BLDC Efficiency at different voltages

[Jeremy Harris]

Motor efficiency is governed by how low the losses in the motor are (resistive losses in the windings, core losses from hysteresis and eddy currents and frictional losses in the bearings) when compared to the output power that the motor is actually delivering.

It's impossible to say whether or not running at a higher voltage will improve or reduce motor efficiency, as it depends entirely on the actual motor and the operating conditions. In general terms, motors are at their most efficient around the point where eddy current and hysteresis losses equal winding resistance losses. Eddy current and hysteresis losses are proportional to rpm (which is proportional to voltage) whilst resistance losses are proportional to current (which is proportional to torque).

For example, if you had a motor that was running at point where the resistive losses in the windings were twice the eddy current and hysteresis losses then increasing the voltage (and rpm) and reducing the current (and torque) would probably deliver the same amount of power at a better efficiency figure.

In practice life isn't this simple, as motors have to run over a wide speed range so rarely get to run at the 'best' speed. What we tend to do (particularly with low speed hub motors) is run them in the area where core losses are reasonably low and put up with the fact that resistive losses are often the dominating factor. The opposite may apply to higher speed RC type motors, where core losses may be dominant.

Sorry this isn't a simple answer - I'm afraid that there are so many interdependent factors that it's not really possible to give a 'yes or no' type answer to this question.

Jeremy

 

[Desertprep]

Thanks! You r right...not a simple issue.

 

[sico]

Hi all,

I'm building up a 9x7 9C to run on lipo with a Lyen 12 FET controller. My in-frame battery box holds 6 6S packs (zippy 5ah 15c) and I'm looking to use the bike to travel 18km each way to work. I believe if I run the motor with batteries wired 2s3p (nom. 44.4V) I will do about 40kph, which is a good speed for my commute. My question is: If I set up the batteries as 3s2p (nom. 66.6V) and back off the throttle to run at the same speed (i.e. 40kph, but pick any speed really) will I use more or less Wh for the total journey?

Thanks in advance,

Simon.

 

[adrian_sm]

Technically higher volts means better efficiency. But in reality, your throttle hand will not hold back, and you'll use the extra power available. The only way I could get back to similar efficiencies I had at lower voltages, was to set my controller speed switch to a lower setting. Ohterwise I would just accelerate harder, and cruise faster.

- Adrian

 

[gtadmin]

... Ohterwise I would just accelerate harder, and cruise faster.

- Adrian

Lead under the finger nails syndrome on display

 

[sico]

Technically higher volts means better efficiency. But in reality, your throttle hand will not hold back, and you'll use the extra power available. The only way I could get back to similar efficiencies I had at lower voltages, was to set my controller speed switch to a lower setting. Ohterwise I would just accelerate harder, and cruise faster.

- Adrian

Ok, thanks for that Adrian. I read on the forums that higher voltage meant higher efficiency, but I wasn't sure if that included a throttled back high voltage system compared to lower voltage system at maximum rpm. I know what you mean with the temptation to use it though ... lead throttle syndrome indeed!

 

[John in CR]

Higher voltage at lower throttle will make the controller imitate a lower voltage through PWM. With the same motor and identical speed what the motor "feels" in terms of voltage and current will be virtually identical, so there will be only a tiny difference in losses in the controller. Full throttle acceleration up to that same cruising speed is going to be a bit less efficient, but stronger. You will have far greater passing power, so it will feel like a completely different bike using a 50% increase in voltage.

As others said, being heavy handed is virtually unavoidable, so I hope you're not at your pack range limit already. Electricity is dirt cheap for our ebikes, you will love the difference and use it, so plan on buying another 3 packs and have more fun with increased range and performance...a great combo. In the meantime, figure out a way to charge at work so you can have fun blowing an extra $.05/day in electricity. It's the cheapest amusement park ride of all time.

 

[sico]

Hey thanks John. I'm not sure what kind of range I'm going to get out of this thing, so I don't know if I'll have to do the charge at work option. I'd prefer not to. I can't fit any more batteries in the frame triangle, so more batteries is not an option without a complete redesign at this stage. I feel encouraged to try my luck with the 66V setup now and see how it goes. My main concern is going too fast in front of the law and attracting unwanted attention. Unfortunately my commute goes directly through the middle of town and alternative routes are 50% further or more.

This bike is meant to be a stealth version of my ICE zbox bike, which is a cop magnet.

