Part throttle efficiency

[Simon Chaddock]

How does an ESC control the current to a brushless motor?

I assume it does so by a pulse proportional method but does the motor actually see the pulses or do the capacitors even them out?

Of course if I had an oscilloscope I could see it directly.

 

Question.

Is there an efficiency penalty in installing a higher voltage battery than you actually need and use only part throttle or would it be better to install the same battery weight but with a lower voltage and use nearer full power?  

[Bert]

Brushless ESCs have to switch (commutate) at all throttle settings, however they are less efficient at part throttle.

 

Bert

[birdy]

To my understanding, the power is always supplied to the motor at the batteries voltage, but for varying amounts of time. The higher the throttle the longer the pulses / the faster they come. Err... is that any help?

[Myron Beaumont]

I too can't understand how "clipping" pulses whether voltage or current or maybe a combination alters the actual speed of the motor . Maybe I'm just trying to apply too much of my logic to something I can't get my head round . I appreciate that what we need is probably a highly technical explanation . OK I can handle it ( I hope! ) . Might give a few of us some answers to "lektrikkery " .What do you think ?

Where's Timbo?

[Myron Beaumont]

PS

I keep thinking of the usual water/ electricity analogy & think of a water wheel combined with something that restricts the water flow (hence-not completely full ) to the buckets or whatever you call them (scoops maybe) In other words a mixture of potential energy & kinetic energy OR  Voltage& current '.

Am I making sense I wonder ?

[Tim Mackey]

I'm here, but not likely to satisfy your advanced  technical thirst I am afraid.

I aint that tecchy, or indeed that bothered - all I know, and need to know, is that the ESC effectively works like a very fast on/off switch, supplying full voltage bursts to the ESC at the correct timing spaces in order to rotate the motor ...the more the throttle is opened, the longer this power is applied for, so the motor spins faster. Think of it like a light dimmer, full power, but in very short pulses, so the filament doesnt get too hot and barely glows. The longer the on/off pulses are applied the hotter the filament, and the more it glows IYSWIM

I am an electrric flier, with some basic electronic knowledge obtained in tech school a long time ago, far far away in another galaxy - I am not, and have never claimed to be, an electronic engineer or similar.

I like KISS

[Myron Beaumont]

Timbo

Light dimmers don't get hot !

I,like yourself am not bothered about it .Just curious, inquisitive, wanting to know answers etc .

[Tim Mackey]

Well actually they do get warm - but the currents involved are much higher in ESCs so they get hotter.

[birdy]

I liken it a petrol engine with on being a blip of the throttle, and so if you blip more often (say 5000 times a second) then it will spin faster then blipping 1000 times a second.

 

Now I have confused everyone...

[Myron Beaumont]

Timbo

I thought heat was caused by Watts !

A dimmer at 240 volts at I suppose about any where from 0 to 5 or so amps ( 1200 watts !) is controlling a lot of power -Isn't it . Then again I realise it's a whole different ball game - which is precisely what I 'd like to understand better.

Forgive my inquisitiveness  .Can't help it .Maybe someone else can explain ?

Nothing personal but there are electronic wizards out there somewhere aren't there ?

eg The guys that make 'um

[Myron Beaumont]

Birdy

I'm no more confused than I am / was

[Tim Mackey]

I think you will find that high current is the main cause of heat?

One of the reasons electricity is fed to the domestic circuit at 25Kv or higher is so that the required wattage can be obtained at relatively low current. Dimmer swiches usually control relatively low power lamps, and I doubt they will be pasing much more than an amp or so.

100 watt lamp / 250 V = .4A

Edited By Timbo - Administrator on 16/01/2010 16:34:03

[Phil Brooks]

Heat is generated when a current flows through a resistance. (Watts=Current x resistance x resistance).  A perfect switch either has infinite resistance, and therefore zero current, when it’s open, or zero resistance, when it’s closed. Either way it does not generate any heat. This is why digital techniques are now so widespread. Electric light dimmers and ESCs are not perfect switches, all solid state devices take a short but finite time to switch from one state to the other, and during this period a small amount of heat will be generated. Domestic dimmer switches are not over constrained by size limitations, and are fitted with a big enough heat sink to control the heat build up. A brushless ESC delivering 40 or more amps for several minutes obviously gets warm, but if it tried to control this sort of current by analogue means it would melt within seconds.

