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  1. #76
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    Quote Originally Posted by jack620 View Post
    I think the RHS of the circuit diagram is cut off John.
    Hi Chris,

    Yes it is ! I just took a screen grab from an article that I'm also writing. Plus some of the component values for the trigger circuit are not firm yet.

    All there is off the RHS is the DC power source and trigger components.

    I'll post a full circuit later.
    Best Regards:
    Baron J.

  2. #77
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    Default Latest circuit.

    Hi Guys,

    I've decided to post the latest circuit diagram !

    Final Circuit.png

    This circuit includes the thermal switch ! I'm currently having problems with sourcing a suitable item. So if you want to use this circuit just short out the 68 Ohm resistor. The Thyristor that I've used is a cheap BT285-800 sensitive gate device, although any device that can handle 7 or 800 volts at 8 amps should work OK. I used one of the salvaged 220 uf 400 volt capacitors. All the diodes were 1N5408 3 amp 1Kv devices except D4 and 5, they were 1N1007 devices but are only 1 amp devices. The spike suppression capacitor was again a salvaged 0.01 uf ceramic 1Kv device, though almost any high voltage rated capacitor will do here.

    If you use a different Thyristor the 68K gate feed resistor value might need adjusting. I just used the nearest standard value to get the amount of gate current to trigger the device that I used. About 5 ma.

    NOTE: The DC supply cannot be automatically switched off, so the machine stop must be activated as soon as the motor stops turning. Otherwise DC current continues to flow through the motor windings which would cause overheating if left flowing. It also prevents the motor from being turned by hand.

    Thanks Guys.
    Best Regards:
    Baron J.

  3. #78
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    Without fully understanding exactly how its doing what its doing. Does the brake side of things need to be powered before the motor is turned off?

    Assuming you have a machine that has at minimum a no voltage switch, couldn't you have the supply to that switch going through NC contacts of your press button?
    Press the button, no voltage switch opens, motor goes off, your relay closes, brake comes on. Let go of the button brake goes off.

  4. #79
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    Hi Stuart,

    Thank you for your post.

    Does the brake side of things need to be powered before the motor is turned off?
    Yes ! The relay disconnects the motor without disconnecting the DC supply.

    The DC supply is kept powered because using just the capacitor as originally there simply wasn't enough charge to bring the motor to a complete stop. Small motors stop very quickly, the bigger motors discharged the capacitor very quickly and just braked for a second and then carried on coasting.

    In one experiment I increased the capacitor value to 880 uf and welded the relay contacts, using a Thyristor gets around the contact welding. So rather than use a very large value capacitor I used a constant DC feed, so the actual high capacitor value is no longer required.

    Also using a series resistor to feed the DC supply allows the severity of the braking to be controlled, but at the expense of a high wattage resistor.
    Best Regards:
    Baron J.

  5. #80
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    Hi Baron,

    I get that the DC needs power after the motor is turned off. I just meant could it be turned on as the motor was being turned off?
    Using a no voltage switch could it be wired up like this?(most of your circuit removed as I wouldn't want people to think you were responsible for my scribble).
    The brake would only be on while you were pressing the button but it turns the main switch off so you cant forget to.
    Could you turn the brake on and off while the motor was spinning down or would that let the smoke out of something?
    Attached Images Attached Images

  6. #81
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    Hi Stuart,

    Thanks for your post.

    Yes that circuit would disconnect the motor but wouldn't disconnect the DC supply. It is the DC supply that needs to be removed after the motor has stopped.

    A time delay relay might work ! But then the circuit starts to get complicated.

    I'll give it some more thought !
    Best Regards:
    Baron J.

  7. #82
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    Quote Originally Posted by Stustoys View Post
    The brake would only be on while you were pressing the button but it turns the main switch off so you cant forget to.
    That is exactly what I suggested earlier in the thread. Simply a pushbutton switch that applied rectified AC to the motor after it is switched off. The operator holds the button pressed until the motor stops or slows to the point s/he is happy with. It would be wired so it couldn’t be energised when the motor is powered.

    It would be perfect for my table saw, as I have to sit and watch it coast to a stop before I grab the workpiece anyway. It would just mean waiting for a much shorter time.
    Chris

  8. #83
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    Hi Baron,

    Doesn't my schematic show the relay latching removed?

    Not exactly what you had in mind but as Jack says may suit better in some cases.



    Quote Originally Posted by jack620 View Post
    That is exactly what I suggested earlier in the thread.
    I did have a recollection it may have been brought up but didn't get around to looking.

  9. #84
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    Hi Chris,

    Quote Originally Posted by jack620 View Post
    That is exactly what I suggested earlier in the thread. Simply a pushbutton switch that applied rectified AC to the motor after it is switched off. The operator holds the button pressed until the motor stops or slows to the point s/he is happy with. It would be wired so it couldn’t be energised when the motor is powered.

    It would be perfect for my table saw, as I have to sit and watch it coast to a stop before I grab the workpiece anyway. It would just mean waiting for a much shorter time.
    The problem with DC and Thyristors is that once they have been turned on it is very difficult to turn them off. The only reason for using one is to protect the relay contacts.
    Best Regards:
    Baron J.

