Limit on number of servos ... Why?

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Chris - 21 Oct 2008 13:22 GMT

I'm thinking of putting an AXI 2808/24 with a Jeti Advance 30 Plus speed
controller into a Multiplex Parkmaster.

In the instructions for the speed controller it says that if you use a
three-cell LiPo then you should use no more than three servos.

Why is this?
And what happens if you do?

I actually did use four servos with the said motor and speed controller
on an own-design model ... and it seemed to work.
What was I risking?
What might have happened?

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Chris

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The Natural Philosopher - 21 Oct 2008 13:37 GMT

> I'm thinking of putting an AXI 2808/24 with a Jeti Advance 30 Plus speed
> controller into a Multiplex Parkmaster.
[quoted text clipped - 3 lines]
>
> Why is this?

It usess a linear regulator to supply power to them, and this will have
a fair chance of overheating and sh.tting off all servo power if you do;.

> And what happens if you do?

You lose complete control of the model, and depending on exactly how the
receiver BEC and servos behave, some or all of them may go towards the
end stops. Leastways that's what happened to mne.

Fortunately a full aileron/rudder and UP elevator and no throttle was a
repairable spin only.

> I actually did use four servos with the said motor and speed controller
> on an own-design model ... and it seemed to work.
> What was I risking?
> What might have happened?

I've told you.

Its by no means a done deal..I had been fine till I stayed up longer..on
a warm day.

If you are regularly using 4+ servos on 3s+ cells do yourself a favour
and go to dimensions engineerings web site, and order some
PARK/SMART/SPORT BECS to replace the builtin ones in your ESC.

For a few dollars, you may save a few hundred. One hot day this year, I
watched a friends EDF jet pile in at probably over 100mph. The fan
broke, and so did the airframe - completely. The pack wasn't the same
shape either, though it still sort of works.

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Chris - 21 Oct 2008 21:03 GMT

>> I'm thinking of putting an AXI 2808/24 with a Jeti Advance 30 Plus
>>speed controller into a Multiplex Parkmaster.
[quoted text clipped - 33 lines]
>broke, and so did the airframe - completely. The pack wasn't the same
>shape either, though it still sort of works.

Thanks for that - although it leaves me in something of a quandary.
Also, my limited experience of electric flight means I don't fully
understand all you said.

So - if I may prevail upon you to expand ...

Are you saying I should use a different speed controller?
Or a different motor and speed controller combination?

Or do you mean I should modify my existing speed controller?
(That doesn't look possible, at least on the face of it.
Certainly not for the electronically challenged!)

Or should I buy the recommended power pack from Multiplex, which I
sincerely hope wouldn't have this particular problem?

I will be grateful if you do have time to explicate further ... and URLs
would also be much appreciated.

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Chris

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The Natural Philosopher - 23 Oct 2008 07:53 GMT

>>> I'm thinking of putting an AXI 2808/24 with a Jeti Advance 30 Plus
>>> speed controller into a Multiplex Parkmaster.
[quoted text clipped - 42 lines]
> Are you saying I should use a different speed controller?
> Or a different motor and speed controller combination?

Not exactly, no.

This is not easy to put into simple terms, but I will do my best.

Most speed controllers, with a couple of exceptions, come with a simple
linear regulator, which give reasonable cooling works extremely well at
low current draws.

You can say, for example, that you will almost *never* run into problem
with 2 or 3 servos on a 3s pack.

However beyond that, you enter into a zone of uncertainty. Where if
circumstances conspire against you, you may run into overheating regulators.

Things that affect this are:-

- how much voltage is in the pack.
- how many servos there are
- the type of receiver and its current draw
- How long you fly starting with a cool regulator.
- how hot the plane and electronics was before you started it - was it
left in the sun?
- how much cooling the speed controller has
- how much extra heat the controller is generating from being run hard
driving the motor
- how busy a flyer you are on the sticks. (hovering is especially heavy
on servos)

Put all that together, and you have no simple rule. All you can say is
that once over the 3 servo, 3 cell limit, you have a rising chance of
failure the more of the above are in the wrong direction.

> Or do you mean I should modify my existing speed controller?

Basically yes. What you do is bypass the speed controllers regulator
altogether and either use a separate battery for the avionics, or a
'switching regulator'

The advantage of switching regulators is they don't get much hotter with
extra current drawn. So the simple rule 'don't exceed this voltage or
this peak current' can be applied.

SOME newer ESC's have this built in, but after market ones are very
inexpensive.

This is my favorite in the smaller sizes

http://www.dimensionengineering.com/SmartBEC.htm
because it also monitors the battery voltage, and cuts throttle if that
gets too low. Normal ESCS do that, but this one is very nice for twin
engines with tow controllers, where you don't want one motor slowing
down before the other.

For bigger models use the SPORTBEC. That handles up to 3.5A. But
installation is slightly more complex..download the instruction PDF to
see how to do that.

There are other manufacturers, with cheaper and pretty much as good a
product, but I like these guys as the information and service is pretty
good. Especially if you are in the USA.

