Lets start out with the easy stuff.
KV actually means kilovolt, in our appliation it means revolutions per volt. So a 3500Kv motor will rotate 3500 times per volt applied to it.
As for the amp ratings, what ever motor you choose will have a max amp rating. Thats how much power it can potentially draw from your batteries. Keep in mind Ohms Law on this topic. A 3S battery at nominal voltage is 11.1 volts. This means your motor of 3500KV will spin at 38,850 RPMs. On 6S that same motor will spin at 77,700 RPM. I only stated that for the porpouse of this question. You wouuld not use a 3500Kv motor on 6S.
I do not know what heli you have so I will keep this as generic as possible. Now an airframe designed for 325mm blades is typically a 450 class heli. This size historically has been a 3S setup, 45 amp ESC, 3500Kv motor and a 3S 35C lipo pack. This setup will get in the range of around 2700 to 3500 RPM at the head.
Next are the 360 / 380 class helis. Currently the 360 class is the same airframe as the 450 but a little longer to accomidate the longer main blades and thats it. This size heli is usually flown on 6S but can be flown on 3S just as well. This setup requires a slightly bigger ESC, motor and battery setup. Your battery on this setup for this size frame is going to be a 6S 1800 35C pack. Thats going to be an estimate based on what the Guai X3 and Goblin 380 can handle. Next the ESC, Goblin recommends a 60 amp esc for this setup and I dont know why but they do. Your motor for this setup is going to be based on what you are trying to do. First you need to know what target head speed you want, what does your gearing need to be at based on motor kv for that head speed to be acheived. It sounds complicated but you will understand it the more you work with it.
Can a 3S pack draw 50 Amps also? Yes it can. You need to know the capabilities of your batteries. a popular 3S pack is the 2200 35C pack. These are by far the most popular packs for a 450 class heli. If we take the 2200 mah (capacity) and multiply it by the c rating (discharge rate) then divide by 1000 we will get the constant discharge rate of the pack( (mah * C rating) / 1000) . Now with this info we can find a motor and esc combo that ideal for the pack in question.
No 2 packs are made the same. Because of the variance in manufacturing you typically will not see the labeled output of the packs. So when you build your electrical system for your model you want to make sure your not going to over work any one part. Its bettery to be on the high side of things and have all the head room you need. While the ESC and motor can handle peaks well above their ratings your packs may not. I hope this answers your questions. If not ask me again and I will try and do better.
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