Brushless DC motor current / voltage

[Tech_boy]

Hi guys,

hope this is the right section for this question.

I have this electric scooter which is powered by a pack of 48V batteries feeding a 350W Burshless DC motor. The DC Motor has 3 phases, each having it's respective position sensor (photo attached shows the wires coming out of the motor).

Now I am creating a datalogging system using an arduino Mega 2560. I need to measure Voltage and Current between the controller and the hub motor across one phase and rpm.

RPM - No problems there
Voltage - I'm using a POT to lower the 48V to 5V to input the arduino and that works fine as well

Current - THAT IS THE PROBLEM!

First I placed my multimeter in series with one of the phases, but didn't get any readings strangely enough!

So far I used ACS758 LCB-050B following this tutorial: https://www.dfrobot.com/wiki/index.php/50A_Current_Sensor(SKU:SEN0098)
I managed to read current drawn by a 12V car heater which draw approx. 7A but no success with the brushless DC motor.

I tried a CT but I guess it was faulty .. but still no success

Now I just bought lts 15np by LEM. Didn't have much time to test it but it's the only component that is giving me some kind of readings. However, they don't seem to be consistent .. For example if I try to stop the wheel from turning, I'm don't get a proper increase in current and if I leave the "throttle" at constant speed, the current value varies sometimes. I also introduced a low pass filter.

Anyone can guide me on how to measure current drawn by a brushless DC motor? Maybe someone already did a similar project using arduino or any other microcontroller? Is there anything I should know about controller and brushles motor in electric scooter?

 

[D.A.(Tony)Stewart]

Be sure to read all the FAQ's

The output stage is designed for use with a bipolar power supply and diodes must be inserted in series with the output , to allow a minimum output bias voltage without creating a measurement offset.

**broken link removed**

 

[kam1787]

why not measure the current directly from the battery? just subtract the current [at idle] used by the control circuit

 

[Tech_boy]

Be sure to read all the FAQ's

The output stage is designed for use with a bipolar power supply and diodes must be inserted in series with the output , to allow a minimum output bias voltage without creating a measurement offset.

**broken link removed**

Thanks, will go through that in detail later on

 

[Tech_boy]

why not measure the current directly from the battery? just subtract the current [at idle] used by the control circuit

didn't think of that stupid me :O .. I did a quick test and it seems to have worked .. at idle the current is about 1A or even less .. stall current turned up to approx. 7A .. so I guess I'm on the right track.

I'm using the arduino and the LEM sensor calculating:
current = ((sensorValue * 5.0 / 1023.0) - 2.5) / 0.0416;
(-2.5 is due to an offset and /0.0416 is the sensitivity)

.. I will check better tomorrow coz everyone is sleeping here and don't want to wake them up haha

Thanks a lot

 

[FvM]

Are you aware of the fact that the motor phase current (as well as the voltage across each winding) is an AC quantity? I presume, it can be well measured with ACS758 LCB-050B, but the problem is probably with your method of processing the sensor output.

 

[Tech_boy]

Are you aware of the fact that the motor phase current (as well as the voltage across each winding) is an AC quantity? I presume, it can be well measured with ACS758 LCB-050B, but the problem is probably with your method of processing the sensor output.

Yes I am aware of that ..
what do you mean exactly by "the problem is probably with your method of processing the sensor output." ?
thanks

 

[FvM]

what do you mean exactly by "the problem is probably with your method of processing the sensor output." ?

E.g. measuring DC instead of AC current.

 

[Tech_boy]

E.g. measuring DC instead of AC current.

no no .. i'm getting those right .. I did as Kam1787 said and got some reading which seem to make sense.
for ex: Idle aroud 800mA .. switch on the lights, 1.12A .. turn the motor 1.85A .. load the motor 3A .. stall current up to 7A etc