I'm new to working with motors. Recently I was working on 3 phase BLDC motor. I needed to build a gate driver + 6 MOSFET circuitry but instead, I have BDC motor drivers from Hindustan Dynamics called Hercules Driver. Now, I need three BDC drivers to drive one 3Phase BLDC motor. I did all the connections and using Arduino mega 2560 I got everything working but without using feedback from either hall sensor or back EMF. I'm applying following sequence to the motor phases

 A      B      C
 1     -1      0
 1      0     -1
 0      1     -1
-1      1      0
-1      0      1
 0     -1      1

where 1 is VCC and -1 is GND and 0 is OFF(open circuit). Surprisingly motor starts spinning without any problem. Then I moved rotor manually to a random position and again tested with code even then motor starts spinning in right direction and at the right speed. I tried moving motor rotor to a random position different from where it stopped and each time motor rotates perfectly. As I understand from the literature that it takes some kind of feedback to drive BLDC motor but I don't observe that here. My motor has the following configuration:

Number of poles: 16
Number of winding slots: 18
Motor voltage: 24V
Current cap: 3 Amp 

What is the role of feedback and how to use it? Thanks in advance.


closed as unclear what you're asking by ThreePhaseEel, Marcus Müller, Voltage Spike, Daniel Grillo, jonk Nov 30 '16 at 23:43

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    \$\begingroup\$ What is your actual question? \$\endgroup\$ – PlasmaHH Nov 30 '16 at 12:46
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    \$\begingroup\$ Assuming the actual question is: "what does feedback give you?" - 3 things : (1) your current scheme will probably fail at higher speeds, and stall with any kind of load on the motor (2) it won't track instantaneous speed changes at all and you may have to ramp speeds up and down gradually (3) it'll be hopelessly inefficient, like a simple stepper motor drive, because you can't tune the drive voltage (PWM duty cycle) to match the motor's actual requirements. \$\endgroup\$ – Brian Drummond Nov 30 '16 at 14:21
  • \$\begingroup\$ @PlasmaHH I have updated my post, I want to know what is the role of feedback if a motor can rotate on its own and how to use it(feedback measurements)? \$\endgroup\$ – Vinay Joshi Nov 30 '16 at 14:42
  • \$\begingroup\$ @BrianDrummond Thanks for your answer. So using feedback one can maintain synchronization between electrical and mechanical rotations and helps in avoiding problems listed in your comment, right? \$\endgroup\$ – Vinay Joshi Nov 30 '16 at 14:51
  • \$\begingroup\$ BLDC motor control is a topic on which books can be written, the questions needs to be more specific. That being said you can drive bldc motors open loop, but they won't be able to react to torque loads like a motor in feedback will. There are many ways to get feedback from the motor. Back EMF is one of them. \$\endgroup\$ – Voltage Spike Nov 30 '16 at 17:02

A 3-phase BLDC motor can be operated open-loop as a permanent-magnet synchronous motor (PMSM). If the motor is started at a low frequency and accelerated at a controlled rate by limiting the rate of frequency change, it can be operated over a reasonably wide speed range. The applied voltage must be adjusted to maintain a relatively constant ratio of voltage to frequency. This is the basic method of variable frequency drive (VFD) control used for induction motors. Maintaining the V/Hz ratio, even open-loop, can keep the motor from being "hopelessly inefficient" as @Brian Drummond put it. It will also allow the motor to develop full torque over a useful speed range. An open-loop VFD controlled PMSM will not have high starting torque or the ability to cope with rapid load changes. It could also experience torque-angle oscillations if the load inertia is too high.

I believe it would be fair to say that the emf and hall-effect feedback techniques provide everything that V/Hz control can do plus the things that it can't do. However I believe that sensorless-vector VFD control techniques can provide most if not all of the dynamic performance and perhaps more torque for a given size motor and higher efficiency. The controller cost and complexity may be higher with the VFD approach.

  • \$\begingroup\$ The key difference is that a (permanent magnet) brushless DC motor cannot "slip" in the manner of an induction motor. Hence, it cannot itself handle a condition where the drive is insufficient to maintain synchronous rotation under the load, but must rather have the lag handled by adaptive drive electronics watching something such as sensors or back EMF cycling, or else fail when the lag accumulates to the point of cogging to a different pole matching. In essence, you can operate it as a "stepper motor" only in conditions where it does not loose steps. \$\endgroup\$ – Chris Stratton Nov 30 '16 at 16:17
  • \$\begingroup\$ In the context that I presented, the motor can operate as a synchronous motor only in conditions where it does not lose synchronism. \$\endgroup\$ – Charles Cowie Nov 30 '16 at 16:39

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