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Single phase induction motor won't stop rotating

Discussion in 'Troubleshooting and Repair' started by XGamer23, Sep 9, 2019.

  1. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    Hello folks,
    I have an electric saw that is being powered by ~2kW induction motor. Everything works fine besides that it won't stop rotating after you shut it off. It used to stop automatically, but, one time it just stopped doing so, and now keeps spinning more than a minute after you shut it off.

    I can't realize if the the motor has a fault, or the mechanism, since every electric induction motor I have stops under 5 seconds, and even if there's no load attached to them, and I mean every but not this one anymore.

    I checked out if the motor contained any additional electronics such as some circuit boards or such, or some capacitors, but it doesn't, it has just one large starting capacitor, besides it, nothing else.

    Also, if you need more information on the motor, just say, I didn't really find it useful since there's no information about it online.

    Thanks everyone in advance!
     
  2. 73's de Edd

    73's de Edd

    2,678
    1,085
    Aug 21, 2015
    Sir XGamer23 . . . . . . . . .( and why do XGamers needs hi horsepowder moders ? )


    How about some mo' info ?

    120 or 240 VAC supply and I'm heavily leaning towards 240 with that hefty wattage consumption .
    If there is any fine sawdust involved . . . . I'm thinking of it drifting in / penetrating into contacts and a film of it disrupting contact action.
    Braking action could be initiated by either a contact action of a centrifugal switch upon power down.
    Or, a DPST power switch shorting out the motor winding, upon its switching into power off condition.
    To test the first condition , you need to be able to turn on the motor, wait for full speed, and then rapidly unplug the AC power cord and place a screwdriver short across the plugs blades.
    If that then causes it to then experience a braked condition presence, work on seeing if the switch is not creating a short internally on flipping into its power off position.
    If that first test was unfruitful, see if you can use a BRIGHT light to see inside of the motor, thru its ventilation holes, to be able to see the internal centrifugal switch assembly surrounding / and nearby to one motor shaft end.

    Use high power compressed air to clean out sawdust, micro-caked upon it, followed by a wet spray degreaser , followed up by more air to dry it out.
    Usually the switch is located at the end of the motor closest to the power input. Put an ear close by and you should be able to hear its click action upon power up or again at power down, to be able to further zero in on its physical location.


    73's de Edd

    There are two types of pedestrians . . . the quick and the dead.



    .
     
  3. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    [​IMG]
    Hey there, thanks for the answer!

    So, firstly, a few corrections which I'm terribly sorry for, I got the wrong info about, but after checking it a bit deeper I realized a few things:
    Firstly, it's 1.1kW, 230V AC. Second, there IS a circuit board on the main power switch, I really have no idea for what it is, can't be a soft start circuit since it dims the lights when it starts. Also, after opening it I found out that a ceramic capacitor fell of, I can't realize how it fell of just like that. Afterwards, a glass fuse (10A) burned out. I still can't figure it out how the motor was started and working fine with missing and dead components.

    I fixed those things, I soldered the ceramic capacitor back in place, I can't really confirm if it works, I did test it with a multimeter, it did show a value. I replaced the fuse with a 11.25A one since didn't have a 10A one. I found out that on the circuit board there's a place for a component, the pcb drawing confirm it, also there are holes for it, but there are no other components, I didn't find it like I found the capacitor lying around. Anyways, I fixed the issues, put everything back, and the motor started, again with dimming the lights (which confirms it's not a soft starter, and the components on the board confirm it too...), and yet it doesn't brake again, but it does work fine.

    I tried shorting the two blades on the cable while it's unplugged but the motor is spinning, and didn't get any results, maybe because it's not the way to stop the motor, but maybe that the switch is electronically controlled since the two pushbuttons on the main box are just push buttons, momentary ones, so there's one for ON and OFF, and maybe that's the reason for the circuit, but, I confirm that it was working even before fixing the components. Anyways, I couldn't short the blades properly because after shutting down the power the switches are useless, and thus couldn't do anything. I could've tried removing the cables, getting normal male plug and re-do, but the box is sealed in a way that you can't do that.

    Next I will try is to clean the engine for the sawdust, but there's one thing I don't understand: how come that sawdust could get into the engine? Also, is the centrifugal switch located inside the engine, or outside behind the vents? Since I can't really figure out how could sawdust get into the engine which should be sealted agains it.

    Anyways, I will try cleaning it next and locating the switch.

    (note on the image, the missing cap on "R8" - no idea why "R" but it was the capacitor place, also the burned fuse, and the missing component at "041". Also, the one with the heatsink appears to be a transistor (2N6388))

    Thanks for the answer!
    Please, add input if you have any!
     
