Brushless DC Motor Investigation

[Raag90]

Hey guys,

I'm new to this site, and am a university student studying Electrical and Electronics engineering. I just completed a design project controlling a BLDC motor (pretty low powered at 60V, and giving output rpm of about 6000rpm). Now I had an idea where instead of using a single BLDCM for an application, would it make sense to put two identical BLDCMs on the same shaft, in order to give double the output torque? I would operate the two using the same drive as I would use for one, but I'm still unsure of how to take into account a lot of my efficiency calculations. I have a feeling that this would just draw double the power (if not, then only provide half the output power to each motor).

Please help me out. I need some answers ASAP for an important project.

Cheers

 

[DragonPetter]

Hi Raag90, I'm also working on a BLDC motor projects but have no experience and have not gotten as far as you.

I am curious, when you say the same shaft, do you mean the motors are coupled to a shaft, or do you mean to have two 3-phase coil/magnet groups around a shared rotor?

Also, would you be willing to answer some questions I have about driving a BLDC motor?

 

[Raag90]

hey DragonPetter...

yeh I'll be glad to answer any questions that I can help out with...not an expert but I'll definitely tell you why we did certain things the way we did in our design...

and yes so basically on a single shaft, you would have two separate motors...I really don't think this idea is viable, as theyre not doing anything at all...I mean, for the same drive, theyre getting half the applied power, and therefore would be working at half the torque..I wanted to see whether anything like this or on the same lines had been considered...

 

[DragonPetter]

hey DragonPetter...

yeh I'll be glad to answer any questions that I can help out with...not an expert but I'll definitely tell you why we did certain things the way we did in our design...

and yes so basically on a single shaft, you would have two separate motors...I really don't think this idea is viable, as theyre not doing anything at all...I mean, for the same drive, theyre getting half the applied power, and therefore would be working at half the torque..I wanted to see whether anything like this or on the same lines had been considered...

I think it is viable. Like you said, half the torque for the same load as full torque on 1 motor, but when they are both at max torque, there will be twice the torque.

I think you may need a mechanical device, analogous to a free-wheeling diode, that allows both motors to run the shaft independently, even if one motor stops running. Kind of like in a tandem bike as an example, where it is using 2 pairs of legs to move the same gears, so I think this concept is similar.

See this: http://en.wikipedia.org/wiki/Free_wheel

 

[DragonPetter]

Also, you could probably use a motor controller that has feedback for both of the currents of each of the motors and close-loop control the torque in each motor and also control the RPM. This way, you wouldn't accidentally drive one motor more and then think its an increased load, when you're actually driving the other motor as a generator.

Actually, I can point you towards a project with 2 motors coupled together at the shaft that I did where I modeled a common shaft system in MATLAB, but they weren't BLDC. Do you have a messenger service? I'd rather not give the project information out here since it has my name all over it and I'd like to keep my identity private. I could also ask my BLDC questions more easily.

 

[cncmachinemat]

You could couple the stators of each motor together however they would have to be perfectly aligned with one another. The phasing is most important. If you are paralleling the motors together you will get half the torque per motor output. Coupling a bldc motor with another is quite tricky as opposed to just a normal dc motor. Is there any particular reason for the coupling or just a proof of concept idea?