Today's Message Index:
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1. 03:54 AM - Re: Re: Woodcomp after Sales Service (Kingsley Hurst)
2. 05:23 AM - Re: Re: Woodcomp after Sales Service (Frans Veldman)
3. 06:13 AM - Testing your Woodcomp micro switches and diodes (Frans Veldman)
4. 07:01 AM - Re: Re: Woodcomp after Sales Service (Jan de Jong)
5. 07:40 AM - Re: Re: Woodcomp after Sales Service (Frans Veldman)
Message 1
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Subject: | Re: Woodcomp after Sales Service |
----- Original Message -----
From: "Mark Burton" <markb@ordern.com>
Hello Mark,
Thank you very much for going to the trouble of logically going through the
different failure modes with explanations that go a long way to helping
those with only basic electronic knowledge like myself.
In the main I am confident I understand what you have explained but there
are a couple of points where I am not so sure so I would be much obliged if
you could see fit to clarify my queries below please.
You said:-
> Assuming that the switch has operated as it should, the pitch will now be
> fully fine. When the controller next commands the pitch to go coarse, the
> switch is already open so the diode will have to carry the current. That
> current can be large as it's basically the pitch motor stall current
> (initially around 25-30A, falling quickly as the motor spins up). Now the
> diode is only rated for 5A continuous current but it has a good pulse
> capability (check the data sheet) so it probably won't fail (we will talk
> about what happens if the diode fails next) - assuming the diode did not
> fail and the pitch motor is actually moving, the switch will close shortly
> afterwards. Exactly how much current will flow through the switch is
> difficult to predict because we don't know how much the motor has spun up
> so it could be quite a lot - mind you, there's only around 1V across the
> switch (the diode drop) so I should think the switch could hack it OK
> (perhaps with a reduced life).
As I understand it, the 1V across the switch would only be present while the
switch is still open and on the closing of the switch, is it correct to say
that ALL of the current (whatever that might be) would then flow through the
switch (assuming closed switch has minute or zero resistance)
notwithstanding the diode is still forward biased?
Secondly . . . .
> Now, let's consider some failure modes:
> 2 - the diode fails short circuit - in this case when the switch opens as
> the prop reaches fully fine, the pitch motor keeps going and will drive
> the pitch all the way to reverse position (and the reverse switch would
> operate). If the controller subsequently tried to coarsen the pitch, it
> would succeed unless the diode decided the current was too much and died
> in which case the pitch would freeze. Now as it has been reported that the
> pitch of the prop was not in the reverse position this probably didn't
> happen. Again, the C/B would not blow.
In this case the diode fails short circuit . . . . . later you say that the
controller would succeed when trying to coarsen the pitch "unless the diode
decided the current was too much and died"
In this case, I would have considered the diode to have already died when it
short circuited . . . . . am I correct in my understanding that what you
are saying is that given enough current, the already shorted diode may then
go open circuit causing the resultant "freeze" ? OR is the reference to
the diode dying (dying open circuit) another diode across the reverse switch
?
Sorry if I am a bit thick Mark but I'd rather not understand at all than
only half understand hence my plea.
Best regards
Kingsley in Oz.
Message 2
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Subject: | Re: Woodcomp after Sales Service |
On 06/29/2011 12:51 PM, Kingsley Hurst wrote:
> As I understand it, the 1V across the switch would only be present while
> the switch is still open and on the closing of the switch, is it correct
> to say that ALL of the current (whatever that might be) would then flow
> through the switch (assuming closed switch has minute or zero
> resistance) notwithstanding the diode is still forward biased?
That is also my thought.
But let me explain a bit about switches and AC and DC currents.
It is often said that for DC applications the switches need to have a
higher current rating than for AC applications.
The reason is this: If we look in slow motion to the process of opening
a switch, a small arc will form just at the moment of opening. With AC,
the arc extinguishes automatically when the current goes through the
null when alternating. With DC, the arc will continue to exist until
either the contacts are spaced out far enough, or when the source of the
DC is taken away.
While the contacts are closed, or during closing, the current rating is
the same for AC and DC. It is the opening of the contacts which is the
limiting factor for DC applications.
So, in the scenario Mark is describing, excessive current over the
offical DC rating of the switch has no consequences.
Furthermore, in the application of Woodcomp propellers with either
feathering or resversing options, it isn't actually DC which is fed to
the controller. The controller uses PWM (Pulse Width Modulation) to
control the prop, and it is the ratio between the pulses and pauses that
determines how fast the pitch changes. This pulsing DC behaves, as far
as the micro-switch is concerned, exactly as AC, because the DC is
interrupted periodically and any arc when opening will extinguish in a
few milli-seconds.
So, in this application with PWM you can use the micro-switches to their
max AC-current rating. The lower max DC-rating doesn't apply here.
And finally, I like to restate that the controller and prop form
together a RPM-controlled closed loop, and the limiter micro-switches
will never open as the controller will cut the power to the prop when
the target RPM has been established. Only in faulty setups the
micro-switches will open. Such a fault could be a setup error, like a
too powerfull engine which overrevs even with the prop fully coarse, and
the controller will subsequently try to move the pitch coarser than the
design limit of the prop. Another failure mode would be a faulty
controller, i.e. a controller that keeps feeding power to the prop after
the target RPM has been reached. A faulty micro-switch will consequently
only reveal itself when something else goes wrong in addition.