    Last edited: Sep 10, 2019
  4. Minder

    Minder

    2,834
    586
    Apr 24, 2015
    Certainly does not look like an induction motor controller, in fact the 1ph version does not control well at all, like its 3ph cousin does using a VFD.
    Even using a Triac does not work well with a induction motor over a certain HP, only the shaded pole and PSC motor does.
    Are you sure it is not a Universal/series motor?
    What is the power device show as Q1, is this the 2n6388?
    A motor of this type/application should be controlled by some kind of contact closure, not semi conductor for off control..
    M.
     
  5. 73's de Edd

    73's de Edd

    2,678
    1,085
    Aug 21, 2015


    Sir Xgamer . . . . .



    Ahhhhhhhhhhhh-sooo. . I wasn't expecting that units, additional, most honnable "black box" baggage.

    I was basically only perceiving of just 3 or 2 wires going from a wall outlet, plus a power switch and then wiring going into a bare naked motor.

    In the situation now, I can see the breakdown of the use of the discrete components in the top half quadrant if the PCB as being two small signal transistors , rectifier diodes for a power supply, with its associated filter caps and a 1watt zener regulator diode.
    That circuitry all results into a created DC drive signal to be sent over into the base of the heat sinked Q1 POWER transistor, down just below.

    This all leads up to our need of making an analysis, and thereby ascertaining if the then freewheeling motor, after turn off, is being dependent upon a short being initiated across the motor winding for braking OR if a timed out DC voltage is applied to the motor for braking.

    OK now . . . . in filling you in on that loose "ceramic capacitor " that you are dealing with .
    Au contraire . . . . .I do believe that device is ACTUALLY being an Negative Temperature Coefficient AC inline surge thermistor . . . . and with it being of the resistive family . . . .it is thereby acquiring the R-8 board designation.
    Plus, some are capable of getting hot enough to melt solder***, whereas, that is NOT being the situation of a common ceramic disc capacitor.
    *** Plus, its falling loose to the bottom of the case, if its mounting plane involved some gravitational downward pull.

    If you are not familiar in seeing these devices different case profiles, you might be finding / seeing one as a bare naked unit as in A or with a thermoplastic covering as at B and a final and optimal ceramic fired coating at C.

    # Kodachrome . . . . . . . thermistors
    [​IMG]

    Now, that units RED device at center PCB should be an varistor, for chopping voltage transient peaks. You might also mistake that unit as being a covered disc ceramic capacitor.

    Now with your mentioning the unit using a set of pushbuttons for power on and off function lets make our first test as seeing if our heat sink mounted " Q1 " device is being an on-off power switching transistor . . . I'm not expecting so, with its !! 80volt !! rating.

    Its emitter is the lead closest to the RED Varistor and the center lead is the collector and the extreme far lead is being the base.
    So get DVM in hand and switch to its DC voltage mode and negative probe to the emitter and the positive probe to the base . You want to then power up the motor and see if there is being in the order of ~<1vdc on the metered emitter to base connection.
    If you don't have an autoranging meter . . .to be on the safe side . . . .start at 500VDC and switch on down ranges, until eventually being readable.
    Then press the off button and see if that voltage doesn't dissappear .

    If this holds true, that is your switching for the motor and our braking action must be accomplished on the bottom half of that board and is dependent upon rotor derived voltage . ( Still skeptical )

    Now confirm this . . . . . notice that you have a BROWN wire that loops from a bottom boards push on terminal to a top of the board push on terminal .
    Then there are the paired center BLUE push on wires that join at the PCB foil, as being one connection.
    Finally the BLACK push on wire.
    Lets now have you take the possible AC voltage readings between those possible combinations, as quickly as you can , just after a power off.

    Brown to Black
    Brown to Blue . . . . . thinking that this will be the high reading one
    Blue to Black

    Find the combination with the highest initially developed decaying AC voltage and then that will be the pair that we will be wanting to short together with a double clip ended test lead just as soon as is possible, after power turn off.
    Confirming that short is then intiating a braking action.

    More info is needed . . . .particularly the resistance reading across your R8 ersatz "ceramic disc capacitor " . . . I'm expecting it having to be taken out of circuit to get a true reading . Also be VEWY-VEWY carefull in not bending the leads, if being a naked / side lead soldering visible unit.
    I'm expecting a sub 100 ohm reading.

    I needed that part number of the 2N6388 POWER transistor . . . . as it seems like I could only make it out as a possible 2N**99/88 on the top line.
    It turns out to be an NPN POWER and its 80 V -10 A -40W- β 1000, specs would not equate with being a power switcher.
    BUT, possibly, merely eing used for switching in the braking function with a period / threshold being set by the BLUE trim pot + its associative timing capacitor.


