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  1. #1
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    Thumbs up Motor Braking using DC injection !

    Hi Guys,

    I mentioned in another thread about braking a single phase motor using a direct DC voltage feed into the lathe motor to cause it to rapidly stop rotating. I've found the circuit that was used and offer it here. I don't see any reason why a similar injection of a DC voltage could not be used with a three phase motor, though I admit that I've not used any of this circuit. Three phase braking would normally be one of the functions of a VFD.

    DC_Injection_Breaking.jpg

    The circuit shown above consists of four components in addition to the lathe motor. The only expensive component, assuming that you purchased it, would be the capacitor. In the original circuit it was an electrolytic one of about 350 uf at around 350 volts DC working. A large value motor start/run capacitor could be used instead. I believe that the one originally used was intended as a smoothing capacitor in a valve amplifier or radio.

    The diode rectifier is shown as a 3 amp 1000 volt device but almost any high voltage diode rectifier could be used here. The Thermistor is one that is specially specified for anti surge purposes and can be either salvaged or purchased very cheaply. You could find one in almost any power supply used in a TV, or similar device.

    The relay is a very common AC relay with two or three sets of changeover contacts. The circuit only uses two poles, and is shown in the UN-energised condition with the contacts connected to the capacitor. Everything inside the red box outline is part of the relay.

    NOTE: Electricity is dangerous unless you know exactly what you are doing. I haven't built this circuit but will make a test unit at some point. The relay uses all solder joint terminals and all the components except the capacitor can be secured directly to them. I personally would place all the components into an insulated plastic box with just the two motor leads and the two power supply leads plus earth coming into the plastic box.
    Best Regards:
    Baron J.

  2. #2
    jatt's Avatar
    jatt is offline Always within 10 paces from nearest stubby holder
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    Not something I recall playing with.

    So half wave (pulsed DC) is sufficient for this applic I guess.
    Frisky wife, happy life. ​Then I woke up. Oh well it was fun while it lasted.
    From an early age my father taught me to wear welding gloves . "Its not to protect your hands son, its to put out the fire when u set yourself alight".

  3. #3
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    Quote Originally Posted by jatt View Post
    Not something I recall playing with.

    So half wave (pulsed DC) is sufficient for this application I guess.
    Hi Jatt,

    Actually the capacitor is charged up from the rectified positive half of the incoming AC, since there is no load on the capacitor until the relay drops out, the capacitor gets charged up to the AC voltage peak. Approximately 1.414 times the AC voltage. The actual braking is pure DC from the charge held in the capacitor. But yes it is half wave rectification.

    You could use a similar scheme to brake a three phase motor, but that is usually handled by a VFD.
    Best Regards:
    Baron J.

  4. #4
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    Hi Guys,

    I've found the relay that I had in mind for this application ! I will look for the other components tomorrow.

    17-12-2020-001.jpg 17-12-2020-002.JPG

    This is the box information for the relay. Originally supplied by "Keyswitch Relays" manufactured by "Omron" in Japan. This particular one is an MK3, which has three change over poles with contacts rated at 10 Amps at 330 volts AC. I don't recall the DC ratings at the moment. The coil is 230 volts AC. They also do a 380 volt coil one.

    Its quite likely that these relays are no longer made, at least not open frame ! Most likely they will be enclosed types nowadays. I've had these in stock for a few years. A comparable relay can be had for a few UK pounds.

    17-12-2020-004.jpg 17-12-2020-003.jpg

    The relay occupies about 1 cubic inch. The fastening screw and hardware that comes with it is in that little plastic packet. There is also a version with spade terminals. This is the relay that I will be using to make up a test unit.
    Best Regards:
    Baron J.

  5. #5
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    Quote Originally Posted by BaronJ View Post
    Hi Guys,
    The relay occupies about 1 cubic inch.
    Hi John, I'm watching this project keenly. I would love to fit motor braking to my table saw.
    But do you think that relay is robust enough to cope with the likely huge inrush current that will be dumped into the windings?
    Chris

  6. #6
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    Quote Originally Posted by BaronJ View Post
    This is the relay that I will be using to make up a test unit.
    That is the relay you are about to put on the sacrificial altar of the prototyping gods..

    You’re about to create a big L small R short time period emf, the contacts of that relay won’t last very long.

  7. #7
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by racingtadpole View Post
    That is the relay you are about to put on the sacrificial altar of the prototyping gods..
    You’re about to create a big L small R short time period emf, the contacts of that relay won’t last very long.
    It depends on the HP rating of the motor but from memory those relays have a rating of about 300 (if you are lucky) switching operations for a 2HP inductively loaded, switching operations.
    Even going up to 15 or 20A won't make much difference to the relay lifetime.
    I have a similar one on my not very oft used 3HP table saw (it that switches mains on when a dust collector micro pressure switch is switched on) that I have to replace every couple of years = fortunately they cost about $6 on ebay and its a plug in type so its not biggie but I should really replace it with a proper contractor.

