What does 250W mean legislation wise?

[andyh2]

Considering a Stokemonkey kit for my Xtracycle. (Toying with the idea is probably more accurate as for the same money I could keep the current Tongxin on the Xtracycle and equip my roadster and Brompton with Tongxin or Bafang motors and controllers and just move my Ping battery to whichever bike I wanted to use).

The Stokemonkey (SM) is a 500W rated motor which comes with a Cycle Analyst (CA) set up for 20mph max. The CA max speed could be reset to 25kph max and can also be set to limit max current which would limit motor power. I think this means I could set the max current to 6.94A and then max power would be 6.94A x 36V = 250W. But motors rated at 250W are usually capable of drawing well over that at peak times. I saw Dans' post where he measured his 250W Bafang at, I think, 360W peak, which would imply a 10A limit should be acceptable.

What is normal and legal in this context? Would this method of limiting speed and power be acceptable or is the legal definition 'what is written on the side of the motor by the manufacturer'?

I know I'd be unlucky to get caught out using a system which is illegal on the road without registering as a moped, but firstly I wouldn't want those potential consequences and secondly my new job is with an organisation which promotes among, other things, low carbon transport and it wouldn't seem 'right' to use a solution which is illegal.

 

[NRG]

First thing, use the search function and go through the threads that have covered this before IE: http://www.pedelecs.co.uk/forum/electric-bicycles/5192-250w-48v-10ah-off-road-bmx-2.html

500w is illegal in the eyes of the law in the UK today as is a 20mph assisted speed. 200w is the maximum rated continuous power allowed but the law makes no ruling on the peak power allowable and in practice most manufactures ignore the 200W limit anyway and use 250W...in practice it isn't going to cause you an issue.

My recent DIY conversion uses a Bafang 250W motor with a controller set for 20+ amps...I've found it can pull over 870W of power under load but it's still legal in that assistance via Pedelec is only available up to about 15~16mph...so I wouldn't really worry about it.

 

[flecc]

In effect the power rating amounts to a measure of average power over time in working circumstances. I'm posting below a power graph of a well known nominal and legal 250 watt motor, graph author and motor confidential. You'll see that although there's peaks to near 500 watts, the mean of the measurements is about 250 watts. I'm adding further notes below the graph:



As ever with the law, imperfect drafting means that no-one really knows how the legal requirement should be measured. In reality it's arguable that the only true measure of the legal power is that imparted to the road for effective use, meaning a dynamometer measurement, lower than the gross or net motor power.
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[andyh2]

NRG I'd had a look through that and the summary seems to be that the law is as clear as mud

Flecc that graph helps.

Looks as though if CA set up to limit max power to 400-450W (around 11-13A) AND most importantly speed limited to 15-16mph then system would be within spirit of the law. And at least if there were ever a court case it could be shown that the system had the same power characteristics as 1,000s of legal bikes already on the roads.

So it seems the SM system could be used without too much concern, but maybe worth waiting until changes to current law are finalised following consultation and I decide if my actual usage really would benefit from through the gears power delivery or of my hub will suffice. At the moment I expect it will for 95% of the time, but that could change.

 

[flecc]

Looks as though if CA set up to limit max power to 400-450W (around 11-13A) AND most importantly speed limited to 15-16mph then system would be within spirit of the law. And at least if there were ever a court case it could be shown that the system had the same power characteristics as 1,000s of legal bikes already on the roads.

That's about dead on. Most mainstream e-bike motors have peak powers in the 400 to 450 watt region whether hub motor or not, a smaller number have around a 500 watt peak and a minority push the boundaries with around 700 watts, all these gross powers.

The motor type affects these though. Brush motors tend to have a more "peaky" power characteristic with a clearly definable optimum climb speed, for example the traditional Powabyke motor with a sharp peak of 700 watts at 7 mph when in a 26" wheel, power dropping away quite sharply either side of that.

