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Design and Realisation of Battery Powered Drivetrains

You have mentioned that your run the karts for a maximum of 8 minutes between charges.
What will be the effect of running them for longer at the same controller setting (i.e. 90 amps)? Will there be a drop in performance into the longer (say, 15 minute) ride? Or will the only impact be a shorter battery life?
As long as the batteries are new there will be no difference in performance during 15minutes but if you run longer than 10 minutes charging time becomes longer too (longer than running time). If the battery gets older it will have less capacity, so running time will drop constantly. Now if you are running 10 minutes only instead of 15 you won't feel that the battery is emtyafter 14 minutes. Emty means that the kart is getting slow.
If this is the case, by how much will battery life reduce?
This question is difficult to answer as it depends on several factors, as track design, driver habits etc.  But if you discharge an OPTIMA Yellow Top for example 10% you can do that over 10'000 times. If you discharge the battery 50% you can do that about 1'000 times and if you completely dischatrge the battery (100%) you can only do it about 350 times.
As you see it is not a linear function it is a logaritmic one and the goal is to discharche the battery only between 10% and 50%. That is how kart tracks can make between 2500 and 4500(max 5 min) rides per battery set.
With the controller setting at 90 amps, please give me an indication of the acceleration time (i.e. the time taken, in seconds, from rest to reach a top speed of 25 mph and 30 mph).
About 5 sekonds
By how much would acceleration improve if the controller is set at 120 amps?
At the start acceleration will be ecxactly the same until you reach about 1/3 of top speed then it will stay 25% higher until top speed.
At this higher setting, what kind of running time is possible before charging and how will battery life be affected?
Running time will be reduced by about 25%. 8 Minutes would still be fine 10 possible, this has to be tested.
One of your competitors uses 6x12v batteries, but each having a lower capacity (26 Ah) as compared with the Optima Yellow Top that you seem to prefer. What are the advantages / disadvantages of running your competitor's battery combination on your motor?
The energy stored in the battery is voltage times Ah. So if you have 72V (6*12) times 26 = 1.87 kWh compared to Optima 60Ah; 48V (4*12) times 60Ah = 2,88 kWh which is 35% plus energy or ruinning time or nearly50% more battery life (as it is over proportional to the depht of discharge).
Our Motor is probably one of the most efficient motors (over 92%) on the market. Each % higher means about 2% more (life/runningtime) out of your battery. Important is also that our motor has the high efficiency over a wide range.
What is the difference between a Single Phase Charger and a Three Phase Charger? Is one better than the other? If so, then how is it better (e.g. faster charging time? less risk of over-charging? better efficiency?) - please explain.
A singel phase charger is less powerfull (max. 60amps per charger)as it takes the current only from one of the three available phased chargers. Electricity consumption may also become more expensiv as the power supplier charges you by the peak consumed per phase. Where as a three phase charger takes the equal amount of power from all three phases and has therefor more power available and can charge faster (max. 120amps per charger).
The charger specifications mention: Power Supply == 400+/- 15% VAC.
The power supplier has to provide 400 V 3 phase with a tolerance of maximum 15% (min voltage 340V maximum voltage 460V).