Torque

Tiberius · Dec 9, 2008

Hi James,

Be careful about what speed the motor torque is quoted at. They may be giving you a figure for low rpm.

Motor torque is proportional to current (for an ideal motor, and close enough for a real one), so it's easier to generate high torque at low speeds, where the back emf is lower. In practice the controller usually has a current limit built in, but that works on the battery current not the motor current.

As for power, the 250W rule is for max continuous rated power; motors are capable of much higher power for short periods or in different circumstances. It's a bit of a minefield. I am one of the few people on here to have burned out a motor.

Edited to add: The limit in USA is 750 W, so ebikes.ca may be working to that rather than our 250 W one.

Nick

PS. I prefer to work in w = radians/sec rather than revs/sec.
Then P = Torque x w for rotating systems, directly analogous to P = Force x Velocity for linear ones.

flecc · Dec 9, 2008

Following on from Nicks comment on power outputs, e-bikes peak powers are typically between 300 Watts and 700 Watts in the UK, with only a couple below 400 Watts.

The 200 Watt or 250 Watts figures are largely notional, a legal nicety for legislative purposes, but having almost no design meaning.

Data on these motors is difficult to come by, but some give a fair amount of information. Here's an example of a web page that may help, that of the Cyclone drive through the chain motors.
.

torrent99 · Dec 10, 2008

James1986 said:
Tiberius: Does that mean that there is something wrong with my math?

Yes, there is. You haven't enough mathS for the UK!

flecc · Dec 10, 2008

James1986 said:
Flecc: Thanks for the web page, are there any PM pancake motors that can boast 96%+ efficiency? As I am designing my own Hub motor, it would be nice not to loose all the power to the motor, and have it overheat =(.
The golden motors are 82% efficient for 36v, which I used to think was high!

One might now argue the case that it is far better to add one of these to the drive chain of a normal bike, as then you can use the derailleur gears to achieve better hill climb, and have much less power loss.

I always ignore those claiming foolishly high efficiencies. For the best of todays internally geared hub motors like those from Suzhou Bafang, around 85% efficiency is typical, some older designs falling well short of that though, in the 75% plus region.

The problem with drive through the bike's chain with one of these powerful motors are the limits of bicycle gears. Hub gears in particular can't stand too much, typically ridden by utility cyclists outputting well under 300 watts peak, when presented with up to 700 watts of motor power plus 100 to 300 watts of cyclist they can just give up quickly. Even with a moderate system like the Lafree's Panasonic chain drive unit (390 watts peak), the Nexus 4 gear hubs failed at such a rate that Shimano discontinued them, and Giant switched the Lafree to the SRAM 5 speed hub.

Though derailleurs are less likely to fail outright, the wear rate with high inputs can be very high.

Bike transmissions just aren't intended to continuously transmit in the 500 to 1000 watts range long term, well over double what might be expected from a cyclist alone, one reason for the popularity of hub motors in more powerful applications.
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Miles · Dec 11, 2008

James,

You might like to play with this:
http://www.pedelecs.co.uk/forum/electric-bicycles/50-spreadsheet-calculator-2.html

Scroll down for the latest version.

Tiberius · Dec 11, 2008

Hi James,

I didn't see anything wrong with your mathematics, just that I prefer to use rad/sec for angular velocity - it makes a lot of things easier, I find.

BTW, it wasn't me complaining about the missing s, though if you persist in dropping it, we will expect you to work in ft-lbs and BTU.

Nick

Andrew harvey · Dec 11, 2008

It's not your maths that's the problem it's the physics, now I stand ready to be corrected but the power equation you use
P = V x I
is for power consumed by a cicuit.
The power output for a motor should I belive be
P = back EMF x motor current
were the motor current is not equal to the battery current.
Why?
Because we are using pulse width modulated controllers. These incorporate capacitors which charge up when the motor is not drawing current, .ie. between the current pulses then discharge throught the motor windings when the controller outputs open reinforcing the current drawn from the battery.
These capacitors act as smoothing devices that smooth out the peaks in the current drawn from the batteries giving an effective continuous flattish DC current. Batteries do not like giving out there power in short high current bursts, the circuitry helps to protect them as a side effect of the their design.
The torque equation is also inappropriate in this instance.
Electric motors give out most torque at low revs, but maximum power at mid revs, the equation you use only relates to the torque at the speed for maximum power. There's even more complexity to this, consider ;

at zero RPM the back EMF ( induced voltage ) in the circuit is zero,
therefore it follows that

P = 0v x I = 0 watts therefore using your equation the torque = 0

Does the torque equation you have used comes from the Golden Motor site,
if so start again, The best reference for the information we need I have come across since going on line is at

ebike.ca

Hope this helps, if I've made any mistakes well it's along time since school.