Hall effect brushless motors have a flatter and broader power curve giving a better spread of optimum climb speed, and are able to have a lower peak power for around the same overall performance giving slightly better economy.

There is one simple reason for the legislative "cop-out" of specifying motor power only with no definition. In the EU, and soon in Britain, it's pedelec only with torque sensor control. That makes it completely impossible to measure the e-bikes power intelligibly, since the rider contribution has to be made to achieve the power and that becomes inseparable from the motor power.

Also, what is a riders power into a torque sensor? Once again, it's an indefinable factor.
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[Jeremy]

As others have said, the law is rather unclear, in terms of practical power measurement, but very clear in terms of maximum power assist speed.

The legal power restriction is 200 watts of mechanical power at the output shaft of the the motor (at the moment), something that is far from easy to measure at the roadside. Motor efficiency will vary considerably with load, rpm and current, but will generally tend to be between about 60% and 80%.

Some of the most efficient motors, with the flattest torque/efficiency curves, can be brushed types (axial flux PM motors, for example) and some of the least efficient can be brushless three phase PM types (direct drive hub motors, for example). Generally, brushless motors (if correctly and efficiently geared) tend to be slightly more efficient than brushed motors, although the difference between them is quite modest. The shape of the efficiency/rpm curve has little to do with whether or not the motor uses brushes or not, it's primarily a function of the winding resistance, frictional losses and eddy current losses in the winding cores, plus the resistive losses in the controller and wiring.

In terms of staying legal, then the most significant point is ensuring that all power assistance has ceased by 15mph. This is the parameter that is very easily checked at the roadside, should anyone ever want to, so provided that the bike complies with this aspect then it's quite likely that the authorities wouldn't look any further.

The power issue is more complex, as although the current EAPC law (which is in the process of being changed) states that the power at the motor output shaft shall not exceed 200W, most motors are rated in terms of electrical input power. This is fairly easily measured, but not easily translated into the legal power limit. For example, any electric motor run at low rpm, but high torque, might only have around 50% efficiency, so 400W of input power would be perfectly legal. The same motor, spun up to it's most efficient rpm might be around 80% efficient, so the maximum legal input power would then be reduced to 250W.

As efficiency is difficult to measure with the motor on the bike, it would be extremely difficult for the police to determine whether or not the bike was within the 200W motor output limit at the roadside.

My own view is that, provided you act sensibly and don't whizz along at high speed on level ground without pedalling, then there is virtually no chance of being caught for having too much motor output power. Having said that, the police seem less than well aware of the subtle nuances of the EAPC regulations, so it may well be that even a perfectly legal ebike might attract attention, or a completely illegal one might be ignored by them.

I've never heard of a case being brought for a breach of the EAPC regulations, but that doesn't mean that a sufficiently bored or vigilant policeman won't choose to take action. Bear in mind that the bench is made up of ordinary folk, who, in my experience, will usually look to apply common sense when there is any doubt as to whether or not a breach of the law has taken place.

 

[flecc]

A warm welcome back into the forum Jeremy, you hadn't posted for a long time and I for one had missed your valuable contributions.
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[eddieo]

A warm welcome back into the forum Jeremy, you hadn't posted for a long time and I for one had missed your valuable contributions.
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you can relax a bit.......

 

[flecc]

you can relax a bit.......

I had already been Eddie, my posting rate well down on what it had once been. I wouldn't mind it going rather lower yet.
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[lemmy]

It seems to me that the EU legislation could be simplified to 'you can do what you want with electrical power so long as it is cut out by 25kph'.

Road speed is perfectly easy to ascertain and a cut out would be easy to implement. Then if you wanted a 1kw motor because your daily commute took you up Porlock hill you could.

Ths is so simple I'm obviously missing something.

 

[flecc]

I think most of us agree Lemmy, it makes perfect sense. I firmly believe that only the speed is a matter for the legislators, the power required being properly the province of the designer and no business of the bureaucrats.
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