dismantling procedure for tachogenerator 2

[panter]

Hello , everyone !
I have a request to change the bearings and treatment of collector on 30 year old Hubler tacho-generator with permanent magnet.I do not have dismantling instruction . I'm afraid unable to stop interruption of magnetic circuit and my colleagues say 25 percent lower output voltage of the generator after installation .I was wondering if anyone has similar experiences and a certain way to not spoil functionality of the magnetic circuit .
Thanks in advance .

 

[mikekilroy]

some older hi mag field staor would instantaeously loose demag some amount if mag path opened. mostly was on alnico magnets.

we had 'keepers' that we would put on side of stator to keep the mag path complete to not harm it while rotor was out.

perhaps u can google alnico, keeper, and find enough info to make one urself

 

[mikekilroy]

since no other replies, I will add to mine... yuur friend's GUESS of 25% magetism lost is a good GUESS, but realize it is simply a GUESS....

Unless he knows the exact coercivity and specs on the magnets used, as well as the magnetic path design, it is just a GUESS. Lacking any other info, I would agree with his "25% loss," but know it is just a GUESS.

IDD google and I think you would have a good chance to make a keeper to prevent the loss.....

 

[panter]

Thanks Mikekilroy ,
for your replay .
My colleagues admitted that they had made an oversight several years ago and they left a few days stator of similar generator with open magnetic circuit. On the internet searching I did not found good idea of ​​keeper construction and I'm thinking to use an old bearing, which outer diameter is a few millimeters smaller than the inner diameter of the stator and I hope it could replace the rotor while it is being processed. The stator is with 6 magnetic pole with 300mm inner diameter and if someone has an idea how to keep magnetic circuit I'd be grateful.
Thanks in advance .

 

[mikekilroy]

our keepers all were just same OD & ID as magnetic stator and just stuck on SIDE of the magnets to make a path. I used to ask my cusotmers with demaged motors, tachs if they had them "apart for more than 35 minutes." Invariably they would say "NO! Only about 10 minutes max!." the demag is INSTANTANEOUS so they got no warranty repair.

I will look for some example keeper pictures in old catalogs or on old motors around here.....

 

[mikekilroy]

I found an old brochure with keep described and pictured: it is 100% as I described above: just a soft magnetic steel ring that goes on stator.

 

[waross]

Lps Mike.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[panter]

Thanks,
mikekilroy

 

[mikekilroy]

LPS? I just got my first 'smart phone;' it is winning so far.

It just hit me I could upload cc of the keeper pix/descrip I found - will try to remember to do tomorrow morning.

 

[electricpete]

mikekilroy,

Welcome to the 21st century (first smartphone).
lps = coveted "Little Purple Star" award (result of pressing "like this post... star it")

I've nver worked with these things - you guys obviously know the subject
Am I understanding you're saying significant fraction of AlNiCo magnet strength is lost in a matter of 30 minutes?
I wouldn't have thought that would be the case fwiw. Would be glad if someone could explane it to me

=====================================
(2B)+(2B)' ?

 

[waross]

It happens instantly Pete. I don't know if the tach generators are magnetized with the rotors in place or if they are magnetized with keepers in place. The rotor forms part of the magnetic circuit or path. If the rotor is removed without keepers in place a relatively large air gap is created in the magnetic circuit, dropping the flux density and the magnets loose some of their strength.
A known issue with older tach generators.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[mikekilroy]

and motors. When we magnetized them in our magnetizer fixture, a keep was added to the side of the stator before it was removed so a mag path remained. sorry for my confusing statement about 'how long it takes' to loose magnetism - sometimes when our customers were suspected of taking a motor or tach apart w/o the keeper, I would just ask that kind of question as they did not realize the loss is instantaneous like I said in my post above; so asking if they had it apart for more than 30 minutes misled them into believing a shorter time was ok and they would then admit taking it apart - but not for 30 minutes!

anyway, here are a few pages from older catalog showing the use of a keeper.

 

[electricpete]

Thanks Bill. I'm not doubting that what is said is true but I'm still trying to reconcile it with rate of demagnetization of permanent magnets at room temperature.

Google leads to several results which talk about "Fast" and "slow" rates of loss of magnetization.
AlNiCo certainly lose faster than rare earth for example.
I haven't seen any that talk about rate of demagnization in seconds/minutes for AlNiCo at room temperature

Here's an example of a contrary statement:

http://www.billlawrence.com/Pages/Pickupology/magnets.htm

Before the introduction of alnico magnets in 1935, permanent magnets were not quite that permanent. During a certain time, they lost a good amount of magnetism till they finally reached a stable condition. The process to accelerate this decay was called in the industry, "magnetic aging." In modern science, it is called "stabilizing." Since the '50's, we use Alnico 5 magnets which lose, under normal conditions, less then half a percent per 100 years

So there is something that I'm missing if the problem is rapid demagnetization of AlNiCo at roomo temperature in seconds/minutes. Maybe a different type of AlNiCo which acts completely different? (does anyone have link showing demag in seconds/minutes?) Or maybe it has something to do with magnetization of the iron (rather than the permanent magnet) in the flux path?

