PMA Alternator: Metal Face Plate on Coil

Electrical and electronic topics for small boats
Oldslowandugly
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PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Thu May 12, 2022 10:16 pm

[On an older Evinrude outboard engine the permanent magnet alternator (PMA) stator assembly has a peculiar coil called a] "charge coil" that has a stainless steel [later found to be magnetic steel] faceplate attached to the front of the coil assembly as shown below. [Some other Evinrude outboard engine permanent magnet alternators have a] ''charge coil" that does not have the face plate.

coil.jpg
Fig. 1. A Charge Coil with stainless steel faceplate.
coil.jpg (74.16 KiB) Viewed 526 times


What is the purpose of the face plate?

On my outboard engine, the face plate came loose and was causing damage to the coils and flywheel. I found the faceplate jammed against the flywheel magnet.

If my engine came with a stainless steel faceplate attached to the front of the charge coil, can I use a charge coil without a stainless steel face place?

jimh
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Re: PMA Alternator: Metal Face Plate on Coil

Postby jimh » Fri May 13, 2022 9:05 am

I do not know the intended purpose of the metal face plate on the charge coil, however I will make a guess.

There are two realms in which the purpose may lay:
  • mechanical
  • electro-magnetical

Considering the mechanical realm, the purpose of stainless steel faceplate for the charge coil may be to protect the pole or laminate structure from damage if there is any debris in the the gap between the coil and the rotating flywheel.

Considering the electromagnetic realm, the purpose of the stainless steel faceplate for the charge coil may be to reduce or increase the influence of passing magnets to exert magnetic force onto the pole to a smaller area, based on the assumption that stainless steel is a martensite steel of type 316, a steel with very low magnetic permeability.

I really don't have a great understanding of magnetism and steels. Perhaps the stainless steel faceplate helps to focus the magnetic field onto the coil.

If you could explain further about the FUNCTION of the coil you refer to as "the charge coil", it would help me better understand its purpose.

Test the faceplate with a magnet to see if the steel can be held by a magnet. I think you will find it cannot. This would be important. If the plate were magnetic steel, the passing magnets in the flywheel would tend to pull it way from the coil and onto the flywheel.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Fri May 13, 2022 10:54 am

I am totally confused now.

I tested the shards with a magnet and while at first they appeared to be stainless steel, they are not. In fact they are extremely magnetic like mild steel.

Would [the face plate being made of ferromagnetic steel] help to expand the exit-able [perhaps excitable] area of the laminated core?

Underneath the steel plate is a plastic plate of the same size. The function of the Charge Coil is to produce 200 to 300 Volts-AC when the flywheel magnet passes by. That voltage is then sent to the CDI unit.

A trigger coil [sensor] is then excited by the flywheel to let the CDI know when to charge the ignition coils to fire the spark plugs.

OMC changed up the parts year by year for some reason. My 1993 48SPL has the trigger and lighting coils all in one unit. The Charge Coil is separate. The 1996 48SPL has separate charge, trigger, and lighting coils. My 1985 30hp has the same ignition parts as the 1996 48SPL but no plate on the Charge Coil.

jimh
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Re: PMA Alternator: Metal Face Plate on Coil

Postby jimh » Fri May 13, 2022 12:09 pm

Thanks for explaining that the "charge coil" acts to produce a voltage sent to the spark coil primary windings, under control of some mechanism actuated by the "trigger" coil. I presume the "trigger coil" must be located such that only certain magnets in the flywheel pass over it.

On that basis, my guess for the purpose of the faceplate is that it (perhaps) helps to conduct the magnetic flux of the magnets to the charge coil more efficiently.

I would ask your question to CDI, if they are the sellers of the coils with and without the steel faceplate.

I believe that the "lighting" coil is intended to create 12-Volts for powering a lighting circuit and to recharge an attached 12-Volt battery.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Fri May 13, 2022 2:29 pm

Yes, the lighting coils are for producing 12 volts to run accessories and recharge the battery. Rope start motors usually lack them but it is not unheard of.

The Charge Coil sends voltage to the Capacitor Discharge Ignition(CDI) module. Electronic switching determines which cylinder gets fired. It then energizes the spark plug coil when determined by the sensor [timing] coil to time the spark correctly. That is why the entire timing plate turns when the throttle linkage is advanced.