  8. #8
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    Quote Originally Posted by BobL View Post
    I have a similar one on my not very oft used 3HP table saw (it that switches mains on when a dust collector micro pressure switch is switched on) that I have to replace every couple of years
    Why does it fail?

    The 3HP motor should be AC – switching AC should be the relay's forté – it is not as if it has a big DC shunt?

  9. #9
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nigelpearson View Post
    Why does it fail?

    The 3HP motor should be AC – switching AC should be the relay's forté – it is not as if it has a big DC shunt?
    Rather than reinventing the wheel maybe a have a read of this?
    https://www.hw-group.com/cs/podpora/...estroy-a-relay

    The relay on my 3HP TS switch is rated at 16A/230V
    But reading the fine print in the data sheet says it should not be used on an AC motor that draws more than 500W. I have several relays rated at 10A/240V and they have a max inductive (motor) loads of of 375W

  10. #10
    BobL is offline Member: Blue and white apron brigade
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    Quote Originally Posted by nigelpearson View Post
    Why does it fail?

    The 3HP motor should be AC – switching AC should be the relay's forté – it is not as if it has a big DC shunt?
    Rather than reinventing the wheel maybe a have a read of this?
    https://www.hw-group.com/cs/podpora/...estroy-a-relay

    The relay on my 3HP TS switch is rated at 16A/230V
    But reading the fine print in the data sheet says it should not be used on an AC motor that draws more than 500W. I have several relays rated at 10A/240V and they have a max inductive (motor) loads of of 375W

  11. #11
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    Quote Originally Posted by jack620 View Post
    Hi John, I'm watching this project keenly. I would love to fit motor braking to my table saw.
    But do you think that relay is robust enough to cope with the likely huge inrush current that will be dumped into the windings?
    Hi Chris, Guys,

    I had a search on Omron's web site for the MK relay type, without success I might add, all the ones I found were enclosed ones. The nearest one to the MK was the MKY with 10 amp AC contacts, the DC ratings were 30 volts and 3 amps.

    Unfortunately I can no longer go back and see how the original device which was fitted to a lathe with a 1 hp single phase motor has fared.

    My gut feeling is that whilst the circuit will work as designed, the failure point is going to be the relay contacts welding together ! As they say in Black Adder, I have a cunning plan...

    I'm not going to say what my cunning plan is until I've worked out the details and done some testing. Unfortunately the biggest single phase motor I have is 1 Hp, so that will be the test machine.

    More later.
    Best Regards:
    Baron J.

  12. #12
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    Quote Originally Posted by racingtadpole View Post
    That is the relay you are about to put on the sacrificial altar of the prototyping gods..

    You’re about to create a big L small R short time period emf, the contacts of that relay won’t last very long.
    Hi Adam, Guys,

    Yes I agree ! The surge current will kill the relay contacts very quickly, which I am aware of. I do recall that this was the relay that we used originally. I've no idea if Bill made any alterations to the device we built. But we did find that the braking effect could be controlled by putting a resistor of a few Ohms in series with the relay contacts.

    At one point we found that you could make the chuck start to unscrew by stopping the motor dead.
    Best Regards:
    Baron J.

  13. #13
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    Quote Originally Posted by BaronJ View Post
    As they say in Black Adder, I have a cunning plan....
    So cunning you could put a tail on it and call it a weasel?
    Chris

  14. #14
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    Quote Originally Posted by nigelpearson View Post
    Why does it fail?

    The 3HP motor should be AC – switching AC should be the relay's forte – it is not as if it has a big DC shunt?
    Hi Nigel, Guys,

    The part of the circuit that causes the problem is the charge on the capacitor which is a DC component. The relay contacts are not designed to handle the enormous surge current that will be produced by the capacitor discharging through the relay contacts into the motor windings. That is an issue that needs to be addressed or the relay isn't going to last very long.

    The surge current into the capacitor when charging it up, is taken care of by the 5 Ohm Thermistor, though that could simply be replaced by an ordinary resistor. It is what was used originally.
    Best Regards:
    Baron J.

  15. #15
    BobL is offline Member: Blue and white apron brigade
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    Now that I think about it the 500W max relay I use on my 3HP TS has only failed when I have turned the saw off under load eg jammed the blade and then turned off the motor. Free running the saw draws about 4A (~1000W) but turning the motor off when free running has never blown a relay, The last blow was back in Sept but I cannot remember how far back it happened before that - at least two years.

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