Noise issue in measuring current from dc motor through current sensor!!!

[siva88]

Hi everyone,

I am working on a project where I control PMDC motor by switching on/off through mosfet (PWM control) by DSP controller, also I measure real-time current of the motor using LEM (LTS-15-NP) current transducer.

Well, the problem is I get noise in the current sensor output line when the PWM(or switched voltage) is given to the motor!!:sad:.

While the motor is turned-on the input voltage to the current sensor picks-up noise(see the below scope image).:???:
The output voltage of the current sensor has also have the same level of noise.


I also had a doubt that there could be some switching noise (some kind of transient peak current in the motor line while switching)!!. So I have measured the line current (while running) by putting a SERIES RESISTOR and measured with a scope, and it doesn't look like it has noise in the line.:|(below is the scope image for line drop voltage).


The ultimate problem is when I read this values through ADC, and plot the current graph all I get is some jumpy values so I do some software low pass filter, but still I have some noise, due to which I can’t able to interpret actual rise in current.

Instead of getting a “almost steady current “ I get some “jumpy steady current” which gives me lot of issues!!!:-x

Series resistor and some calculation:

Resitor = 0.5E

Voltage = 0.366v

Current(I) = 0.732A

Additional details FYI:
Motor = 180vdc PMDC motor.
Switching freq = 10Khz
Control: PWM control.

Added the circuit, It's not in full detail but I hope this will be of some help!!


If you can suggest or give an opinion on this, it will be of great help.
Thanks in advance.

 

[chuckey]

I think its noise due to the brushes in the motor. De-couple both brushes to the motor case with 1 NF capacitors (make sure that the case is earthed) and connect a larger capacitor across the motor terminals - 10/100 NF?.
Frank

 

[siva88]

Thanks frank.I will sure check that, also I was more hoping on suggestion for the current sensor part like some filter circuit or fundamental mistake in the circuit etc.But anyways like you said there could be some noise induced by the brushes too.I will check on your suggestions.
siva