Question: which should be the "weakest link": Lipo - ESC - Motor ?

[Familyman67]

Hi everyone,

I am thinking about upgrading my Blade 300X with a new ESC and motor. While ordering parts I asked myself how the three components -LiPo-ESC-motor- should be aligned with each other, or in other words: Which should be the "weakest link in the chain" in order to avoid the other components from beeing stressed out.

My theory is, that the ESC should be the weakest link.
Background for this:
- if the ESC pushes more Amps into the motor than it can handle, then the motor could get damaged. In return: if the motor can handle more Amps thant the ESC can provide then this is not an issue.
- if the ESC can "suck" more Amps out of the LiPo than the LiPo can handle based on its mAh and C-rates, then the LiPo will constantly be running at the edge of its capabilites and would get hot and bulgy and soon get damaged. In return: if the LiPo can provide more Amps than the ESC can handle, then that's a non-issue since the ESC will only draw what it needs and is capable of.

Is this a fair statement ?

Thanks in advance.


My plan:
1600mAh 3S 30C/40C Lipo (constant max = 48A, peak = 64A)
ESC: Robby Roxxy 940-6 (constant max = 40A, peak = 50A)
Motor: Turnigy Helidrive SK3 T450 4300 (max = 60A)

[paulsouthport]

Nope, ESC's don't push amps, nor do batteries.
The motor will "demand" amps, the ESC will deliver those amps if it can, and the battery will deliver them, if it can.

So, the lipo needs to be able to produce the amps the ESC demands, and the ESC must be rated to deliver the amps the motor may demand.

[bulldog5046]

ESC doesnt PUSH amps to the motor, the motor PULLs from the ESC.

So, your ESC should have sufficent amp rating for the motor, and the battery should have sufficient amp rating the for ESC.

to answer your question, the motor would be considered the 'weak link' or start of the chain if you like...

Eg, 30amp motor, 40amp esc, battery that can handle over 40amps constant. This is capacity X C rating. Eg. 2200mah X 30C = 66amps continuous

Ryan

[NoFixedAbode]

Re: Question: which should be the "weakest link": Lipo - ESC - Motor ?

Interesting question...I have limited knowledge of electricary but I was under the impression that things were pulled down the line. Ie the motor pulls out the esc and esc pulls from battery. If too many amps are pulled from esc then esc will get hot and fail and the same with lipo.

[Familyman67]

Wow you guys are quick - thanks a bunch.

OK - so the chain starts with the motor, not the ESC - got it thanks.
But one part of my assumption was correct, the battery should be capable of handling at least as much Amps as the ESC will transfer to the motor - right?

So in my case: The motor might demand more Amps from the ESC than the little bugger is capable of providing. in this current example, let's say the Motor demands the full 60A. What happens to the ESC? Will it get damaged right away, or would it just get hot as long as I don't overdo it ?
I mean what's the risk of linking a 60A motor to a 50A (peak) ESC on a Heli as small as the 300X ?

Thanks again.

[paulsouthport]

If the motor demands 60A and the ESC can only provide max 50A the outcome is likely to be bad. How bad really depends on the quality of the ESC, some have more safety headroom than others, but it's quite possible the ESC will cook itself. That may just mean a few melted FETs and the end of the flight and some lovely white smoke, or it could result in a full blown fire.

Is it even possible to fit a 60A motor to a 300X?

[Familyman67]

Thanks! and yes it is possible to put that motor into the 300X but it is very tight and you have to adjust the canopy to fit it - a buddy of mine had it in for a few flights but replaced it by the stock motor after a heavy crash (pilot error not ESC failure) - in hindsight its great he crashed it and put the stock motor in again because otherwise he would have toasted his stock 25A ESC.

So now it's up to me - I "inherited" the motor and wanted to put it in, but after the great feedback from here I will think twice. I am going to replace the ESC anyway, since the stock ESC has some trouble providing constant BEC currant to the Rx, which caused my second 300X to crash heavily. (remainders can be admired in plastic bags).

I am not willing to put an even bigger ESC in, even though Robby Roxxy offers an ESC of the same size that provides a constant 60A currant. The reason for this is my LiPo. I am already using a larger (1600mAh @ 30C) Lipo than stock (1350mAh @ 30C) and the heli is already a bit nose heavy. Of course I could try to get a 1600mAh LiPo with higher C-rate. 40C constant would solve that problem - I have however not yet seen such a 3S LiPo being offered anywhere over here and even if, this would widen the "money drain" even more.

