trilerian

iTrader: (51)

If you look at Fantom Racing's website under the Fantom Facts Machine 3s, they list inductance as a contributor to motor performance. Basically they say to look for a "spread", which is the difference in the inductance reading and the resistance reading. They say that if you have two motors with the same resistance reading, the higher inductance motor will be a better motor. Now I have been wracking my brain on this and have done a lot of reading about inductance and why a higher inductance could lead to a better motor.

So let's start with the basics. Inductance is the resistance to change in an electrical motor. The very EMF that is created by charging the coils creates an inductance that resists it. Now inductance isn't like resistance, it isn't wasted energy as heat. From what I have read, and I have no professor to go ask about it, the inductance of a motor is essentially what creates the back emf. Back emf is what causes a motor to lose torque as the rpm increases (and other things that I am not going to get into). An ideal motor would have no back emf, so I have been trying to rectify a higher inductance with a better motor.

They only thing that makes sense is that a higher inductance motor would be higher because the emf of the motor is higher. So in this instance saying the inductance is directly proportional to the torque constant of the motor. And this makes sense as the inductance of a motor goes down as the windings of the motor goes down. So a 13.5 motor should have less inductance than a 21.5, and a lower torque constant, which we know the last to be true.

Measuring inductance? I have an LCR meter, pretty cheap one, but it does seem to do its job. If you measure the inductance of the stator without the rotor you will get a much larger value than if you measure the inductance with the rotor. Also, if you measure with the rotor the value changes depending on which phase the rotor is aligned with. So if you align the rotor pole with the phase you are reading you will get the lowest reading. The strength of the rotor affects the reading as you can see that the poles when aligned may have a slightly different reading to each other.

Based on the above, it seems to follow that you would want to take the reading without a rotor, since the strength of the rotor affects the reading.

If what Fantom says is true and what I have reasoned is true, then as the inductance goes up so does the torque constant. Which given the same winding of the motor should mean that the torque is higher in a motor with higher inductance so long as the resistance is the same.

I guess the question is, am I way off?

Roelof

Look up some formula's about coils you will see that:
- the amount of windings determine the magnetism strength
- the current is determine the magnetic strength
- the amount of windings determine the inductance
- the amount of metal placed in the coil determine the inductance
- the impedance of the coil is frequency related

Magnetic field - Wikipedia

So yes, people try to gain performance to rewind the windings to get a shorter total length of wire to keep the resistance and so impedance low to get more current for more magnetism. ROAR has set stator resistance values for certain windings to avoid these modifications to motors.

trilerian

iTrader: (51)

But none of that states why a higher inductance would be better for a given resistance value. IE, let's say I have two different 21.5s with the Roar limit on resistance, 55.5 55.5 55.5 milliohms on the phases. One has inductance of 59.4 59 58.4 microhenries and the other has inductance of 49.7 48.5 48.5 microhenries across the phases. Both of these motors have been released in the last 5 years and both motors are very good motors for their time. Fantom says the motor with the higher inductance will be the better motor. I'm trying to reason out why that would be. However if I listed the models of each of these motors, the overwhelming response would be that the lower inductance motor would be the better motor.


That Wiki is some heavy reading...

Edit: So it does seem that I reasoned out that inductance is back emf or causes it.
Edit: What is the frequency? Is this the PWM frequency?

gigaplex

iTrader: (2)

Actually it kind of does. "the amount of windings determine the magnetism strength" plus "the amount of windings determine the inductance" gives "the amount of windings determine the inductance and magnetism strength". You want magnetism strength. You don't want resistance. So that stands to reason that reducing the resistance without reducing the magnetism strength/inductance is a good thing.

Back EMF is caused by a spinning magnet (rotor) inside a coil (stator).
Frequency is whatever is changing the input current. Both PWM frequency and motor RPM will be relevant frequencies here, since the stator phases are energising and de-energising on each rotation.

Strictly speaking it's not the inductance that makes the stator better. But some of the factors that make the stator better also tends to increase the inductance.

You mention two different models of motors though. Stator length is another factor in performance. Generally shorter stators perform better but their inductance is lower. When Fantom said that a higher inductance stator was better, they were probably talking about comparing two of the same model. The longer stator will likely have higher resistance, you didn't actually specify what their resistances were, you just said they were above the ROAR limit.

trilerian

iTrader: (51)

Yes, you have to infer from the reading that a higher inductance is good. There isn't anything that states it. And the motor models were a Trinity X-Factor 21.5 and a Trinity Slot Machine 21.5. Since they are both 21.5, use the same diameter wire and are bound by the same resistance rules in place by Roar, you can make the assumption that the wire is the same length which should lead to the same length stator. I'm not sure if the laminations in the stator affect inductance, probably, since I think that the core material of the stator does affect it. The resistance of the motors is actually under the Roar limit. 55.6 milliohms is the resistance limit for the 21.5. The X-Factor was labeled with 27.4 27.5 27.2 and the Slot Machine is labeled as 27.7 27.8 27.7. The X-Factor always reads lower on my milliohm meter than the Slot Machine.

When I first took the reading for inductance on the X-Factor vs the Slot Machine, I attributed it to the X-Factor being an RPM motor whereas the Slot Machine is a torque motor. This seemed to follow a couple of other motors I took readings on. Much More Fleta ZX V2 also had a high inductance, but it also had a higher resistance and the same for the Motiv MC4. But the MC4 doesn't seem to be either an RPM or Torque motor. Also to note, USGT motor had an inductance of 65 65 65 microhenries, which is higher than the others, but the wire is probably longer or smaller diameter since the resistance of mine is right about 62 milliohms per phase.

But regardless, what I am kind of seeing is that inductance acts like Newton's 3rd law in classical mechanics.

gigaplex

iTrader: (2)

At what temperature? That matters significantly. Also the ROAR limit is 54.6 milliohms for 21.5 between any two phases. I'm not sure how Trinity is labelling those, but those numbers look like they're almost exactly half of the limit.

trilerian

iTrader: (51)

They must have changed it! It used to be 55.6, at least that is what a google search pulled. Anyway, I'm pretty sure either Trinity is halving or measuring from the tab to the ring. And yes, I know temp matters. But if you are just looking for resistance of A compared to B, then you can set A to read whatever you want then take B reading with the settings from A. Also, I have an Instek GOM-804 that has temperature compensation.

HarryLeach

They're measured from the tab to the collector ring.
Tech is between phase tabs, but if you're "grading" stators, the individual phase resistances (measured tab to collector) being near the limit and very close to each other, they are graded higher.

As for the inductance question, higher inductance tends to generate a tick more torque on the dyno, and doesn't seem to impact kV too much. Net result is a motor that can pull more gear for the same amp draw and/or less heat.

With the way motor batches are these days, they're pretty small differences though, the "spread" was a bigger focus to find those really good performing motors several years ago. It's still checked when sorting and assembling a stock class motor, but the unicorns are fewer and further between to find those outlier higher impedance lower resistance cans.

Source: BamSpeed tuning and talking.

alpenglowquasar

That's awesome! It's always great to dive deep into a topic that you're passionate about. From what you've explained, it seems like a higher inductance motor could potentially lead to better performance, all else being equal. And it's cool that you have an LCR meter to measure the inductance of your motor. As for whether you're way off or not, it's hard to say without being an expert in the field, but it seems like you have a solid understanding of the basics.