  10. #85
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    Quote Originally Posted by BaronJ View Post
    The problem with DC and Thyristors is that once they have been turned on it is very difficult to turn them off.
    How about just a diode? Here's what I'm thinking. The diode creates half-wave rectified DC. It may be possible to use a bridge rectifier, but I haven't looked into it.

    Here's how I envisage it working:
    1. The NC contact of the momentary pushbutton provides power to the NVR ON/OFF switch. When the ON button of the NVR is pressed the motor runs.

    2. When the braking pushbutton is pressed it removes power from the NVR which 'trips' off. Now the NO contact of the pushbutton sends power to the motor via the diode and current limiting resistor.

    3. When the motor stops the operator releases the pushbutton. The half-wave rectified DC is removed from the motor and the NVR switch receives power again. However, the motor will not start again until the ON button is pressed thanks to the safety feature of the NVR.

    Obviously the braking pushbutton needs heavy duty contacts or it needs to drive a contactor.
    Attached Images Attached Images
    Chris

  11. #86
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    Quote Originally Posted by jack620 View Post
    How about just a diode? Here's what I'm thinking. The diode creates half-wave rectified DC. It may be possible to use a bridge rectifier, but I haven't looked into it.

    Here's how I envisage it working:
    1. The NC contact of the momentary pushbutton provides power to the NVR ON/OFF switch. When the ON button of the NVR is pressed the motor runs.

    2. When the braking pushbutton is pressed it removes power from the NVR which 'trips' off. Now the NO contact of the pushbutton sends power to the motor via the diode and current limiting resistor.

    3. When the motor stops the operator releases the pushbutton. The half-wave rectified DC is removed from the motor and the NVR switch receives power again. However, the motor will not start again until the ON button is pressed thanks to the safety feature of the NVR.

    Obviously the braking pushbutton needs heavy duty contacts or it needs to drive a contactor.
    Hi Chris,

    You will find that it doesn't work ! The motor will continue to run on the rectified half cycles !

    Which is why you need a proper DC source. The two diodes D2 & D3 in my circuit provide about 320 volts on that capacitor, which is only used to smooth the DC. You may also find the the switch contacts don't like the surge current. Measured at about 38 amps for 100 ms or just under.
    Best Regards:
    Baron J.

  12. #87
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    Quote Originally Posted by BaronJ View Post
    You will find that it doesn't work ! The motor will continue to run on the rectified half cycles !
    I guess that makes sense. How about a full bridge rectifier?
    Chris

  13. #88
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    Quote Originally Posted by jack620 View Post
    I guess that makes sense. How about a full bridge rectifier?
    Hi Chris,

    Thanks for your notes.

    The other problem with your circuit is you cannot use a smoothing capacitor on the output of the rectifier. That point has the full AC on it. So it becomes impractical. ( unless you have a big paper capacitor )

    This is why you see the DC circuit I used and also why I used a Thyristor !

    You could turn the circuit around and use the normally open push button to trigger a Thyristor, but you are still left with not being able to turn the thyristor off.

    That is why in my circuit I was looking at a thermal switch to break the supply for a few seconds to allow the capacitor to discharge and the Thyristor to turn off. At that point the amount of current flowing into the motor windings would have dropped to almost zero.

    Then you would have to press the stop button in order to be able to restart the motor.
    Best Regards:
    Baron J.

  14. #89
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    Quote Originally Posted by BaronJ View Post
    The other problem with your circuit is you cannot use a smoothing capacitor on the output of the rectifier.
    Wouldn’t a 400V cap be OK John? And are you sure the cap is needed? Does the motor winding inductance have a smoothing effect?
    Chris

  15. #90
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    Quote Originally Posted by jack620 View Post
    Wouldn’t a 400V cap be OK John? And are you sure the cap is needed? Does the motor winding inductance have a smoothing effect?
    Hi Chris,

    Thank you for your questions.

    To answer the second question first: No not in this instance, the peak voltage at that point could reach 700 volts.

    The rectified half waves will continue to drive the motor, abit at approximately half normal power.

    Adding a capacitor across the rectifier output in parallel with the motor will make very little difference because the back EMF from the motor will tend to restore the sine form but with a shifted phase. This technique is used to restore the power factor of inductive loads connected to the AC mains to nearer unity.

    In my circuit the DC voltage produced is effectively independent of the motor only being coupled to it via the Thyristor, which until it is triggered on isolates the motor. The AC supply to the motor being removed by the relay contacts before the Thyristor is triggered on. All the diode D1 and small value capacitor is doing, is spark suppression across the relay contacts when they open. You could omit those two components if you wanted to.

    The 220 uf 400 volt capacitor that I used was simply one that was convenient, you could use almost any value above 64 uf since most of the current supplied by the capacitor and being replaced by the pair of diodes has been absorbed by the motor windings in the first 100 ms or so.

    The thyristor is also handling the very high initial current surge which caused relay contact welding with very large capacitor values.

    The circuit that I posted works just fine ! As long as you press the machines own stop button when the motor has stopped. This will remove both the AC and the DC voltages and reset the relay.

    My issue with my circuit is that I haven’t found an automatic way to remove the DC voltage from the motor. The danger is if the DC current is left flowing, it might cause the motor windings to overheat and damage them.

    Would you like me to talk through the rational of the circuit design ?
    Best Regards:
    Baron J.

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