> (That doesn't look possible, at least on the face of it.
> Certainly not for the electronically challenged!)

All it normally involves is cutting the red wire from the ESC to the
receiver, and plugging in another battery or the new regulator. The
SMART BEC goes in between the reciever and the speed controller, so
needs no wire cutting.

It's less *modification*, than simply bypassing.

> Or should I buy the recommended power pack from Multiplex, which I
> sincerely hope wouldn't have this particular problem?

Dream on.. I don't know what that is, but I think there is no substitute
for getting to the bottom of problems, understanding them, and
proceeding from there.

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Chris - 24 Oct 2008 12:27 GMT

>>>> I'm thinking of putting an AXI 2808/24 with a Jeti Advance 30 Plus
>>>>speed controller into a Multiplex Parkmaster.
[quoted text clipped - 119 lines]
>substitute for getting to the bottom of problems, understanding them,
>and proceeding from there.

Thanks NatPhil - and I totally agree with your last comment.
I reckon my main motivation is to understand things.
In fact, I do things in order to understand, rather than understand in
order to do things!

Your link was very useful. Thanks again.

I am not in USA but rather in UK.
(I guessed that you were, in that this is a UK newsgroup.)
Are there better alternative products available in UK?
Or is it best to buy the one you recommend from the states?
Or maybe you know of an importer for it?

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Chris

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The Natural Philosopher - 24 Oct 2008 13:33 GMT

> I am not in USA but rather in UK.
> (I guessed that you were, in that this is a UK newsgroup.)
> Are there better alternative products available in UK?
> Or is it best to buy the one you recommend from the states?
> Or maybe you know of an importer for it?

I'd buy direct. I did.

Mind you the FX rates have gone weird..

IF you google SBEC and UBEC you can find similar parts from elsewhere,
but I prefer these boys.

Much more electrotechnical chat on www.rcgroups.com. Try power systems
forum.

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Red Scholefield - 21 Oct 2008 20:38 GMT

The BEC circuit of your ESC is conservatively rated. I have exceeded the
number specified on a number of models. One in particular has 6 standard
Futaba servos and I have had no problems in over 2 years of flying it. Of
course 2 are on flaps so not being driven in conjunction with any others.

The only thing that can happen is that the BEC cannot support the voltage
with the drain that multiple servos can impose and you might seem to get a
glitch. On 2.4 gHz systems you could lose your lock on long enough to be a
serious problem.

The new line of FMA ESCs have a switch mode BEC that will handle more
servos. This is for the 40 Amp and larger units.

Red S.
Red's R/C Battery Clinic
http://www.rcbatteryclinic.com
Check us out for "revolting" information.

> I'm thinking of putting an AXI 2808/24 with a Jeti Advance 30 Plus speed
> controller into a Multiplex Parkmaster.
[quoted text clipped - 9 lines]
> What was I risking?
> What might have happened?

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The Natural Philosopher - 22 Oct 2008 09:37 GMT

> The BEC circuit of your ESC is conservatively rated. I have exceeded the
> number specified on a number of models. One in particular has 6 standard
[quoted text clipped - 5 lines]
> glitch. On 2.4 gHz systems you could lose your lock on long enough to be a
> serious problem.

This is dangerously wrong.

Yes if you momentarily exceed the CURRENT rating of a linear regulator,
you may see a momentary loss of voltage, but in fact as anyone who knows
how these beasts are designed knows, this is not the worst hat can
happen. The chips are deigned to protect themselves against overheating
the only way they can: By shutting down until they cool down.

You may have not had any problems. Perhaps at your age, you are not a
busy twiddler of sticks, but believe me I've seen at least three BECS
shut down and result in a crash, two complete destruction of the model.

In my own case my crash was very gentle: i was able to walk over to the
model and note that all the servos were gently buzzing at their end
stops, that I had no control, and the motor would not start. It was a
memory I had of setting a model up on a flat receiver battery that gave
me the cue: There too the servos slowly moved to the end of the travel.

I removed the battery, plugged it back in again, and everything worked
perfectly.

Unusually for a BEC failure, I still had most of the model to test..

Heat is very difficult to analyse and predict. A typical reciever and 4
small servos probably only draws 40mA or so when the controls are idle.
About double that with a 2.4Ghz receiver. You can fly forever as long as
you don't touch the sticks.

> The new line of FMA ESCs have a switch mode BEC that will handle more
> servos. This is for the 40 Amp and larger units.

At last, some facts. So also do some JETI ESCs.

However, for the rest of us, the answer is no more than 3-4 servos or
3s. More than that and you need to buy a separate switching BEC, and
Dimension Engineering make the best.

You may get away for a time, if you don't fly too long, if the weather
is cool, if you are gentle on the sticks.if the ESC is in a cool
breeze... but one day you will get bit - hard!

Then it will be less a case of what needs repairing, more a case of what
you can salvage.

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Model Forum / Radio Controlled / Air Models / October 2008

Limit on number of servos ... Why?