# The limits of torque production for a given weight of motor

#### Miles

##### 100 TW
Ref: https://endless-sphere.com/forums/viewtopic.php?p=1165115#p1165115 and preceding ramble....
Miles said:
If we accept the premise that, for a given weight of active material, we can alter the torque:velocity ratio yet maintain the same power output, by manipulating the form factor. What are the obstacles to practical implementations? What are the present limits?

I don't really get it... Is it constrained to a particular type(s) of motor?

Yes, there is a constant upper limit to the EMF X-amount of winding and iron can produce (iron saturates, or winding over-heats). Increasing radius from the axle simply increases torque.

The potential trade-off is the corresponding reduction in rotational velocity for a given commutation frequency. If the ERPM is fixed (due to core losses - hence question on motor type), then yes, power should be constant?

Punx0r said:
I don't really get it... Is it constrained to a particular type(s) of motor?
I suggest PMSM only, for now.

Air-core?

Are there not "models" ( formulae) that can predict the critical factors ?

The commercial motor design software has automatic scaling of motor designs. I'm not sure how useful that would be. Once you start altering pole count etc.....

Miles said:
Punx0r said:
Air-core?
Why? I thought we need to maximise torque to weight?

You're quite right. I was straying from the thread subject by thinking of the core losses of a large diameter, high-pole count motor.

Back on topic, I think if torque (perhaps considering stall/holding torque simplifies things) is the only goal then increasing diameter is likely the answer. Light-weight engineered structures tend to be large: Structural aluminium tubes are thin walled and large diameter. Trusses are similarly bulky. Human-powered aeroplanes and helicopters use massive wings and rotors. There's a general theme that going larger isn't inherently heavier, but the structure requires much more careful design as it becomes more open space, more flimsy and the strength more anisotropic.

Carbon fibre might be an obvious improvement for motor packing. The fly in the ointment might be the backing iron: If it needs to be a particular thickness then it's weight will grow disproportionately with motor diameter. Besides a Halbach array, can anything be done with the motor design to reduce the back iron requirement?

subed

Punx0r said:
The fly in the ointment might be the backing iron: If it needs to be a particular thickness then it's weight will grow disproportionately with motor diameter. Besides a Halbach array, can anything be done with the motor design to reduce the back iron requirement?
Let's assume that we increase the pole count to maintain the same fundamental frequency. Obviously, we can't increase the fundamental frequency because that wouldn't be a fair comparison.

Punx0r said:
[Structural aluminium tubes are thin walled and large diameter.
That's a different requirement, though. What happens with primarily radial forces?

I guess there's another assumption here. That we're dealing with a radial flux motor.......

Miles said:
Punx0r said:
The fly in the ointment might be the backing iron: If it needs to be a particular thickness then it's weight will grow disproportionately with motor diameter. Besides a Halbach array, can anything be done with the motor design to reduce the back iron requirement?
Let's assume that we increase the pole count to maintain the same fundamental frequency. Obviously, we can't increase the fundamental frequency because that wouldn't be a fair comparison.

In the tesla example we were looking for a equally high torque solution with less or no reduction gearing losses and better if possible top end. We know that the larger gap diameter will produce more torque and be better at running direct to the wheel. I would think we need to look at torque and losses at both the bottom and top of the rpm-speed range to compare how the size options would compare in real world performance. Some insisting that larger diameter would be heavier than its geared down counterpart. Not understanding why we would not look at different frequencies in either or both designs if they would accomplish significant performance advantages possibly on one more than the other.

I thought it would be good to analyse a reasonably simple case - that we can swap speed for torque and keep motor weight constant. Is it not relevant as a first step? Any other discussion can carry on on the original thread......

speedmd said:
Not understanding why we would not look at different frequencies in either or both designs if they would accomplish significant performance advantages possibly on one more than the other.
If there really was an advantage for either case, we could. The most important thing is that they are both equally optimised for the task.

Simple is good. Optimized yes. How easy is it to quantify the torque - efficiency - power over the rpm ranges with some confidence. Mass seems like the easy part.

Miles said:
I guess there's another assumption here. That we're dealing with a radial flux motor.......

Good point. Just a hunch, but I suspect it will be easier to package a large radial motor than an axial one, as I imagine the main loading in a radial motor is hoop whereas the axial would primarily require lateral stiffness.

We could take a commercially available motor like the Joby JM1, that is pretty well optimized for power, as the reference and do some simulating to try and achieve the same power at lower nominal speeds, using the same weight of active material?

It will need to be a motor that you have access to all the relavent reference data .
Dimentional, performance , electrical, details of the winding plan, weight of individual components, ( active, non active) etc etc ?

Yes, I've got (or can calculate) all that for the JM1. Toolman2 has dyno tested it, too.

Any other candidates?

The joby is already a enlarged gap diameter version of a solid built style motor and already somewhat optimized from a power to weight stand point. Something like a astro may be more of a fair comparison to the car motor-gearbox setup.

Who said anything about a car gearbox set up?

I'm happy to use any motor as a reference though. No problem using the Astro.

We can run the Astro 3220 at 10,000 rpm.

It can make about 5Nm of torque continuously.

If we assume a reference wheel speed of 500rpm that means a reduction of 20:1

So, we're looking at a hub motor of 2 kg, plus whatever the weight of reduction gear for the Astro is, capable of putting out about 100 Nm continuously.

Is that fair?

What leads you guys to believe that the smaller diameter motor won't always have an advantage? I ask because it's the non-productive weight in the stator and rotor supports that will kill the the large diameter motors in a direct comparison based on weight.

I'm not believing anything. I'm an agnostic.

Who said anything about a car gearbox set up?
I did. Got me. Interested in what motor configurations may limit the torque band range. We know the gearbox is used to increase the torque of a given size /weight motor. We were effectively comparing the size / weight /gearbox options for maximizing the power band range of the various options. I will hold my horses for now.

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