Building an Electric Bike, Part 2
So, how do we wire the motor to the batteries? Here’s where it starts getting interesting.
The cyberbanking gurus amidst you will by now be abounding phrases like ‘speed ascendancy through the beating amplitude accentuation ascendancy of boilerplate motor current’ and being like that…but take a step back. Remember what was said above about best results coming from Keeping it Simple? (Oh, you skipped that bit – able-bodied go apprehend it again!)
There are two credibility to realise.
Firstly, a motor of the sort that we’ve been describing is basically a constant speed design. That is, back fed a specific voltage, it wants to circle at the one speed. Of course, as it is loaded down, it will circle added boring – but importantly, it will again additionally draw added accepted as it attempts to get aback to its ‘natural’ speed.
Secondly, in any practical electric bicycle, the natural speed of the bike (say, on the flat) will be way less than the speed limit – any speed limit!
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So let’s say that you’ve got the motor powered directly from the battery, using just an on/off switch for control. The aftereffect of the accessible power, the gearing and aerodynamic and frictional drags is that back the motor is on, the bike campaign at (say) 20 km/h. No one in their right mind would want to go slower (and if you need to, just turn off the motor and pedal!), while when you reach a hill or run into a headwind and the bike slows, the motor will automatically draw more current and so help you along more strongly.
In effect, the motor self-regulates its current draw – taking more as more is needed, and reducing its consumption as less power is required. Hey, that’s fine – and all just done with an on/off switch!
But what about regenerative braking? Lots of electric bikes don’t use any form of regenerative braking, so in hilly areas they’re wasting plenty of power in heating the brakes – power which could otherwise be going back into the battery. So how do we accomplish regen braking happen?
Regen braking occurs when the bike’s speed exceeds the ‘natural’ speed of the motor. In other words, if the bike normally does 20 km/h on the flat and you go faster than that down a hill, the motor automatically turns into a generator and the current flows back into the battery, helping to recharge it. But there’s a ambush that can be active to get regen occurring at abundant slower speeds: abate the array voltage that the architect sees!
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This diagram shows how it is done. Two batteries are acclimated with a bifold pole, bifold bandy (DPDT) about-face active to configure the batteries in either alternation or parallel. (A added about-face acts as an on/off control.) Let’s accept that they’re 12V batteries. When the batteries are configured in series, 24V is accessible at the motor. When the batteries are switched to parallel, 12V is accessible at the motor.
The natural speed of the electric bike in 24V mode might be 20 km/h, but in 12V mode it’s more likely to be just a quick walking pace. So aback you about-face to 12V approach aback rolling bottomward a hill, the motor tries to cull you aback to that apathetic speed. As a result, regen braking is very effective – in fact, in this example, the regen will work at all speeds down to a walking pace… at which point the 12V batteries again alpha active the bike forward.