Theory of electrical noise in boats

StarTed

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I looked for something already discussed regarding electrical noise and the theory behind it. I either missed it or failed to see it (its happened before). I have a background in metering AC electricity and see lots of similarities especially when there is a form of AC on the DC system. Alternators, ignitions, transmitting radios, depth finders and other added devices. What recently caught my attention was the addition of a PWM device to control a trolling motor. Pulsated DC has similar issues as AC so resistance is no longer the only current restriction, now impedance raises its nasty head. I am very familiar with Ohm's law and its various forms as well as Kirchhoff's laws but I could use some additional thoughts on this. I'm hoping this post will raise some discussion on theory as well as solutions tried and trued.

I hope to add a trolling motor to my boat at bottom feeder price and am interested in a PWM to control it. Here is a question - can the output from a PWM be filtered using a capacitor, an inductor or will it affect the output because of interference with the oscillator? Could I put a rectifier on the ground lead between the filter and the PWM to prevent any negative feedback? I'm assuming that the TM is not grounded causing a problem. I understand that the electrical system should be only grounded at one spot to prevent ground loops and that the hull of an aluminum boat should not be used as a ground conductor.

I'm looking for suggestions and the supporting theory. Feel free to chime in even if you don't have both the experience and theory, either will help.

I plan to later add thoughts about component additions and wiring methods to help reduce or eliminate electrical noise.
 

StarTed

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My computer did a weird thing. It posted this twice. Maybe a moderator can remove my prior post.
 

Grub54891

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Have had a few that get "noise" on the screens, or in the vhf, or stereo over the years. Yeah you are supposed to run wires to different components separately, but that is not always possible. Usually a ferrite will clean up the signal.
 

gm280

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Oh this is going to generate a lot of interesting replies. Let's start with the noise sources first. Yes ignition, PWM trolling motor circuits and depth locaters are such sources. But any pulsating or oscillator circuit can add to it.

Ignition noise it the first issue to deal with. But with good filtering back to the battery source, most of that can be eliminated. A battery actually acts like a filter in itself. However, if the ignition noise is coming from actual transmission in the air, then radio circuits will be affected more. And that can be a little harder to eliminate as well.

A lot of inline filtering circuits can be added to the power supply lines to help those pulse noises from reaching other accessories in a boat. Coils and capacitors properly selected can form a band-reject filter, and are the best. Coils induce inductive reactance (a type of resistance) and that is a resistance to changing voltage frequencies. Capacitors induce capacitive reactance (again a type of resistance) that tries to keep voltage levels constant. Both are effected by changes in voltage levels and frequencies changes. So selecting them properly will be the best filtering you can select.

PWM (Pulse Width Modulation) circuits as well deal with pulsating on off voltage to control the trolling motor. That changing on off time allows the trolling motor to change speed(s). If you put filtering after the PWM circuit to filter the voltage going to the trolling motor, you just defeated the PWM idea. Filtering in the power source lines is the best you can do...again. The off on times change to make the trolling motor "think" (I know not the best word) the voltage is changing levels when in fact the voltage remains the same total level of the supply. When it turns on and off at different amounts the effective output will change the motor speed. Neat isn't it?

That brings us to depth locators/fish finders. Yes those neat sonar type equipment gives us a crude 2D picture of what is going on in the 3D water. And those capabilities are achieved via pulsed transmissions and receptions. So once again pulsating voltage levels are introduced. But you can't filter the power going to the transducer or you loose the capability of that equipment. So as usual, the best you can do is filter the source power supply again.

In all the efforts to cancel interfering noise issues, it really boils down to filtering the power source to everything that induces frequency and voltage level changes. And there are very good filtering circuits available for each and every type problem already on the market. Inventing new ones is a long hard complicated trial and error efforts with a lot of calculations and a very thoroughly knowledge about electronic filtering components. Not impossible, but why do that when there are lots already design for such things.

I will finish with this. Most today's offerings of any boating accessories have very well designed filtering included in their offerings. Does that mean you don't need other filters, NO. But it does mean that there are a lot less issues with noise in the newer boating equipment. A great way to determine if the noise is coming from the supply source or the air, is to disconnect the questionable equipment, and use a separate battery power source to see if the problem still exist. If you still have the noise issue, it is not the power source. and then you have something transmitting that noise over the air. JMHO!
 

bruceb58

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The PWM frequency of a trolling motor is so low that you really likely will not have an issue with it interfering with other electronic equipment. A filtering circuit on the output to the trolling motor is not practical either.
 

StarTed

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Thanks Grub, GM and Bruce for your input. I think your reply GM will be the most useful for others searching for information about electrical noises. I agree that it is most important to filter at the power source. That means that an alternator should be filtered near it. A battery is a great filter for lower frequencies but high frequencies could just fly by it so if a noise on an FM radio or possibly a fish finder (sonar) or radar (I don't know much about these) might be filtered by a high frequency reject type filter. I'm sure such products are on the market since ham radio operators know a lot about such stuff.

That leaves a PWM that should be filtered at it's input. I would think a large capacitor should work great. If one is included in the PWM unit another shouldn't hurt. Any current caught by the capacitor will be put back into the lines later without any apparent energy loss. The PWM is a great circuit to reduce energy loss. I would think that about 0.7 volts would be about right for a silicon junction. The down side is that its pulses create pulses in the current and if that travels down the power lines from the source it becomes a noise source due to the changing magnetic fields that can get induced into other wires. The square waves do contain a high frequency component at the leading and trailing edges so storing energy at the input of the device is most important. Separate battery and wires run away from the others is one way but it should be possible to use the main battery if thought out and set up properly.

