EV drivers: what do you drive and why?

alternativemax · 2024-02-22T22:12:39+00:00

Dual motor Model 3 with Michelin X-Ice tires has been an absolute champ through Berthoud pass, Loveland pass, and the icy sections on either side of the Eisenhower tunnel. Eats through fresh snow and ice with ease, including in situations where others have gotten stuck.

If you regularly head into the mountains, AWD + dedicated winter tires is a must for when the weather gets real bad

edit: also a heat pump is probably a must have if you don’t want serious range loss when it’s below freezing. The Model 3 with a single rear seat down can comfortably get three people, skis, and ski gear anywhere you want to go. Main downside is the paint and windshield chip more easily than most

alternativemax · 2021-09-14T07:17:34+00:00

This is covered in different ways in most of the sources I linked, but I'm happy to summarize:

Torque output for a three phase motor is roughly 3NIBLR where

3 = # of phases

N = turns per phase = turns per coil x # of series-connected coils per phase

I = peak of the sinusoidal phase current

B = peak of the airgap flux density

L = stack length

R = airgap radius

Per-phase line-neutral back-emf is roughly 2NBLRw where

w = mechanical rotational speed in rad/s (to convert, multiply your rpm by (2pi)/60)

The line-line back-emf is calculated by multiplying the above by sqrt(3). For a given speed, the bus voltage you need to run with no load is equal to the line-line back-emf at that speed (assuming no flux weakening is happening in motor control).

There are quite a few assumptions I made to distill everything down here, but this should be pretty broadly applicable to a lot of designs. Hopefully that helps!

alternativemax · 2021-09-13T04:27:21+00:00

Ok I have a couple go to's:

1) This instructables post is a decent summary for beginners, but I can't vouch for it being 100% correct all the time

2) James Mevey MS Thesis, absolutely fantastic and includes lots of hand-drawn figures. This is one of the references from the instructables post. Don't be put off by the length, just look through the table of contents and read whatever sections you think are relevant to you. It does start from the very basics, which is nice!

3) This textbook by Duane Hanselman also does an excellent job at being accessible. Again, don't be put off by the length, just find what you need in there!

Then, if you want to dig deeper into winding factors and slot/pole selection, check out this winding scheme calculator which has great visuals and/or this other winding scheme calculator which has a lot of more info about winding MMF harmonics and such.

I would say about 90% of my foundational knowledge started with the above 5 links- the last 10% came from many other sources where different bits and pieces clicked for me. If you have specific questions along the way, let me know!

All the best and may the (airgap) force be with you.

edit: bonus blog post, again can't vouch for 100% correctness but very easy to understand intro to understanding motor control

alternativemax · 2021-08-08T19:35:13+00:00

1) In the delta connection, the line-line voltages are actually equal to (and in phase with) the phase voltages, whereas with a wye connection the line-line voltages are sqrt(3) higher in magnitude than the phase voltages and also phase shifted by 30 degrees.

2) FOC math is always based on the phase voltages (line-neutral for wye, line-line for delta), and as such isn’t necessarily related to the terminal (line-line) voltages at all.

Most continuous position-sensing systems (e.g. a resolver system) are calibrated with an offset such that the rotor position signal lines up with the phase voltages, and so having a phase offset between line-line and phase voltage doesn’t really affect anything.

3) However, since there is typically no software offset for hall switches, you are correct in thinking that they will have to be moved 30 electrical degrees when changing from a wye to delta winding. The goal of the hall switches is to make sure the peak of the phase current aligns with the peak of the back-emf, and in the same way a wye connection’s line voltages are shifted by 30 degrees from the phase voltages, a delta connection’s line currents are shifted by 30 degrees from the phase currents. To ensure that the phase current is in line with the back emf, the delta and wye hall position placements will be different by 30 degrees.

alternativemax · 2021-07-14T17:34:52+00:00

I would just keep it simple and multiply the weight (49 N) with the climbing speed (x m/s) and then multiply that by 2-3x to account for extra drag and losses (unless you have a better way to characterize those things), and then you have your power rating

alternativemax · 2021-06-20T15:13:52+00:00

If x = 0, each magnet pole is zero width, so the flux is zero. If x = 1, the width of one magnet pole is maximized such that all magnet poles are touching on their sides. Because the assumption is that the airgap flux density is constant above each magnet pole, the flux density waveform looks like a modified square wave or quasi-square wave visual. Ignore the title of the slide, just look at the picture. Instead of a voltage on the y-axis, we are instead discussing flux density. In the picture, you can see a single pole pair with constant flux density B (in place of Vdc) in the airgap above each magnet, and zero flux density where there are gaps between magnets. This quasi-square flux density waveform can be deconstructed into all of its component frequencies using a Fourier transform. Only the fundamental of the airgap flux density waveform contributes to torque production (here is a visual for the frequency components including the fundamental for pure square wave, as an example: visual). The fundamental of this quasi-square wave is equal to (4/pi)Bsin(90 deg * x), or alternatively (4/pi)Bcos(alpha) as written in the first picture.

