explanation of electrical current needed, *smart people look*
#1
explanation of electrical current needed, *smart people look*
after doing some research about ohms law on google i am wondering how it all applies to rc cars, when you have a 120 amp esc does that mean it is the total ammount of amps that can pass through it at any given time? and for an esc with a lower amp rating does that mean it just has more resistance? im all very confused by this. any help or clarification is appriciated
Last edited by Subhuman2178; 03-10-2013 at 11:43 PM.
#2
Tech Master
iTrader: (3)
ESC's usually have two current ratings listed (continuous and burst)
Generally speaking they are referring to the components used in their construction (capacitors, transistors etc....)
Select a motor appropriate for your task (motors also have similar ratings) then choose an ESC that will handle the expected load, best to allow some leeway
You are correct in that a higher rated ESC will generally have a lower resistance
An Example might be ..........
100 amp burst rated motor (740 watts runnning 2s lipo)
120 amp continuous rated ESC
200 amp continous rated battery (5000mah 40c)
Cheers
Generally speaking they are referring to the components used in their construction (capacitors, transistors etc....)
Select a motor appropriate for your task (motors also have similar ratings) then choose an ESC that will handle the expected load, best to allow some leeway
You are correct in that a higher rated ESC will generally have a lower resistance
An Example might be ..........
100 amp burst rated motor (740 watts runnning 2s lipo)
120 amp continuous rated ESC
200 amp continous rated battery (5000mah 40c)
Cheers
#3
Tech Adept
i'm also looking for on resistance for 17.5 blinky class racing. does the difference between .003 and .006 make more of a difference or can a low end revention or juststock be just as good as a high end esc.
#4
So it bascically means that the higher the amp rating the higher the c rating lipo it can handle?
#5
Tech Champion
Assuming the values are comparable, say from the same manufacturer, then yes theoretically the lower resistance esc would have less power loss. How much practical difference that makes on track might be debatable though. Possibly worthwhile in oval racing, less so in offroad, not sure.
#6
Tech Champion
What is needed is: Battery amps > ESC amps > Motor amps
Having a higher C or larger amp battery generally isn’t a problem for a RC car. (In specific cases it can be an issue for flight RC, as they can run more constant high throttle with more limited battery and esc sizes than cars typically have)
Last edited by Dave H; 04-13-2013 at 07:10 AM.
#7
Super Moderator
iTrader: (2)
Not really, at least for most RC cars. It’s more about how hot of a motor it can support (low turns or high kv).
What is needed is: Battery amps > ESC amps > Motor amps
Having a higher C or larger amp battery generally isn’t a problem for a RC car. (In specific cases it can be an issue for flight RC, as they can run more constant high throttle with more limited battery and esc sizes than cars typically have)
What is needed is: Battery amps > ESC amps > Motor amps
Having a higher C or larger amp battery generally isn’t a problem for a RC car. (In specific cases it can be an issue for flight RC, as they can run more constant high throttle with more limited battery and esc sizes than cars typically have)
#8
Tech Adept
I have used the hobbywing justock 45a esc (apparently really good esc for blinky stock) and a reventon pro 120a esc and noticed no difference.
The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.
Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.
C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.
The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.
Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.
C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.
#9
Tech Fanatic
iTrader: (13)
Everything about an ESC is largely dependant on the load and the resistance it presents to the circuit.
If your motor maxes out at 80A draw (say [email protected] Avg), that's all the ESC is ever going to give it, and that's all your battery will ever have asked of it.
Obviously, motors will draw surge currents during acceleration, as they are now being tasked with generating a significantly larger magnetic field.
Match your motor's surge current ratings to your ESC's surge current ratings. As long as the motor doesn't exceed your ESC's capabilities, you'll never have an issue. Just make sure your batteries can keep up at the end of it all.
Edit: One thing you'll see typically on motors is a maximum watt count. To work that into Amps, it's Watts/Volts=Amps. So a 800w motor is only good for [email protected] Nominal.
Here's some heat numbers for you, too!
Say you've got a motor resistance of 0.8 Ohms. That's 81.6W of heat thrown off. Now here's something neat. Double the voltage, assuming a 4S pack. Now we're only using [email protected], throwing off a measly 40.8W of heat. This is why you'll hear people talking about HV setups being more efficient. Because they are!
If your motor maxes out at 80A draw (say [email protected] Avg), that's all the ESC is ever going to give it, and that's all your battery will ever have asked of it.
Obviously, motors will draw surge currents during acceleration, as they are now being tasked with generating a significantly larger magnetic field.
