BEC output voltage

[Tim Donald]

Hi all, another electric question if I may?

My Pilot-RC Laser 67 has all high voltage servos and a high voltage receiver. The ESC output can be set to anything up to 8 volts. I have set it to be 8v as the servos give the best performance at that voltage on paper. Is there any benefit to using 8v or is this a bit risky? Doesn't seem much point in slowing servos down by lowering the voltage if this is not necessary? It's a 6S 5000 LiPo so battery drain should not be an issue.

[Nigel R]

Have to say, myself, I'd not use a BEC up at this size. And without knowing much more I'd say put it back to 4.8V.

If that is a BEC on an ESC I'd want to be very certain it was capable of genuinely supplying sufficient current to the servos at that voltage. Most budget ESCs are, how can I say, "generously described" as compared to real ability.

I'd want to know how much juice the servos will draw at high loads (see e.g. stall current rating) and work backwards; you may well find your servo wiring might not be up to it, the RX block may not be suitable, that's before getting to the BEC itself.

Just some guesses and fag packet maths, you're flying a 20cc class 3d model, so some fairly chunky digitals, Hitec HS5625 or similar - they can pull 2.5A at 4.8V - so scale up for 8V; 5A per servo, that's 20A across four servos as a headline figure to work with from the block the servos plug into back to the BEC.

[Tim Donald]

Thanks Nigel. It doesn't seem much point in paying for hv servos / receiver then running at 4.8v? The servos are MacGregor MG5921HV but I can't find current draw specified. The ESC is a Hobbywing Platinum 120A which includes a BEC with 10Amp continuous and 25Amp peak output. I am told that these are very reliable but have no first hand experience. I think the general idea is that the rx and servos can all run directly from a 2S Lipo so 7.4v (max 8.4 fully charged). Having said that - if the performance difference is too small to make a difference then I could set the BEC to 6.6v to be on the safe side but this is what I'm trying to establish.

I'm thinking I might add an Optipower Ultra Guard 430 just in case the ESC does fail. There is quite a lot to loose if it does fail !

[Engine Doctor]

Make sure the Bec delivers adequate amps. If you set it too low and servos are hungry you risk the Rx browning out.

[Trevor]

FWIW, in my experience HobbyWing ESCs are very reliable. I have a dozen or more in regular use and have never had any sort of issue.

Trevor

[Tim Donald]

Thanks Doc / Trevor.

[Nigel R]

OK less need for guesses now !

Id be inclined to go with a separate lipo myself.

That said I have had no problem with hobbywing escs.

Edited By Nigel R on 19/10/2020 19:00:30

[Chris Walby]

If everything is rated for 8V I would select 8V after all that's what its designed to do.

Not much point paying money for HV servos, RX capable of 8V and not using it

If you are concerned about browning out the RX then with the model tethered and throttle cut on, rapidly (as fast as you can thrash the TX sticks in every corner) drive all the servos for a few seconds on high rates. Admittedly there is no wind loading on the control surfaces, but then again most people don't thrash the sticks in the corners either when flying. Then check the RX is not showing signs of loss of signal.

PS - I am not convinced that doubling the BEC voltage will double the current as there are efficiencies in running higher voltages (are the servos twice as powerful and twice as fast?).

[Nigel R]

High speed tumble are sticks in corner at speed.

Broadly speaking in a dc motor a doubling of voltage results in double the current and quadruple the input power. Output power increases less so because of inefficiency, the difference has to go somewhere... heat.

[Frank Skilbeck]

Posted by Nigel R on 19/10/2020 19:07:09:

High speed tumble are sticks in corner at speed.

Broadly speaking in a dc motor a doubling of voltage results in double the current and quadruple the input power. Output power increases less so because of inefficiency, the difference has to go somewhere... heat.

The current will depend on the force it's required to apply, so if the load remains the same then the work the motor has to do is the same, so if the servo torque output is the same then the high voltage servo would draw a lower current.

[John Lee]

I'm with Frank on this, the higher the voltage the lower the current for any given load.

But if you want to check the draw at various settings this little gadget is a useful tool.

And finally I would highly recommend the Optipower Ultraguard, mine saved one of my large scale models following a power failure.

[MattyB]

Given you have the Ultraguard I would say just set the ESC BEC at 8V and go fly. I would definitely use telemetry to monitor if/when the Ultraguard is kicking in though (easy to do as it always supplies 0.5V less than the primary supply voltage detected at startup).

