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BLDC Notes/Cheat Sheet

Misc. notes on brushless motor controllers

  1. Magnetic Force - Unlike poles attract and Like poles repel

  2. Left Hand Rule - F = BILsin0

B = Magnetic Force I = Conductor current vector L = Length of Conductor (Length of wire) 0 = Angular difference between B and I (Highest at 90degrees)

Usually we have to deal with coils so …

Usually the lengths of the coils act in the opposite directions as the current passes through the magnetic field.

Also coils usually have multiple turns (N). Higher coil turns > Torque Constant > Torque

So the torque (N.m) is 2rFN = 2rBILN

Torque constant = 2rBLN = N.m/A

  1. Right Hand Rule

The movement of the conductor in the magnetic field causes a BEMF -

E = BLvsin0

E = Voltage B = magnetic field v = velocity of conductor 0 = Angular difference between B and L

  1. Right Hand corkscrew

Gives the direction of the magnetic field created by a straight current carrying wire

Important Parameters:

  1. Torque Constant -> N.m/A
  2. Rated speed? Higher speeds by a technique called flux weakening
  3. Rated Current
  4. Cogging torque - Max torque it has when not being driven
  5. Mechanical Time Constant - Time needed to spin up to full speed. 4 - 10 ms.
  6. Electrical Time Constant
  7. Winding Inductance
  8. Pole Pairs - the number of poles is just pole pairs/2

The phases of a BLDC motor are A,B,C or U,V and W

Cogging torque is due to the variation in airgap or relucatance of the stator teeth and slots above the magnet as the motor rotates

Ripple torque is torque produced by the interaction between the stator and rotor MMF. Ripple torque is mainly due to fluctuations in the field distribution.

The motor windings can be wound to give either trapezoidal or sinusoidal feedback.

Alrighty, so the standard ways of winding coils is either to produce a trapezoidal shaped BEMF or producing a sinusoidal shaped BEMF. Sinusoidal motors have lower torque ripple but suffer higher switching losses and greater drive complexity. So the trapezoidals are more common

In a sinusoidal motor, current travels through all three windings at any point, while in a trapezoidal motor, current only flows through 2 of the 3 windings.

3 Ways of controlling brushless dc’s are:

  1. Hall effect sensor
  2. Encoder
  3. Zero-crossing (sensor-less)

Hall sensors are spaced 120 degrees from each other

Trapezoidal control Quick and easy LUT involves different driver inputs for different hall sensor changes outputs

Commutation Table
Sensor Output Driver Input
HS1 HS2 HS3 A B C
0 0 1 X HI LO
0 1 1 HI X LO
0 1 0 HI LO X
1 1 0 X LO HI
1 0 0 LO X HI
1 0 1 LO HI X

Sinusoidal control

Less cogging???

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