Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

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ferret
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Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by ferret » Dec 27, 2017 7:01 pm

Hi.

Rising a vehicle's performance can be achieved using several different methods:
Increasing pack voltage
Changing gearing or wheel size
Field weakening
Advanced timing, static
Advanced timing, dynamic (only at high RPM)

How do these methods compare to each other in terms of efficiency, torque and power?
What are the pros and cons of each?

Avner.
Last edited by ferret on Dec 28, 2017 10:41 am, edited 2 times in total.
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Re: Higher top speed question for motor gurus

Post by liveforphysics » Dec 27, 2017 7:08 pm

Field weakening only has any penalty in efficiency while you're at the speed range that uses it.
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Re: Higher top speed question for motor gurus

Post by motomech » Dec 27, 2017 8:04 pm

Start with the highest pack Voltage that fits into your "comfort zone", then select the motor speed that will give your desired top speed.
Example: My Mean Well power supplies that I use for charging have a top charge Voltage of approx. 58 Volts. Since I am not a novice nor an expert, I select 54 Volts, which allows me to use "off the shelf wiring connectors and adapters, fogo pre-spark precautions and won't blow my fingers off if I make a wiring mistake.
The sm.mini-motor I intend to use has 3) avail. speeds;
Low-speed (201)
Mid-speed (260)
High-speed (328)
W/ 26 inch wheels, the high-speed motor is not practical for my usage, so I have two choices;
Low-speed gives a top speed of 22 to 23 MPH.
Mid-speed motor gives top speed of 24 to 25 MPH.
This an "assist" bike and has gearing 11T X 48T, which allows me to pedal into the low 20's MPH range, so I select the low-speed motor that will go as fast as I want to go, with the benefit of climbing a bit better.
The higher Volts/lower speed motor system will always be more efficient than the lower Volts/higher motor speed system(for a given speed).
Other considerations when choosing a system Voltage are;
Motor-What is a "safe" level of sustained Wattage that the motor can handle? The Amps/Volts ratio can be adjusted to limit total Watts, but too high Volts/ too low Amps can "power(Current) limit" the motor reducing top speed. A optimized system will have the motor "speed" (RPM) limited at, or slightly past the top speed.
Controller-Obviously one does not want to exceed the high Volts rating of the controller. Also, if the controller's LVC (Low Voltage Cut-off) is desired, raising the Voltage will lower the battery"s cell Voltage value at cut-off.
The short answer to the original question-I.M.O., raise the pack Voltage.
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Re: Higher top speed question for motor gurus

Post by fechter » Dec 27, 2017 8:13 pm

Dynamic timing advance is the same as field weakening.
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Re: Higher top speed question for motor gurus

Post by Papa » Dec 27, 2017 9:54 pm

ferret wrote:
Dec 27, 2017 7:01 pm
Rising a vehicle's top speed can be achieved using several different methods:
If you're talking speeds in excess of 18-20 mph, then improving aerodynamics should be near the top of your "to-do" list.
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Re: Higher top speed question for motor gurus

Post by Triketech » Dec 27, 2017 10:04 pm

Papa wrote:
Dec 27, 2017 9:54 pm
ferret wrote:
Dec 27, 2017 7:01 pm
Rising a vehicle's top speed can be achieved using several different methods:
If you're talking speeds in excess of 18-20 mph, then improving aerodynamics should be near the top of your "to-do" list.
Absolutely. Why throw watts into the wind?

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Re: Higher top speed question for motor gurus

Post by ferret » Dec 28, 2017 7:42 am

Thanks for all the responses.

The reason I posted this question was to gain deeper understanding of the various methods and the differences between them, not to reach a specific top speed.
liveforphysics wrote:
Dec 27, 2017 7:08 pm
Field weakening only has any penalty in efficiency while you're at the speed range that uses it.
Of-course, but when it is used, how does its efficiency compare to the other methods? For example, suppose a vehicle reaches a certain speed at full throttle. The same speed could be reached using field weakening and %90 throttle. Which way would be more efficient?
fechter wrote:
Dec 27, 2017 8:13 pm
Dynamic timing advance is the same as field weakening.
Good info! :) I suspected that they were different names for the same mechanism, but I was only guessing.

