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filter for low voltage 3 phase BLDC motor - Help!

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nikeplato

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bldc motor wire length

Hello experts,

I have a 3 phase BLDC motor controller board: current is limited to 3A.
I am PWM ing the FETs at 20 KHz.
Motor inductance 4.5 mH, resistance 2Ohms.
Ta the radiated emisiions tests, the whole assembly is over the limits at 20 KHz, 700 KHz, 190 MHz and possible higher (the test is not over).
I have limited experience with the 3 phase BLDC motor.
Could anyone put me on the right track to design a filter to suppress motor noise, please?
I forgot to mention - when the motor is not activated (not driven) but just in stand by, the module passes the test, which would mean that the electronic circuit does pass but only the motor fails.

Thank you a lot.
I would apppreciate a timely answer.
 

bldc freewheel

in stand by, the module passes the test, which would mean that the electronic circuit does pass but only the motor fails
Click to expand...

Yes I had guessed that, but the electronic circuit itself is the problem anyway as well as probably the wiring. Most likely it's not particular related to BLDC motor, could happen with any similar switched circuit.

The low frequency (20k and 700k) limit violations indicate to my opinion, that motor wiring and shielding/grounding is inappropriate. Depending on the application (cable length, motor mounting) this may acutally be difficult to achieve.

The 700k component should be reducable with LC filters at the controller, 190 MHz(?) with small RF chokes. Most likely the 190 MHz isn't generated by the switching transistors istself but some (too?) fast freewheeling diodes in a bad designed PCB.
 

bldc motor and wiring

Hi FvM and thank you for your quick response,

The low frequency (20k and 700k) limit violations indicate to my opinion, that motor wiring and shielding/grounding is inappropriate. Depending on the application (cable length, motor mounting) this may acutally be difficult to achieve.

The 700k component should be reducable with LC filters at the controller, 190 MHz(?) with small RF chokes. Most likely the 190 MHz isn't generated by the switching transistors istself but some (too?) fast freewheeling diodes in a bad designed PCB.

The freewheeling diodes I am using are those internal to the FETs - FETs are IRF 7341Q (http://www.irf.com/product-info/datasheets/data/irf7341q.pdf).
The traces between the power supply (12 v car battery) to the FET drain to the FET source to the current shunt to the GND are as short and wide as they can be (they are restricted by the connector position).
So the freewheeling current wouldn't need to make a big loop. The PCB is a disk with 2 inches diameter.
So I hope the PCB is not that bad - could you give more details about what you mean by bad designed PCB - maybe it didn't cross my mind. Otherwise I think our PCB designer did the best he could.
Oh, and that frequency was not 200 MHz, but rather 150 MHz - where we are over the limit.
Interesting is that the duty cycle of the PWM command for the FETs has some importance - for example at 80% DC the results are not so bad as at 30% DC.
Could that be because the rippple current would be lower in case of the higher DC?

Let me know if you have some time for me - I could pm you with more details.

I greatly appreciate your help,

nike
Click to expand...
 

bldc filter

Hello,

the freewheel diode and bad PCB consideration was just a first guess. I may not apply at all. The VHF component could also be generated from control signals. You would need some differential diagnostics, e.g. operating PWM without motor supply to see the actual source.

Obviously, generated interferences depend on PWM duty cycle, you can't conclude much from that fact. Cause you have three independant phases, you could even get nearly zero interference at some duty cycles.

I think, it's useless to guess further without knowing all design details, circuit as well as geometry, test setup and results.

Regards,
Frank
 

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    nikeplato

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3 phase bldc controller pcb

FvM,

I sent you a pm, please let me know if you got that.

Thank you,

nike
 

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