BLDC motor measurements

[Terp]

Hi all out here:

Greetings from Maryland.

I am wondering if someone here has done some electrical measurements with BLDC motors? Actually, I want to measure the current coming into the motor, and the rpm. I also want to figure out the torque. I know the theory and the physics behind all this. However, I am going through my first experience with lab setting, and real time engineering measurements. Any suggestions and help will be great.

Regards. :)

[Peter Seddon]

You are dealing with quite fast currents and voltages on a BLDC and so you need to decide what you are trying to measure - average, instantaneous etc.
Making good current sensors for rapidly changing signals is not easy but you could start with the Allegro current sensors that use Hall effect.

regards Peter

[Terp]

Hi Peter,

Your input is greatly appreciated.

I think I will go with instantaneous.

I was thinking along the lines of a DMM for measurement of voltage across the BLDC motor at full speed every minute. Since I will know the resistance of the motor using the DMM, I can calculate the power being consumed by the motor (V^2/R) at every 1-minute instants -- voltage measured with the DMM. Then, using

Power(instant) = torque(instant) * ang. speed(instant),

I will get the torque values. For the ang. speed, I will have a camera take a video of my ornithopter's flapping-wing during the whole measurement duration. Then, I will have the number of revolutions every minute, from the flapping frequency data I collect from the shot video.

I might consider the sensor for current measurement later in my project; but, for now I just want to use the equipment that is currently available in the lab.

[malibus]

Hi, Terp
In this formula Power(instant) = torque(instant) * ang. speed(instant)
for "Power" is intended the output power on the shaft, or the power consumed by the motor?

[tobbeanton]

Hi,

I'm trying to do nearly the same thing. I have rebuilt a Tower Pro 50A BLMC for i2c communication according to one of the other threads here in the forum. I have also added an allegro ACS756 current sensor so I can calculate the torque from the measured current. However I figured out that this isn't an easy task after watching the current sensor output on a scope. The current is highly correlated to the PWM signal, meaning almost zero current during PWM off time and a lot during on time. To get a good torque measurement should I add a low pass filter to “flatten” it out, or maybe I should measure the current during the PWM on time and then calculate the average current knowing the PWM duty cycle? The intended application for this is a RC car with in-wheel motors, so I would like to do measurements as often as possible, hopefully around 500 Hz.