Brushed DC motor filter design - Page 1
Forum section: electronics / projects

author: brumbarchris
author_rank: Regular Contributor
timestamp: August 15, 2016, 08:37:23 pm
content: Hi all,I am currently designing a microcontroller based motor control system. The controlling element will be a micro, the motor driver will be a Toshiba TB67H303HG for it (datasheet attached) and the power supply will be a DC-DC boost operating off a 12V battery.Unfortunately, the motor is fixed and cannot be changed, and there is no datasheet available for it (unknown part number). However, I can do some measurements of the motor in the system it currently operates in. As it is a brushed DC motor, there is a lot of noise that is currently induced on the power supply of the existing system. I know I will need to design a proper power filter to reject this noise (owed most likely to the back EMF) but I am a novice in that, and I would need some pointers.To help us in designing the proper filter, I have made the following measurements:- electrical resistance of the motor winding is 1.7 Ohms- RMS voltage on the motor supply terminals (when measured with a voltmeter) during operation is 29V- RMS current through the motor winding (when measured with an ammeter) during operation is fluctuating between 2.5A and 3.7AI am a bit puzzled because 29V over a 1.7Ohms resistance would yield something like 17A! But I suppose that would be really the stall current, when the speed is 0 and back EMF is also nil. But when the motor is spinning, the EMF induced in the winding actually counters the voltage applied to the motor resulting in a much smaller current (which is what I measured as fluctuating between 2.5A and 3.7A). I hope I did get this rationale right. I do not have the means of capturing the current waveform through the winding.Regarding the noise owed to the back EMF, I made three screenshots of the voltage applied on the motor (with respect to GND) and these are also attached below. All three screenshots represent the same motor supply voltage, but they are all made with different timebase, in order to emphasize the noise level and frequency. The cursors in the first picture have no meaning, I just forgot to turn them off. They do have meaning in the second and third screenshot, where they reveal the high frequency noise as being 3.623MHz (third screenshot), coming in bursts with a repetition rate of about 31.64kHz (second picture). For some reason, the last picture does not seem to be visible inline, but if you click on its name you will get the option to download it. It is a .png file.The power supply of the current system is based on a transformer and operates off a 220V/50Hz mains, taking voltage down to some 29-30V.So to sum up my questions/concerns:- I would need some confirmation that my judgement with regards to the RMS current is correct (I have barely worked with motors until now)- I am a bit puzzled about the low frequency oscillations in the first picture; I initially thought these were due to the rather poor design of the on-board transformer based power supply of the current system, which as I mentioned before is transformer based, and then rectified with what seems to be a group of diodes in bridge configuration. But then the period of the oscillations would have to be some 20ms (due to the 50Hz of the mains) but I see it as being 10ms (exactly half of what I expected)- Finally, as mentioned before, I would need some starting points for designing the filter for the 3.623MHz oscillations, possible with a low enough corner frequency so as to kill also the 31.64kHz bursts.- Which would be the best place for this filter? Between the Toshiba driver and the motor (as I suspect it is) or between the power supply (which I must have in my design) and the Toshiba driver?Regards,Cristian

author: LabSpokane
author_rank: Super Contributor
timestamp: August 16, 2016, 04:12:32 am
content: I made a PCB with an X2Y capacitor to help tone down the interference on my project. It worked well for me. Others have...well...other opinions.https://www.eevblog.com/forum/projects/brushed-motor-emi-reduction-with-x2y-caps/msg621138/#msg621138