Some [CDI] modules incorporate a rev limiter, too. I was looking at the schematics and parts lists for quite a few engines. It seems OMC would use one style of coil for a few years and then switch back to the other kind. This applies to all of the two cylinder-engines from 15-HP to 50-HP. I have asked my Electrical Engineer cousins about this and I will report back.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Sat May 14, 2022 6:44 pm

Just great. I purchased an entire timing plate complete with all the coils, and the charge coil is bad. I contacted Green Bay Prop to see what they can do. One of my cousins got back to me. He feels the steel shield may be to "concentrate, magnetic field energies". His words. Makes some sense.

I am looking at every charge coil I can and the only models that use a coil with the steel shield are some 40-48-50-HP engines from 1990 to 1999. Prior models as well as some 40-HP used a non-shielded coil.

I wonder why the change was made.

All other charge coils look the same with the exception of the shield.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Sun May 15, 2022 10:12 am

My other cousin said:
All I can think of is a metal core increases the inductance of a coil.

Lenz law states and inductor will provide an opposing voltage to a changing field.
Where v = -L di/dt.

I assume this coil with more [inductance] provides a bit more kick to the ignition pulse.

[You can] fabricate a piece of metal of the same composition and save $70.


Further investigation shows that OMC used this shielded coil right up to the change-over to the E-TEC line. I guess [the shield] had some value.

I will get one of the later versions which are more available and much less expensive.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Wed May 18, 2022 6:19 pm

Green Bay Propeller contacted me. They apologized for the dead coil, and they were ordering me a new one. No charge. What service!

I highly recommend this outfit as I have always gotten excellent parts at a very fair price. You hardly ever find such great customer service anymore.

jimh
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Re: PMA Alternator: Metal Face Plate on Coil

Postby jimh » Thu May 19, 2022 8:30 am

a cousin wrote:... a metal core increases the inductance of a coil.
Yes, but this metal plate is not the core of the coil. Your other cousin's opinion about focusing the magnetic flux of the magnets into the coil's pole piece sounds better to me.

I can only guess at the purpose, but my two guesses are:
  • the face plate helps to focus more magnetic flux from the magnets in the flywheel onto the coil winding, or
  • the face plate acts to stop the magnetic flux from reaching the coil windings until the magnets are directly over the end of the pole piece.
Because this design was used in more recent models, to assume it is an improvement is a reasonable inference.

Designing permanent magnet alternators is not really a textbook topic for electrical engineers, but more of a specialty area. Apparently someone at Evinrude found that the steel face plate changed something in a way that was considered an improvement.

Oldslowandugly wrote:I tested the shards with a magnet and while at first they appeared to be stainless steel, they are not. In fact they are extremely magnetic like mild steel....Underneath the steel plate is a plastic plate of the same size.
You remark that the bright steel face plate is made of magnetic steel and there is a plastic insulator behind it. I assume the plastic insulator is to prevent any electrical contact between the bright steel face plate and the ferromagnetic steel of the laminated coil core.

When you got a new coil with a face plate, have you tested that face plate material to see if it is magnetic?

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Fri May 20, 2022 8:32 pm

jimh wrote:
When you got a new coil with a face plate, have you tested that face plate material to see if it is magnetic?

I tested it. It is very magnetic. Just plain steel but it must be chrome plated. It is very shiny.

jimh
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Re: PMA Alternator: Metal Face Plate on Coil

Postby jimh » Fri May 20, 2022 10:12 pm

Oldslowandugly wrote:
jimh wrote:When you got a new coil with a face plate, have you tested that face plate material to see if it is magnetic?
I tested it. It is very magnetic. Just plain steel but it must be chrome plated. It is very shiny.
That is interesting. With that information, I think the purpose may be related to focusing the magnetic field into the coil--but here I am really guessing. I don't have background in Permanent Magnet Alternator design. I think making a PMA perform better is more more of a craft or art, probably honed with some trial an error.

ASIDE: I recall a rather sophisticated technical paper published by the company I worked for that explained the basis for how one of their products worked, an Eddy Current based Hardness Tester, which employed a very unique and unusual technique. While the paper explained in theory how the device worked, it acknowledged that the method was discovered by experimentation. The theory only came later to support the experimental discovery as a true scientific measurement method of hardness testing.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Sat May 21, 2022 6:03 pm

Green Bay Propeller sent me the CDI-brand replacement coil; they sent just the coil, not the coil and its laminated core. The the original laminated core must be used with the new coil winding assembly.

