Brushless DC Motor Control

DC motor, BLDC motor, Motor control

A BLDC motor may have a trapezoidal Back EMF, there is something called Trapezoidal Motor Control Methodalogy (TRZ). TRZ is an extension of 6-step control with additional switching states generated to allow winding current to ramp up/down in each phase prior to/after each commutation instant. Trapezoidal motor winding currents, achieve a low noise performance.

Brushless DC motors are generally, although not always, three phase devices. They are wired in either a Y (Wye, Star, T) or Δ (Delta) configuration, but in either case there are three connecting wires, and the current input to any two coils must be output through the third. That is,

C = –(A + B) where A, B, and C are the current flowing through each leg of the 3-phase brushless motor.

The back-emf in a BLDC motor is linearly proportional to the rotational speed of the shaft. The back emf is proportional to the speed of the motor and its direction is given by Flemings right hand rule.

C = –(A + B) simply means that since there is no neutral used the sum of three phase currents must add up to zero

A+B+C=0

Although the drive system is three phase it can be easily simplified and viewed as a Buck Converter fed PMBLDC drive.

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I have a doubt about the sensor less control of BLDC motor. Usually Hall sensors are used to detect the position of the rotor and based on that feedback, the switches of the inverter are turned ON in order to energize the windings but in sensor less control we will not be using Hall sensors, so we are dependent on BEMF waveform (trapezoidal shape) Now since the motor manufacturer do not provide the neutral and the BEMF must be measured per phase so exactly How this sensor less control works...??

<- - Comment made by: Moghe - ->

Unlike the motor with hall (or equal) sensors, you first align the rotor by applying current to two of the windings. After a short time, a time you have to program in, you run the motor like it was a stepper motor. You have to decide how fast you can accelerate the motor in this mode/phase. You can then measure the actual BEMF and let the controller decide when the BEMF is large enough or decide a time or speed when you can switch to allow the BEMF to control the switch sequences.. There are limitations where you can use the sensorless motor. Driving a fan is OK but if, for example, you have several motors joint together, such when you have several propulsion motors driving a railroad locomotive you can not use sensorless motors.

I have wondered if it is possible to determine the rotor position by measuring the small variation in the winding inductance due to the variation in the magnetic field. You will be at different parts on the hysteresis curve depending on how the magnetic fields go through the magnetic material in the different windings. It should be possible by applying a DC current together with an high frequency AC. I leave it to you to figure it out. It should be possible though. It might not be more economic than install hall sensors.

<- - Comment made by: Hans - ->

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