> In this case, I would have considered the diode to have already died
> when it short circuited . . . .
There are multiple ways of dying... I'm pretty sure that Mark meant that
after short circuiting the diode might blow like a fuse and go open circuit.
Frans
Message 3
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Subject: | Testing your Woodcomp micro switches and diodes |
Hi Folks,
After the misleading information here about underrated micro switches
and diodes in Woodcomp propellers, here is a way to establish the health
of both the switches and diodes against all failure modes, without
disassembling anything.
I do this test periodically (I found no reason to distrust these
components to justify a daily check), but you could even make this test
part of the standard startup checklist if you wish.
Note that I designed this test for Woodcomp props with feathering and
reversing options, but it probably also works on the basic Woodcomp
models. (Maybe someone with such a prop can confirm this?)
The props with feathering and reversing options have a more powerful
pitch motor and are probably easier to provoke to overrun the pitch,
which is an essential part of this test.
1) Switch on the electrical power, but do not start the engine. The
entire test is conducted with the engine switched off.
2) With the controller on manual or disabled, use the pitch control
switch to drive the prop all the way to the fully fine (or coarse)
setting. Keep the switch steadily engaged until the pitch won't change
any further. With no aerodynamic load, and no target RPM for the
controller, the pitch will change much faster than while flying and the
pitch will overshoot. This will open the micro-switches. Opening of the
micro-switches is indicated by an interruption of power and subsequent
stop of the pitch motor. (If the motor stops very abruptly and current
continues to flow, this will indicate that the pitch mechanism jammed
against the physical end stop and the micro-switches didn't open. In
this case your current-gauge will indicate that a lot of current
continues to flow and possibly the CB will pop. The following steps will
further confirm that this is not the case but the limiter switches
instead cut the power, as it should).
3) Now very briefly move the switch in the opposite direction. Just long
enough to give the pitch a very small kick.
4) Now move the switch in the same direction as in step 2. (this is the
opposite direction from step 3). The pitch motor should not run. If step
3 was short enough, the prop is still in the overshoot position, and the
micro-switch is still open. Hence the pitch will not change and you
don't hear the pitch motor running. This will confirm that the diode is
not open circuit (otherwise step 3 would have failed) and has not
shorted (otherwise step 4 would fail). Furthermore this will confirm
that the pitch mechanism didn't jam against the physical end stop,
because if this were the case, step 3 moved it away from the stop and
you would be able to jam it once again against the stop in step 4. If
the pitch motor doesn't run in step 4 after moving it slightly back in
step 3, it confirms that the micro-switch has opened and the motor
didn't halt just due to reaching the physical end stop.
5) Although not necessary, you can repeat step 3 and step 4 a few times
in succession. At some point after having kicked the pitch a few times
with step 3, the pitch will get into the normal operational area again,
the micro-switch will close, and the pitch motor will start to run in
step 4. You have now fully established that both the micro switch and
diode are working correctly.
6) Now repeat this test on the opposite pitch limit. This will confirm
that the other set of micro-switches and diodes are functional.
It looks like a complicated test, but once familiar with this procedure,
it only takes a few seconds: Hold the switch engaged until the motor
stops, repeatedly toggle the switch rapidly back and forth and note that
the pitch only changes in one direction, and after a few toggles the
pitch changes in both directions. Any failure will reveal itself because
the behaviour of the pitch motor during this procedure will change
visibly and audibly when a diode or micro-switch malfunctions in any way.
Hope this helps,
Frans
Message 4
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Subject: | Re: Woodcomp after Sales Service |
Hi Frans.
In the section below you describe PWM with a pulse width of less than
100% - thus running the motor at less than full speed.
This surprises me as electric full speed pitch change is slow compared
to hydraulic pitch change already.
Is there ever a reason to slow the electric motor down?
> Furthermore, in the application of Woodcomp propellers with either
> feathering or resversing options, it isn't actually DC which is fed to
> the controller. The controller uses PWM (Pulse Width Modulation) to
> control the prop, and it is the ratio between the pulses and pauses that
> determines how fast the pitch changes. This pulsing DC behaves, as far
> as the micro-switch is concerned, exactly as AC, because the DC is
> interrupted periodically and any arc when opening will extinguish in a
> few milli-seconds.
> So, in this application with PWM you can use the micro-switches to their
> max AC-current rating. The lower max DC-rating doesn't apply here.
>
Jan
Message 5
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Subject: | Re: Woodcomp after Sales Service |
On 06/29/2011 03:58 PM, Jan de Jong wrote:
> In the section below you describe PWM with a pulse width of less than
> 100% - thus running the motor at less than full speed.
> This surprises me as electric full speed pitch change is slow compared
> to hydraulic pitch change already.
> Is there ever a reason to slow the electric motor down?
I'm sure Mark Burton can answer this question better. But as I
understand the operation is that with large RPM mismatches the motor is
fed with a higher duty cycle, and with smaller RPM mismatches the motor
is run more slowly, to avoid pitch hunting and overshoots. At full speed
the motor runs out for a considerable time after cutting the power, so
correcting small RPM changes can only be done with a very slow running
pitch change motor.
Do not underestimate the speed of the woodcomp pitch motor: It will go
from fully fine to fully coarse within one second. This only applies to
the propellers with reverse or feather options.
I have not much experience with hydraulic props, but the one I have
flown was considerably slower and more instable than my Woodcomp/Smart
combination.
Frans
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