Noob Electronics Question: Idle Current

At any throttle value, is idle current part of the total current? e.g. is the total current = idle current + current due to work on prop? or does the idle current get ‘absorbed’ into useful work when a prop is on?

also how does idle current vary with voltage / throttle?

according to this site: Brushless Motor No Load Current -

10v is chosen as the test voltage because the idle current ‘doesnt change much’ from this value. is it reasonable to assume at 5v the idle current will be close?

this 272W motor has idle current of 0.8A: S500 V2 Kit,S500 Kit
and this 2915W motor has an idle current of 0.5A: KDE7215XF-135 | UAS BLDC Brushless Motor | KDE Direct

the 10x more powerful motor has almost 40% LESS idle current LOL. what a difference quality makes.

edit: or maybe its Kv (920 vs 135)

Idle current, or no-load current is always part of the total current drawn.Two similar motors with a different no-load current would definitely give a different hover current.
You wouldnt be able to directly compare two motors of totally different kv rating and use-case. The two motors you list will have a difference number of poles, windings, wire size and may other specifications.

I suggest some hours working in ecalc.

I suspect that the “idle current” you speak of will be most noticeable for making the motors beep while the system is disarmed. The no-load current will probably pale into insignificance once props are on and motors are doing actual work.
On the other hand, every bit of efficiency and battery life can be worth it. I’ve no doubt the no-load current will contribute to the total current in flight and flight time.

Modern ESCs are capable of running the motors very efficiently and will only supply the required current and voltage to the motor windings in order to operate the motor reliably at the demanded RPM. I believe older ESCs operate much more simplistically and may compound the no-load current problem.

I would say it will be just as important to choose modern ESCs designed to be efficient and get the most out of the chosen motors. Some ESCs actively recover energy whenever possible, like when decelerating the props.
No point spending more time and effort and money choosing an efficient motor, only to pair it with an ESC that is years old running some ancient/unknown version of firmware.

1 Like

can you elaborate? how would an old esc waste additional energy when running a prop?

issue with most motors, escs, props made in china is that there is no data or documentation. thats why im going with kde direct for when efficiency is important. i wont have to worry about badly engineered motors/escs then.

The APD ESCs have actual data and you can call or email them for further details.
Old ESCs were never designed to return energy to the battery, and were lucky if they did anything more than very basic braking

1 Like

just to clarify, the 0.8A no-load current in the first motor i linked S500 V2 Kit,S500 Kit

is always present? if so, that is a very substantial portion of load current. for example, 30% throttle is 1.44A. unless no-load current drops at 30% throttle, most power is wasted.

Only when the motors/ESCs are armed and spinning, but at that stage the no-load current will be eclipsed by loaded current draw if you have props on.
It is almost a useless measurement.

To clarify, once the motors are spinning props, there is essentially no such thing as no-load current.

kind of contradicts this though?

the discussion here and apparently ecalc disagrees with your latest statement: how does no-load current affect motor? - RC Groups

are you sure you know what you are talking about?

I’m not sure exactly what you’re trying to figure out but here’s my two cents worth. I just went and plugged my X500V2 through my multimeter. On the bench with all hardware connected but no blades turning it draws between .15 and .2 amps. There was a peak of about .4 amps while the ESCs sang their startup tones. I know when it’s flying in it’s current configuration it draws about 20amps. So basically the current drawn on the ground is insignificant compared to flight.

The amp-meters on most flight controllers are useless below 3 or 4 amps so don’t pay much attention to what your telemetry is saying on the ground.

And the link you posted is 10 years old. ESCs, along with all the other parts we’re using having changed a ton since then.

I guess my point is that the no-load current really doesn’t matter, unless it’s something crazy like 10% or more of what the motor draws under load.

Do you know what you are asking? Because you compared 2 motors that are worlds apart.

Although there’s good explanations of no-load current within that RC Groups thread, much of it is out of date due to far more intelligent ESCs these days.
Just read the BLHELI doco to understand how complex the inner workings if an ESC are these days. And keep in mind BLHELI can work on a wide variety of hardware from many manufacturers. Then take a very specifically designed ESC with customized firmware for commercial and industrial use, high reliability…

A few of the comments mentioned heat, and that’s probably going to be a good indicator.
In our case we can check our logs periodically for RPM, temperature and current. Particularly at idle and hover - that may tell us if bearings need replacing, probably a significant contributor to motor temp and reliability.

I’m probably drifting off topic too much.
The problem is we’ve started talking about idle current (implying props are on, ready to fly), then morphed into no-load current (an isolated motor measurement).
Some of that is much more under control with modern ESCs, and some of it doesn’t matter if flight performance and reliability satisfies your requirements.

Look up copper losses and iron losses,

the question isnt whether idle current is or isnt significant portion of current draw under load. the question is ‘is idle current part of total current draw under load’. im not sure why ppl are trying to impute whether it is negligible or not. it COULD be non-negligible if throttle is very low, and you have a high kv motor. so for clarity ill repeat the question with some qualifiers ‘is idle current, as specified, part of total current draw under load at same idle current test voltage.’

its a really simple basic question (i thought) based on physics. doesnt matter if a response is 10 years old.

i just gave you an example. 1.44 under load (30% throttle 16V), 0.8A no-load current (10V). assuming no-load current holds at all voltages, does this mean most power (at this particular throttle) is mostly turning the motor and not the prop?

thats why im asking the basic question is no-load current present when the motor is under load. im NOT asking if it will or wont always be a significant amount (everyone knows at peak operation it should be negligible unless its a bad motor). but when you are off-peak such as at really low throttle can the no-load be most of the power consumed, like in my example.

i was just pointing out how 2 motors of vastly different power ratings can have similar no-load currents. it was a tangent that had nothing to do with my question, but you are trying to make it seem like i am making no sense or something?

the no-load current question is very basic, and has more to do with motor losses than with ESC quality. you are just spouting generalities and random details about old ESCs. if you dont know the answer, you dont have to muddy the waters.

ill repeat your quote again…

you imply they are same, now you are saying they are different?

Are you trolling me?

yes, that is what no-load is due to. so my question should be really simple. but @xfacta keeps bringin in ESCs lol

Anyways I am going to assume that if you measure no-load current at all throttle values, you can subtract those values from prop-on current measurements to get a better idea of the actual power consumed by the prop. The relationships within one motor (e.g. throttle at 30% vs throttle at 60%) can then be more accurately transferred to another motor, accounting for no-load current at low throttle values. yes, i know motors and escs are different but if you are using modern escs and motors across the board this is as accurate as you can get.