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  1. #46
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    Quote Originally Posted by BaronJ View Post
    I doubt that the capacitor life would be much affected by discharging into what is basically the impedance of the motor inductance. Ignoring motor back emf, it wouldn't be as damaging as putting a dead short across the capacitor.
    Agree about discharge. Motor winding resistance is significant (i.e. safe).


    I was more worried about the charging cycle.
    User switches circuit on, and 10Amps of half-rectified AC gets dumped into cap in 1/100 of a second?

  2. #47
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    Quote Originally Posted by jack620 View Post
    How hard are we talking John? Roughly how many seconds to come to a stop?
    In any real world scenario there's also going to be the rotating mass of the machine to bring to a stop (pulleys, spindle, chuck, saw blade, etc).
    Hi Chris,

    This is a difficult one ! The bandsaw motor stopped within a couple of turns, but I put the very rapid stop down to the load imposed by the rest of the saw mechanism. That is why I took the belt off for further testing. Then I pulled out another motor with the same power ratings but only about half the physical size, 9 mm shaft compared to the 15 mm shaft on the bandsaw motor.

    I suspect that if your 2 Hp table saw motor is very free then it will just slow it down to maybe a tenth of its speed before coasting to a stop.

    I say this based on the behaviour of the physically smaller motor I tested with. It brakes hard and then runs on for half a dozen turns.

    I doubled the capacitor value thinking that it might assist but my conclusion is that the capacitor simply gets discharged too quickly. Adding a 47 Ohm resistor makes almost no difference. Tests with a Thyristor behave in exactly the same way without the sparking across the closing contacts.

    I need to give this some more thought !
    Best Regards:
    Baron J.

  3. #48
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    Quote Originally Posted by nigelpearson View Post
    Agree about discharge. Motor winding resistance is significant (i.e. safe).


    I was more worried about the charging cycle.
    User switches circuit on, and 10Amps of half-rectified AC gets dumped into cap in 1/100 of a second?
    Hi Nigel,

    Yes I agree ! That was the purpose of the Thermistor, to limit the current surge through the rectifier.
    47 Ohms is about 7 amps, the BY127 has a half cycle rating of a little over 20 amps. Having said that the 1N007 is much higher.
    Best Regards:
    Baron J.

  4. #49
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    I just had another look at this..
    I promise not to reply to things on nightshift ever again...

  5. #50
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    Quote Originally Posted by racingtadpole View Post
    I just had another look at this..
    I promise not to reply to things on nightshift ever again...
    Well you did confuse me for a bit

    I'll let you off this time, tiredness doesn’t help.
    Best Regards:
    Baron J.

  6. #51
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    Hi Chris, Guys,

    I've been doing some playing about ! I had forgotten that I have a 2 Hp circular saw in the garage, it been buried under all sorts of rubbish. So I've had a quick play with trying the braking circuit on it. Partly because of your mention of your saw and just to see exactly what effect it had on stopping the blade from rotating.

    As I have found out from the experiments with the other motors, you get a great big kick as the motor attempts to stop dead and then it free wheels for a few seconds before stopping completely. After measuring the motor winding resistance (about 12 Ohms) I tried 840 uf. It made very little difference, but after half a dozen attempts the relay contacts welded together.

    My conclusion is that the low resistance of the motor windings causes the capacitor to discharge far too quickly for it to be an effective method of stopping the motor from rotating completely. The reason that it was more effective when I tested using the bandsaw was because of the load on the motor from the bandsaw mechanism.

    I'm going to have a look at another braking scheme.
    Best Regards:
    Baron J.

  7. #52
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    Nov 2007
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    melbourne australia
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    Quote Originally Posted by BaronJ View Post
    I had forgotten that I have a 2 Hp circular saw in the garage, it been buried under all sorts of rubbish.
    Hi John,
    Thanks for checking it out for me. To me a circular saw is hand-held and has a universal motor. Did you mean a table saw (with an induction motor)? My TS is 3HP and has a 300mm blade. Based on your testing so far it will require a very robust braking circuit.
    Chris

  8. #53
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    Hi Chris, Guys,

    Yes my 2 Hp saw is a table one with an induction motor ! I never thought about trying a universal motor.

    A hand held circular saw is sometimes called a "Skill Saw" here, because you need skill to prevent yourself from cutting your own body parts off ! Particularly fingers. I know a surprising number of people that have injured themselves with various styles of power saws.

    I have a couple of small universal motors I can try out.
    Best Regards:
    Baron J.

  9. #54
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    Hi Chris,

    Adding to my previous post, since universal motors will run on DC the braking circuit actually causes the motor to run on slightly longer than just removing the power ! You do get a little judder at the point that the DC is applied, but that depends upon where in the AC mains cycle it occurs.

    So that one is a no no !
    Best Regards:
    Baron J.

  10. #55
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    Thumbs up New Design !