Thanks again,
Avner.
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Re: Higher top speed question for motor gurus

Post by dogman dan » Dec 28, 2017 9:46 am

if its a proper experiment, then the bike weighs about the same, weather is about the same, grade the same, and aero drag is same, etc.

20 mph is 20 mph, and it will take around 400w on flat ground, and ordinary bike. Oh sure, one setup might pull 395, another 410, but the areo drag and weight insure that the laws of physics say 20 mph will take about 400w. Differences in efficiency that small mean little. you can save 10w by pedaling half a block, or less. A 10w difference over an hours ride is 10 watt hours. 10 wh is a puny amount of battery. I call it no detectable difference, in my world.

Where motor wind choices really effect efficiency, is when you are not cruising 20 mph. If what you do is stop constantly, or on a twisty single track, constantly brake for corners and then grab full throttle, then a high rpm motor in a large wheel will punish your efficiency up to 20%.

Speed will do about the same thing. you will be greatly punished in efficiency for even one mph more speed. Its literally unbelievable how much farther you go at 18 mph, vs 20 mph.

But lets say you have the slower wind, and you are cruising 23 mph. If you have the fast wind and still go only 23 mph, efficiency will be very very very close to the same. You will of course, be at a lower throttle setting on the fast motor, but you will still be pulling what the laws of physics say 23 mph takes, on that road, in that weather, at that weight.

Choose the fast motor of course, if you want to go 27 mph. But if you plan to ride off road a lot. lots of short but very steep hills, or ride in the city with tons of stop signs, then go for the medium motor.

The slow motor, is only for an extreme case, such as an adult trike that gets stupidly dangerous to ride fast, but you still want full wattage to get up hills heavily loaded. Still have 1000w, but need to go very slow.

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Re: Higher top speed question for motor gurus

Post by ferret » Dec 28, 2017 10:46 am

Dan,
thanks for taking the time to write the detailed response.

I guess I didn't title the topic correctly, because I'm not getting the type info I'm looking for.
Something like a motor simulator that allows you the mess with the motor's timing.
I'll try to steer the discussion to motor parameters' effects on performance rather than the topic of top speed by renaming the thread to

"Discussion: Motor voltage, timing and gearing's effects on performance and efficiency"

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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by liveforphysics » Dec 29, 2017 3:49 am

It depends on the motor and the % of base speed you're looking to multiply. A Nissan leaf motor goes to 250% base RPM speed for just a couple percent efficiency hit. A Zero motor also only takes a couple percent hit in exchange for a large jump over normal Vpack/BEMF limited base speed.

This is partly because they have IPM rotors leveraging the rotor irons reluctance, and partly because well designed motors often handle 'field-weakening' really well IPM or surface PM types alike.

I can't speak for how a given hubmotor may be impacted by it, but every automotive production EV uses it for good reasons (but also generally design the motor around it).

Most motors I've played with loved a ton of field weakening and it gave them top end that feels incredible, but I'm also only working with excellent magnetic designs, and many ebike motors I've seen look like someone just sketched a stator tooth out VS FMEA optimizing.
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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by ferret » Dec 29, 2017 9:22 am

Thanks,

my applications are RC inrunner motors, my Astroflight 3215 (sensored) powered bike and actual RC race cars.
On the bike I can either use the Phaserunner's field weakening or increase cell count for more top end power and in the RC cars advance timing, change gearing use a lower wind motor.
I am trying to understand the pros and cons of these approaches.

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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by Toorbough ULL-Zeveigh » Dec 29, 2017 6:28 pm

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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by liveforphysics » Dec 29, 2017 7:04 pm

First run whatever max voltage the controller supports, as not doing that doesn't make best use of the parts, and you eat the same resistance of the FETs.
Then max out field weakening settings, run it and see what happens. I've personally not yet been impressed by the Astros, but they might work amazing with field weakening, or might get hot fast and not like it. You won't guess it's behavior in advance of just testing it.

