Hello.
I have been reviewing the AN12435 document for BLDC motor control, and I have a question regarding 3.3.1 (BEMF zero-crossing principle)
1) Un is used in Equation 3, but Un is not present in Figure 7. What is Un? (Does Un mean Ua, Ub, Uc, and the voltage applied to phases A, B, and C respectively?)
2) How was equation 5 derived? (I think Un means Ua, Ub, Uc and Uc are already the same, so how come -ec is added?)
3) Shouldn't it be 2/3ec -> 1/2ec in Equation 6?
4) What the below sentence means is, ‘Measure the BEMF voltage(ec) in a phase where power is not supplied and find the moment when BEMF voltage(ec) reaches 0V'. That is, finding the point where Uc = 1/2Udcb. Am I right? (or Does it mean comparing BEMF voltage and DC bus voltage?)
I mean What voltage is measured? regarding the following sentence 'by measuring voltage at the unpowered phase (ec) and comparing it to half of the DC bus voltage,'
5) Considering Figure 5 and Figure 30, it seems that all three DC bus voltage (Udcb), BEMF voltage (ec), and Phase voltage(Uc) are measured. And it seems that the three measured voltages are compared as in Equation 6. Is that correct?
[Figure5. 3-phase DC/AC inverter with shunt resistors for current measurement]
[Figure30. System block diagram]
Best regards,
trunie
Hello,
1) Un is used in Equation 3, but Un is not present in Figure 7. What is Un?
neutral point voltage or natural voltage.
2) How was equation 5 derived? (I think Un means Ua, Ub, Uc and Uc are already the same, so how come -ec is added?)
At the time of the BEMF zero-crossing, the BEMF voltage (ec in this case) is zero as the name implies.
The unpowered phase has the following voltage equation, since there is no current flowing:
uN = uC – ec
3) Shouldn't it be 2/3ec -> 1/2ec in Equation 6?
4) What the below sentence means is, ‘Measure the BEMF voltage(ec) in a phase where power is not supplied and find the moment when BEMF voltage(ec) reaches 0V'. That is, finding the point where Uc = 1/2Udcb. Am I right? (or Does it mean comparing BEMF voltage and DC bus voltage?)
I mean What voltage is measured? regarding the following sentence 'by measuring voltage at the unpowered phase (ec) and comparing it to half of the DC bus voltage,'
You measure induced voltage on unpowered phase.
5) Considering Figure 5 and Figure 30, it seems that all three DC bus voltage (Udcb), BEMF voltage (ec), and Phase voltage(Uc) are measured. And it seems that the three measured voltages are compared as in Equation 6. Is that correct?
Sorry, I dont understand your understanding.
In case you never worked with sensor-less motor and you are going to design control circuit. I strongly recommend you to undergo some basic training and also have a look at our reference examples.
We have posted dozens of application notes on this problematic.
Best regards,
Peter
