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12 V motor loss of power

Discussion in 'Sensors and Actuators' started by Donald Helm, Sep 4, 2017.

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  1. Donald Helm

    Donald Helm

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    Sep 2, 2017
    Hello, my name is Don , my hobby is model engineering and I have a Myford ML7 lathe. My problem is -
    In 1990 (yes 27 years ago) I fitted a readily available power cross feed.. This consists of a small geared 12 V motor connected to the cross travel lead screw , powered by a power pack. This consists of a PCB with a transformer , rotary variable speed control , a forward/stop/reverse switch and small resistors and other bits.
    Earlier this year , it had been running fine but when I came to use it , the motor had lost both speed and power..
    A electrician I know had a look at it and thought it was the motor so I bought a new one (they are still available) , fitted it but it was no better . I then renewed the transformer but still no better. One item I recognised was a capacitor and remembering from old car days how these could give trouble , I replaced it but still no better.
    I am afraid I have now exhausted my limited knowledge and wonder if anyone can advise what the power loss could be.
     
  2. duke37

    duke37

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    Jan 9, 2011
    Ey oop, welcome to the forum.

    Show us a picture of the motor and power supply. We need more information.
    A 27 year old supply should be fixable - no microprocessor.
    I could help, hands on, if you are not too far from Derbyshire.
     
  3. Irv

    Irv

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    Jun 7, 2017
    If there are no obvious signs of smoke on other parts, my first suspect would be the "rotary variable speed control" - probably a potentiometer, which can get dirty, corroded, or noisy, or change value due to various things. There is probably a transistor or scr on board, I would check that also. Need a pic.
     
  4. Donald Helm

    Donald Helm

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    Sep 2, 2017
    Thanks for reply. will try and get photo . I am a bit far from Derbyshire , I live at Scarborough.
     
  5. Donald Helm

    Donald Helm

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    Sep 2, 2017
    Thanks for reply . Did think of that but was unable to find one that looked like mine . Will try and get Photo.
     
  6. Donald Helm

    Donald Helm

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    Sep 2, 2017
    IMG_0378.JPG IMG_0380.JPG IMG_0381.JPG IMG_0382.JPG IMG_0383.JPG
     
  7. Donald Helm

    Donald Helm

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    Sep 2, 2017
    I have hopefully added some photos.
     
  8. Alec_t

    Alec_t

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    Jul 7, 2015
    If the (4-pole 3-way?) rotary switch is the forward/stop/reverse one, then it looks like the full motor current goes through the contacts on that switch. Use some switch cleaner on the contacts and see if that improves things.
     
  9. Minder

    Minder

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    Apr 24, 2015
    The board looks very simple design and would not take very much to reverse-engineer it, try drawing the schematic up from it, If your DC supply voltage is ok, it could be the power semi.
    M.
     
  10. duke37

    duke37

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    Jan 9, 2011
    Please tell us what components are on the board.
    You have four 1N4002 diodes and another, perhaps the same.
    You have two small transistors, what are they labelled?
    You have one big transistor or scr, what is this?

    The diodes can probably be tested with a meter on a resistance range without taking them out.
    The small transistors will need to be taken out to test.
    I would go after the big device which will be passing all the motor current, it has no heat sink so is probably a SCR.
     
  11. Minder

    Minder

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    Apr 24, 2015
    If it is a Triac inside the bridge, they are a little noisy at low rpm, a simple upgrade would be one of the $5.00 PWM versions off ebay, just connect the output to your reversing arrangement on the board.
    M..
     
  12. 73's de Edd

    73's de Edd

    2,871
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    Aug 21, 2015
    Sir Donald Helm . . . .

    Can you control the speeed of the mtor at all with the pot ?
    If the motor is turning what would you estimate that the current speed is as compared to maximum speed ?
    Need the numbers from the two small plastic transistors.
    Also across the board from there is a larger "power semiconductor " that is being beside the only black electrolytic on the board.
    The heat sink tab has been clipped off short on what I believe will be a SCR that I just mentioned.
    Also there is a diode just beside them that feeds in pulsating DC to the "SCR".
    If you can fire up the system and get any movement from the geared motor, if you will then place the tip of a mini pocket screwdriver . . .standard blade . . .between the closest terminals . . .referenced to that nearby diode.
    The motor should then run at full speed if the basic power supply is operating at its normal capacity . It just needs our further attention and analysis of the few small parts in the frontal control electronics portion.
    Need the capacitance and working voltage rating of that electrolytic also . . . . did you replace it ?

