ScottLaroc

I have a hpi sprint to flux with stock motor and mmp esc with 2s 5300 30 to 60c , my question is why when I put all the same elctronics and battery in my direct drive 2wd pan car it cuts off when I give it full throttle(when I punch it)? .. Thanks

mikpiz

iTrader: (7)

Most likely gearing , if the motor turns too many rpms the ESC will shut off , try a bigger pinion gear and see if that fixes it.

mkiiina

iTrader: (71)

???? Not likely. 1) A bigger pinion will not change how many RPMs the motor turns and 2) the esc doesn't know if you have a 25.5 or a 5.5 motor hooked up so there isn't a way for it to "limit" the number of rpms it will turn.


The only thing I can think of is amp draw causing shut down perhaps? What motor are you using?

BlueGlowBoy

iTrader: (2)

MMP? Browning out under full throttle? If it were my money, I'd bet that either a glitchbuster capacitor or an external BEC may solve your problem.

Are you running a Spektrum radio?

ThePanda

iTrader: (68)

The castle motors have a 100,000 rpm motor limit though.

mkiiina

iTrader: (71)

That's an engineering limit for the rotor, not the esc. Also since he was only running 2s it will never hit that limit. Lastly, castle motors are sensorless so there is no way for the esc to know how fast it is going (sensored is a different story)

BlueGlowBoy

iTrader: (2)

True


True. At 8.4v full charge, you would need a 12000kv motor to get 100,000 rpm. Only a few of their motors are near that, and they are airplane motors. Ground stuff is usually 8000kv or less for 1/16 scale, and 5000kv or less in 1/10 scale.

Not true. The sensor isn't there to tell what your RPM is, it's there to get a more accurate reading of the position of the rotor. The ESC MUST know which phase the rotor is in, or it wouldn't know when to switch to the next phase, and your motor would not run. When a motor is sensorless, the ESC can tell where the rotor is by inductance, plus even without that, it would know each time the rotor has completed one rotation because the ESC has complete one full sequence of energizing A,B,C,-A,-B,-C . If the motor does anything different from what the ESC tells it to do, your car will not drive well at all!



See this video for how a brushless motor works.

ThePanda

iTrader: (68)

That's what i meant. I was disagreeing to this post. Its not that the motor is Going to fast, 2s is not enough voltage.

RogerDaShrubber

iTrader: (6)

Of course it does, acceleration is rpm / time, so a bigger pinion will change the acceleration curve, thus giving less rpm over time initially and also will lengthen the duration at which peak rpm are met. Also, RPM is not a static constant for any system, it is an exponential which declines as gearing is increased.

Geezatec

Recalibrate your TX & RX. Things probably change between them. Even though I the same component.

mkiiina

iTrader: (71)

I am speaking strictly about peak rpm at the motor, not how long it takes to get there. Theoretically you can run 1:1 and with a long enough track you will still hit the same RPM (ignore aerodynamics, friction, etc), will take you alot longer to get there but the motor will not be spinning any faster at peak than if you had a 10t pinion installed.

mooby64

iTrader: (3)

+1

RogerDaShrubber

iTrader: (6)

And this still will not happen. As you increase gearing, you increase load on the system, which has a two fold effect on rpm, firstly it will reduce the max rpm and because the system has to work harder to overcome the resistive force of the load, it becomes less efficient and wastes more energy to heat, all of which reduce the rpm potential of the motor.

All this can be seen when you dyno a motor under load, take any motor and dyno it with and fdr of 8 and 4 and watch the rpm drop. Sure a motor might have a potential max rpm of voltage x kv but after that physics takes over to limit that max rpm all over the place.

mkiiina

iTrader: (71)

This will not fix the OP's issue and we are getting way off topic. If you were to increase the size of the pinion you would only generate more heat as the motor could not overcome the load. If you read the post I was responding to initially you will see he was talking about the ESC thinking the motor was over-revving. If you have documentation on the MMP having this capability I will gladly take a step back and admit I made a mistake.

Be helpful or be quiet is the bottom line.

RogerDaShrubber

iTrader: (6)

Well there is not enough information to make a call one way or the other. Really need to know what the FDR are for both cars, as that is the only difference here, same esc, same motor, 2 different cars and one produces this issue, the only difference is gearing, and if i knew the FDR of both i could give him a solution, but for argument sake, it is either under geared or over geared. Assuming the same radio gear is being used, as that is the only other place this issue could creep in.