Thursday, January 28, 2016

A Ming-Inspired Cabinet (28)

Post 28 in a continuing series, and 900th post since this blog was started back in 2009.

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I got tired of using a screwdriver across the terminals of the otherwise dead start switch on my Zimmermann, and got around to replacing both push buttons and their mounting plate:


The original push buttons were a larger size and in replacing one of them with a 22mm push button operator, I needed to make a new mounting plate, so I ended up getting a new 22mm 'stop' switch as well . The new 'stop' switch is larger and a little easier to hit if you need to shut things down in a hurry. Safety first!

While doing that work, I accidentally grounded a light circuit conductor and blew a 6 amp fuse. The fuses on the Zimmermann are ceramic, a bit like the ones you find in an electric stove, and fortunately I had one spare 6 amp fuse on hand. There are a dozen fuses in the machine and when a fuse blows there is no obvious visual indicator that I can see on the fuses themselves.  You have to test across each one for continuity, which is a little tedious.

Looking in my factory operation manual to dig into the wiring schematics and electrical component specifications, I discovered that the fuses are supposed to be Siemens 'Diazed' 25amp units, however almost all of the ones in the machine are 6amp. That leaves me slightly puzzled. Either the ones in the machine are wrong, or the operations manual (from 1980) is wrong, or for some reason the specification was changed between 1971 and 1980. I also have a wiring diagram from another year, and it is identical to the first one but there isn't an electrical parts list included in the other year's paperwork to confirm which model of fuse was used.

I am wondering then about the correct fuse amperage:  25 or 6A?  They have all been working fine so far, but then again, other than the recent accidental connection, there has been no event which has caused them to be tested. They could be fine and they could also be inadequate. I'm not sure what to do. I'm seeing no reason to change anything, but also wondering if the fuses in place are undersized.

I bought a package of 6amp fuses from Germany, and they were pretty cheap actually, and shipping was less than $10.00. Hopefully they will be here soon as I feel nervous having no back up fuses.

This is where I wish I was a bit savvier about motor electrics, and could possibly work out what the correct fuse size should be. I need to study this more.... I confess I am a little fuzzy in my understanding of momentary switch electrics. I do understand that the starter switch is a normally-OFF circuit, meaning the button push connects the circuit, while the 'stop' switch is normally-ON, and thus pushing the 'stop' button disconnects the circuit.

Motor controls are not direct in the same way a when you turn on a household light switch. There are magnetic controls between the switch and the motor. When the "start" button is pressed, the motor is not powered directly, rather the electromagnet in the contactor is energized. The magnetic switch in the contactor then engages, simultaneously switching current to the motor and providing self-sustaining current to maintain its own state. Thus when the start button is released, the magnetic switch remains engaged and the motor remains running. I find this fact slightly counter-intuitive, but it is rather similar to the use of relays in an automobile. Instead of routing 460volts through the start/stop switching, the power comes from an internal transformer putting out 30 volts. A low voltage circuit controls a high voltage motor.

I spent the past couple of days doing some CAD work, as my car has been in the shop. The client got back to me and is fine with the design revision to change the steps in the middle portion of the cabinet for shelves. He also wants me to devise a system to attach the cabinet mechanically to the wall in case of earthquake. He lives in LA, so this is a reasonable concern of course. I've got some ideas about that and we'll see where things end up.

Last time in the shop, I was trimming leg tops to a double-bevel or compound angle:


The legs only slope 0.5 in 10, so the slope is hard to see in the above picture.

The legs after the compound bevels have been cut on top - a little more obvious to see the bevel now:


The post tops were the tenoned in one axis:


Another view:


 Another view:


Further reductions lay ahead....

Thanks for visiting the Carpentry Way. My car should be fixed this morning so I will be back in the shop as well. On to post 29

9 comments:

  1. When in doubt about fuse size, under is always better then over. And there isn't much more to know about the switches. If you use the nameplate data on the motor (it does have one?) Power (watts) = current times voltage. Plug in what you have and you'll get an idea of fuse size. There is also another formula for getting power using HP but I don't remember it.

    ReplyDelete
    Replies
    1. Good to hear from you Mr. Boumenot,

      I think you're right that it is probably best to keep to the same fuse that is in there presently as it has been working properly.

      The motor data plate is not part of the equation here since this fuse was on the low voltage 30v circuit coming out of the transformer.

      ~C

      Delete
  2. As Ralph said, if it works properly with 6A fuses, don't change them for higher values.
    For single phase power=tension X current X cos(Phi). Were cosPhi is lower than 1.
    For 3-phases power= SQRT(3) X tension X current X cosPhi. So with 3 phases operation [with the same power], the current is divided by 1.732...
    Six A gives you up to 4780W (about 6.5 hp)in 3-phase which is already a lot of power.
    Of course a motor takes much more power at the start-up but fuses may be of a slow type accepting a much higher current for a short time, allowing the starting.
    Some motors are using a star/triangle starting to reduce the sarting current.

    There should also be a plate on the 460/30v transformer.
    But I guess all the magnetic switches together don't use much power.
    Sylvain

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    Replies
    1. Yes, the low voltage circuit is where the blown fuse was associating, so given a 30v supply from the transformer, it seems unlikely that a 25amp fuse would be involved with that.

      Thanks for your input Sylvain.

      ~C

      Delete
  3. What if you increased to only 10 amps? If all the circuits have been working fine with 6 amp, Why take a risk? if they are "burning out" because of loading the circuit then step up slowly, check the wire gauge..that would give an indication if 25amps would be too much. remember that some of these circuits are 220 (or more if three phase), so a 6 amp circuit could be quite "Big".
    I would stay with the 6's until more information is found.
    The ceramic fuses that I've come across have a burn indicator on the end of the fuse usually can be seen with the retaining caps on....but the color change (showing burn out) is not always apparent because of location/lighting etc. They are sometimes a pain to test for sure.

    ReplyDelete
  4. after looking at the web...some of these fuses are limited in the capacity used, by the diameter of the fuse base/holder... eg a 10 amp fuse will not fit a 6 amp fuse holder. I might not be explaining it right so here is a spec sheet with further explanation. look under the "d-type fuses".
    https://en.wikipedia.org/wiki/IEC_60269
    also here:
    http://www.altechcorp.com/PDFS/FUSECAT3.PDF
    Thanks again for your posts, love seeing your precision joinery!

    ReplyDelete
    Replies
    1. Joe,

      many thanks for the links. I didn't realize that these fuses might be available in the US. I may have ordered the wrong size from Germany, now I see there are several sizes and models.

      ~C

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  5. Newbie question here, wouldn't it be structurally stronger to have the mortise angled rather than the tenon?

    -Aaron

    ReplyDelete
    Replies
    1. Aaron,

      thanks for the question. The answer is 'no', it would not be structurally stronger to angle the mortise. The mortise, in fact, is not simply in one piece of wood, but is a space passing through 2 pairs of lap-jointed components, and angling the mortise through a lap joint is going to weaken a portion of that joint. Better to have the mortise centered in a lap joint. besides, if the tenon were angled, assembly of the components above would be much more awkward. With the tenon angled, it is in a vertical position, so the post and stretcher assembly can be put together as a unit, and from there any timbers which locate on top can be pre-assembled and slid straight down onto the tenon.

      ~C

      Delete

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