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I have some dilema with commutation on BLDC motor. I have driver for BLDC (MP6540) and I can commutate BLDC motor but motor it is heating up and speed is not appropriate (datasheet says that motor must have higher rpm). Here is picture of one sequence of commutation (Enable A, Enable B, Disable C, pwm on A, gnd on B): enter image description here

My question: Will complementary pwm (complementary pwm A) resolve this problem?

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  • \$\begingroup\$ The MOSFET you point as always OFF should be off, when green arrow MOSFET is on. It is the way they work. You need to show how you have connected the chip and how you drive it with the 6 input lines. \$\endgroup\$ Mar 4, 2020 at 1:20

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According to its datasheet, the MP6540 has 'automatic' synchronous rectification:-

When both the HS-FET and LS-FET are turned off and the voltage on an Sx output pin is driven below ground, the LS-FET is turned on until the current flowing through it reaches near zero or until the HS-FET is commanded to turn on. Similarly, if Sx rises above VIN, the HS-FET is turned on until the current reaches near zero or the LS-FET is turned on.

The benefit of this is lower power dissipation in the upper FET, since when it is turned on the body diode is bypassed and voltage drop is lower. However this should not greatly affect motor efficiency or running speed. Forced complimentary PWM is a bit different in that it will actively brake the motor when reducing speed, but otherwise the effect is similar.

You do not show what method you are using to detect rotor position, or the commutation sequence and timing. Incorrect sequencing or bad timing can cause rough running, excessive motor heating and failure to achieve full speed. Another possible cause of low rpm and excessive heating is high mechanical loading.

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  • \$\begingroup\$ Hello, I think that problem is in timings/sequence of commutation. This is sequence: 1. pwm on A , gnd on C, enable A and C, disable B 2. pwm on A, gnd on B, enable A and B, disable C 3. pwm on B, gnd on A, enable A and B, disable C 4. pwn on B, gnd on C, enable B and C, disable A 5. pwm on C, gnd on B, enable B and C, disable A 6. pwm on C, gnd on A, enable A and C, disable B \$\endgroup\$
    – subavet995
    Mar 4, 2020 at 9:57
  • \$\begingroup\$ There are plenty of examples of 6 step commutation on the net that you can check it against. How are you synchronizing the commutation? \$\endgroup\$ Mar 4, 2020 at 14:35
  • \$\begingroup\$ With integrated Hall sensors. I follow commutation table. Now I think my driver is burned down. I concluded that because ENA is disabled and on PWMA is high signal so should be obtained on SA 24V (but there is only ~1V). \$\endgroup\$
    – subavet995
    Mar 4, 2020 at 14:59
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When the motor is unusually hot while not reaching the expected torque, first thing to look out for is anything related to the electrical angle and commutation timing (starting with position sensing).

A wrong electrical angle will cause current flow with wrong timing. Current flow with wrong timing will not have optimal contribution to torque, but still cause ohmic losses in the windings, of course.

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