    Hi Guys,

    I've been doing some more playing about with the design of a DC braking system for single phase induction motors. Based on previous experience with the original circuit (the second drawing posted), the conclusions drawn are that the capacitor simply does not have the ability to store enough charge long enough to be effective. Even using a limiting resistor the current flow doesn't last long enough to bring the motor to a halt.

    In order to remedy this shortcoming I have redesigned the circuit so that there is a continuously applied voltage that will cause a much larger current through the motor windings. I have yet to test this new circuit and calculate a few values. The components used excluding the relay, are all cheaply available or can be salvaged.

    There is one component that might need to be purchased, that is a normally open push button switch. This switch only has to carry the relay coil current and then only for a fraction of a second whilst the relay locks itself closed. The relay will release when the machine stop is activated or the power is removed. If I've got the circuit correct the motor cannot be restarted unless power is removed from the relay allowing it to release.

    DC_Injection_Braking_Thyristor-2.png

    The relay contacts 1B and 2B are wired to the points 1B and 2B and are used to turn on the Thyristor. Again the current at this point is very low, only enough to turn on the Thyristor, which for the one I am going to use is about 0.015 Amps.

    The resistor in series with the Thyristor is going to have to withstand the whole of the motor current during braking and depending upon the motor could get quite warm. For the 1 Hp motor that I will be using for testing I estimate about 5 amps, but I will know more about that when I test the circuit.

    It should be stated that the Thyristor will turn itself off automatically when the incoming AC reverses, and this turning off can only happen when the gate voltage has fallen to a level where the Thyristor is no longer triggered on.

    Comments and opinions appreciated. Thanks Guys.
    Best Regards:
    Baron J.

  11. #56
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    If I'm reading the circuit correctly John, you will be injecting half-wave rectified AC into the motor winding via a high power resistor? So an approximation of DC injection? I'm keen to see the results.
    Chris

  12. #57
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    Hi Chris, Guys,

    Thank you for your comments.

    Yes the idea is that the much higher rectified current availability from the mains supply will stop the motor very quickly as desired. I still have to work out the details and do some testing.

    The circuit also avoids the contact damage caused in the previous circuit due to the very high current from the capacitor discharge through them. The largest current being that of the motor and that set of contacts is normally closed when power is applied anyway, only the current from the running motor is seen when the contacts break, which would be quite low.
    Best Regards:
    Baron J.

  13. #58
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    Thumbs up Collecting Parts !

    Hi Guys,

    Just an update ! I have been doing some playing about with the circuit and collecting some parts for making up a test unit. I've also modified the circuit a little and done a new revised drawing.

    DC_Injection_Braking_Thyristor-2.png

    This is the modified circuit. The relay contacts labelled (B) are used to switch the Thyristor trigger voltage from the charging circuit to the Thyristor gate.

    The circuit works by using the diode and resistor to charge up the capacitor directly from the mains supply along with a zener diode across the capacitor that is used to limit the capacitor voltage and to keep it below the value of the Thyristor gate voltage. Using the capacitor to provide the trigger voltage on the Thyristor gate.

    Because the Thyristor is being fed from the incoming AC supply voltage, when turned on, the Thyristor will rectify and supply a DC voltage directly to the motor windings. The current only being limited by the series resistor and the motor winding impedance.

    The Thyristor will turn off when the incoming AC reverses polarity ! However since the Thyristor gate is still being held on by the capacitor voltage, the Thyristor will immediately begin conducting again as soon as the AC cycle becomes positive.

    This action will continue as long as the Thyristor gate remains triggered. The trigger voltage on the capacitor will decay quite quickly allowing the Thyristor to stop conducting and stop supplying current to the motor.

    The push button is used to trigger the relay and start the braking behaviour. The relay will only drop out when the mains supply is broken by the main switch on the supply line. The braking cycle cannot commence until the incoming mains supply is present or restored.

    Thanks for looking ! As always comments are appreciated.
    Best Regards:
    Baron J.

  14. #59
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    Quote Originally Posted by BaronJ View Post
    The circuit works by using the diode and resistor to charge up the capacitor directly from the mains supply ....
    Looking at your circuit John, I can't help thinking the capacitor will not charge, since both ends of the cap will be at the same potential when the motor is running.
    Chris

  15. #60
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    Quote Originally Posted by jack620 View Post
    Looking at your circuit John, I can't help thinking the capacitor will not charge, since both ends of the cap will be at the same potential when the motor is running.
    Hi Chris. Thank you for your post.

    Yes you are quite right ! the bottom of the capacitor should go to the neutral connection.

    Thank you for spotting that error, its nice that someone understands circuitry and it saves me from chasing my tail when I get around to testing it.

    At the moment I'm chasing around in my electronics junk for a suitable sensitive SCR. All the older ones that I do have are either too low a voltage rating or not a high enough current rating. I need something around 600 volts and 6 to 12 amps. The other requirement is a 2 to 5 milliamp trigger current, hence the desire for the sensitive ones. Most of the ones that I do have require 50 ma or more to trigger them, which doesn't hold them switched on long enough.

    I'll post a corrected drawing later.
    Best Regards:
    Baron J.

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