I've just checked out the ebikes.ca site, the bit you'll be interested in is on their Hubs page below the pictues.

Orraman · Dec 11, 2008

A horse walks at 2 1/2 miles per hour for an 8 hour day pulling a load of 150 lb vertically out of a shaft.
2 1/2 mph pulling 150 lb relates to 33,000 ft lb per minute = 1 horse power according to James Watt.
Electric bikes were permitted 1/4 hp in ft lb and that was the power you got continuously out of your motor.
Later this was raised to 200 W and recently to 250 W.
1 horsepower = 746 watts and nowadays motors are rated in Watts but is that input or output?
Motors can have an efficiency as low a 40%

Please, can I have a 1/3 horsepower motor in my bike?

The French are a little kinder to horses, force de cheval = 0.9863 British horsepower.

Dave

flecc · Dec 11, 2008

Orraman said:
Electric bikes were permitted 1/4 hp in ft lb and that was the power you got continuously out of your motor.
Later this was raised to 200 W and recently to 250 W.
1 horsepower = 746 watts and nowadays motors are rated in Watts but is that input or output?
Motors can have an efficiency as low a 40%

In practice the so called gross (input) peak powers of 300 to 700 watts are continuously available from our bike motors and none have an efficiency anywhere near as low as 40%. From 75 to 85 % is commonplace on hub motors.

For example the Suzhou Bafang motors of nominal 576 watts gross on my two bikes are about 85% efficient so yield about 490 watts nett (output), some two thirds of a horsepower, and the real world speed/climb performance easily bears that out. In fact these motors have been shown by some of us to have peaks of over 700 watts, well over the manufacturer's declared nominal peak power.

The legal niceties of 200 watts (UK) and 250 watts (EU) are just that, having no useful purpose in design or performance assessment terms.
.

Orraman · Dec 12, 2008

Greetings flecc,

In a thread discussing the performance of motors I thought that some members might find the origins of measurement and calibration to be of some interest.
I then lamented the passing of a system that I feel gave the customer more useful and accurate information.
If any part of my post was inaccurate in the slightest degree I will willingly eat humble pie.

Agreed, good motors can attain high efficiencies, but this is normally within a restricted load and rev band. When nearing the stall the efficiency of even the best motors plummets.

flecc said:
In practice the so called gross (input) peak powers of 300 to 700 watts are continuously available from our bike motors and none have an efficiency anywhere near as low as 40%. From 75 to 85 % is commonplace on hub motors.
..........................................

The legal niceties of 200 watts (UK) and 250 watts (EU) are just that, having no useful purpose in design or performance assessment terms.
.

I am unable to agree with your first statement, surely if a peak power is continuously available from a motor then that must be the continuous power of that motor.

While can I accept the that non European makers will continue to ascribe to your standards I am firmly convinced that Heinzmann and any other EC makers will continue to offer motors and cycles totally within EC regulations.

Dave

flecc · Dec 12, 2008

Orraman said:
If any part of my post was inaccurate in the slightest degree I will willingly eat humble pie.

Hi Dave, no humble pie necessary of course, but as you'll see below, I'm just correcting some common misbeliefs.

Orraman said:
Agreed, good motors can attain high efficiencies, but this is normally within a restricted load and rev band. When nearing the stall the efficiency of even the best motors plummets.

Fully agree, and I'm only quoting for motors run within their optimum band. For UK and EU legal hub motor bikes that's generally in the 8 to 15 mph region, give or take 1 mph at each end.

Orraman said:
I am unable to agree with your first statement, surely if a peak power is continuously available from a motor then that must be the continuous power of that motor.

While can I accept the that non European makers will continue to ascribe to your standards I am firmly convinced that Heinzmann and any other EC makers will continue to offer motors and cycles totally within EC regulations.

Dave

No they don't Dave. The legal 200 watt nominal Heinzmann motors used to pump out around 600 watts with ease and would do this on a continuous basis quite happily, the only restriction being the battery's ability to supply current. With Heinzmann's original NiCad batteries that was never a problem of course, current delivery with NiCad being almost unconditional. The Heinzmann would be unable to climb as it does if it's maximum continuous power were 200 watts.

