Question About A Power Supply For A Brush DC Motor

Discussion in 'Power Electronics' started by PGB1, Jun 5, 2019.

  1. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    102
    9
    Good Day To All!

    I have a leaf blowing tool & its circuit board burned after about 3 decades of use. It has a 120 volt brush type DC motor. The speed control with which the user interacts is a variable resistor.
    For the education I'll gain, I'd like to try to repair the machine.

    While studying what is left of the circuit board, I drew what I believe is the correct wiring, but something does not look correct. I can't put my finger on what is wrong- if anything. I am not very familiar with quadracs, so perhaps that is the source of my confusion.

    Attached are two sketches. One is wiring with the components laid out in the arrangement they are found on the board and the other drawing shows where I think the the circuit traces should be reconstructed behind the components. (Some traces are still present.)

    Do you all mind to look at the drawings and tell me where I went wrong (if I went wrong) while trying to determine the correct wiring for this control board?
    I sure appreciate the education!

    Enjoy Today!
    Paul
     
  2. mvas

    Active Member

    Jun 19, 2017
    538
    129
    The Bridge Rectifier circuit is not drawn correctly.
     
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  3. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    102
    9
    Thank You Mvas for taking time to reply.
    I goofed up the easy part!

    Does the attached new drawing look proper?
    It is kept in circuit board view, instead of the normal schematic diamond shape so I can plug it into the earlier drawings.

    Thanks Again!
    Paul
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    That is a common Triac controller, unusual to see the bridge rectifier as most of these motors are Universal type.
    There are not many components to trouble shoot, and the obvious firsts would be the Triac or the bridge.
    But a cheap replacement can also be had from one of the dimmer style controllers for power tools, if not certain if it is in fact a Universal motor, (wound field as well as rotor) then a bridge could be added to the dimmer.
    Max.
     
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  5. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    The triac M1 connection is not drawn correctly.
     
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  6. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    102
    9
    Thank You all for replying with information.

    Actually, upon opening the machine for the first time, my first thought was the same as yours, MaxHeadRoom, that the motor was universal. But the motor runs only on DC.

    Trying to learn, I re-visited the schematic, textbook in hand. I think that I have the wiring figured out and "Drawing Version 2" is attached. (The way I'm going, we'll be at Version 10 soon.)

    From the new drawing, do I have the circuit understood?
     
  7. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    No, still a long way off.
    Can I suggest you attach good pictures of both sides of the board here.
     
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  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    It is odd to have speed control for a leaf blower!
    They usually operate at very high rpm using a Universal motor.
    You could connect direct to the 120vdc and see if it runs full rpm.
    Max.
     
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  9. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    This could be the circuit for the Universal motor, or DC type.


    Permanent-Magnet-Motor-Control.gif
     
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  10. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    102
    9
    Thanks Again All for helping me learn. I sure appreciate your help.

    Thanks for asking, AlbertHall, for the photos.
    Attached are photos of the component and trace side of the board. It's got some missing traces due to the melt down. Also, one diode was burned apart & is now gone. The capacitor marked C-1 internally shorted.

    MaxHeadroom, when I first opened the machine, I was surprised to see a permanent magnet motor instead of a universal type. (Assuming I still remember the difference. Universal has wiring to the stator windings as well as to the brushes.) I guess the manufacturer went as cheaply as possible.

    Knowing better, I still had to convince myself that it would not operate on AC. Sometimes the only way to learn is to wreck stuff.
    From the isolation transformer, I very briefly tried the motor on 120 vac. It ran quite slowly & got instantly hot. Rotation was coincidentally opposite what is required for the impeller. A re-try with a push-start got it rotating in the correct clock, but still slow and hot.

    On half-wave rectification it runs faster, but gets hot quickly.
    On full wave, it runs cool but I think (and don't know for certain) that it is in run-away mode. If I can find data, I'll check the rpm versus the specifications.

    Back to the learning- I tried once agin to draw a somewhat simplified circuit for the speed controller. The Q4015LT is a Quadrac, so I didn't need separate diac and triac.

