In the previous blog, we discussed that deriving the right commutation table plays an important role in controlling 3-phase BLDC motors using hall sensors. As a refresher, here are some surprising facts about the drive system:
We also previously covered a method to generate the commutation table based on three-phase motor back EMF waveforms and their relation with respect to hall sensor output. This is based on the assumption that you are able to get the required information from motor manufactures. The reality is that in almost more than 60-70% of cases, the information is not easily available.
If you’re puzzled on how to derive the right commutation sequence and how to get your motor rotating in either direction, please read-on where we’ll cover the steps to generate a commutation table for a given BLDC motor using a DC lab power supply on the actual motor sample without depending upon manufacturer’s data.
The good thing about this approach is that once you follow and understand it, you are able to visualize how a motor actually rotates and subsequently, how to reverse the rotation. The below method assumes that hall sensors are displaced at 120⁰electrically apart from each other.
Figure 3: Commutation sequence as per configuration-A
Figure 4: Commutation sequence as per configuration-B
Table 1: Six state commutation table
Using the above method, you can derive the commutation table for reverse direction of rotation. By the way, table 1 is also valid for configuration-B types of motors or hall combinations in reverse (i.e.: clockwise) direction.
TI supplies various motor control kits which supports hall sensor interface including the 24 V, 3.5 amp DRV8312-C2-KIT and the 400-V, 1hp high voltage motor control kit supplied with Piccolo MCU. Both the kits are supported with BLDC sensor based software through control-suite.
Click here for a complete guide to motor control solutions.
Thanks for reading and your comments are welcome.
Can you confirm magnitude of the vectors on Figure 3 and 4 are marked to scale ? . What are the magnitudes of dotted line and normal line.
The magnitude of 3-phase current vector (vector sum of 3-phase currents) would be 3/2*Idc.
Actual torque devloped will depend upon the motor torqure constant. In the blog I mentioned to apply Idc around 10-20% of motor rated current.
In actual conditions this may vary deepening upon the shaft inertia and load applied. If motor is on no-load or partially loaded, 10-20% current would be sufficient to move the rotor to next state on the other hand if motor is fully loaded, current may need to increase to 30-40%. For the proper measurement, you should make sure sufficient current is pumped such that rotor moves to next state to get align with it, continuous free rotation is not required.
I am trying to understand the difference between the amplitude of vectors when two phases conducting vs three phases conducting.
My analysis shows assuming constant current (I) from power supply vector amplitude when two phases are conducting ( solid line )is bigger than the amplitude of the vector when three phases are conducting (dotted line ) , I am confused as the figure shows the amplitudes other way , and trying to get clarification whether the figure is drawn to scale ??
For example when only two phases are conducting ( Let Ia = I , Ib = -I , Ic =0 ) resultant vector amplitude is sqrt(3)* I
when three phases are conducting (( Let Ia = I , Ib = -0.5*I , Ic =-0.5*I ) resultant vector amplitude is (3/2)* I
First of all thanks to you for reading the blog meticulously and bringing out this point. You are absolutely right. The magnitude of resultant vector with two phase excitation is higher than 3-phase excitation.
Definitely, it is not reflect in the diagram published with blog because main purpose is to show the 90 deg angle difference between the 3-phase excitation with respect to six hall sensor positions which coincides with vectors corresponds to 2-phase excitation.
I can understand that diagram creates confusion but if you follow the real theme of blog, you may not mind this error very much!!
Dear Milan ,
would you explain the fact that there is only one right connection for both direction of rotation and the remaining 5 are wrong. Why???Is there any literature about that ???
The closest one I could find on web. Refer to section 7.4.1.
It is a very good article
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