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Posted by scirocco | Aug 05, 2014 @ 06:38 AM | 1,189 Views
In Part 1, I briefly outlined a process of determining overall power, how to deliver the power, how big a motor is required, and finished with suggesting last of all choosing a motor with the Kv to do the job.

This entry will talk a bit more about how to deliver the power, and how to find the elusive Kv that supports the power system design goals.

Pick a preferred cell count (voltage) and pack capacity for how to deliver the power

Voltage. The easiest rule of thumb to apply in power system design is watts input power per pound all up weight as it gives a single number in Watts, eg 6lb at 100W/lb = 600W. But 600W can be delivered by a 3S pack with nominal 11.1V at 54A, or by a 6S 22.2V pack at 27A, so how many cells to use? And what about pack size? Unfortunately, it's hard to give a good rule of thumb; rather some considerations. One driver is that ESCs bigger than 60A capacity start to get bulky and expensive. So allowing for some headroom on ESC capacity, it can be cost effective to add a cell and drop current. For example, using the 600W target, 40A on 4S is comfortable for a 60A ESC, while 54A on 3S is close to the limit. But going up the scale, ESCs capable of handling over 6S tend to be classified as 'high voltage' with a step change in pricing. Maybe re-using an existing ESC sets an upper current limit. Existing charger capabilities or commonality with existing packs may also be valid considerations. So choosing cell count up front might be somewhat arbitrary and...Continue Reading
Posted by scirocco | May 16, 2014 @ 06:23 AM | 1,668 Views
Kv, while an absolutely critical part of the system, is actually the item one should choose last.

Decide your peak power requirement based on the weight of the model and how you want to fly it
Pick a preferred cell count (voltage) and pack capacity for how to deliver the power
Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate
Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.

Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as Ecalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.

The reason I suggest picking Kv last is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. OTOH, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, eg limited prop diameter if it's a pusher, or you already have a bunch of 3S packs and don't want to buy more, and so on.
Posted by scirocco | Jan 01, 2011 @ 02:57 AM | 3,486 Views
I've found a very common thread in question in power systems, so I though I'd persist with blogging some of my relies, if for no other reason than being able to find em myself!

Motor nomenclature seems pretty much all over the place; however within that mess you can usually find the Kv for the motor and how much power the seller claims it can safely handle.

Power ratings: Note 'handle' is deliberate, not create or generate as the motor doesn't have an inherent power rating like the horsepower of an IC engine - it just converts the electrical energy from the battery with varying degrees of efficiency and delivers it to the prop. So think of power ratings as the max you can jam through a motor without it getting so hot it kills itself. Also, don't feel you have to run a motor at its max rating - most 300W 'rated' motors will be very happy handling say 200W. In other words, the power a motor has to handle is not determined by its claimed rating, but rather by the combination of voltage supplied to it, its Kv, and the load from the prop it is trying to turn. The ability of the battery to deliver the demanded current is also a limitation.

Typically also running significantly below the rated power probably means running closer to the motor's peak efficiency power handling point (which varies depending on voltage btw). That hypothetical 300W motor handling 200W delivered by the battery might be converting 75% to the prop, but up around 300W, maybe only 50%. When you realise...Continue Reading
Posted by scirocco | Jan 01, 2011 @ 02:47 AM | 5,056 Views
I've found people often getting mixed up by power ratings. The problem seems to me that people see a rated power for an electric motor and not unreasonably assume that that is the max it will deliver, like horsepower for a car engine. But it's not that way at all - the motor will happily destroy itself trying to turn whatever prop is attached at a speed depending on the Kv of the motor and the voltage of the battery. If the prop is too big for Kv times volts or the Kv is too high for that prop at your voltage, then in trying to turn it the motor is going to want to draw a lot of current from the battery - maybe way over the rated power if the battery can deliver the current and the ESC doesn't fail first.

So what's the use of the power rating? The manufacturer is trying to tell us that the motor at the rated max input power is capable of operating at some reasonable (but almost never specified) efficiency level that won't generate so much waste heat that you cook the motor. For cheap brushed motors, that efficiency might be as low as 50%. For cheap outrunners, maybe 60-66%. In a roundabout way, the power rating is almost more about how much wasted heat the motor can cope with without getting so hot that magnets fail, etc.

So given that the motor won't actually limit itself to the stated power rating, it's up to us as the power system designer to set things up so that we provide enough power for desired performance and that the motor won't want to absorb more than its...Continue Reading