CDI calls the metal face plate a SHUNT PLATE. If the coil being replaced used a shunt plate, the replacement coil MUST be installed with the included shunt plate. If the coil being replaced did not have a shunt plate, the instruction emphasize to NOT install a shunt plate on the new coil. Use of the shunt plate seems to be application=specific.

If the installation has been done properly, the new coil, laminated core, and shunt plate should appear as seen below.

CDI coil.jpg
Fig. 2. The original laminated core with a new CDI coil with Shunt Plate installed.
CDI coil.jpg (85.36 KiB) Viewed 346 times


My problem: the original coil melted and I can't get it off without destroying it.

jimh
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Re: PMA Alternator: Metal Face Plate on Coil

Postby jimh » Sun May 22, 2022 6:41 am

Thanks for providing the term "shunt plate."

I found an old article about magnetic phonograph pick-ups with an illustration of their components. In the illustration, a component is identified with this comment:

Shunt Plate
Completes the magnetic circuit of the pole pieces


In another reference I found this text:

Magnetic Shunt
Magnetic shunts are often used to adjust the amount of flux in the magnetic circuits found in most electrical motors.


And in a third reference, I found this text

What is a magnetic shunt?
Magnetic Shunt. The conception of a magnetic circuit being formed, the shunt is a corollary of the theory. It is any piece of iron which connects points of a magnet differing in polarity, so as to divert part of the lines of force from the armature or yoke.


On the basis of that information, I infer that the purpose of the magnetic shunt or "face plate" is to affect the amount of voltage generated by the charging coil. The shunt plate alters the distribution of the magnetic flux from the permanent magnets as they pass across the coil in some manner that is desirable for certain applications, and apparently not desirable in other applications.

As I said earlier, the design of permanent magnet alternators is something of an art.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Sun May 22, 2022 9:39 am

Thanks for that info Jimh. It is still way above my pay grade.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Mon May 23, 2022 10:47 am

My other cousin replied with this: "Oh you mean that disk in front? OK that is what they call a shunt plate. It is shielding the coil windings from the passing magnet. Hence ‘shunting’ the field as it goes by. That is to sharpen the timing pulse at the center pole. The other two poles are just completing the magnetic circuit. That plate keeps the ignition timing from being sloppy." That makes the most sense of all, even to me.

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Re: PMA Alternator: Metal Face Plate on Coil

Postby jimh » Mon May 23, 2022 11:26 am

Earlier:
jimh wrote: Perhaps the stainless steel faceplate helps to focus the magnetic field onto the coil.

Later:
Oldslowandugly wrote:My other cousin replied....that disk in front--is to sharpen the timing pulse at the center pole.


When something is "sharpened" you could also say it was "focused."

Earlier:
jimh wrote:...the face plate acts to stop the magnetic flux from reaching the coil windings until the magnets are directly over the end of the pole piece.


Later
Oldslowandugly wrote:My other cousin replied....That plate keeps the ignition timing from being sloppy.


Perhaps, but is this coil we have been discussing the "timing" coil or the "charging" coil? Or both?

I was thinking that perhaps the name "charge coil" was used because this coil is supposed to provide the electrical power to charge-up the capacitor in the capacitor-discharge-ignition (CDI) system used to create the spark. This coil charges the capacitor. Another coil is the timing coil and tells the CDI when to discharge the capacitor.

ASIDE: CDI systems created very high-voltage but very short duration spark. Later outboard engines tended to change to the opposite approach, using an inductive-discharge ignition. A voltage is created across an inductor, producing a current that creates a field around the inductor. When the circuit is broken, the field of the inductor collapses, creating a much longer duration voltage for the spark coil primary. The longer duration voltage was deemed more desirable because it created a longer duration spark. The longer duration spark helped to insure better igniting of the fuel-air mixture, particularly when the fuel-air ratio might not have been at the optimum value to create the best ignition conditions.

Oldslowandugly
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Re: PMA Alternator: Metal Face Plate on Coil

Postby Oldslowandugly » Mon May 23, 2022 11:55 am

This is the charge oil. What confuses me more is why the larger two cylinder motors need that shield but the smaller ones don't. You would think any benefit would be applicable to all models.