Simon.

 

[John in CR]

Simon,

My heart feels for you guys having to go stealthy. Only the ICE's are regulated here, so as long as I don't have an ICE larger than 49cc, and I have pedals, then I am perfectly legal as a bicycle and only have to obey the traffic laws. I run 5-8kw without a care in the world and ride right by traffic cops regularly. It's high power ebike paradise.

 

[sico]

Here's a few build pics.

Hanging from the ceiling makes it easy on the back:
View attachment 6


9x7 9C motor with customised dingotookmybaby torque arms (still some shaping required):




6 lipo bricks fit nicely in there:


Aluminium covers will be black, I thought about some "urban camouflage" so that the frame looks see-through from a distance out of the corner of your eye:




Lyen 12 fet and some mess:



Double torque, arms of course:


The Peugot stink-pot that's being replaced:



Cheers,

Simon.

 

[dogman dan]

Because of the "bad right hand" syndrome I also suffer from, you will be more efficient at 12s (44v) than at 18s. But having the extra speed is considered life saving to some. Personally, I just stay out of situations that would need it.

The other consideration is how hard the fets work in the controller. At part throttle you have the fets working hard and making heat. Maybe not an issue at low wattages, but mabye a big issue at higher power. In any case, controllers keep thier cool when used at full throttle and the fets don't have to work as hard.

 

[sico]

Hi dogman, thanks for the reply. That brings me back to the original question: If the motor is seeing essentially the equivalent voltage amd current in either case (due to PWM) then its efficiency is not effected by the choice of source voltage. But if the controller is heating up due to extra switching of the fets (in the high voltage case) then that sounds like a source of inefficiency. If all other things are equal, will the lower voltage therefore be more efficient (in a steady state at 40kph)?

Simon

 

[wineboyrider]

With a cycle analyst you can dial in your amps and speed and keep the higher voltage pack under control. I prefer the higher voltages now even for longer trips. I just tame the motor and controller for the ride. I use a 9c motor too with my CA and it's an awesome tool to have, especially for longer trips. 8)

 

[dogman dan]

The lower voltage should be more efficient. But the difference between the two is not worth worrying about. Really, if the diff was even bigger, it still wouldn't amount to enough to care about. It's like wanting your penny when you spend $999.99 On other large EV's like cars this difference might add up, but on a bike, it's chump change.

Differences between two different kinds of motors now, that could make enough difference to care about. Some motors are a watthog, like a 5303.

What I've seen in the real world, in three years of riding at least 2000 miles a year, is that the difference in efficiency that matters is much more related to other factors. How fast are you riding, how steep are you riding combined with how much you weigh, and is the wind in your face. Those are the huge variables. You want more range per watthour? Simple, slow down by a mere 1 mph. Range at 15 mph is double the range at 25 mph. Even small decreases in speed yeild amazing increase in efficiency. 1 mph slower will get you more range than any fussing about the voltage.

Do go for the higher voltage if you need it, like to get up a hill, or whatever. Many really dig voltage above normal 36v. But 12s is a nice voltage too, keeps the bad hand in controll, and allows speeds that don't need big chainrings to keep pedaling along. If you like 20-25 mph, then the 12s voltage is perfect. Try em both, and just pick the one you like, and don't worry about efficiency unless you can't control that throttle hand.

Wineboys advice to use a CA to limit the controller is good, gives you the ability to make changes while out riding if you want. So you can get some efficiency by using a lower amperage than usual when you need it.

 

[jag]

Hi all,

I'm building up a 9x7 9C to run on lipo with a Lyen 12 FET controller. My in-frame battery box holds 6 6S packs (zippy 5ah 15c) and I'm looking to use the bike to travel 18km each way to work. I believe if I run the motor with batteries wired 2s3p (nom. 44.4V) I will do about 40kph, which is a good speed for my commute. My question is: If I set up the batteries as 3s2p (nom. 66.6V) and back off the throttle to run at the same speed (i.e. 40kph, but pick any speed really) will I use more or less Wh for the total journey?

This is the kind of question that the http://ebikes.ca/simulator/ is good to answer. I assumed you have the IRFB4030 FET version and put in the 44V at 100% throttle and 66V at 66% throttle.

At 40km/h cruising any difference in efficiency is difficult to spot in the diagram. I'm assuming it is 1% or less.