    If you all don't mind, would you please look at my drawing and see where I goofed this time?

    Thanks, too, DodgyDave for the schematic. Following it was easy, but I don't (yet) understand the reason for the 100 ohm, 15 kw & 3.3 kw resistors. Once I learn that, I'll perhaps also understand the 0.1 uF capacitors.

    You all can probably tell that I am trying to learn my way through this interesting project. (I can actually still purchase a replacement circuit board, but that would take the learning away.)
     
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  11. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Q4015LT is a diac/triac combo.
    If repairing the board, it may be wise to use a 60w-100w filament lamp to test with, and save the motor from any possible stress.
    Max.
     
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  12. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    Your circuit is the same as i posted, it's a simple motor speed controller, the extra resistors ( 3.3K, 250K) and the 0.1uF capacitor are for Variable speed control (like a dimmer) the 100 ohms and 0.1uF are for spike suppresion.
     
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  13. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    A slight correction to your diagram. The red X marks a connection that shouldn't be there (between M1 and G) and the red line indicates where M1 should connect. It goes the long way round just to save me from drawing hump back bridges.
    Clipboard01.jpg
     
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  14. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    102
    9
    On The Way To Success! Thanks to you all, that is!

    That's a great idea, MaxHeadroom, about using an incandescent lamp for a test dummy. Sure beats wrecking the motor.

    Thank You for the correction Albert Hall. I appreciate your sharing of knowledge.

    At first I didn't understand what you said about your circuit being the same as mine, DodgyDave. But after more studying about the Quadrac, I now see it plain as day. I knew what the 250 ohm was for, but the 3.3 k confused me until, but now I think I get it. We start with 3.3 k for the initial resistance and 'trim' it with the 250.

    I have been called away, so there will be a delay until I can get back to the project. I'm excited to find out if it works. I think I'll breadboard it first so I can experiment around. (With MaxHeadroom's light bulb.)

    The Box-O-Stuff didn't provide a diode to replace the bad one, but I do have a 15 amp, 600 volt bridge rectifier salvaged from a TV or something similar. (I gut dead electronics for their treasures.) Perhaps that will work instead of individual diodes- with a large heat sink.

    Electronics fascinate me, but in Detroit schools such classes weren't available back then. Perhaps in the fall I can take a beginning electronics class for seniors (or the young & smart).

    There must be some electronics skills buried deep inside me from my Father. In the 1930's and 40's he designed and installed radio broadcast systems for the underground in his home country of Germany. After escaping to USA, he went right back as OSS and continued. He was involved in some circuit design for Eniac and, later, laser guidance circuits. He mostly worked & lived in Germany doing the government stuff while I was growing up, so I didn't get to pester him too much. I do remember on a couple of his home trips "helping" him build a television and a HiFi receiver & amplifier for our home. So, some of that must be have sunk in- perhaps only the fascination.

    Thank You All again for helping me learn. I do appreciate your sharing of knowledge and igniting the spark to explore such circuits, Diacs, Triacs, Quadracs and more!

    Enjoy This Day!
    Paul
     
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  15. PGB1

    Thread Starter Active Member

    Jan 15, 2013
    102
    9
    I wanted to insert a quick note to say Thank You to each of you for helping me with this project and learn. It was fun to explore triacs and diacs.

    Finally, I was able to return to it and build the circuit board.

    One glitch I ran into was that the quadrac was bad (M1 to M2 permanently closed). For some reason, I didn't check it earlier, assuming the burned diode and circuit board were the only problems. Lousy troubleshooter, huh? So I built one with a lower rated triac and a diac which I had in my parts group. Testing was with a lower voltage and amperage DC motor and all worked well.

    I also experimented with different resistors and capacitors to help me learn about gate triggering. It was very interesting & hopefully will become permanent knowledge.
    My plan is to get the proper component(s) and make this circuit work for the leaf blower.
    (Having a leaf blower was secondary to learning about the circuit.)

    Thanks Again to all for helping me learn. I sure appreciate it!
    Enjoy Today!
    Paul
     
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