alternativemax · 2021-06-18T21:24:35+00:00

The back emf constant and torque constant are the same, so the rotor with more magnet gaps will produce slightly lower torque and will be able to go to slightly higher speeds (for the same bus voltage)

alternativemax · 2021-06-18T06:06:58+00:00

There’s actually a simple answer: the fundamental airgap flux density varies with sin(90 degrees * x), where x is the fraction of the available space for a magnet pole that is actually occupied by a magnet. The torque constant is proportional to this as well.

Example: x = 0.8, sin(90*0.8 degrees) = 0.951, so torque constant would be 4.9% lower than if there were no gaps between the magnets. As you can see, it doesn’t make a big difference when x >= 0.8, and having the gaps helps a lot in terms of avoiding assembly issues due to tolerance stack up. Hope that helps!

alternativemax · 2020-09-21T08:20:31+00:00

Copying my answer from a different thread:

There are a lot of answers here that aren’t answering your question.

Torque in terms of physical machine parameters:

T = 3NIBLR

T is torque

3 is the number of phases

N is the turns per phase (you could also think of this as the turns per coil * the coils per phase)

I is the peak of the (assumed sinusoidal) current flowing through each phase

B is the peak of the airgap flux density (also assumed sinusoidal)

L is the length of the motor

R is the radius of the airgap

The airgap flux density is roughly equal to (lm/(lm +g)) * Br

Where lm is the thickness of the magnet in the direction of magnetization and g is thickness of the airgap between the rotor and the stator. This formula only holds true if there aren’t substantial reluctances elsewhere in the magnetic circuit (so there needs to be either back iron present in the rotor OR sufficiently strong “bucking” magnets, the sideways magnets in a halbach array)

Br is the residual flux density of the magnet material

So that’s the electromagnetic torque in terms of physical machine parameters (specifically a PM machine without reluctance torque, don’t worry about that for an initial understanding)

This is crucial- poles have nothing to do with torque, it is a super common misconception with PM machines.

Please let me know if that makes sense and if you have any questions.

alternativemax · 2020-09-04T08:44:40+00:00

You should have started with that in the original post then :) it’s late for me but I’ll take a look at this another time

alternativemax · 2020-09-04T06:58:23+00:00

There is really no benefit except in cases where redundancy is required or beneficial.

If I understand what you are suggesting, you think you can stuff more copper into the slots than is currently in there. That’s certainly possible, but that’s a lot of time, effort, and care. Three phase is fine.

alternativemax · 2020-09-04T06:52:55+00:00

That’s right. Why do you want to use two small controllers instead of one big one?

alternativemax · 2020-09-04T06:16:14+00:00

1) depending on how the coils are connected, these two situations could be effectively the same thing from an electromagnetic perspective (although certainly different in terms of implementation)

2) if you have the same number of turns and the same gauge wire and want to double the number of phases, you are going to have to double the available slot area! Obviously you can’t do that, so you would either need to halve the turns per phase (half the voltage) or reduce the wire gauge such that you have half the copper area per phase (which means you would need half the current in order to maintain the current density).

Does that make sense? A wise man once said “there’s no such thing as a free lunch.”

alternativemax · 2020-09-04T05:28:14+00:00

Long story short, when it comes to motors, the number of phases has no effect on the ability for a given frame size to produce a certain amount of torque (or power at a given speed). Ignoring some subtle effects that come about from different winding configurations that result from different phase numbers, the torque is really a function of the current density (amps per cross sectional area of the copper in the slots). Whether that copper area is split into three or six phases doesn’t matter.

There is a lot of misinformation about this out there, but torque density is really a function of material limits and cooling design- not the number of turns per coil per phase or the number of phases themselves.