For every RC car and ebike and EV I personally would max field weakening settings (this is also what OEMs all do).


ferret wrote:
Dec 29, 2017 9:22 am
Thanks,

my applications are RC inrunner motors, my Astroflight 3215 (sensored) powered bike and actual RC race cars.
On the bike I can either use the Phaserunner's field weakening or increase cell count for more top end power and in the RC cars advance timing, change gearing use a lower wind motor.
I am trying to understand the pros and cons of these approaches.

Avner.
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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by ferret » Dec 30, 2017 4:29 am


Thanks for pointing out this thread, I will check it out.
liveforphysics wrote:
Dec 29, 2017 7:04 pm
For every RC car and ebike and EV I personally would max field weakening settings (this is also what OEMs all do).
I would have guessed that at some point too much timing/field weakening will reduce efficiency and even performance. Does the controllers' designs prevent it by limiting the max field weakening settings?

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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by liveforphysics » Dec 31, 2017 2:29 am

ferret wrote:
Dec 30, 2017 4:29 am

Thanks for pointing out this thread, I will check it out.
liveforphysics wrote:
Dec 29, 2017 7:04 pm
For every RC car and ebike and EV I personally would max field weakening settings (this is also what OEMs all do).
I would have guessed that at some point too much timing/field weakening will reduce efficiency and even performance. Does the controllers' designs prevent it by limiting the max field weakening settings?

Avner.

It only reaches the penalty of the % at that speed it achieves. So if your max field weakened no-load speed for your motor is 80mph or whatever, but you only have the power to achieve 50mph, you will never be penalized for what it would be doing at 80mph, because it only advances timing as much as it needs to keep it making torque at whatever speed it's able to achieve. Essentially, if you max it out, it just means if you had a really long steep downhill or whatever that you might still get to be applying some motor thrust, but it's no penalty while climbing hills or moving at lower speeds etc.
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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by ferret » Jan 01, 2018 6:23 am

liveforphysics wrote:
Dec 31, 2017 2:29 am
It only reaches the penalty of the % at that speed it achieves. So if your max field weakened no-load speed for your motor is 80mph or whatever, but you only have the power to achieve 50mph, you will never be penalized for what it would be doing at 80mph, because it only advances timing as much as it needs to keep it making torque at whatever speed it's able to achieve.
This example can also be viewed differently: for example, if at 80 mph with field weakening the efficiency drops to 10% of the efficiency at 50 mph, than the "penalty" for using field weakening becomes too big and I would look for other methods to reach 80 mph.

So while it can be said that I will "never be penalized" for going 80 mph with field weakening, it is also possible to say that between 50 and 80 mph I will always be penalized (compared with a setup built for 80 mph without field weakening).

Would this view also be true?

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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by liveforphysics » Jan 02, 2018 2:10 am

The efficiency penalty would almost certainly be smaller than running a larger diameter wheel, and static/dynamic timing advanced is just field weakening effects done more crudely.

Really you need to just try that individual motor and see how to does. It might double its useful RPM range only changing from 91% efficiency to 90% efficiency while it's at double speed. It might drop from 90% the 80%, but I've never seen any amount of 'field weakening' achieve a double digit efficiency penalty on any motors I've tested to date.
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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by dogman dan » Jan 02, 2018 9:45 am

Sorry about oversimplifying, but some members really need that.

Luke has it dead on as usual, whatever your motor and controller can stand for voltage, begin by increasing volts to that level.

oversimplifying again, watching the motor temp with a thermometer will instantly tell you when you went too far, and now have designed a space heater that rolls. If so, lower amps, but keep the volts up.

And oversimplifying some more, if you need more big gains, make the motor bigger. so it can handle more power without overheating.

RE gearing, with hub motors reducing the gearing ( smaller wheel) helps a lot more than you might think. A smaller wheel can still go very fast. I would expect that this also applies to non hub as well. Too tall a gear to get 1 mph more top speed might not be worth its effect on acceleration.

