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Question on going from 3S to 4S

Old 02-14-2021, 12:44 PM
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Wenox
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Default Question on going from 3S to 4S

If my ESC is rated for 3s or 4s, will my engine handle the increase? I guess my question is how are motors rated? If someone could point me in the direction to a article that explains electric power systems I would appreciate it.
Old 02-14-2021, 07:42 PM
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Sorry - I don't have any links for the type article you are looking for, but if you look up the spec's for your motor (it is not an engine - engines burn fuel), then you should be able to find out if it will handle 4s batteries.

For example, this shows that this particular motor is rated for 2-3s batteries: https://www.altitudehobbies.com/prod...50c2f78d&_ss=r

Those spec's tell you other things that could be really handy to know. I suggest looking those up for your particular motor.
Old 02-16-2021, 08:53 PM
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Motors don't really care for voltage, they can handle any voltage in RC land.
As long as rpm, current drawn and power drawn don't get too high, those are the ratings that matter bigly.
Insulationlacquer is tested up to several hundred volt.

When a manufacturer/suppliers lists voltages and propellors, always use highest voltage with lowest propellor load listed, and vice versa. Suppliers tend to mess those two colums up One column should increase, the other should decrease.

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So going from 2s to 3s, current wants to increase by factor (3/2)²=2.2, 120% extra.
Ignoring extra voltage losses in battery, wiring, controller and motor itself due to higher current.

The 'full' story.
Even a seemingly small change/error in setup can result in quite an unexpected increase.
Because current and power drawn are proportional to
  • pitch
  • #blades
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  • kv³ Name:  eek.gif
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  • diameter⁴ Name:  eek.gif
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So even a small change/error in setup can have huge effects.
Increase in current with one or two cells added, simple table - RCG

Expensive examples.
Going from 2s to 3s, a 50% increase, would (3/2)²=2.2 fold current, more than double, 120% extra.
A 25% increase in velocity konstant kv³ would (1.25)³=2 fold current, 100% extra.
A mere 10% increase in diameter would (11/10)⁴=1.1⁴=1.4 fold current, 40% extra.

Extreme examples.
Doubling voltage would 2²=4 fold current.
Doubling velocity konstant kv³ would 2³=8 fold current.
Doubling diameter would 2⁴=16 fold current.

Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Last edited by ron_van_sommeren; 02-16-2021 at 10:17 PM.
Old 02-16-2021, 08:58 PM
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Originally Posted by Wenox View Post
... If someone could point me in the direction to a article that explains electric power systems I would appreciate it.
E.g.
www.stefanv.com/electric-rc-airplanes
→ Power System Selection

Below an excellent quote about motor selection.
From
brushless motors Kv?.
Originally Posted by scirocco View Post
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. 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.
  4. 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.
  5. 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. On the other hand, 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, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS
Old 02-16-2021, 09:04 PM
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An e-flight calculator will get you in the ball-park, provided the input is correct.
E.g. www.ecalc.ch (several languages)
Sticky: e-flight calculators (compilation) - RCG

Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC
Old 02-16-2021, 09:08 PM
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Just ignore most off the above posted be me, for now, if you are completely new to e-flight.

Training/trainer suggestions:
Advice for getting into flying RC airplanes, and how not to - The Ampeer

Some well-structured reading for rainy/windy days, including e-flight_101 & 102, and some handy e-tools. Will save you, and us a lot of questions. Notably the 'what went wrong?' kind of questions Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

And pleasepleaseplease, do your RC equipment, wallet, ego, battery, controller, motor, house/garage/car a big favour ... get a watt-meter. It will more than pay for itself, will save you at least one fried motor and one fried controller. Will also help you finding the best setup.

Vriendelijke groeten en wees voorzichtig, Ron
• Without a watt-meter you're in the dark ... until something starts to glow •
E-flight calculatorswatt-metersdiy motor tips&tricksCumulus MFC

Last edited by ron_van_sommeren; 02-16-2021 at 09:44 PM.

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