[Simon Chaddock]

Questions.

When does a servo draw the most current, assuming it is not actually stalled?

When it is travelling or when it is accelerating up to speed?

Will that acceleration be greater, and thus the current drawn, with a higher applied voltage?

[Frank Skilbeck]

All electric motors draw the most current when they first start, but this would also depend on the electronics that control them. On large industrial motors it is common to put in a "soft start" to reduce the starting current, and if possible start the machinery with no load on it.

Bit more tricky on a servo as the resistance on a control surface will increase as it deflects more, so for an unloaded servo the current will be greatest as it starts to move, but on a plane that is when it will have the least load, so for say flaps the largest current may be as they approach maximum deflection.

BTW on an electric plane, you will see the highest currents as you bang the throttle open and the motor accelerates to speed. You can also see the motor current increase and decrease at a constant throttle setting as the load on the motor changes depending on whether the model is climbing or diving. 

Edited By Frank Skilbeck on 20/10/2020 08:22:22

[Former Member]

[This posting has been removed]

[FlyinFlynn]

Posted by Chris Walby on 19/10/2020 18:58:35:

PS - I am not convinced that doubling the BEC voltage will double the current as there are efficiencies in running higher voltages (are the servos twice as powerful and twice as fast?).

A certain Mr. Ohm might beg to differ with your assertion.

[Former Member]

[This posting has been removed]

[Bob Cotsford]

Posted by FlyinFlynn on 20/10/2020 11:35:55:

Posted by Chris Walby on 19/10/2020 18:58:35:

 

PS - I am not convinced that doubling the BEC voltage will double the current as there are efficiencies in running higher voltages (are the servos twice as powerful and twice as fast?).

 

A certain Mr. Ohm might beg to differ with your assertion.

Doubtful as Mr Ohm was referring to fixed resistance loads whereas a servo motor is a variable inductance load. A higher voltage means that there is more torque available approaching a stalled state but I'd hope no-one here is running servos that near the limit.  For the same load a higher voltage will need less current - think of it in terms of work done, they both need the same rate of energy supply (watts) regardless of whether that energy comes as low voltage high current or high voltage low current.

Edited By Bob Cotsford on 20/10/2020 11:50:18

[Nigel R]

"The current will depend on the force it's required to apply, so if the load remains the same then the work the motor has to do is the same, so if the servo torque output is the same then the high voltage servo would draw a lower current."

Erm. With that I must take issue. Separate the ideas of holding a constant position against load, and moving there from a very different position.

In the first case, yes, with constant load then higher voltage would mean lower current and same overall power (ish).

In the second case, no, the movement would be faster, draw more current, would use more power, albeit for a briefer period. Total physical work done would remain the same. The rate at which it is done is quicker.

" For the same load a higher voltage will need less current - think of it in terms of work done, they both need the same rate of energy supply (watts) regardless of whether that energy comes as low voltage high current or high voltage low current."

As above.

Work is done when the servo moves. It is performed at two different rates with two different voltages. The energy is transferred faster with a higher voltage. They don't 'need the same rate of energy supply' - the higher voltage will mean higher current drawn by the motor and thus a higher power - for a shorter time. Total work is the same. Power, i.e. rate of work, or watts, is different.

[FlyinFlynn]

Think of it however you like....I am going to stick with proven electrical theories that have stood the test of time. The assertion was that if you double the voltage the current wouldn't double.

Edited By FlyinFlynn on 21/10/2020 10:55:07

Edited By FlyinFlynn on 21/10/2020 10:55:29

Edited By FlyinFlynn on 21/10/2020 10:58:01

[Nigel R]

Motors are not resistors - that's correct it won't double; for double voltage, somewhere approaching double current, say 175%.

[Frank Skilbeck]

Posted by FlyinFlynn on 21/10/2020 10:50:48:

Think of it however you like....I am going to stick with proven electrical theories that have stood the test of time. The assertion was that if you double the voltage the current wouldn't double.

Edited By FlyinFlynn on 21/10/2020 10:55:07

Edited By FlyinFlynn on 21/10/2020 10:55:29

Edited By FlyinFlynn on 21/10/2020 10:58:01

what like Power = Volts x Amps? so if you double the volts and amps you get 4 x the power.

[Tim Donald]

Thanks for all your help. Interesting reading. My friend has just bought a cheap onboard watt monitor to measure servo power. If it works ok I might get one just out of interest and to do some experimenting.