2 hall sensor and 3 phase BLDC motor

[boydli]

Hi all,I get a brushless permanent magnet motor used on autobike, with 2 hall sensor, but 3 phase of delta connection.
its back EMF is sinusoidal well. And I know most of BLDC motor use 3 hall sensors to detect the rotor position and sent the signal into controller. also its back EMF is trapezoidal. But this motor is much different, could anyone give a answer about it?
Thanks

 

[TurbineGen]

I've seen many 3 phase BLDC motors with 2 hall effect sensors. All it is doing is sensing the rotor position. You can control a sensored motor with a sensorless controller. Sinusiodal controllers are smoother and will have reduced cogging effects, however trapezoidal seems to be the standard for applications in vehicles such as yours. The motor back EMF will still be sinusioidal. I haven't seen any that were trapezoidal.

What is so different about the motor that has you concerned? Also, if not already in place, when using longer battery leads , it is a good idea to place some extra capacitors at the input terminals of the controller. Varying speeds with a trapeziodal controller tends to be hard on the controller perhaps due to the inductive effect of the longer wires.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[boydli]

Thanks for your suggestion. Actually I'm not sure that when I change the 2 sensors to 3 sensors and use a trapezoidal controller to drive the motor, the result would be what? its out power increase some,torque ripple increased,etc? sensorless controller may could drive the motor also. But,and you mean that these 2 sensors are used ONLY to detect the rotor positon for the starting time, after motor speed is high enough then switched to sensorless controller mode?

 

[benta]

Wow, this must really be a cost-optimsed design.
Yes, you can get away with using only two sensors, however starting the motor is a problem, as you don't know exactly where the rotor is.
I would expect the controller for this motor to apply a rotating three-phase field to get the rotor turning, and as soon as this is established then use the signals from the two sensors to synthesise the third phase.
If this is some kind of constant-speed application it would be OK.

Benta.

 

[blacksea]

Guys,
this isn't cost-optimized design - I hope this is 2 sine analog Halls instead 3 digital Halls! Some sine servo amp may use them for sinusoidal commutation of PMBL motors.