1976 airplane motor filter, how much does ferrite bead effect this circuit? - Page 1
Forum section: electronics / beginners

author: NaDobraNich
author_rank: Regular Contributor
timestamp: June 20, 2024, 09:08:21 pm
content: I have parts from a 1975 light aircraft, heater blower motor is 24VDC. Has this "electronics plate" between it and the plane.See diagram.I can't find a part number for the ferrite, but I think it can only be a ferrite because the screw terminal goes completely through it.What I'm trying to figure out is why in the 1960s when they were designing this plane, did they put the 2.2uF axial 13mm diameter caps in this config?The signal isn't changed with the ferrites, just the EMI right? So why make a big steel plate with the caps on it in what I had figured was an input/output "side"?Also... The motor is blown. Is it likely that a new mfg motor would need this same config?- I know none of what I think is airplane advice, just doing the research for the person that will eventually fix and sign off on it.

author: Benta
author_rank: Super Contributor
timestamp: June 20, 2024, 09:20:54 pm
content: Pictures would help.

author: jpanhalt
author_rank: Super Contributor
timestamp: June 20, 2024, 09:29:31 pm
content: 1970's light aircraft of 1960's design with 24V supply in North America. Is it a light twin? That cannot be the original schematic. Are you sure of the schematic? If it is a Beechcraft (e.g., Baron series), I may have the manual, or is that copied from the original manual.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: June 20, 2024, 10:18:06 pm
content: Not a Beechcraft.It's a PC6.Of course it isn't the original schematic. I just took it apart and measured it. Picture posted.

author: Postal2
author_rank: Frequent Contributor
timestamp: June 20, 2024, 10:50:25 pm
content: Quote from: NaDobraNich on June 20, 2024, 09:08:21 pmI have ferrite beads for this. This is a special ferrite, the key difference is that ferrite conducts current like metal, but looks like regular ferrite. I assume very big magnetic permeability. Can also be used to suppress the noise of a 50Hz thyristor circuit.

author: David Hess
author_rank: Super Contributor
timestamp: June 20, 2024, 11:04:02 pm
content: My guess is to suppress the considerable noise from the brushes which would otherwise interfere with the AM aircraft band.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: June 20, 2024, 11:44:08 pm
content: Quote from: David Hess on June 20, 2024, 11:04:02 pmThat is what I figured.I'm just not sure if it would be as required with a modern blower motor. Or if this contraption could be modernized a bit to be smaller/lighter/cheaper and more effective at the same time.The USED blower motor quote I have is $6000. It's a completely average 24VDC motor.... Airplanes, man.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 21, 2024, 03:44:41 am
content: Yeah, that's pretty normal, brushes make hella arcing noise, not something you can have with your radios, especially LW RDF. Even voice can be affected (>100MHz), sparking really goes up there.Here's another example from my bin:Not at all uncommon to see a small PCB here with just a couple caps, maybe ferrite beads, and mating the terminals to a cable, nothing more.Tim

author: jpanhalt
author_rank: Super Contributor
timestamp: June 21, 2024, 09:04:51 am
content: I have no experience servicing turbines. Single-engine (recip) in the US were usually 12V . The PC-6 is a pretty good sized airplane. (10,500#) and still qualifies as "light." Janitrol was a popular brand of heater used in the twins I have seen. Here's a link to its service manual from a little later:https://www.csobeech.com/files/Janitrol-MaintenanceManual.pdfThat's just FYI.Anyway, as others have mentioned, noise suppression filters are used on every motor and other potential sources of noise.

author: NaDobraNich
author_rank: Regular Contributor
timestamp: June 21, 2024, 06:50:31 pm
content: All good info.I guess something I'm still not clear on, is do those two caps make a difference where they are?Is this really just decoupling with 4-4.4uF (they are 2.2uF each, but measure 2.01)?Or does it matter that 2 are before the ferrites, and two are after?Is there any benefit to the + and - lines both having separate ferrites compared to being in a shielded cable with a single ferrite?Twisted pair helpful?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 22, 2024, 12:26:41 am
content: I would suggest maintaining or reproducing the mechanical assembly, including the general layout of conductors and grounds, component placement and value, and preferably component size. (Similar component lead + body length will matter more than identical size. Most likely, smaller is better anyway, but I would further air on the side of similar, rather than risking improvements, for something my life depends on.)Capacitors before and after, matters.Ferrite type, matters. Most ferrites, even of that size, saturate with a few amperes running through them; saturation means the AC impedance drops considerably. Not clear what type they have here, or if they're just kind of letting it happen and not caring, but reproducing that behavior would demand replacing them with equivalents; but good luck identifying it without drawings.A shielded cable, with the shield tied to the motor housing, would be one way to confine brush noise. A differential filter is still required to eliminate noise conducted within the cable. A shielded cable could not be isolated, though. It's not clear to me whether this is a metal-chassis environment (like a typical automotive scenario) or if other practices or standards apply. The single wires may suggest they could be wired in either order, or that the electrical system is isolated or elevated in some way from the chassis, or that no chassis is available to matter (fiberglass hull?), which will all affect the ideal design of a filter.Even for the two-line case, some capacitance to the motor housing (and probably by extension, ground/chassis) would probably help for reducing common mode noise, and maybe additional ferrites (both wires, common mode) or a choke proper would be used as well. But again, it all depends on the setup.Tim