A preface
The rectification prototype for a customer’s vehicle motor control project, mainly through the PWM signal for motor speed adjustment, the actual test process, there is a low frequency band part of the radiation seriously exceeded the standard problem, the following is the test and rectification analysis process.
Two motor controller architecture and problem points
The above figure is a simple schematic diagram of the motor controller, through the PWM signal to control the motor negative power MOSFET tube on and off, adjust the size of the duty cycle of the pulse width of the PWM signal output from the control board, you can achieve control of the motor speed, PWM signal switching frequency of 16KHz. preliminary analysis can be obtained as follows.
(1) EMI can be divided into conducted interference and radiation interference, conducted interference refers to interference energy along the cable in the form of interference voltage propagation, radiation interference refers to interference energy in the form of electromagnetic waves through space will be its signal coupled to another electrical network.
(2) in order to effectively limit the value of EMI, must figure out the source of interference and coupling path in order to suppress electromagnetic interference. In a variety of products driven by DC motors, the sources of EMI mainly include.
Sparks of the motor, sparks are commutator area near the air medium ionization, the formation of charged particles in the air, the formation of electromagnetic interference.
non-linear devices, silicon controlled, rectifier diodes, and the operating characteristics of transistor switches on/off generate high frequency harmonic interference.
PWM control signal of the high harmonics caused by the low frequency (pole antenna, current method and voltage method test band) radiation exceeds the standard.
Three test data
The test standard is CISPR25-2018 level 3, the data above is the rod antenna test data (0.15-30MHz), the most serious radiation exceeds the standard in all test bands, is the focus of this correction. By analyzing the data, we can learn that the data shows a regular distribution, the frequency interval between two is about 16KHz, which is the switching frequency of PWM signal.
Four corrective measures
The three elements of electromagnetic interference are: interference source – coupling path – sensitive equipment, the common means of rectification is also based on these three elements, can be broadly divided into shielding, grounding and filtering.
Known radiation source for the PWM signal, exceed the frequency point for the signal of the higher harmonics. Considering the radiation exceeds the standard frequency band for the low frequency from 150KHz, if the means of filtering, on the one hand, the filter device parameters will be relatively large, such as large capacitance inductance; on the other hand, after filtering, the output drive voltage waveform will be affected, which may affect the function. Therefore, the rectification gives priority to the shielding of the radiation path.
As shown above, the green box 1 is the transmission path of the PWM signal from the control board to the driver board. The test data after first disconnecting the motor part is as follows.
The data after shielding of this harness is as follows.
From the two test data comparison can be seen that the PWM signal to the drive board harness is one of the path of radiation. Since the negative terminal of the motor power supply is controlled by the PWM signal, the voltage waveform at green box 2 will be the same as the PWM signal, which is also 16KHz. The actual measured motor voltage waveform near the driver board end is as follows.
At the same time, due to the motor, the voltage waveform at both ends of the motor will have obvious overshoot and ringing phenomenon, the closer to the motor end the more obvious, the following figure is the actual measurement of the voltage waveform near the motor end.
As can be seen from the above graph, there is an obvious overshoot and ringing phenomenon of the voltage at both ends of the motor, where the rate of change of voltage is greater and the radiation is stronger. The motor is connected back on the basis of the shielded PWM signal line, and the test data are as follows.
From the above figure, the entire low-frequency band of radiation exceeds the standard is still very serious. We continue down the rectification, also in accordance with the previous method of motor harness shielding treatment, test data are as follows.
As you can see, shielding can indeed play a significant effect, but here it should be noted that the shielding needs to be double-ended grounding process, the motor end of the grounding is connected to the metal shell of the motor, the effect of connecting and not connecting a big difference.
Five summary
The above is the entire content of this sharing, mainly related to speed control motor low-frequency radiation exceeds the standard problem of rectification ideas and the actual rectification effect, rectification measures to shielding. Considering the impact of various factors in the actual application, the feasibility of shielding is low, the author will continue to try other more feasible programs, but also look forward to generous advice from readers, to provide us with valuable advice in this regard.