[Bert]

Brushed motor ESCs and dimmer switches simply pulse the current to the load. They are most effiiciient when fully on - wide open throttle or maximum brightness.

 

Brushless ESCs not only pulse the current to control the speed but also commutate the supply to the coils in the motor. In brushed motors the commutation is done by the brushes and the commutator.

 

Because they have to commutate the supply brushless ESCs are always switching the supply to the motors and as a result waste some energy in the form of heat at all throttle settings.

 

They waste less energy at full throttle than at part throttle because at full throttle they are only commutating the supply and not pulsing it to control the speed.

 

Bert

[Simon Chaddock]

Unless of course the ESC simply alters the duration of the commutated pulse, then the switching efficiency would not be effected by the throttle setting.

 

Actually what I was trying to understand was the overall efficiency.

The total power in a battery is measured in Watt Hours. So a 1Ah 12V battery has the same power capacity as a 2Ah 6V one. Each can deliver a total of 12Wh and should weigh more or less the same.

Say you needed 24 Watts to fly would you get the duration with the ESC in the 12V set up throttled back to show 2 amps or the 6V one running at 4?

 

The conventional answer is the 12V 2A set up would more efficient as it would have only 1/4 the resistance losses. However is this really what is happening or is the ESC actually providing a full voltage pulse, and thus an 8A current, but for only 25% of the time?

Looked at it like this the 12V resistance losses would actually be 4 times greater!

 

I appreciate it is rather obtuse but I am seeking the most efficient part throttle set up.

Edited By Simon Chaddock on 16/01/2010 19:20:37

[Peter Beeney]

    Phil,
        Perhaps I can add some of my confusion to your excellent synopsis. As you said, ESCs are not perfect switches, because of the switching periods. One of the judgements of the quality, and thus in some measure the expense, of a MOSFET is the value of it’s forward, or ON, resistance. Or rather, the lack of it. The lower, the better; but, consequently, a proportional higher cost. So I’m a bit suspicious that the cheaper brands of ESCs do not always use the most expensive, and thus lower resistance, FETs. Thus the ON resistance might have a significant part to play when on full throttle, adding to the heat generation. This could be another good example of how doubling the resistance increases the power dissipated fourfold. Making your little formula earn it’s keep, let’s say, hypothetically, the resistance is 0.1 Ohm. The current 40A. Then, 40 × 0.1 × 0.1 = 40 × 0.01 = 0.4W. Doubling the resistance to 0.2 Ohm, then 40 × 0.2 × 0.2 = 40 × 0.04  = 1.6W. A fourfold power increase. I think this might account, in some small part, the reason why I seem to see probably too many cases of obvious overheating. In one or two cases, very spectacular, the ESC exploding in a shower of sparks, nothing left!
    Going right back to the top of the thread, Simon’s efficiency question, this might be a thoughtful discussion. I’m not always convinced that we actually pay enough attention to the propeller/motor/battery combinations for maximum efficiency, and thus maximum performance. There might be a trade off here. Again, a dynamometer might be a useful tool.
    Some experiments with the amount of watts used might be a start.     PB         

[Peter Beeney]

   Simon,

             Apologies, a crossover posting by me, I fear!

[Brian Parker]

Connected a 'scope probe to one phase of a brushless inrunner (cheapo unmarked 2000kv) powered by a 3S lipo. Motor is fitted to an X-ite wing, prop removed for test. Excuse balsa dust on screen.
 Traces start from a very slow idle, low throttle, high throttle and at max revs.

Scope set at 2volt/div at 5ms/div.

Digest. 

 

I may set up a test rig (to enable connections to be made without the risk of shorting and meltdown of model) and do some proper testing. 

 

[Simon Chaddock]

Thanks Brian

I hoped someone would have a scope!

The breaks in the traces suggest that the ESC power modulation is at a fixed frequency independent of the commutation although I am not sure why the trace appears to vanish rather than drop, or perhaps it does and its simply beyond the definition of the screen to be visible.

 

With a prop load, thus slower speed, would it be possible to set the scope to examine the voltage over a single commutation? I suspect the time scale would have to be set to a fraction of a ms per division to show it.

