If you're set on obeying the specific word of the legal limits, and your locality uses the controller's output as the rating (rather than the simple labelling of the motor), you'll need to have a controller that has a current limit that, at the battery voltage you are using, does not let the system exceed the wattage limit you wish to obey.
If that is 500w, then you divide that wattage by the maximum full charge battery voltage to get the maximum amps the controller can be allowed to output. Then find a controller that either is already limited to that, or is adjustable down to that limit. If your locality does not allow adjustable limits, and you wish to obey that rule, then you will need to get one that is not adjustable and already has the correct current limit.
If your locality uses the labelling of the motor as the rating, you'd need to purchase a motor that is specifically labelled and rated for or below the limit you wish to obey.
Note that the motor itself does not limit power at all, regardless of what it is rated for, which is why a legal limit based on a motor rating is silly. You can label a motor with any number you want, but it does not affect how much power that motor uses or produces in a given scenario--the controller does all of the limiting of power to the motor.
If your locality doesn't state what part of the system the rating is actually limiting, such as saying "the bike cannot exceed 750watts" then to legally obey it you would need to consult legal counsel to find previous case law that gives a legal interpretation of this limit that you can apply to your system in a practical way.
Most systems simply limit the current at the specific voltage they run at so that the effective system maximum power usage cannot exceed the power limit created by this. Most controllers have a momentary peak allowable that is much much higher than their limit, before the limit kicks in, but once it does, it wont' exceed that limit again until the load drops so that power usage reduces below it, and then spikes again from a high load.
If you slow motor heat loss to the environment, the motor will likely overheat and be damaged fairly quickly, if you are using it anywhere near it's ratings. *Increasing* heat loss to the environment is what is typically done, so that excess heat does not build up and damage parts inside. Motors do not use less power just because they are hotter. (the wiring does increase in resistance so current flow does decrease, but this is not significant and simply causes more waste heat buildup inside the system by reducing it's efficiency, and if you block the heat from escaping it just increases the likelihood of system damage).
If you slow controller heat loss then the FETs and capacitors will overheat. Capacitors then degrade until they no longer perform their function, and voltage spikes then cause the FETs to fail. The FETs, even without capacitor damage, will overheat and fail, once the heat reaches sufficient levels, or they degrade over time from the excessive heat.
If you slow battery heat loss to the environment, and it builds up enough, it can cause a battery fire. If you are in a very cold environment, then it's often beneficial to slow the heat loss from the cells, but otherwise it is detrimental to the lifespan of the cells, and can damage them in ways that can make them a fire hazard.
If you want an acceleration and speed limit, you can do that with some controllers, and for those that can't you can use systems like the Cycle Analyst v3 from http://ebikes.ca to do whatever kind of limiting you wish. Acceleration would be limited by ramping of throttle output, and/or limiting current or power, or using the feedback loops (P/I) to change how the whole system ramps up when demand is placed.
endlessWork said:
Europe, Britain, North America. They all have legal requirements for max wattage.
peak watts,
nominal watts,
Continuous watts,
just plain watts.
Local law is rated continuous watts not exceeding X. The G060 is rated for 750 watts; I mean 750 Watts and 80Nm on the spec sheet. What type of watts, I don't know. Maybe cow watts.
Continuous watts in 100F temps is different than continuous watts at 32F (I don't know by how much).
I don't think the power supply is considered part of the motor by law; Bafang sells a motor that inlcudes a hub and everything inside of it.
I figure if I put something to slow heat loss to the environment I lower continuous wattage and make it an unrated motor. The law says I am not allowed to modify it to have more wattage but not so it has less. So let me make it less.
It would be so much easier to have an acceleration + speed limit.