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Old 03-20-2011, 09:42 PM   #4506 (permalink)
Join Date: Mar 2010
Location: New Zealand
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Originally Posted by bennelson View Post
I think many of us would love to hear more about Harlequin2's dual motor setup.

Could you tell us more about it? (In a new thread please! )
Not sure if it warrants a new thread! Maybe I'll give it a go and stick some photos up as well, but basically the space for motors in the BMW was long and thin so a single, fat motor wouldn't fit whereas two long skinny ones, end to end, just went in nicely. And give about the same total power and torque.

There does seem to be a lot of ignorance about controllers here, so let me see if I can shed some light:
The controller is merely a high speed switch with a variable on-to-off ratio. At zero speed, the controller is on for a very short time, applying full battery voltage and the current limiter switches it off as soon as the current reaches the limit, 500A in this case. So you get a series of short pulses of high current. This means the average voltage across the motor is quite low and it slowly increases as the motor speed comes up. At full speed, the controller is on 100% and the motor back emf limits the current. But the point is, the full battery voltage is applied to the motor every time the controller switches on. You can sort of think of volts being proportional to power and amps being proportional to torque.
A series wound dc motor will absorb whatever you force through it until it melts or flys to bits!
The faster a motor turns, the higher the "back emf" it produces and so the more volts you have to apply to force current through it and make it turn faster. This means that your top speed is limited by how many volts you have available and at full speed the controller is switched on all the time, connecting the motor directly to the battery. In my case, two motors in series across a 144V battery "see" 72V each and this gives me a top speed of 110 kph. When I had them in parallel, each saw the full battery voltage and I got 130 kph.
Next, capacitors. You don't need much capacitance to absorb the inductive spikes generated when the controller switches off, what you do need is the lowest effective series resistance ESR that you can get because the current - which at the instant of turn off is the same as the motor current - flows through this ESR and hence generates a voltage. If you have 500A flowing, you only need an ohm to generate 500 volts!
Also, the ripple current capability of the caps is important. If you have only 10 caps, then each must carry 50A which is asking quite a lot. So, to choose capacitors, don't worry so much about the capacitance but look for low ESR and high ripple current capability. Of course, high C values generally give lower ESR and higher ripple current capabilty.

Hope this was not too boring and maybe helpful!
Sometimes I sits and thinks and sometimes I just sits.
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