Help! How to find the rigth ESC?

[Magne]

Hi everybody, I want to build the migthy mini arrow. I can see the battery recommended for this build is a 3s lipo battery which has a 45 c-rating and max 90 c. That means it maximum can produce 76,5 amps. 0,85 x 90 = 76,5. But why are they then only using a 20 amp ESC.
How does that work?
Magne.

Link to the battery: https://store.flitetest.com/tattu-850mah-11-1v-45c-3s1p-lipo-battery-pack-with-xt-30-plug/

 

[quorneng]

The ESC has to supply the amps the motor requires not what the battery is capable of delivering.
The 2204 2300kv motor normally only needs 12A at full power so a 20A ESC is quite adequate.
Remember a 90 C rating means the battery would be completely discharged in less than 40 seconds and would almost certainly be wrecked in the process.

 

[Merv]

...a 3s lipo battery which has a 45 c-rating and maxim 90 c...

The 90C is the burst rating, what the battery can do for a short time, maybe 10 seconds or so. The 45C rating is what you want to use for your calculations.

Your point is correct, the recommended battery is more than you need. Something like a 850mah 30C would suffice.

 

[Magne]

Thanks everybody, this is something I have had a lot of problems with

 

[Merv]

Thanks everybody, this is something I have had a lot of problems with

All of the amp ratings in the hobby are DO NOT EXCEED numbers.

Most sellers will give the max amp draw for a motor. Select a prop that will not overload the motor, that is cause it to draw too many amps. Then select a battery & ESC that can supply amps the motor is asking for. It's always good to have a bit of overhead as the Rx & servos will draw a few amps as well.

If you exceed the amp rating of any component, the motor, battery or ESC, it's the weakest link that will burn out first.

 

[Inq]

12A at full power so a 20A ESC is quite adequate.

I've read here not to run an ESC up to its rating. Is there some rule of thumb of what is prudent?

1. Like for a 30A ESC can I run it at 90% or do I need to keep it under 50%?
2. I'd guess it time dependent. (for #1) are the answers different for a 30 second burst and for full flight duration.
3. If I were to put a thermistor and keep it under some max temperature, would this keep it from failing? Or are there other things beside burning up?

 

[Merv]

...Is there some rule of thumb of what is prudent?...

The motor is not the only thing drawing amps. Your servos also draws amps. A typical 9g servo, when stalled out, will draw approximately 3/4 of an amp. In other words 4 servos will draw about 3 amps. If you are doing lazy circles in the sky, the amp draw will be significantly less. But if you are flying fast, doing high G manures, it’s something to keep in mind. The Rx will also draw a bit of power as well as anything else you might have hooked up.

As long as your total amp draw is below the ESC rating, you should be OK. Most of my fleet, the motor will draw 40-50 amps, I run 50-60 amp ESC’s.

You also need to be aware of the BEC rating. I fly hard and only buy ESC’s with a 4 amp or larger BEC’s

I buy the cheap ones, occasionally one of my ESC’s fail. I always keep a couple of spares on hand & just pop in a new one. Then place an order for a replacement and keep on flying while I wait on the slow boat to deliver my replacement.

...I'd guess it time dependent. (for #1) are the answers different for a 30 second burst and for full flight duration.

Yes to some extent, but it’s more on how accurate is the manufacturers rating. Believe it or not some manufacturers will overrate their ESC’s.

...If I were to put a thermistor and keep it under some max temperature, would this keep it from failing? Or are there other things beside burning up?

I don’t know, maybe, for me, that would be far more trouble than is is worth. If you have those kind of skills, try it and report back so we all can learn something new.

 

[quorneng]

It is actual MOSEFT chip inside the components on the circuit board that are temperature restricted. Obviously at a steady state an ESC external temperature will give an indication of the actual MOSFET temperature but for most flying a true steady thermal state is never reached. If the MOSFET chip exceeds it temperature limit it fails virtually instantly.
Given the thermal delay period between the actual MOSFET chip and any measurable temperature point does rather explain the need for some amp "headroom" for an ESC.
A higher priced ESC with good quality components? may be able to run closer to its claimed amp limit than a cheaper one.
Really effective ESC cooling can reduce the thermal delay a bit allowing continued operation closer to an ESC's advertised amp limit but as with all ESCs get the load wrong, overload the ESC, it can fail and you lucky not to have a serious crash.

 

[Inq]

Really effective ESC cooling can reduce the thermal delay a bit allowing continued operation closer to an ESC's advertised amp limit but as with all ESCs get the load wrong, overload the ESC, it can fail and you lucky not to have a serious crash.

Has anyone tried doing things like would be done on a Raspberry Pi micro computer? For instance stripping the typical plastic ESC shrink-wrap cover and adding the little aluminum heat sync to the Mosfets.

 

[Merv]

Has anyone tried doing things like would be done on a Raspberry Pi micro computer? For instance stripping the typical plastic ESC shrink-wrap cover and adding the little aluminum heat sync to the Mosfets.

I have never added a heat sink to an ESC, I have bought several ESC's with heat sinks from the factory.

 

[Xamith]

Has anyone tried doing things like would be done on a Raspberry Pi micro computer? For instance stripping the typical plastic ESC shrink-wrap cover and adding the little aluminum heat sync to the Mosfets.

View attachment 233548

I would be very interested in seeing someone try that. I may even take a crack at it on my cheap Chinese ESC/motor combo. I was surprised how warm it got idling without a motor running while setting up servos.

 

[Piotrsko]

Added heat sinks are a "thing" on ESC units in electric car racing. Those units tend to be full on or off for a couple of minutes until the pack is drained. If you want, I have a 2 pound igbt rated at 600 volts, 500 amps used for only 5 years that wouldn't necessarily need a heatsink

Come to think of it, I also have a 200 volt 20C lipo pack that does 16 million mah that I use for traction. It's water cooled, too.

 

[Inq]

I would be very interested in seeing someone try that. I may even take a crack at it on my cheap Chinese ESC/motor combo. I was surprised how warm it got idling without a motor running while setting up servos.

I burned up the motor in one of those A2212-1000KV / 30A ESC / 10x4.5 kits before it burned up the ESC.

 

[Merv]

I burned up the motor in one of those A2212-1000KV / 30A ESC / 10x4.5 kits before it burned up the ESC.

Sounds like you should put the heat sink on the motor.

 

[quorneng]

You mean like this.

A 'fingered' heat sink (twice as efficient compared to solid lines) fixed directly to the ESC MOSFETS with heat conducting paste. Although the ESC is inside the airframe the fingers stick out into the airstream.