    Thassssssit

    73's de Edd
     
  13. Donald Helm

    Donald Helm

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    Sep 2, 2017
    Thanks for the info but as I explained I am a complete novice in electronics but learning all the time , so here goes .
    The motor runs . Difficult to tell how much it is down on speed , just sounds as though it is running slower than it used to . It is definately down on power as I can stop the much geared down output shaft quiet easily. It is turning at about 25 RPM..
    Numbers on small plastic transistors - one nearest the end M2N 3904 EBC1
    one towards the centre 2TX 651
    Numbers on large "power semiconductor" or SCR (?) C10662 CXRCA844
    Diode next to it 1R 2004
    Other 4 diodes 1N 4002
    Placing screwdriver tip accross the 2 terminals mentioned makes no difference.
    I assume the electrollytic you refer to is the black capacitor which I replaced . It is 63V 1uF . Which is identical to the one that was originally on.
    Hope this gives you a few more clues to work on.
     
  14. duke37

    duke37

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    Jan 9, 2011
    The SCR is probably a C106 or maybe an armoured cat fish! 4A. Sensitive gate, very common.
    The transistors are npn, The 2N3904 is 200mA, The ZTX651 is 2A. Both common.
    Could the 1R2004 be another 1N4002?

    It looks as if the 30V AC is rectified with a bridge rectifier and if the motor speed and hence voltage is low, the SCR is triggered to raise it. The reference voltage will be controlled by the potentiometer.

    Be careful of the mains voltage.
    Measure the voltage across the outer terminals of the potentiometer.
    Measure the voltage from the centre terminal of the potentiometer to the most negative terminal. Does it go up and down as you twiddle the pot?
     
  15. kellys_eye

    kellys_eye

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    Jun 25, 2010
    As far as motor speed controls go the circuit you are using is pretty poor and the earlier recommendation to change to a PWM driver is highly recommended.

    Have you checked the gearbox grease?
     
  16. BobK

    BobK

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    Jan 5, 2010
    Have you tried supplying power directly to the motor to see if it runs correctly?

    Bob
     
  17. Minder

    Minder

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    Apr 24, 2015
    He did mention that the motor was replaced, so unlikely it has ruined another as I would expect to see some ill effects as soon as it was connected.
    M.
     
  18. Bluejets

    Bluejets

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    Oct 5, 2014
    Is the motor/gearbox on your bench or still connected to the carriage?

    If so, probably get your electrician mate to check that the transformer is working ok on both legs.

    Something does not look right....you have a 15+15 transformer which appears to be connected in series on the output yet motor is only 12v.
    Transformer is only 3va, seems very small for a motor driver.
     
    Last edited: Sep 7, 2017
  19. duke37

    duke37

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    Jan 9, 2011
    I disagree. I made such a supply to drive a coil winder. It measures the speed by voltage 100 times a second and gives a boost if necessary. It will cater for motors with strong cogging. The larger the inertia the better for low speed use.
    It is awaiting being put into a box so may have to wait a few years unless I am put into a box first.:)

    A PWM voltage supply may be good enough or this purpose, It gives a controlled average voltage but no speed control. It would normally have a big electrolytic input capacitor which is not necessary since the motor inertia will smooth the speed variation.
    I have used a battery charger and Variac with a motor well on top of the job.which is good enough for a rough speed control.

    Edit: What does the led do?
     
    Last edited: Sep 7, 2017
  20. Minder

    Minder

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    Apr 24, 2015
    You just have to compare the popular KB DC drives, the simple version is the 2 quadrant SCR bridge which is OK but a little primitive in that it is noisy at low rpm, Their PWM version is much superior, more parts and a bit more money, but worth it. IMO.
    M.
     
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