Today's Message Index:
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1. 06:58 AM - Re: Re: Can a low voltage situation cause a fuse to blow? (Alec Myers)
2. 07:55 PM - DC motors (Robert L. Nuckolls, III)
3. 09:58 PM - Mysterious Solution to Radio Noise (Ron Springer)
Message 1
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| Subject: | Re: Can a low voltage situation cause a fuse |
to blow?
In the past, the 14V avionics product and its 28V cousin were distinct items. These
days its the same unit, nor does it need configuring for voltage. Its therefore
possible, even if not likely in practice, that a low supply voltage could
cause an overcorrect trip.
On Jun 26, 2018, at 1:28 PM, Robert L. Nuckolls, III <nuckolls.bob@aeroelectric.com>
wrote:
Breaker size isn't necessarily a 'tell' on an appliance's
behaviors during a low voltage event. It has ALWAYS been
the case that 28v appliances were feed with smaller wire
and breakers than their 14v cousins.
Message 2
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I've had a couple of queries about observed motor
behaviors wherein the writer was certain they
observed situations where motors struggling under
low voltage caused protection devices to operate.
It's easy to characterize the physics of a DC motor.
A motor reveals most of its secrets when you measure
three characteristics: (1) The motor's DC resistance;
it's torque constant (Kt) wherein you measure the
output torque when excited by some value of test
current. From this you can derive In-Oz of torque
per amp of current draw; (3) the counter EMF constant
(Ke) in RPM/Volt by spinning the motor at some known
RPM and measuring the motor's output voltage as
a generator.
Once you have these three numbers you can construct
a plot of motor performance at various voltages. An exemplar
plot is shown below:
Emacs!
Applying 10V to this hypothetical motor with a locked rotor
would produce a stall current of 20 amps. If a torque watch
were attached to the immobilized shaft it would read 60 in-oz.
This experiment yields a motor resistance of 0.5 ohms and
Kt - 3.0 in-oz/amp.
Okay, now spin hte motor with a drill motor. Measure the
RPM and voltage at the power leads. In this case we would
find that the motor 'generates' 1.0 volts per 300 RPM
of the shaft. This number lets us put a counter-emf scale
alongside the RPM scale.
Note that the family of voltage lines are parallel
to each other. The SLOPE of these lines is proportional
to the motor's DC resistance. If the motor's resistance
were 1.0 ohms, then stall current at the lower end of each
voltage line would be 1/2 that presently illustrated.
I.e. a new family of lines having 2x the slope of the
0.5 ohm lines shown.
Let's take a hypothetical application where we operate
this motor at 15.0 volts applied and a load of 38 in-oz.
The plotted characteristics tells us that the motor will
run at 2600 RPM and draw a current of about 12.6 Amps.
15 Volts applied across an Rm = 0.5 ohms yields an inrush
current of 30A . . . which could be added to the plot
above if I extended it another 5A to the right.
You can pick any spot on the face of the plot, draw a 0.5
ohm load line thorough it and you have a new applied voltage,
running current, RPM and stall current.
But without increasing the load on the motor shaft, there
is no way we can make the motor draw MORE current by
reducing its applied voltage.
Bob . . .
Message 3
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| Subject: | Mysterious Solution to Radio Noise |
I recently started flying with a new Garmin GTR-200 radio. My antenna design and
placement is new and highly experimental. I made a full wave loop from 22 AWG
and installed it one foot in front of my instrument panel. Since the fuselage
circumference is too small there, it is more of a zigzag loop.
Initial results are good. I transmitted from 10 miles away from the airport and
the Unicom reported loud and clear. When low on final, I heard a radio transmission
from and airport 70 miles away. And, I had no problem talking to ATC in
the DC SFRA.
BUT, on many frequencies I get continuous noise on the radio. Increasing the squelch
configuration didnt help. I noticed that when I changed screens on my GRT
Sport EX EFIS the noise changed its character. And, sure enough, with the EFIS
off, the continuous noise goes away. I did discover that the back of the case
of my EFIS is not grounded like everything else on the panel. But, connecting
ground to it didnt change anything.
However, the noise completely goes away if I just touch the radio or radio rack
with my finger. The radio case and rack are both grounded. I checked that. So,
I grabbed a two foot length of 22 AWG wire and taped one end to the radio rack
and connected the other end to nothing. The result was the same as when I touched
it. The noise is gone and the radio works normally.
So, theres the fix, but how and why does it work?
Thanks,
Ron
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