Basic Motor Questions - Torque V. RPM

[modern_messiah]

Hi all,

Although I am currently in the process of building my own low power coreless axial flux motor from scratch as a test I am, like most people, already thinking of bigger and better things. But I have a few questions looming large that have me scratching my head. I have theories – but it’s always nice to hear others opinions before jumping in the deep end. The only thing holding me back thus far was making a complete ass of myself – time to bite the bullet and have faith in the anonymity of the internet. :wink:

So here goes!

1) If a hub motor was mounted directly to the wheel (so a 1:1 ratio between motor and wheel rotation), is the speed of the vehicle merely limited by the maximum rpm of the motor, given various other factors such as the torque required to maintain velocity, heat generated and power required? The way I see it, electric motors are (depending on design of course) capable of rpm’s much higher than any wheel would ever be required to spin.

2) Naturally the higher the rpm a motor can achieve, the less torque the motor is able to produce? Having large gaps between magnets on the rotor or between teeth on the stator will result in lower maximum torque, but should result in higher achievable rpm?

3) Further to point 2, if a designer was after higher rpm’s they would have the rotor moving further with each phase shift – so larger distances between magnets (less magnets if you will). If you want more torque, the rotor must move shorter distances between phase changes so there would be shorter distances between magnets (more magnets). The same effect could be achieved by reducing/increasing the number of stator teeth instead.

4) Is having more coils on the stator a detriment to the design in any way other than increased heat? I’m assuming more coils will consume more power in order to produce more magnet fields. Why not build a motor with for example 30 teeth and only 4 magnets? Obviously the more magnet pairs you introduced the more torque the motor will produce but the slower it will be able to spin.

Pretty much every question above will result in palm-to-face like reactions, and even though I believe most them are fairly self explanatory when you think about the way a BLDC motor works, but I’m the kind of guy that always second guessed himself on university exams.

So yeah, any insight and clarification would be very cool.

Regards,

Matt.

 

[Jeremy Harris]

Here goes at some answers:

1) If a hub motor was mounted directly to the wheel (so a 1:1 ratio between motor and wheel rotation), is the speed of the vehicle merely limited by the maximum rpm of the motor, given various other factors such as the torque required to maintain velocity, heat generated and power required? The way I see it, electric motors are (depending on design of course) capable of rpm’s much higher than any wheel would ever be required to spin.

In essence, yes. Provided that the motor can take the current required to deliver the torque needed to maintain any given speed, then the motor speed is dependent on the motor voltage (for a permanent magnet motor with a fixed value for Kv).

2) Naturally the higher the rpm a motor can achieve, the less torque the motor is able to produce? Having large gaps between magnets on the rotor or between teeth on the stator will result in lower maximum torque, but should result in higher achievable rpm?

Not really true. PM motors have a torque characteristic that is a function of current, up to the point where the core reaches magnetic saturation. This means that the torque is related to current at any rpm, in practice. For example, if 50 amps gives 10Nm at 10 rpm the same 50A will give pretty much the same 10Nm at 5000rpm. There is a torque constant, Kt, that is directly mathematically related to the speed constant, Kv. In SI units, Kt = 9.5478 / Kv, where torque is in Nm and Kv is rpm/V.

Increasing the air gap will increase Kv and reduce Kt. It will also tend to reduce the maximum torque the motor can deliver.

3) Further to point 2, if a designer was after higher rpm’s they would have the rotor moving further with each phase shift – so larger distances between magnets (less magnets if you will). If you want more torque, the rotor must move shorter distances between phase changes so there would be shorter distances between magnets (more magnets). The same effect could be achieved by reducing/increasing the number of stator teeth instead.

Partly true. Reducing the number of pole pairs/stator slots will increase Kv, so the motor will spin faster. Kt will reduce, as a function of the increase in Kv. However, if you increase the current the motor would still be able to produce a similar ultimate torque, as that's a function of the magnet strength and the field strength of the stator windings, not the number of pole pairs or slots in the motor.

4) Is having more coils on the stator a detriment to the design in any way other than increased heat? I’m assuming more coils will consume more power in order to produce more magnet fields. Why not build a motor with for example 30 teeth and only 4 magnets? Obviously the more magnet pairs you introduced the more torque the motor will produce but the slower it will be able to spin.

More stator slots/poles doesn't have to mean more heat, as they can be wired in parallel to reduce total I²R losses. More poles/slots does mean a higher commutation frequency for any given motor rpm, and also means the motor will tend to have less torque ripple. There are good and bad configurations of the number of slots in the stator and the number of pole pairs in the rotor, some work better than others. Somewhere there is a table of all possible combinations showing which ones work well and which either don't work at all or are less then optimal.

Maximum torque is a function of the radius of the rotor (the moment arm, in effect) and the force that the magnets and windings can exert on each other. There is a finite limit to this force, set by the strength of the magnets, the current through the windings and the number of turns in each winding. There's also the ultimate limit of the saturation flux density of the stator core, but that should normally be greater than the flux density from the magnets, at least with present neodymium magnets. As the maximum force is pretty much fixed by the magnet strength and current that the winding will take without overheating, the most effective way to increase torque is to increase the motor diameter.

Jeremy

 

[modern_messiah]

Wow thanks for the excellent reply...I'm going to have to take some time to let this digest but more or less I think I have enough information to satisfy my mind .

Is the table you are referring to this one: http://fast-results.com/lnl/nutpol_e.htm ?

If so do you think it's posible to determine the effectiveness of certain combinations on paper, or is it likely that the above table was constructed from trial and error with physical motors? From the looks of it it appears that a good combination of teeth to poles is as close to unity as possible (obviously 1:1 is not possible with a 3-phase motor) - I blieve this was discussed in KingFish's Maths thread...maybe I should review that part.

 

[Jeremy Harris]

Yes, that's the table. Thanks for finding it, I'd seen it a while ago and not bookmarked it................

I'm not sure how that table was compiled, my guess it was mainly experimental data, but I have a feeling that it was connected to the guys that first came up with the LRK and dLRK winding patterns for brushless motors, back when the RC guys were first playing with them.

Jeremy