Brushless DC Motor Configuration

Discussion in 'The Projects Forum' started by Herman.C, Jul 30, 2010.

  1. Herman.C

    Thread Starter New Member

    Jul 29, 2010
    I have recently acquired a 3 phase 4 pole BLDC motor that I would like to get working bi-directionally. The BLDCM has 3 hall sensors that will provide the feedback to a MCU that will control when the current flows etc.

    The phases are located in such a way that it is from top CCW U,V,W,U',V',W'

    With U',V',W' all connected to common.

    I know about the concept of an H bridge that will provide 4 states of operation, however that is for an 'H' design. As I have 3 phases, I will have 3 half bridges.


    However as U', V', W' are all tied into Common the current will enter U or V or W around the windings to U', V', W' to common and to ground?

    So I was wondering what are the uses of the Low Side Transistors in the Half bridge? Since I could have 3 transistors that will control when the current enters the U,V,W phases and as they all 'exit' via common theoretically I don't need the low side?


    Is that correct? I see many 3 phase BLDC motors online that still employ all 6 Transistors?

  2. BillB3857

    AAC Fanatic!

    Feb 28, 2009
    A Brushless DC Motor (BDCM) is, in actuality, a special design of a standard 3 phase motor that happens to have permanent magnets in the rotor. Your second drawing of the 6 transistor H bridge configuration is a standard for BDCM controllers. If you were to draw a plot of the three phases of a standard 3 phase power source, then draw a vertical line through all 3 phases, the intersections would represent the individual phase voltages (both positive and negative) for a particular shaft position (assuming a 2 pole configured motor). Commercial conrollers simulate these instantenious voltage values and polarities by means of PWM techniques.

    To determine the number of poles of your motor, apply a low level DC between any two lline leads (Typically, +U. -V) The shaft should rotate to a given position, and have holding torque at that position. Then, force the rotor a full 360 detrees. Count the number of times the rotor "locks up" and multiply by 2. That is the number of poles for your motor. While the motor is in one of its lock-up positions, you should be able to read the outputs of the Hall devices. They will be in a particular pattern and will change as the shaft is rotated out of lock-up position. You can make a table of these changes as you rotate the shaft whic will help you determine the programming requirements of your controller. A 2 pole motor will have one shaft rotation for 360 electrical degrees. A 4 pole motor would require 720 electrical degrees for one full shaft rotation.

    Hope this helps.