 

[Brian Parker]

  Simon,
I did experiment with various time scales but only recorded/posted the set at 5ms.
The problem with fractions of a ms. is holding the 'scopes syncro long enough to take a photo (probably because of the low quality of the ESC) although its not impossible.
The motor tested is a bit of a screamer, so will test a slower outrunner under load and post results, perhaps also including Power, RPM etc per trace.

Might be a couple of days or so before I post results.

[Jimbo565]

Just seen this on Google

 

 

Regardless of the type used, an ESC interprets control information not as mechanical motion as would be the case of a servo, but rather in a way that varies the switching rate of a network of field effect transistors, or "FET's."^[1] The rapid switching of the transistors is what causes the motor itself to emit its characteristic high-pitched whine, especially noticeable at lower speeds. It also allows much smoother and more precise variation of motor speed in a far more efficient manner than the mechanical type with a resistive coil and moving arm once in common use.

Most modern ESCs incorporate a battery eliminator circuit (or BEC) to regulate voltage for the receiver, removing the need for receiver batteries. BECs are usually either linear or switched mode voltage regulators.

DC ESCs in the broader sense are PWM controllers for electric motors. The ESC generally accepts a nominal 50 Hz PWM servo input signal whose pulse width varies from 1 ms to 2 ms. When supplied with a 1 ms width pulse at 50 Hz, the ESC responds by turning off the DC motor attached to its output. A 1.5 ms pulse-width input signal results in a 50% duty cycle output signal that drives the motor at approximately half-speed. When presented with 2.0 ms input signal, the motor runs at full speed due to the 100% duty cycle (on constantly) output.

[edit] Brushless ESC

Brushless motors otherwise called as outrunners or inrunners have become very popular with radio controlled airplane hobbyists because of their efficiency, power, longevity and light weight in comparison to traditional brushed motors. However, brushless DC motor controllers are much more complicated than brushed motor controllers. They have to convert the DC from the battery into phased AC (usually three phase) in order to produce the changing magnetic field.

The correct phase varies with the motor rotation, which is to be taken into account by the ESC: Usually, back EMF from the motor is used to detect this rotation, but variations exist that use magnetic (Hall Effect) or optical detectors. Computer-programmable speed controls generally have user-specified options which allows setting low voltage cut-off limits, timing, acceleration, braking and direction of rotation. Reversing the motor's direction may also be accomplished by switching any two of the three leads from the ESC to the motor.

 

Hope this helps,far too technical for me.

 

 

 

 

 

 

[ericrw]

Ooooo,er! I think I'll go back to IC !! Eric.

[Simon Chaddock]

Thanks Brian

I look forward to seeing the results.

[Brian Parker]

Made up a test rig for a 2812 outrunner.
 Not much success in obtaining a clear and steady trace (budget items).

 Did manage somehow to de-program the ESC though, and have been unable to co-ordinate the beep..beep….beep………….beeps to obtain anything other than wailing banshees. Perhaps the ESC has given up the fight.

 Did have a look at the 'beeps' on the scope, it beeps on one phase only.

 Connected the scope back to the inrunner and ran some tests using the duel trace to show the relationship between the phases.

 Used 2mv/div with X5 magnification of horizontal trace (steadiest wave form).

 The traces with channels Ch1 & Ch2 set at different Volts/DIV is to show the phasing more clearly. Third phase position readily determined from this trace info.

Not enough natural light to obtain RPM.

Pulls around 15Amps at 1/3 throttle and 30+ at full RPM.

First trace 1/3 throttle (unequal Volts/DIV). Second, dual trace at idle. Third, Single trace at Full throttle. Last trace is both channels at equal Volt/DIV.

 

 

 

 

 

Edited By Brian Parker on 19/01/2010 20:53:29

[Simon Chaddock]

Brian

Interesting. Pity you upset your ESC on the 2812 test.

 

The second trace (at idle) does seem to suggest that the ESC controls the power by pulsing within  each commutation. What of course is not clear, as this is at a single speed, is whether the power is varied by altering the pulse duration or by actually limiting the voltage and thus current in each pulse.

 

Altering the pulse duration seems logical to me, but then I am not an ESC designer, as the pulse frequency can remain the same but when their is no gap between pulse you have full power, however from an efficiency point of view at part throttle each short pulse would incur full power resistance losses.

 

Would it possible to get an equivalent single commutation trace at low power to see if the pulse gap ratio has altered?