Nov 03, 2014, 09:40 PM Registered User Joined Oct 2014 103 Posts Discussion General explanation of electronics Hi guys, I am having trouble really grasping the max current that you can put through a motor. Would it work to run the Sunny Sky 2204 2300kv motors with kiss 18 amp esc on a 4s? If anyone could explain to me how it all works I would really appreciate it. Does the esc step the power down enough to make that plausible? Im good on the vast majority of all things involving multirotors besides the electrical aspects. Thank you so much
 Nov 03, 2014, 10:58 PM Registered User USA, MO, Florissant Joined Nov 2010 2,066 Posts Motors usually have two ratings: maximum continuous current, and maximum continuous Watts, both of which are there to keep the motor from overheating, and damaging itself. While these are related, they are not generally equivalent. For example, a motor limited to 10 Amps could only develop a bit over 120 Watts on 3S, because a 3S battery, at full charge, is 12.6V, and the volts drop some when current is drawn. That same motor, drawing 10 Amps on 4S would develop a bit over 160 Watts, because that battery might hold ~16 Volts at 10 Amps. However, assuming that same motor is rated at 140 Watts, you have to reduce the current some to keep it from overheating; in this example, that would be about 8.75 Amps. But, you can still get about 20 more Watts from it on the higher voltage. There are motors that cannot reach anywhere near the maximum power rating when the current rating is reached on a lower voltage, and the only way to reach it is to use a higher voltage. A warning, before randomly increasing your battery voltage: the motor will try to go up in rpm by the same proportion as the increase in voltage, so it will try to spin 33% faster on 4S over 3S. The power to turn a prop goes in proportion to the cube of the rpm increase, so going from 3S to 4S, if the prop is not changed, the power will try to increase by 2.37 times. You have to change prop size (or whatever other load the motor is driving), to take this into account, and, to verify this, a good Watt meter is required. There is also a limit to the safe rpm of a motor, which is why you often see a rating for voltage, as well, often expressed as the cell count of the battery that may be used. A 2300kV motor is going to spin really fast on 4S; assume about 75% of 2300 times 16, or 27,600 rpm. Oh, and the ESC doesn't step anything down; it will try to provide as much power as the motor needs to reach the rpm that the Voltage wants it to turn; if it can't, it melts.
Nov 04, 2014, 02:43 AM
Registered User
Staffs, UK
Joined Nov 2003
11,249 Posts
Quote:
 Originally Posted by tdilwo Hi guys, I am having trouble really grasping the max current that you can put through a motor. Would it work to run the Sunny Sky 2204 2300kv motors with kiss 18 amp esc on a 4s?
You've missed out the critical factor. The amount of current that will be used depends on the PROPELLER SIZE.

I don't know that particular motor but I'd guess that any 2204 size 2300Kv motor on 4S could only handle about a 4 or 5" prop before exceeding it's maximum current or power ratings.

Steve
Nov 04, 2014, 06:01 AM
homo ludens modellisticus
Netherlands, GE, Nijmegen
Joined Feb 2001
11,451 Posts
Quote:
 Originally Posted by tdilwo ... If anyone could explain to me how it all works ...
• Motor/battery-current wants to go up with voltage squared and with Kv cubed. Way more than one would expect.
• A motor's Kv constant says nothing about max.power, max.current, efficiency, rpm, quality etc.
• The Kv constant is not a rating, not a figure of merit, it's just a characteristic. More windings will give lower Kv, less windings will give higher Kv, that's all there is to it. No big deal, anyone can do that.
• Motors have just one Kv, not e.g. 1450Kv. It is physical quantity (length, weight, time, current, ..,.), not a physical unit (meter, kg, s, ampère ).
Therefore: Kv = 1450rpm/volt.

For a rainy day(s)
E-flight 101 by RCG member Ken Myers, will at least save you a ruined LiPo (or worse!), a burnt motor and a fried ESC:
The Ampeer
-> Electric Power Basics

followed by
The Ampeer
-> Everything youw wanted to know about e-flight (Ed Anderson)
• Things To Check On a Ready-To-Fly (RTF)
• What function goes on which stick?
• Battery Basics
• Amps Versus Volts Versus C
• Lithium Battery Chargers
• Balance chargers versus external balancers
• Extending Flight Times While Maintaining Balance (proper CG)
• Understanding the Electronic Speed Control (ESC)
• he Low Voltage Cutoff Feature (LVC) Of Your ESC
• The Role of the Battery Eliminator Circuit in Your ESC (BEC)
• Sizing Power Systems for Electric Airplanes
• Prop Versus Amps
• What Do the Kv Numbers On Motors Mean?
• Who Needs a Wattmeter/Power Meter?

www.scriptasylum.com
-> RC calculations
-> electrical
-> system wiring
respectively ...
-> LiPo pack wiring
respectively
The other menu items and demos are also veeeeery handy/instructive for e-folk, add site to your favourites?

www.ezonemag.com
-> FAQ

Vriendelijke groeten Ron
Last edited by Ron van Sommeren; Nov 04, 2014 at 06:14 AM.
Nov 04, 2014, 06:10 AM
homo ludens modellisticus
Netherlands, GE, Nijmegen
Joined Feb 2001
11,451 Posts
Quote:
 Originally Posted by tdilwo ... Does the esc step the power down enough to make that plausible ...
Controller chops battery voltage to lower rpm, Pulse-width modulation (PWM)

Click to enlarge. Pictures from
www.aerodesign.de/peter/2001/LRK350/index_eng.html
-> Speedy-BL diy brushless controller.

PWM chopping

No chopping at WOT
Last edited by Ron van Sommeren; Nov 04, 2014 at 08:21 AM.
 Nov 04, 2014, 08:06 AM Registered User Joined Oct 2014 103 Posts Thank you to everyone that has responded! It has been an enormous help!
 Nov 04, 2014, 09:14 AM Registered User Joined Oct 2014 103 Posts Just one more thing. I was just looking at the assembly that Soma does for his warpquads and he uses the SS2204 motors but he only uses a 12 amp esc. How does he not destroy the esc? here is the data sheet for the draw with different propellers http://www.rcgroups.com/forums/showthread.php?t=2112708
 Nov 04, 2014, 10:35 AM Registered User Staffs, UK Joined Nov 2003 11,249 Posts There are plenty of entries in that chart that draw less than 12A e.g. GF5030, HQ 6x3 etc. If he used one of those propellers what's the problem ? Steve