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Electric Motors

  1. Nov 24, 2016 #1
    Hi guys,

    I recently extracted a motor from an old washing machine and came across some really interesting things when I hooked it up to a D.C. power pack.

    Firstly, I connected a lead to each brush and powered it will 12V D.C. and it span really fast. I cannot work out why because the startor is an iron core electromagnet so how can it work if the electromagnet is not connected to any power source i.e. no magnet!

    Secondly, when I hooked up another 12V D.C. across ONE of the coils of the electromagnet is went a bit slower! At 6V across the electromagnet the motor went very slow and at around 3V it went the fastest!

    Thirdly, when a powered it with 12V A.C. and no electromagnet it went slower than the 12V D.C. with no electromagnet and with A.C. and one coil of the electromagnet powered (with D.C) it juddered and didn't rotate at all.

    Could somebody please explain these observation. Particularly the first one when the electromagnet is not being powered and there is definitely no permanent magnet either!

    Thanks
     
  2. jcsd
  3. Nov 24, 2016 #2

    Merlin3189

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    I can't explain all, but maybe I can suggest some ideas.
    Residual magnetism. Due to the B/H hysteresis in magnetic materials, there is normally some residual magnetism left in an electromagnet when the current is removed.

    It will spin fast on this weak magnet because the free running speed is inversely proportional to the strength of the magnet. This is a result of the dynamo effect. The current through the rotor increases until the applied voltage is equally opposed by the voltage across the resistance of the rotor winding and the back emf caused by the dynamo effect. When free running, the motor is at its maximum speed and most of this opposing voltage is provided by the dynamo effect, with the small current causing only a small V=IR resistance voltage.

    The back emf or dynamo voltage is proportional to the speed, to the number of turns and to the strength of the stator flux. If you reduce the stator flux, you reduce the back emf, so more current can flow in the rotor. Since both torque and back emf are proportional to flux, you may think these would cancel out.
    But, most of the voltage across the rotor is back emf when it is free running: only a small part is voltage across the resistance. So a small reduction in back emf results in a big increase in current. Eg. say for 10V supply, 9V were back emf and 1V was V=IR. Then if back emf fell by 10%, to 8.1V, the resistive voltage could increase to 1.9V, causing a 90% increase in current.
    So a 10% reduction in flux causes a 10% drop in back emf and a 90% increase in current.
    So the new torque is reduced by 10% and increased by 90%, a net increase of 71%, causing the motor to accelerate.

    This is not completely endless, as the current (and V=IR) must end up greater, so further reductions in flux don't have quite as much effect. And the small forces resisting rotation will increase with speed (eg. air resistance.) (But it represents a real problem for series wound motors, which should not be allowed to free run.)

    This is much harder for me to explain. My first thought was obviously that when you applied current to the stator, you increased the stator flux and therefore reduced the free running speed as outlined above.
    But you then provide a range of stator voltages with a non-monotonic relationship with speed. So my second thought was that perhaps the 3V actually opposed the residual field and reduced it to an even smaller value, making it the fastest. Then 6V and 12V actually reversed the field and increased its magnitude in the opposite sense.
    But that didn't work as I needed 12V to be slower than 6V. After a bit of juggling with these, I decided that the only remaining possibility that I could see, is that with each experiment you are altering the residual flux. And you may also be reversing it if you are not consistent in which way you energise the coils. It might be possible to work out a sequence of changes to give the required flux magnitudes, but I gave up at that point!

    As for the AC powering a DC motor, I haven't worked out anything yet.
     
  4. Nov 24, 2016 #3

    Averagesupernova

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    Never heard of a washing machine with brushes in its motor.
     
  5. Nov 24, 2016 #4
    Amazing, thanks. Never had such a detailed reply. Yes I had no idea how the A.C. powered it. I am away for several days but when I return I will try and upload a video to actually show the things I discussed. Could you explain the B/H hysteresis? I have never come across that before. Is the flux from the rotor creating a changing flux in the iron core stator or something? I would have thought that would be an opposing back emf and don't see how it could drive the motor. When I experienced it I was completely amazed that the motor span so fast with no magnet.
     
  6. Nov 24, 2016 #5
    Now I come to think of it I think it came from a pressure washer. Does that change anything?
     
  7. Nov 24, 2016 #6

    Merlin3189

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    The very first point I made, is that there IS a magnet, because the electromagnet does not completely demagnetise when there is no current. It is a weak permanent magnet.

    Here's a link to the B H curve and magnetic hysteresis. There are many others you could find. You could probably find an animated explanationon you tube.

    Essentially: (start at centre of the graph) when you apply forward current to an electromagnet, the flux increases with the current, quickly at first, then slower, until you reach saturation (top right of the graph) and the flux stops increasing even when you increase the current. When you reduce the current (moving left), as you drop below saturation, the flux starts to reduce (following the top curve, right to left.) When the current reaches zero (vertical axis) there is still magnetic flux (residual flux) and you must start applying reverse current (to left of axis) to reduce the magnetism to zero (top graph crossing the horizontal axis.) As you continue to increase the reverse current (go left) the flux increases in the opposite direction (below the axis) until it saturates in the reverse direction (bottom left corner.) When you reduce the reverse current (move right) the reverse flux starts to reduce (bottom graph.) When you have reduced the current to zero (vertical axis) again there is reverse residual flux (below the axis) and you need to apply forward current to bring the flux down to zero (bottom graph reaching the horizontal axis.) Then further forward current produces increasing forward flux (following bottom graph to the right) until again you reach forward saturation.
     
  8. Nov 24, 2016 #7

    Merlin3189

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    I too have not come across brushed motors on a washing machine, but I wouldn't rule it out. In the UK I've only seen AC induction motors used.
    I can imagine the pressure washer might need a high starting torque, which is not a strong point for induction motors. So maybe they use a brushed motor. (In fact, I just found "carbon brushes for Karcher and Bosch pressure washers" for sale!)
     
  9. Nov 24, 2016 #8
    Great, thank you. I had a long read over what you said and have got my head round it. The electromagnet has one set of coils on either side of the stator and each coil winding has two electrical connection. How should these be powered to get it to run the fastest if I have two 12V D.C. power packs (in addition to the one powering the rotor)? Should I simply connect 12V across each coil or should I be connecting wires from one coil to the other? With the electromagnet powered will the motor run faster or slower than when it was running off residual magnetism?
     
  10. Nov 24, 2016 #9
    Yes, it was a definitely a Karcher and has spring loaded carbon brushes.
     
  11. Nov 24, 2016 #10

    Merlin3189

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    Off the top of my head I don't know.
    As you found out, free running speed is probably highest with no power to the field windings at all! But presumably you want speed under load, so you need field current to get torque. My guess would be shunt wound (parallel connection of rotor and field) but I'll give it some thought tomorrow (well nearly today.)
    Having a quick look through some Karcher tech stuff online, I've only found induction motors so far.
     
  12. Nov 24, 2016 #11
    Thanks. When I get back home I will take a video or a load of pictures so its easier to see what is going on. Thanks for all your help.
     
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