Match your motor's surge current ratings to your ESC's surge current ratings. As long as the motor doesn't exceed your ESC's capabilities, you'll never have an issue. Just make sure your batteries can keep up at the end of it all.
Edit: One thing you'll see typically on motors is a maximum watt count. To work that into Amps, it's Watts/Volts=Amps. So a 800w motor is only good for [email protected] Nominal.
Here's some heat numbers for you, too!
Say you've got a motor resistance of 0.8 Ohms. That's 81.6W of heat thrown off. Now here's something neat. Double the voltage, assuming a 4S pack. Now we're only using [email protected], throwing off a measly 40.8W of heat. This is why you'll hear people talking about HV setups being more efficient. Because they are!
#10
Tech Champion
Note from the typical 17.5 dyno chart amps can vary from around 100A at low RPM to around 25A at high RPM. Have to pick a battery voltage, lets assume a nearly full charged 2S can maintain 8V under the 25 amp load and 7.4V under the 100amp load to the ESC (Starting voltage of 8.2V, IR of 8 milli ohms).
At the 100A load, the delta of the voltage drop is 0.015V, the power delta is about 0.41%.
At the 25A load, the delta of the voltage drop is 0.00375V, the power delta is about 0.094%.
If the ESC resistance numbers were as high as the numbers you mention, the power deltas would be about 9% and 1.9% respectively.
#11
Tech Champion
I have used the hobbywing justock 45a esc (apparently really good esc for blinky stock) and a reventon pro 120a esc and noticed no difference.
The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.
Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.
C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.
The amp rating on ESC's tell you how much current it is happy with passing through before things overheat and fail. The WATT rating on you motor determines how much current it will draw.
Lets say your 17.5 motor is rated at 250 watts. It will pull almost 30a at full charge and 39 when the battery is almost dead. So the justock 45a and a 5000mah 10c battery would cover that with a little room to spare for efficiency losses.
C rating isn't really as important as how long your battery will go before voltage drops off. Less decline in voltage will make you go faster toward the end of a race.
A truly higher C battery of same capacity will maintain a higher voltage under load throughout the run. Or it can be looked at in terms of IR, a lower IR battery will provide same.
#12
Tech Champion
Everything about an ESC is largely dependant on the load and the resistance it presents to the circuit.
If your motor maxes out at 80A draw (say [email protected] Avg), that's all the ESC is ever going to give it, and that's all your battery will ever have asked of it.
Obviously, motors will draw surge currents during acceleration, as they are now being tasked with generating a significantly larger magnetic field.
Match your motor's surge current ratings to your ESC's surge current ratings. As long as the motor doesn't exceed your ESC's capabilities, you'll never have an issue. Just make sure your batteries can keep up at the end of it all.
Edit: One thing you'll see typically on motors is a maximum watt count. To work that into Amps, it's Watts/Volts=Amps. So a 800w motor is only good for [email protected] Nominal.
Here's some heat numbers for you, too!
Say you've got a motor resistance of 0.8 Ohms. That's 81.6W of heat thrown off. Now here's something neat. Double the voltage, assuming a 4S pack. Now we're only using [email protected], throwing off a measly 40.8W of heat. This is why you'll hear people talking about HV setups being more efficient. Because they are!
If your motor maxes out at 80A draw (say [email protected] Avg), that's all the ESC is ever going to give it, and that's all your battery will ever have asked of it.
Obviously, motors will draw surge currents during acceleration, as they are now being tasked with generating a significantly larger magnetic field.
Match your motor's surge current ratings to your ESC's surge current ratings. As long as the motor doesn't exceed your ESC's capabilities, you'll never have an issue. Just make sure your batteries can keep up at the end of it all.
Edit: One thing you'll see typically on motors is a maximum watt count. To work that into Amps, it's Watts/Volts=Amps. So a 800w motor is only good for [email protected] Nominal.
Here's some heat numbers for you, too!
Say you've got a motor resistance of 0.8 Ohms. That's 81.6W of heat thrown off. Now here's something neat. Double the voltage, assuming a 4S pack. Now we're only using [email protected], throwing off a measly 40.8W of heat. This is why you'll hear people talking about HV setups being more efficient. Because they are!
#14
Tech Champion
Oops, nevermimd
#15
Tech Adept
Amps do not go up as the battery discharges. Voltage, amps, and power all drop off as the battery discharges.
A truly higher C battery of same capacity will maintain a higher voltage under load throughout the run. Or it can be looked at in terms of IR, a lower IR battery will provide same.
A truly higher C battery of same capacity will maintain a higher voltage under load throughout the run. Or it can be looked at in terms of IR, a lower IR battery will provide same.