---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Wed 06/27/18: 3 ---------------------------------------------------------- Today's Message Index: ---------------------- 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 _____________________________________ Time: 06:58:46 AM PST US From: Alec Myers Subject: Re: AeroElectric-List: 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 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 _____________________________________ Time: 07:55:56 PM PST US From: "Robert L. Nuckolls, III" Subject: AeroElectric-List: DC motors 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 _____________________________________ Time: 09:58:34 PM PST US From: Ron Springer Subject: AeroElectric-List: 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 ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.