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(2B)+(2B)' ?

 

[electricpete]

Maybe there is huge initial loss the first time after initial magnetization that ya permanent magnet is placed in open magnetic circuit.... and the rates that I'm seeing seeing (years/centuries) apply to later loss for magnets that have already undergone that initial loss?

=====================================
(2B)+(2B)' ?

 

[waross]

Yes, that's it Pete. With the keeper in place the magnet may be magnetized to a greater strength than than it may be with a large air gap. If an air gap is introduced the strength drops. The strength with the air gap is where the loss of strength over time starts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Thanks Bill, that makes some sense. I guess these don't act like normal permanent magnets because they are in some kind of virgin state after initial magnetization where they have never seen a magnetic open circuit

=====================================
(2B)+(2B)' ?

 

[dArsonval]

Bearings in Tachometer Generators are replaced on most any day within the Electrical Apparatus Repair Industry.
The Repair Person dismantles the device, removing the brushes first.
End brackets are match-marked for ease in relocating the parts upon reassembly, etc.
The armature is carefully pulled from the Permanent Magnet field frame and a "keeper" installed.
(If the repair technician even chooses to do this.)
In many cases, a keeper may be something as simple as a large cap-screw, or a random length of key-stock to maintain the magnetic flux path.
(Like the bar at the end of a Horseshoe Magnet.)
The small bearings, (608 or whatever they me be) are removed, and new bearings installed.
If the item has issues other than bearings... the device is considered for outright replacement.
Generally speaking, Tachometer Generators have a long useful life. The bearings go first,
followed by the brush wear, and commutator grooving due to prolonged service.
The only photo I've found among my repair archives highlighting a "Hubner Tach" similar to what this thread is discussing is posted with this reply.

Dismantle the item, install new bearings, and put it back in service.
If it does not function as it did... 2 plus DECADES ago when it was brand new...
Install a new tach.

John

 

[panter]

Hello everyone and thanks for your attention
Yesterday I tested the tacho on which they told my colleagues . He has more than 30 percent lower output compared with the data from the data . As it is the same as the generators to be repaired I want to disassemble it, take off the side cover and see the dimensions for making the magnet keeper. I wonder if someone tried to restore strength of alnico magnets .
I was thinking that maybe after removing the side covers and without moving the rotor we could wind round wire around each magnet and try to give a big DC current . .

 

[mikekilroy]

Couple comments.

So its output is down 30%; DOES THIS REALLY MATTER? Most drives have tach input adjustment pot since tachs are analog devices and no two are ever identical anyway. Most drives have input range adjustment jumpers also. So can you just clean the tach up, put back together and simply adjust the pots to this 30% lower value? I cannot imagine a 30% reduction having any significant effect except on a very low speed servo or torque motor system - running less than like 1 rpm for instance.

We have a pancake motor line that has a rotor that was originally made from a PCB but lateer made with copper stampings for single turn coils, glued to like fiberglass separator. The round magnets face this rotating disk and each have a few turns of like #16 wire around them - for magnetizing, after the motor is assembled. Once magnetized, these wires are cut off so they dont show anymore. If the motor is ever disassembled, it immediately looses too much magnetism to be a good motor anymore. But part of the repair process is to wrap new turns around each magnet and hit 'em again when rebuilt. If not good enough, they get hit again, and again, each time with a tad more current. But the specs are fully defined.

Next, know Alnico magnets ARE remagnetized all the time as part of motor/tach repair. Some motor repair shops have this capability. That said, those that do usually have SPECS on how much current, how many turns, etc to hit it with.

I cannot imagine not having that kind of data and trying to remag a magnet to a given level. So I would say if you experiment for the next 12 years over and over again, your guess of how many turns, what current, will get r done.

http://www.KilroyWasHere<dot>com

 

[waross]

I remember seeing an AC magnetizer years ago. It had only one or two turns of heavy wire with a fuse in series. It was energized with 120 Volts. The winding had very little resistance and very little induction. It was basically a short circuit. The Amps and hence the Amp turns were very high before the fuse cleared the circuit. Being over magnetized for an instant did not seem to be an issue.
Disclaimer; If the switch was closed too late in the cycle, the fuse would clear too soon on the next cycle and the maximum magnet strength would not be realized. The operator would re-fuse and hit it again.


Bill
--------------------
"Why not the best?"
Jimmy Carter