    Confirm the marked number on the RED varistor for us.


    Your submitted and "errant" O41 part should properly be read as a D41 . . . as in diode. On this board, if you look at the peep thru foil traces two smaller diodes were being in parallell originally. Now they are using but a single LARGER diode.
    Reflected light on the boards left and central areas, inhibits me from further "reading " of the board foil paths contrasting traces.
    That diode may end up being used to rectify the generated voltage of the unit after power off , by the still spinning rotor, and used for DC braking.
    But. . . IF . . . there is an open circuit by an open R8, that circuit action is never being accomplished.

    # Kodachrome

    youtube]ggPkZ6iEGW8





    73's de Edd . . . . .


    Cannibal Axiom #1:
    Eat the rich. The poor are tough and stringy.





    .
     

    Attached Files:

    Last edited: Sep 11, 2019
  6. Minder

    Minder

    2,834
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    Apr 24, 2015
    Is there no kind of disconnect OFF switch, if not this is not safe and does not conform to code.
    M.
     
  7. 73's de Edd

    73's de Edd

    2,678
    1,085
    Aug 21, 2015
    In now having seen "all of the marbles" dumped upon the table, excepting the motor and its associative capacitor, I can now only see the motors power switching as being accomplished with a simple / conventional Push Button electro mechanical switch . . . akin to . . .

    upload_2019-9-11_17-1-25.png



    73's de Edd . . . . .


    I got myself a new shadow. I had to get rid of the other one . . . sometimes it wasn't doing what I was doing.


    .
     
  8. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    Hello.
    I'm sure too that isn't a controlling unit.
    Also, since it has a large capactiro attached to it, I'm guessing it's a induction motor.
    Here's info about and how it looks like:
    (pics)
    https://ibb.co/vhCtVqv
    https://ibb.co/3WdGM4Q
     
  9. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    There is SOME kind of swtich, never used it before, didn't even realize it's there until I opened the box. It's a momentary switch/button.
    https://ibb.co/MVsvGC6
     
  10. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    Here's how this one looks:
    https://ibb.co/JmzMNdt (sorry, it is reverted, the site screwed it around)
     
  11. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    Well, I surely do feel even dumber now, excuse me for giving dis-information, since I'm still learning, I haven't met such components yet (varistors).

    As I was about to do the test, I opened the case again, and once again the same fuse died again, I don't know what triggered it, the starting or the shutdown of the motor, but it did burn out again.

    Afterwards, I did some additional tests, I tested the varistor that fell of (the name is: "420L20"), the ohm reading is (multimeter set to 200ohm) 00.4 - 5 ohms. Both this one, and the other one, same results, also, upon opening the box, the varistor.. desoldered/felt off again, might be a crappy soldering job.

    Next, I found out that the large diode has died. Using the diode/continuity testing function on multimeter, tests that the diode lets current flow in both direction (beeping no matter of the polarity of the diode). Also I tested out the remaining smaller components on the board, all seem to be fine, the smaller diodes show results in a range of 400-600 something on the right polarity, in short, they do work. Also the resistors all seem to be working just fine.

    Next, I found the that the transistor might be dead/shorting. I used the continuity test on the multimeter to test out possible shorting on the transistor, and the result is:
    B --> E = shorted
    B --> C = shorted
    C --> E = shorted
    C --> B = shorted
    E --> C = shorted
    E --> B = shorted
    All the leads are connected, may casue the fuse burning out also it most likely killed the diode.
    I found the schematic on a datasheet for my 2N6388 transistor:
    [​IMG]
    As it seems, some of the leads should be connected, and some only power is applied, but in my case all the leads are connected even thought no power is applied.
    I might be wrong, but it seems kind of logical to me.

    As for the diode, I have 4 spare diodes, they are rated for 10A and also appear to have the same size as one of the board, but those ones actually work.

    Some links for additional PCB pics:
    (sorry if some are reverted)
    https://ibb.co/0XqSqTx
    https://ibb.co/zXX8F38
    https://ibb.co/qj0pHKD
    https://ibb.co/FwZvkq9
    https://ibb.co/wzy9YLd
    https://ibb.co/8NPwHmK
    https://ibb.co/b6h03Zm
    https://ibb.co/M8NjfN4

    Excuse me for my mistakes, as I said, I'm still learning!
    Sorry for not posting lately, I was busy.
     
  12. Minder

    Minder

    2,834
    586
    Apr 24, 2015
    What type of saw is it?
    I have owned several different kinds and the only one that had any kind of control was a band saw?
    M.
     
  13. XGamer23

    XGamer23

    7
    0
    Sep 9, 2019
    It's a circular table saw, "Elektra Beckum".
     
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