What is limiting my BLDC motor power output?

Question

I am using a BLDC 3-phase hub motor and recently installed a motor controller rated for 3000 W (I originally had a 1200 W controller installed).

The motor is rated for 500 W nominal and 800 W peak, but is currently probably producing approx 1000 W power at the wheel (with the 3000 W controller installed).

I was wondering what design aspects of my BLDC motor would be the limiting factors (in terms of max. useful output power). I think it's one of the following things:

• Stator magnetic core is saturated (I need a wider or bigger diameter stator) **
• Winding diameter too small
• Permanent magnets need to be bigger (not sure if this would do much) ***
• Motor needs better cooling *

*(probably not a big factor since motor performance when motor is cold is not far off from the power output (useful) I got with the 1200 W controller)

**What design changes (for example, more layers, other shape of magnetic core) would positively influence the magnetic saturation point (make it so that this point is reached later/at a higher amount of magnetic flux produced?

***The permanent magnets (36 pieces if I counted correctly) are approximately 3 mm thick, 40 mm in height.

I'm running the motor at 72 V (84 V when the battery is fully charged); it originally was intended for 36 V/42 V.

The power output/torque produced by the motor did increase after switching to the 3 kW controller but not by a lot. It appears most of the extra power going to the motor are going into heating up the motor.

The motor gets significantly hotter (gets hot in shorter amount of time) than with the previous controller.

The motor is running with Hall sensors connected to the controller (motor controller supports Hall sensors).

Wires from controller to hub motor (3-phase wires) are 16 mm² each so probably not a limiting factor.

The performance appears a tiny bit better when the motor is cold, but the output is still quite comparable to the 1200 W controller.

Does the number of windings have any influence on the efficiency of the motor? I'd think longer overall path length of windings within motor would result in more voltage loss thus less efficiency.

The winding wire diameter is approx 1.5-2 mm. I'm not sure how many layers of windings there are unfortunately (have not rewound this motor yet).

Stator's magnetic core outer diameter is approx 25 cm, stator's magnetic core inner diameter is approx 15 cm. See image:

Note: the currently used wires are much thicker than the ones in the photos, it's an old photo.

What changes could I theoretically make to my motor to get more power output at the wheel?

Answer 1

Your correct observation that the stator core is saturating is indeed the main limitation. In a motor design such as this, there is a trade of between the volume of copper vs iron in the stator tooth space. If you make the tooth thicker, it can support more torque, but then there is less space for copper, so it is less efficient and vice versa. The motor will have been optimised for a specific peak torque, and attempting to get more will saturate the iron. Note, that even when saturated, you can still drive higher flux densities through the iron, and that is why you are able to get more torque with the bigger controller, but as you note, the efficiency in the generation of this extra flux is extremely poor.

If you do not want to change the stator design, then the only way to improve the torque output of the existing design is to reduce saturation from the armature reaction by increasing the air gap. You can do this by simply moving the magnet radius outwards, but this will also reduce the stator flux generated by the permanent magnets (which will further reduce efficiency) so it would be better to combine this with an increase in magnetic material thickness. However, you must be careful that you do not saturate the core by adding too much magnet material and to small an air gap. In some cases, it might be beneficial to use a lower performance magnetic material, as if the air gap becomes too large you will have much higher flux leakage effects between the stator poles.

So to summarise, you need to add more magnet material/airgap without changing the stator flux density, and then you should be able to get more torque from the existing stator.

Answer 2

There is essentially nothing you can do to get more torque out of it. Torque controls current, which means trying to get more torque increases the current, which has two negative effects. (1) More I^2*R losses = more heat; this you are observing. (2) instead of increasing magnetic flux to produce more torque, you simply saturate the iron, which reduces the inductance allowing even more current and more I^2*R losses (see 1).

You may get some small improvement - maybe 10 or 20% torque increase - before you run into these limitations - but not much.

The cure for these is more copper (thicker wire, lower R, reduced I^2*R) and more iron to allow more flux without increasing flux density. Both of these mean starting again with a bigger motor.

Get a 100 Amp shunt and ammeter so you can monitor the battery current to establish what it does with the original battery, and where the limits are with the new one.

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However there is no such fundamental limit on speed. (Ultimately there are - insulation breakdown in the windings, or disintegration due to centrifugal force, or bearings limited to 20000rpm, or some such - but these are material or bearing choices not fundamental to the mass of copper and iron)

So, run the motor at a higher voltage to get a higher speed.

The danger here is that you need a higher torque to reach that higher speed, taking you back to (1),(2). As you noticed ... not much improvement, much more heat.

So, run the motor at a higher voltage to get a higher speed - AND gear the motor down so that the torque load doesn't increase. (Test this with the above ammeter)

Doubling the voltage but gearing down 2:1 should double the speed while keeping the torque constant : this doubles the power.

(Of course, if the motor is built into the hub, gearing down may not be so easy. It would probably be an epicyclic gear system, a bit like a Sturmey-Archer.

Answer 3

All of the points mentioned will limit getting more power from the motor than it is designed for. Motors that are well designed are usually close to the design limits in all of the aspects mentioned.

The most likely avenue for getting more power from a given stator and rotor would be to operate the motor at a higher voltage and a higher speed, but it appears that you have already done that.