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Old 06-13-2016, 07:35 AM   #1
in2deep
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Default HV servos and voltage limits

Any electrical engineers in here? You guys seem like a smart bunch, this subject has been bugging me lately.

I only own gasser helis now, and have on board generators set up on both of them. I have full sets of HV servos on both as well. I regulate the generated power down to 8.4v system voltage. The entire system is backed up with a small lipo, so the 8.4v from the regulator keeps the lipo charged as well.

So... If a servo is rated for 2s lipo use (8.4v) then is ok to feed it 8.4 volts at all times? Or is the intention that the nominal voltage (7.4v) should be that standard? The argument against is that a lipo quickly drops from 8.4v under load. The argument for would be that if the components are rated for 8.4v at any time, then they can handle it all the time.

Ive seen this practice questioned in the past, but never got what I felt was a well reasoned answer. Over the weekend a guy at my local field was asking about my heli and eventually we got to my generator and system voltage. He thought that 8.4v was a bad idea.

My reasoning has been that as long as I have the power on tap, why not get the max performance out of my servos? Now if its killing them early, that would be a reason to turn down the voltage, but I guess ive not been convinced of that yet.

Any thoughts?
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Old 06-14-2016, 05:24 AM   #2
extrapilot
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Electronics don’t tend to tolerate 8.4v for a minute, but not 5 min. If the servo can idle at 8.4v under no load for an hour- you are not going to hurt its circuitry by running it at 8.4v for 5 minutes. But you may well overtemp the thing at high voltage, where once you start running the motor- the electronics get hot, and the motor gets hot, and those things tend to become a runaway (with increase temperature you get increased resistance- both in the motor copper, and in the control board FETs). And excessive temperature kills electronics and motors.

If you have a closed canopy with a huge motor blowing 250F air on your servos, operating at max torque in a speed run- those may see low life even at 6v.

Until manufacturers define what constitutes a thermal limit, and what cooling is required, etc- this is just going to be a guessing game.

I understand why manufacturers are hesitant to define specs like these. Temperature rise is a function of power used, but also of cooling. Both are variables that are nearly impossible for a hobby pilot to predict or measure. Heck, there is a massive difference in cooling efficiency at 40mph and 80F and 60mph and 60F… How do you evaluate a mix of 10 seconds of tic tocs followed by a FF pass as regards average cooling flow?

Best option I can suggest would be to write to the servo manufacturers in question- and ask them if you are approved to run to rated torque for 5 min at 8.4v and at whatever ambient temperature you see in your area… At least you will get a response in writing- and if it is approved, you have cause for RMA if you see problems. Doesn’t help what will probably be a crashed machine- but if they are confident enough to approve the servo at that voltage- they have at least some confidence in the thing-
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Old 06-14-2016, 09:00 PM   #3
ticedoff8
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I am an EE.
I suggest you run your 8.4v rated servos at 8.4v for as long as you like.

The electronics are rated at 8.4v without a time limit for a reason.
That is because they are using components that are rated at 10v or higher.
To do otherwise would be ridiculous.
When they designed these servos, they did not try to stick a finger in the air and say "Well, I believe we can survive an input voltage of 8.4v for about 10 minutes. Then the servo smokes".

Heat is not a factor either.
As long as the load does not change, a higher input voltage draws less current (the watts stay the same).
The components are designed to work within that voltage range, so they are not dissipating any additional heat.
The key is the load on the servos.
Sometimes, higher voltage allows a higher load (more torque).
But, if the load is constant, then watts are the same regardless of the voltage.

Personally, I run two helicopters with Scott Gray Reactor HVX.
The HVX has 2 internal, programmable regulators and uses an input voltage range from 2S to 4S LiPo (I use a 3S LiPo).

I run the Align BL815H / BL855H servos that are rated at 6v to 8.4v
And, I set the two internal regulators to 8.4V

If there was any issues running HV servos that are rated at 8.4v at an actual 8.4v, you would see the forums blowing up with comments about the Reactor HVX smoking servos.
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Old 06-14-2016, 09:12 PM   #4
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Yeah I don't see a problem either I run all my helis with becs except one so they all stay at a high voltage without a lipo sag. Though dont quote me but I swear I do remember reading that if you run bks at max voltage you will reduce their life over lower voltage. So if you're worried about longevity and they perform well at a lower voltage you could just reduce it.
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Old 06-15-2016, 12:14 AM   #5
extrapilot
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Quote:
Originally Posted by ticedoff8 View Post
I am an EE.
Heat is not a factor either.
As long as the load does not change, a higher input voltage draws less current (the watts stay the same).
The components are designed to work within that voltage range, so they are not dissipating any additional heat.
The key is the load on the servos.
How did you graduate with insight like this:

“Heat is not a factor either. As long as the load does not change, a higher input voltage draws less current (the watts stay the same).”

That is absolute nonsense. This is PWM mate- when the bridge is on, the current flow for any given motor RPM is purely a function of voltage. You increase voltage, you INCREASE current. Period. Doesn’t matter if the duty cycle drops- the thermal loss is I^2R, so the increase in loss more than offsets the reduction in duty cycle for a given output power.

And all this assumes the same work done. Except- in effectively all cases, a cyclic or tail servo on an FBL machine will do more work at higher voltage.
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Old 06-15-2016, 01:03 AM   #6
extrapilot
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Simpler to show actual data. This is a basic servo (2S), run at 6.18v and 5.15v. The test was simple 0-60-0deg cycling with no external load. Command signalling is the exact same frame rate, rate of change, etc. Signal generator is precise to 10ns. Sample rate is 2 billion samples/sec.

The plotted line is servo current, measured in line (the servo power lead itself).

What is relevant is that at 6.18v, the peak current is 25% higher than at 5.15v. What is more relevant is something you cannot see on these screens- the integration of the current data over the sample window (here, 20ms time). That is- if you add up the current measured at each of (huge number of) samples in the window, you get the total current flow.

When you do that, at 6.18v, the servo eats about 33% more total current. If you multiply the total current by the voltage, you get power used in that time window. Here it is 60% higher at 6.18v vs 5.15v.

Doesn’t mean all servos behave like this. Some will show less difference, some more. But unless you have some bizarre servo with a switching regulator onboard- they are not constant power devices, and throwing more voltage at them effectively always means more power/heat.
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Last edited by extrapilot; 06-15-2016 at 05:13 AM.. Reason: typo
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Old 07-08-2016, 02:23 PM   #7
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Interesting...

On my UAV heli, I found that with the body installed, the motor was getting hot, so I've added ventilation fans to draw more air out. But I wonder if I should be concerned with the servos also being in stagnant air? I'm using Futaba BLS275HV which are full aluminum, and being Futaba, I would assume well engineered. But it's difficult to check the temps with the body installed.

If it's an issue, I wonder if simply using thermal glue to stick some heat sink to the case would be enough to help.
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