This is adapted from a post I made in response to the common question of why isn't there a simple motor / prop / battery ESC/ chart that can be applied to a model to get a workable electric power setup.

It complements my older power system design methodology parts 1 and 2 blog posts but contains a much more specific 'how to' for using Ecalc to make the jump between what you want from a power system and what prop, battery and motor combination will get you there. The watts/lb chart was posted in the original thread by redshift2 - thanks!

If reading this from the link in ECalc help, I suggest first reading the original part 1 post here https://www.rcgroups.com/forums/show...odology-Part-1

Background.

There isn't and I don't think will ever be the simple chart people often are looking for, but instead there are well established methodologies and tools that mean you don't need to be an electrical engineer to succeed, nor does it need to be done by WAG or physical trial and error - anything but that!

But why can't it just be a 'simple' chart? As often pointed out, IC and electric operate so differently that the range of variables for electric solutions compared to the variables in picking a glow engine totally defies straightforward if a then b classification. To illustrate, a given IC engine needs to get to a high and narrow rpm range to develop useful power and its load needs to be controlled so that a) it can get to that range and b) doesn't destructively overspeed....Continue Reading

This entry is adapted from a post I made on an Electric Sailplanes forum thread about Kv https://www.rcgroups.com/forums/show...3-Kv-of-Motors, largely to do with whether Kv is even important to know.

IMO, part of the problem is that in general, we use poor terminology when referring to Kv, which can lead to all sorts of misunderstanding such as Kv being a rating, where more is better.

Kv is a property, not a unit or a rating, just as weight is a property. The problem is that the name of the property has been used to replace the units that describe the property.

For example, with weight, we don't say it is a 2 weight battery, we say it is a 2 lb battery, or a 2kg battery and of course through centuries of usage, it is nearly always clearly understood that the lb or kg refers to a unit of weight. The longhand way to say that if lb or kg were not near universally understood as units of weight would be it is a battery that weighs 2lb/kg.

Just as weight has several units of measure, so does Kv. Rpm/V is most common, but in SI units, it is Radian per volt second or rad/V. s.
So when we are referring to the Kv of a motor and the audience is very likely to understand that the property referred to is Kv it should be "a 1200 rpm/V motor". Or just as with the weight analogy, if the common understanding of which property is being described might not be there, "a motor with Kv of 1200 rpm/V" is an alternative. But never a 1200Kv motor!

Hell will freeze over...Continue Reading

The VR Pro is a linear voltage regulator designed to regulate 2S LiPo voltage to selectable servo voltages. The Duo version adds dual battery inputs.
http://shop.dualsky.com/vr-pro-duo_p0202.html

The manual is a little cryptic, thus this blog post.

The VR Pro has 2 main functions.

First it is a voltage regulator (with adjustable output voltage) so that you can power servos rated at 5 or 6V using a 2S LiPo, which is around 8.4V fully charged.

Second, it provides a much higher current pathway to the servo outputs than a standard receiver can handle.

If it's the VR Pro Duo, then it has the additional function of being able to use 2 independent power sources and automatically use the higher voltage source, providing some redundancy against battery failure. Note that you don't have to use 2 batteries with the Duo.

Putting the key functions together, the VR Pro (Duo) provides (redundant) power to the receiver and servos, and can provide a high current output independent of the receiver power bus to up to 4 servos.

The VR Pro should have been supplied with 4 male to male servo leads.

Choose the 4 highest load or most important channels and use the male to male leads connect each of these to one of the receiver ports on the VR Pro. For example, connect the receiver rudder channel to Receiver port 1 on the VR Pro, elevator to port 2 and so on. The channel to port numbers assignment is arbitrary and doesn't matter, because the servo leads for the 4...Continue Reading

Motor Kv is used to match the power source, assuming it is adequate, to the prop and its power requirement for desired performance. Motor size is set to handle the input power. If you can get both in the same motor, bingo! If you can't - probably need to change voltage. Hence why motor Kv and weight are the 2 key characteristics.

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

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. 100W input power per pound all up weight will always be more than enough to fly sports aerobatics, provided a sensible prop is chosen, and pitch speed is at least 2.5 times stall speed.
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.

Originally posted here: https://www.rcgroups.com/forums/show...57&postcount=6

For a much more detailed guide on applying this methodology using Ecalc, see my later post https://www.rcgroups.com/forums/show...t-we-can-do%21

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

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