You will have 20-30% more power available, but using this will hurt efficiency. e.g. going up a hill at 10km/h 44V gives 500W and 66V nearly 650W. At 10km/h then there's a 3% electrical efficiency advantage to the lower voltage, presumably due to lower percentual losses in the motor.
So at the lower voltage you would have to add 150W yourself by peddaling if the hill so requires (or the bike will bog down)

 

[dogman dan]

For sure, no point in running a voltage too low to get up the hill you need to without bogging down, making heat, damaging stuff. But on the other hand, you will need to be going slow enough to not simply melt down the motor feeding it huge watts up a hill. ( Huge = 2000 or more)

For sure 14 or 15 s lipo is enough power to get most people up fairly steep hills, but I can't say for sure about 12s lipo. It will be enough to get up 6%, but at that grade 36v begins to bog down and need some significant pedaling help. So if you climb much steeper than 4-5% grades much, you may just want to go the 18s route for the hill performance.

Again, getting the bike to do what you need is the important thing, not wheter you do it 1% more efficient.

 

[sico]

Thanks again dogman and Jag. Sound like getting a CA would be a good idea, but I'm already working a second job trying to get this project off the ground (yes, I'm a mathematician by day and a piecework steel fabricator by night) so it will have to wait a while I'm afraid. I can dial in a current limit on the Lyen controller though, so that might be a short term solution. I'll aim to try both, but from what I'm hearing once I set up for 18S I'll be unlikely to go back to 12S.

Simon.

 

[wineboyrider]

sico wrote:I'll aim to try both, but from what I'm hearing once I set up for 18S I'll be unlikely to go back to 12S.

Yes. With the CA I can series my batteries (36v lifepo4 20 ah and 12s lipo 20ah ) together dial the amps down to 10-12 and cruise for 30 plus miles at 25 mph easy over any grade..... 100v nominal is scary even with my 9fet lyen controller without one as I have my 9c on the front, but with the amps dialed down I have all the power that I want or need. 18s lipo is where I'm headed when my 36v ping pack tanks and/or I get enough $$$ for 6s more lipo. 8)

 

[gge5]

I'm planning on building a BLDC hub-motor driven scooter.

The motor is 7kW, 72V, and 11.5Kv.

I plan to overvolt it to 96V. It should theoretically increase my top speed from 47.2MPH to 62.9MPH.

My question: Is motor efficiency a function of (current RPM / maximum RPM)? I'm trying to figure out what it will do to the efficiency curve of the motor, attached below.

 

[amberwolf]

I know your motor isn't listed there, but you could play wiht teh simulator at http://ebikes.ca/simulator to see what different voltages affect a particular motor, to get an idea of what you might expect.

 

[neptronix]

Efficiency doesn't change if you feed it enough amps to stay near the peak efficiency range of the powerband ( about 80% of unloaded RPM for most motors, which would be bout whatever your maximum loaded speed would be. )

power usage goes up the faster you go though. A difference of 20mph over 40mph will require a dramatic increase in power needed.

 

[miuan]

Don't worry about motor efficiency. It may drop some due to greater power needed, but not too much. The eff curve (at full throttle) will remain similar except it will now be longer, like if you broadened it by one third. Getting to 60mph will however drastically drop your total efficiency, so your range will be maybe 2/3 of what it used to be.
96V nominal is typically about 110V fully charged, depending on battery chemistry. Be aware that keeping your stock controller, you may run into problems of overheating and/or voltage spikes killing your (likely) 100V rated mosfets/caps.
You may also need to increase your current limit to avoid cruising in PWM at top speed. And remember, top speed will not increase as much as voltage since the aero drag is not linear with speed.

 

[gge5]

Thanks Amberwolf for the link to the simulator. True, they're not the same hub motors, but the physics should be the same.

I would have guessed that graph #1 would have been the more accurate one:
View attachment 1

But according to the simulator, it appears that graph #2 is closer to reality:


Can anyone confirm this? If it's true, then choosing the lowest voltage that covers your desired maximum speed will give you the most range...

 

[gge5]

I believe I've made a noobish mistake. All of these graphs are sampled at 100% throttle, maximum acceleration. However, decreasing the throttle compresses the efficiency curve, effectively raising the motor's efficiency at lower speeds.

 

[amberwolf]

Yep, because essentially at lower throttle you are running them at a lower voltage. The motors have no clue about what battery voltage there is, only what you apply across the phases by throttling up or down.