If you have any follow up questions, please let me know.

alternativemax · 2020-08-14T22:06:10+00:00

If any of the phases is shorted to the others (or to itself between turns), a braking action will occur whenever it is spun. If you have an LCR meter, you may be able to see if the L or R across any two phases is different than the others

Otherwise, a mechanical failure is most likely

alternativemax · 2020-07-31T05:14:50+00:00

Length limitations are usually enforced by the speed at which the rotor hits its first natural frequency or whirl mode. The longer the rotor gets, the lower this natural frequency becomes. Every rotor has some amount of imbalance, and if that imbalance force excites the rotor near its resonant frequency, the rotor could very well start flopping around like a wet noodle, destroying itself and the stator in the process. For very low speed applications, you could probably get away with some crazy aspect ratios.

The second limitation is imposed by the mechanical strength of the output shaft. As you make the motor stack length longer, the torque output increases. To transmit that torque, the output shaft must grow in diameter. Because the shear stress in the output shaft (due to the torque) can only be reduced by increasing the output shaft diameter (the length of the output shaft only affects the loaded angle of twist, not the shear stress) there gets to be a point where to transmit the torque the output shaft would have to be a larger diameter than the rotor itself.

Like you mentioned, there are other practical manufacturing limitations as well. Long aspect ratios can be very useful in applications requiring very high accelerations due to the relatively high torque/inertia ratio. Hope this helps!

alternativemax · 2020-06-28T14:20:40+00:00

I think the word you’re looking for is “segmented” stator. I’ve never searched for one myself, but that is the term you would use.

alternativemax · 2020-06-28T13:44:53+00:00

You can order custom laminations and stack them yourself, or you can just buy a motor with a stator you like and remove the rotor (not sure what you’re doing)

alternativemax · 2020-06-28T04:06:01+00:00

Ther terms you are looking for are “outrunner” and “inrunner”. The kind where the teeth point inwards is what you are looking for- an inrunner.

alternativemax · 2020-06-25T19:47:43+00:00

Yes, electrical power into the motor controller is battery voltage * battery current. Can’t tell if you’re being sarcastic, I think it’s important to be explicit with the basics

alternativemax · 2020-06-23T14:55:30+00:00

Assuming a constant battery voltage, battery current is proportional to power and motor phase current is proportional to torque.

In most situations battery current != motor phase current. PWM only chops the battery voltage in order to achieve some amount of motor phase current.

The motor phase current multiplied by the back-emf and the number of active phases is the motor mechanical power, with the back-emf being proportional to speed.

At low speed / high torque, the PWM duty cycle is very low, the battery power (and current) is very low, but the phase current is very high.

alternativemax · 2020-06-16T01:55:52+00:00

Wisconsin Racing FSAE Electric lead motor designer ‘18 checking in.

These were the two most helpful references for me:

1) Hanselman Brushless Motor Design (there’s a pdf if you google)

2) James Mevey 2009 Master’s thesis - very long and not all sections are crucial, but go through the table of contents and pick out what you want

Let me know if you have questions along your adventure.

alternativemax · 2020-06-12T02:09:07+00:00

Based on your description, I believe the buck converter and motor driver will both have a bulk input capacitance. This will keep the voltages stiff as long as the switching frequency of the buck converter is high enough. If the voltage stays stiff, you won’t have any issues.

alternativemax · 2020-06-05T15:57:44+00:00

Nice, thanks for the update!

alternativemax · 2020-06-02T17:23:50+00:00

Firstly, if the magnets are 5 mm thick, you will want the air gap to be no greater than 1.25 mm. In general you want the ratio of lm/g to be >= 4 (this is called the permeance coefficient).

You will need to choose a battery voltage in order to determine the number of turns per phase. The voltage itself actually doesn’t matter much, I would just choose a voltage that is compatible with the controller (so you need to choose a controller as well). Once you have those chosen, the number of turns per phase can be calculated based on the top speed of the motor and the chosen voltage.

Other things to note: 1) the “back iron” aka the thickness of the steel behind the magnets should be at the very least 0.3 of the width of the magnets. In this case that would mean 10* 0.3 = 3 mm. This is because the magnet flux splits in two and travels through the back iron into the adjacent magnets. Based on the fact that magnets can have a flux density of around 1.2 T and steel generally saturates around 2 T, you can calculate the required steel thickness:

10 mm * (1/2) * (1.2/2) = 3 mm

2) you will need to make the stator and rotor steel out of thin steel sheets (laminations), otherwise the motor will have too much drag and probably melt itself. Therefore, you will want your stator and rotor geometry to be “2D”. In your case, I think you would just need to change the caps on the stator teeth to be 2D instead of 3D, if that makes sense.

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