Gee - why didn't anyone tell me about this "side effect" of the heli-disease a year ago ?

[bulldog5046]

The amount of current the motor draws will depend on the load that is put on it. So it's possible when your friend ran the motor it was under very little load and ran at low amp draw.

The only way to be sure is to test it with a watt meter.

I tested a heli motor rated for 30 amps with a 6x4 prop on it and recorded a draw of 60amps at full throttle(!!)

[Familyman67]

So it's possible when your friend ran the motor it was under very little load and ran at low amp draw.

That's exactly what I meant - how much load can a little heli such as the 300X ever put on the motor for it to go for the full Amps ?

[bulldog5046]

I cant answer that, you need a watt meter to work it out.

I use one of these: Turnigy 2 in 1 Power Meter/Servo Meter 1.5 TFT

[cjcj1949]

The motor doesn't draw current. The esc produces a voltage. The motor looks like an inductor. Current starts off at zero and increases. Hopefully the motor turns and the esc senses that. It then switches off the volts on that coil and stops the current rising. The current keeps flowing though, but starts to reduce. The next coil is switched on. That's very simplified. The main point is that there is no real 20amp or 30 amp limit. It all depends on how much mechanical load there is on the motor and the voltage of the lipo. The motor will try to turn at a rate given by it's kv factor. That's the numver of rev per volt. So a motor with a kv of 1000 on a 12V motor will turn at 10,000 rpm. 450's use about 4000kv and 12V. The motor is more complicated than just an inductor, as the mechanical load changes it's electrical parameters as seen by the esc.

There is a lot of faith and prayer involved. When this is combined with cutthroat competition we find the occasional esc goes up in smoke. Manufacturers tend to be optimistic in the specs of motors, escs and lipos and buying bigger than they say you need is usually necessory.

[Familyman67]

Thanks CJCJ, so that brings my fomer question back. How risky is it to run a 60A motor on a 40A (50A peak) ESC ?

[Vikki]

ESC should always be capable of supplying more than the motor requires otherwise you risk burning it out. Wouldn't want that when it's 50 feet up.

Think of the ESC as a fuse. If it's rated at 30A and you drag 60A through it, just like a fuse, it'll pop. Now, if you fit a 70A ESC and your motor only ever wants to draw a maximum of 60A then that's all that will be pulled through the ESC. Just because it's 70A doesn't mean, for instance, that the motor is going to be stuffed with more than it can handle. As others have said, the motor will draw what it needs and no more (we are ignoring stall currents here). This way your ESC has a 10A overhead for those *cough* unexpected little surges.

The MOSFETs that power the ESC outputs have very fine wires in them. If you grossly exceed the current carrying capacity they are destroyed and your heli comes down somewhat sharpish! If you overload by a little and sustain that overload then the devices will heat UP. If they get heat up faster than the heatsink can remove the heat then the devices will get hot enough to destroy the silicon inside the devices. Either way, their demise is assured.

Of course, a bigger motor means bigger ESC and bigger battery, makes me wonder if it's wise to subject a more little 300X to that

Mind you, your motor is overpowered for the application so the load may not be as high as in its normal application but I still wouldn't fit an ESC that was underrated against the motor.

Vikki.

[Familyman67]

OK - I guess I have digested all of this in the meantime.

I was just so eager to put this Turnigy 60A motor in which I would have available at no extra cost - nevertheless, have decided to bite the bullet and get a motor that is just a bit better in wattage than the stock one in order to utilize as much as I can from the new ESC.
Scorpion motors are said to be of reliable quality and the types I currently am thinking about are either the HKII-2205-24 with 3590Kv and 21A or alternatively the HKII-2213-14 with 3585Kv and 32A.

Both would run @ almost the same headspead like the stock motor using a 12tooth pinion (stock = 9T pinion) and would be clearly within the standard specification of the ESC of 40A constant and 50A peak.

Thanks again to all of you for your quick and precise consulting.

[cjcj1949]

Glad to see Vikki back. She's much more upto date than I am. It is difficult to know how much detail to put in. A 60 amp motor should be able to work at 60 amps, it won't necessarily take 60 amps if used to replace a 30 amp motor. However if you change the gearing or increase the volts it will survive when a 30 amp motor would have died. Changing things without understanding the engineering is risky, very often it is still risky when you do understand. True understanding often involves a lot of dead things. Sometimes it costs a lot of money, $50 million for LiFe batteries last year, and a dead company.