Brushless ESC to motor voltage question

[AJ2290]

Hi all

I want to understand something I haven't been able to find the answer by googling and looking in various forums.
My setup is a 2S battery charged to 8.4 volts into a Hobbywing G2 stockspec ESC and into a 17.5 motor.

How many volts is the motor getting at full battery charge versus at the end of a race (7.8 volts indicated)? Is the motor getting 8.4 volts from the ESC at the start of the run, and that slowly comes down? Or is it that it can't pass the same amount of amps current to the motor due to less voltage potential? My SkyRC motor analyzer takes whatever volts the battery is at and passes that along to the motor, affecting the RPM shown (e.g. a fully charged pack gives the motor more Kv than the same pack at storage volts). But with the Hobbywing tester, it keeps the volts at 7.4, even if the battery is fully charged.

Can someone help me understand what the voltage and current are at the motor terminals as the pack discharges from 8.4v to 7.8v at the end of a race?

 

[PonkyKong]

The battery will get voltage drop under load. So your motor gets around 7.5 to 7.75 volts at full throttle.

 

[AJ2290]

The battery will get voltage drop under load. So your motor gets around 7.5 to 7.75 volts at full throttle.

so when the pack is at 8.4v, the motor is getting 7.5-7.75v, and when the pack is down to 7.8v its still getting 7.5-7.75v?

 

[PonkyKong]

so when the pack is at 8.4v, the motor is getting 7.5-7.75v, and when the pack is down to 7.8v its still getting 7.5-7.75v?

No it keeps dropping the whole time. Will drop down to like 6.8 and hit cut off

 

[AJ2290]

ok. how does that affect the current draw? when there are less volts, is the ESC passing less current also? Less volts with same amps is less watts, but is the ESC passing the same amps when the volts are lower?

 

[PonkyKong]

ok. how does that affect the current draw? when there are less volts, is the ESC passing less current also? Less volts with same amps is less watts, but is the ESC passing the same amps when the volts are lower?

Current will spike and peak based on the mechanical load total it's pressing against. Amps are delivered to maintain the voltage as needed. Same speed can vary the amount of amps needed. Hard braking using the esc can also cause voltage ripples that are undesirable. Amps and capacitors correct for that also. Consistent power is from the software and hardware working together.

 

[AJ2290]

ok cool I understand much better now. thanks!

 

[RustyUs]

Open up that ESC's data log. Maybe there are "numbers" in there that will help show what is happening as time goes on during a run.

 

[AJ2290]

Open up that ESC's data log. Maybe there are "numbers" in there that will help show what is happening as time goes on during a run.

it doesn't show volts delivered to the motor. only volts pulled from the battery.