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Re: Discussion: Motor voltage, timing and gearing's effects on performance and efficiency

Post by ferret » Jan 02, 2018 12:40 pm

liveforphysics wrote:
Jan 02, 2018 2:10 am
Really you need to just try that individual motor and see how to does. It might double its useful RPM range only changing from 91% efficiency to 90% efficiency while it's at double speed. It might drop from 90% the 80%, but I've never seen any amount of 'field weakening' achieve a double digit efficiency penalty on any motors I've tested to date.
dogman dan wrote:
Jan 02, 2018 9:45 am
Sorry about oversimplifying, but some members really need that.

oversimplifying again, watching the motor temp with a thermometer will instantly tell you when you went too far, and now have designed a space heater that rolls. If so, lower amps, but keep the volts up.
LFP & DD,
thanks for answering.
While I appreciate the fact that some of the readers need "simplifying", I started this thread for members (like me :oops: ) that have have a decent amount of knowledge but seek to deepen it, which can't be achieved by oversimplifying.

Please read this post by Justin, his experiments seems to suggest a substantial efficiency hit from a lot of field weakening, but perhaps it was caused by the design of the motor he used. I could't find any figures for Astroflight or other inrunner motors.

https://endless-sphere.com/forums/viewt ... 25#p984725
justin_le wrote:
Dec 18, 2014 9:50 pm
bspalteh wrote:What happens if you increase the field weakening value beyond that 17%, say 30% or whatever?
We decided to see what would happen if you had a slow winding motor and then used field weakening to increase the top-end speed, since it's a scenario that we've seen quite often where people are mistakenly lead to purchase a slow motor wind under the generally false impression that it has extra torque, only to find that the performance at cruising speeds is quite underwhelming. The first thing we wanted to do was confirm the relationship between the field weakening current and the unloaded RPM.

RPM vs Field Weakening.jpg
Image

In all these tests, we had the defined max motor current at 60A, so a 50% field weakening current is 30A and so forth. The unloaded motor starts off at about 310rpm with no field weakening. By the time we hit 20A of field weakening current, the RPM has increased to about 380 rpm, while the no load current increased from 1A to just under 3A. We don't get a 50% increase in the top speed until the field weakening current is at a full 40 amps, at which point the no load current just to spin the hub is about 7.5 Amps. To actually double the RPM of the motor in this setup required the full 60A of field weakening. The resulting 17A of no-load current shows that it's pretty wasteful, and you could just watch the motor temperature start to skyrocket on the CA readout at this point.

So based on this, I'd say that the field weakening is viable to get upwards of 20-30% boosts to the unloaded speed on a slow DD hub if a higher voltage pack isn't an option, but more than this has rapidly diminishing returns.


As far as the efficiency goes, we then did a dyno test of the hub at both 0% and 50% (30A) field weakening currents, and in both cases with a 25A battery current limit:
Comparison with 50% Field Weakening.jpg
Image

With no field weakening, peak efficiency was around 88%. In the case of the setup with automatic field weakening, it was at 80% efficiency once the weakening kicked in, and then dropped steadily to the 71-72% range. While the power roll-off from back-emf voltage started at about 250rpm with no field weakening, it was able to pull the full 25A right until 340 rpm with the 30A weakening current. So here you are getting a ~33% boost to the top speed, but at 70-80% efficiency levels compared to the 80-88% efficiencies you'd have gotten with a higher voltage pack or a faster wind hub.

Here is a repeat of the same experiment but using a 15A rather than a 25A battery current limit, representative of running part throttle. Comparison with 50% Field Weakening at 15A limit.jpg
Image
There is one thing I'm not clear about: on the first graph at 700 RPM the figures are 21A of no load current and 65A of field weakening current. How much current is actually drawn from the battery? 21A or 21+65=86A?

I have installed a temp sensor in the motor and it's connected to a CA, but my (offroad) riding in characterized by short bursts of acceleration and high speed so monitoring motor heat is not a good way to learn about efficiency.

Thanks again,
Avner.
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