Slaved bldc motor? - Page 1
Forum section: electronics / projects

author: najrao
author_rank: Regular Contributor
timestamp: June 27, 2023, 07:52:03 am
content: Right now this is a theory question, but I want to try it out soon.Take two bldc motors, not necessarily identical*, andparallelthe three phase power windings, no particular order needed. *But with similarvoltageratings.Only one needs to have Hall sensors, and use them to steer the driver bridge.Both motors should run, at possibly different speeds depending on the polages of the two motors. If the one with the active Hall sensors is not loaded, it will draw only a low no load current, and the other can be loaded independently!What other caveats?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 27, 2023, 04:00:36 pm
content: The motors will have a different phase shift, depending on their loads and because of this, the efficiency of the motor without hall sensors will very likely be lower, because phase currents are switched at the not optimal time.

author: mikerj
author_rank: Super Contributor
timestamp: June 27, 2023, 07:46:10 pm
content: The motor without feedback may not self start, or not start reliably, and will have very poor torque characteristics. When a sufficient load is applied to slow the motor it will immediately drop out of phase with the rotating magnetic field produced by the controller.

author: najrao
author_rank: Regular Contributor
timestamp: June 29, 2023, 05:23:40 am
content: I understand my idea borders on harebrained. Thanks to mikerj and Doctorandus-P.I skimmed through the many excellent posts on this forum to learn just that bit more, and won't be pursuing with experimenting.Still, occurs to me that: Imprecise triggering is inherent in hall sensing as the sensors are geometrically at fixed location/s, while the rotor lags the rotating field by a load-dependent angle. How do they compensate for this? If no correction is good enough, the 'master' motor can have it's halls advanced or retarded to get the best conditions for theslavemotor.

author: najrao
author_rank: Regular Contributor
timestamp: June 29, 2023, 03:06:06 pm
content: Oh, I meant to ask: would a (3 phase) 'BLDC' motor run moderately happily on a VFD?Again, what caveats?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 29, 2023, 06:31:41 pm
content: I had been wondering a long time why a BLDC motor was called a DC motor, while it runs on 3 phase AC. The reason is that the commutation has to always be in step with the mechanical position of the motor, or it won't run properly. A BLDC motor has much more on common with a brushed DC motor, then with an AC motor. It's just that the mechanical switching of the brushes on the commutator is replaced with electronic switches. (And because of that you can also do some extra things)There are a bunch of excellent youtube presentations on the workings of BLDC motors. Both ST and TI have made a series of video's about this. You will also find the term FOC (Field oriented control)In general some hall switches are good enough if you just want speed control, while when you want better control over torque and position (especially at low rpm to stand still) then more advanced methods are required.

author: mikerj
author_rank: Super Contributor
timestamp: July 03, 2023, 09:53:17 am
content: Quote from: najrao on June 29, 2023, 03:06:06 pmNo, it would have the same problems I mentioned previously, not starting reliably and poor torque. A BLCD uses permanent magnets in the rotor but in an AC induction motor the magnetic fields in the rotor are created by the rotating magnetic field in the stator. This means the magnetic field in the rotor will naturally follow that in the stator, even if the rotor is turning more slowly than the magnetic fields are rotating (i.e. "slip"). This is not the case when permanent magnets are used in the rotor.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 03, 2023, 11:29:28 am
content: If you just apply voltage to BLDC motor windings, it will create a lot of holding torque, but at a cost of also requiring a lot of current. It's just an electromagnet after all. Now imagine making the applied voltage rotate. The rotor will follow, but again, a lot of current is flowing all the time, with all the related I^2 * R losses in both the motor and the controller. If you lower the current, then the holding torque lowers too and may not be enough to keep the rotor in sync so it jumps. This is exactly how stepper motors are used, and you can use BLDC the same way. The caveat is crappy efficiency. Basically, you use enough current forworst-case torque requirement plus some safety margin! And that margin needs to be quite a lot because exact mechanical load is rarely known in advance. (You could calculate it if you had, say, a simple flywheel with known shape and mass, and nothing else.)Instead, you should be using just the right amount of current to do the job. This is achieved by sensing the rotor position and applying the magnetic field so that it's either 90 or -90 degrees off, for the driving force in either direction. In reality, the thinking is reversed such that you first choose current (torque) and then simply force the 90-degree field. If current is too low for the mechanical load, then the motor just naturally loses speed, and vice versa (speeds up with too much current for the load), but efficiency is always at optimum. You can then use a secondary, slower feedback loop to adjust said current setpoint, to maintain speed.As you can see, this won't work with paralleled motors. The stepper-like crap-efficiency open-loop mode would work, but there is no point using it.