As I've said, the 200 watt and 250 watt ratings are nothing other than legal niceties and do not exist on the real world. Furthermore, no-one has the faintest idea how they could be measured anyway, least of all the legislators, since they are the supposed continuous maximum input power to the road.

The only motor that gets close to 250 watts maximum is the Oriental Powacycle Salisbury/Windsor LPX one, pumping out a little over 250 watts of net power as it's the lowest powered one by far currently.

The main band of hub motors are around 400 to 500 watts, while the highest powered "legal" ones are around 700 watts. The notion of peak power on electric motors is a joke anyway, since any well designed electric motor will run flat out continuously for days, weeks or years, subject to adequate heat dispersal. With efficiencies of around 80 to 85%, there's not much heat to disperse anyway.

In summary, true 200 watt e-bikes would be next to useless for many. Much of the motor's 200 watts of power would be used to propel the substantial additional weight of the e-bike, often equal to the weight of a second normal bike, so the gain for the rider would be too small to be worthwhile. After all, a typical utility cyclist can pump out 200 watts anyway to propel a normal bike, so what would be the gain of adding 200 watts and the weight of a second bike? Very little.

Here's some cycling output figures for average reasonably fit males:

300 watts for about 10 minutes continuous.
200 watts for about 1 hour continuous.
100 watts for about 5 hours continuous.
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johnp · Dec 12, 2008

Maths

All these figures do my head in I only own a electric bike to ride and enjoy
my riding

Tiberius · Dec 12, 2008

flecc said:
Here's some cycling output figures for average reasonably fit males:

300 watts for about 10 minutes continuous.
200 watts for about 1 hour continuous.
100 watts for about 5 hours continuous.
.

Those numbers seem a bit ambitious. I don't have good source of data, but I would have thought you could divide them by two for most members on here.

Nick

flecc · Dec 12, 2008

They are widely published Nick and derived from original NASA research into various human activities. Cycling sources often publish them and they line up well with previous data from other sources.

I wouldn't agree with halving them for members in here, but since this is an e-bike forum, I'd agree that the average for members in here will be rather less, especially given the membership age profiles. I might infer the figures in brackets instead as an average for this forum:

300 (220) watts for about 10 minutes continuous.
200 (150) watts for about 1 hour continuous.
100 (100) watts for about 5 hours continuous.

or alternatively with time changes:

300 watts for about 2 minutes continuous.
200 watts for about 1/4 hour continuous.
100 (100) watts for 3 hours continuous.

After all, sitting in a chair continuously consumes about 50 watts and I think most of us manage considerably more than that, tapping the keyboard for example.

A fair proportion of members in here are still able to cycle unpowered bikes to some extent and many do so, that alone ensuring they are capable of respectable outputs, frequently well over 150 watts. Also, remember all those who get 20 to 30 miles from a 4.5 Ah Cytronex battery and over 40 miles from a Torq 1, not achievable without considerable cycling inputs.
.

Orraman · Dec 12, 2008

flecc,

Thank you for explaining your position but I fear that we are at cross purposes here. In the rating of electric motors continuous means there is no limit of time. One starts the motor against it's rated load and return next week and give the grease caps a wee tweak.

Please excuse me if I take your points out of order and consider your cyclist first. When on the level delivering his rated 100 watts he climbs a hill for 3 minutes at 300 watts then drops to 20 watts down the other side then back to 100 watts on the level, this is repeated over the next 5 hours. And much the same with electric motors.

When working for a City and Guilds as an apprentice mechanic I fiddled with a De Prony brake dynamometer using a single spring balance that measured in pounds against inches marked on a sash cramp holding wooden blocks against a shaft to measure torque. Same would work today but with a watch the answer would be in Brake horsepower.

First week's wage was 1 shilling and 11 pence.

Tiberius,

No leccie ......get taxi.

Dave

Mussels · Dec 13, 2008

James1986 said:
Also, anyone know the typical torque required to get the wheels moving on a flat, in first gear?

Thanks!

Initial acceleration from zero is a very different equation than normal acceleration if I remember right from my GCSE physics and I'd expect very dependant on how much the fat lump on top has pumped up the tyres. If I'm not trying to be picky then probably such little torque is required to get it moving then it would accelerate at a frustratingly slow pace.

Torque

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