I think that most if not all TMs should have PWM circuitry built into their control heads. One reason I asked is that some of us buy old equipment from various sources and may want to add newer circuitry to enhance their performance. An example is TM with a few selected speeds that could be improved with a variable speed control. I would think such a circuit (PWM) would need to bypass the old control or as a second choice run it at the top speed position.

I've found that large capacitors don't catch higher frequencies so a small capacitor should be added parallel to it that catches the higher frequencies. The capacitor is wired across the power source (hot to ground), not as a pass through because we're dealing with DC power here.

Lots of older electronic equipment that used higher voltages (tube types) used lots of twisted pairs to help cancel out noise generation. The same would apply to the boat's wiring but there is a down side. Twisting the wires adequately will make them shorter needing more wire and larger sizes. This all adds up to weight, space and installation issues.

Just some more food for thought. I like the input so far. Maybe it'll save me some problems as I refresh my knowledge. I've been retired for over 9 years now and came from the AC power side. I ran a lab for 26 years and know some about current and magnetic fields. We often generated currents up to 10,000 amps to make thermal tests on connections. We also tested fuses (one time) to determine just how much current they could handle and how long it took to blow the fuse. Let's just say that a common 10 amp fuse can carry 30 to 40 amps for a while. A fuse designed for solid state circuit protection will blow on a 1/4 cycle just above the rated amps so it is possible to get the right fuse for an application. A circuit breaker is handier but is not as reliable as a fuse.

We could affect computer screens in an adjacent room when we generated 10,000 amps and our control computer had to be moved away many feet. We tried shielding but the steel cage was too difficult to use so distance and communications became the desired tools. Even the low frequench 60 cycle magnetic fields can affect other circuitry.

Square waves are another matter all together. The sharp edges consist of many frequencies combined and make motors not designed for them get rather noisy. Laminations in them don't like the square waves. That is why I began thinking of noise in boating wiring from solid state devices that switch the power on and off at some frequencies. I know that 12 volts is a very low voltage but it is the current that makes the magnetic field around the wire and generates the noise.
 

bruceb58

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A capacitor hanging off an output does not make a filter. You need to know the inductance of the circuit to figure out what your filtering frequency will be. Electrical engineer here with 20+ years of signal processing experience.
 
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StarTed

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Thanks Bruceb58 for setting that straight. I've worked with electrical engineers for years and find they're a great bunch of guys. They've always been very willing to share their knowledge and experience with me. I didn't finish my electrical engineering college classes. Life seemed to get in the way but all the college classes I've taken have been helpful, even the extra required ones. Again, thanks for your input.
 

dingbat

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We could affect computer screens in an adjacent room when we generated 10,000 amps and our control computer had to be moved away many feet. We tried shielding but the steel cage was too difficult to use so distance and communications became the desired tools. Even the low frequench 60 cycle magnetic fields can affect other circuitry.
Steel is the wrong material for the application.

It's pretty common for us to run in close proximity of mega-watt, reversing AC and DC drive motors. Sometimes, in between 2 drives. The magnetic fields coming off these motors are mind blowing. To the point the hair on your arms stands on end and they distort fluorescent lamps in the control room. Even inside a steel construction control room, proper grounding techniques and liberal use of Mu metal shielding materials is required to keep things manageable.
 

StarTed

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Actually the cover we had made was from Mu metal.

Even non magnetic materials can be affected by magnetic fields. An example is the disc of a mechanical watthour meter. The disc is placed between 2 AC magnetic fields, one caused by current and the other from the voltage. The magnetic fields cause eddy currents to flow in the aluminum disc. A permanent magnetic is used to keep the disc from spinning like a motor by a counter magnetic field induced by the rotating disc. The result is a rotational force that's proportional to the watthours. The point being that strong magnetic fields can induce eddy currents into aluminum and other non magnetic metals. It's the changing magnetic fields that make this happen, not fixed ones such as from straight DC unless the material is moving within the field.

I was thinking of another posting where a person was having problems with a couple of engines that smoked blue and black when put in gear. It seems to me that he could have an electrical problem with a ground or power to the ECMs. He mentioned fuel injectors so I stuck chokes or floats are ruled out. I don't think a couple or inboard engines would share a single air filter so what's left that is common? Electrical comes to mind. Understanding the theory of how the electrical is used in boats should help with the troubleshooting. I'm short on experience here but I'm sure there are plenty of others that know lots about this. Blue smoke usually indicates oil burning and black smoke usually means too much fuel. Too low intake air flow could make this happen and usually there is some kind of air flow sensing that goes along with fuel injection. That leads back to an electrical problem.

Just thinking.
 

dingbat

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An example is the disc of a mechanical watthour meter. The disc is placed between 2 AC magnetic fields, one caused by current and the other from the voltage. The magnetic fields cause eddy currents to flow in the aluminum disc. A permanent magnetic is used to keep the disc from spinning like a motor by a counter magnetic field induced by the rotating disc. The result is a rotational force that's proportional to the watthours. The point being that strong magnetic fields can induce eddy currents into aluminum and other non magnetic metals. It's the changing magnetic fields that make this happen, not fixed ones such as from straight DC unless the material is moving within the field.
Very familiar with eddy current technology.
I was the lead mechanical design engineer on a joint development project (Aluminum Association and Alcan) back in the 1980's to develop a non-contact thickness gauge to measure aluminum using eddy current. The design and technique worked but performance and physical limitations shelved further development. Fifteen years later, a competitor (ABB) utilized our initial design work and developed a system with limited applications. Radiation gauges (mass absorption) remain the industry standard.
 

gm280

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One thing I find really interesting on all these forums is the number of truly professional well educated folks that supply answers to many interesting questions. Seem iboaters are well educated self motivated individuals that also enjoy building and refurbishing most anything to do with boats. :thumb:
 
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