fast enough to fully define the voltage waveform, and for RMS measurements, that includes the effective frequency doubling inherent in that calculation methodolgy. Typical industry standard power analysers, like those from Yokogawa (who incidentally are NOT class leading for PWM measurement) sample at 2Mhz, for a 1Mhz (ish) voltage bandwidth, despite typical pwm fundamental frequencies being down between 10 and 20 KHz.