January 3, 2019
BLDC Notes/Cheat Sheet
Misc. notes on brushless motor controllers
Magnetic Force - Unlike poles attract and Like poles repel
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
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
Gives the direction of the magnetic field created by a straight current carrying wire
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:
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
|Sensor Output||Driver Input|