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  1. #21
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    Re: AC motor brake system

    Hi,

    Maybe something in this document will help, the BJT circuits only use components that can be e.g. 5V or 500V, etc. rated.

    RAMP GENERATORS.pdf



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    Re: AC motor brake system

    I presume it's an direct drive (controlled by on-off switch) motor. Start up acceleration can be expected higher than DC braking deceleration. In so far, the ramp discussion seems to me like a little bit of overengineering. But if you like it...



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  3. #23
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    Re: AC motor brake system

    Hi and Season's Greetings,

    Quote Originally Posted by FvM View Post
    I presume it's an direct drive (controlled by on-off switch) motor. Start up acceleration can be expected higher than DC braking deceleration. In so far, the ramp discussion seems to me like a little bit of overengineering. But if you like it...
    Have you noticed that in posts #15 and #20 Goldenshuttle wants a ramp or pulsed or whatever kind of similar solution because the braking makes the motor jolt/jump and it seems that bolting it to a table is not a preferred option?



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    Re: AC motor brake system

    Thanks. I think, breaking a 110V motor with the same DC voltage is simply too much, 10 or maximal 20V should brake the motor fast without making it "jump".



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    Re: AC motor brake system

    Diode-capacitor-resistor networks shape bias current to a transistor. Load receives one-time ramp-up ramp-down voltage. This is simulation shows the basic concept. Further revision may result in a better design. A master On-Off switch is needed in the correct position.

    Click image for larger version. 

Name:	RC networks bias 120VAC to NPN load V ramps up to 20V then ramps down.png 
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    NPN gradually turns on as C1 charges. As C2 charges it eventually blocks DC, cutting off bias current.

    Supply is 120 VAC. The load receives half-wave DC since the transistor conducts in one direction (mostly). Amplitude to load is 20V as FvM suggested is adequate. Component values can be adjusted to shape the waveform applied to the load.
    After use the capacitors need to be discharged.


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    Re: AC motor brake system

    I'm no expert when it comes to motors but my first thought was a switchable impedance in series with the AC. Something like a bridge rectifier with a control element across its +/- connections. With no current through the controlled load the motor would stop, with full DC load it would run at full power and by pulsing the load synchronously with the AC it would allow a DC offset to be generated. The same could be done with a triac but control is always easier in DC circuits. Basically, it would drop a varying number of same polarity half cycles to give a controlled DC result.

    Brian.
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    Re: AC motor brake system

    DC braking is an option provided with many VFD inverters. Default DC current is something like 100 % rated AC motor current, DC voltage is respectively low. Applying full rectified AC voltage for braking purposes as discussed above results in a huge overcurrent.


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    Re: AC motor brake system

    Click image for larger version. 

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    This is the timer I used. Its function is to limit the flow of DC to the motor to less than 2 seconds. It worked well, BUT as d123 said, slam brake suddenly on a heavy motor makes the entire table shake. I have 2 ideas to address this. One is to reduce the DC, instead of 110 AV I may try less and less values of DC until I reach the lowest possible that can brake the motion. My expectation is anywhere between 50 and 70 VDC maybe enough. But this is a trail N error approach coz I don't have method to determine momentum of motor under load.
    The 2nd method, I mentioned before, to ramp up the 110 volt DC from 0-110 within the 2 seconds, so it will work like ABS system. Sadly the microcontroller can be programmed to do it, but it needs few seconds to take off.. some logic circuit like the CD4017 maybe giving some ladder sequental that can be built into a ramping..



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  9. #29
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    Re: AC motor brake system

    I still believe that the least effort solution for smooth DC braking is applying a low constant DC voltage (e.g. 10% of rated AC motor voltage) respectively a DC current below rated motor current. That's how a VFD DC brake option usually works. I would try before designing complex solutions. For a short braking time of 2 s, a large power resistor can be used set the current.



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    Re: AC motor brake system

    Motors are basically current devices and the applied DC voltage should be able to deliver sufficient power (energy over 2s) to dissipate the kinetic energy of the rotor. In the worst case, without any load (the load also acts as a brake), the input energy goes into the rotor mass.

    In the steady state the rotor spins at a constant and the motor draws little power (just to overcome the friction) from the line. This may be about 20% of the rated power of the motor.

    As you have sufficient time (~2s), you can use a DC voltage about 20% of the rated line voltage and a capacitor that will have a time constant of about 1s (in combination with the motor resistance). The current will decrease to a small value within 2s (the voltage will reduce to 25% in 2s).

    The large shaking of the table due to sudden braking is understandable and if you try to stop it very fast then mechanical resonance may be a serious problem (bolting is NOT the solution, you will need to use shock absorbers: the motor need to be suspended from the enclosure)



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    Re: AC motor brake system

    Quote Originally Posted by FvM View Post
    I still believe that the least effort solution for smooth DC braking is applying a low constant DC voltage (e.g. 10% of rated AC motor voltage) respectively a DC current below rated motor current. That's how a VFD DC brake option usually works. I would try before designing complex solutions. For a short braking time of 2 s, a large power resistor can be used set the current.
    Applying low DC voltage did not work as brake. I do not have an adjustable power supply, but tried one with fixed output of 50VDC, when applied to that monster motor, it did not brake took few seconds to stop..will try to find an adjustable one, and increase little by little to see the lowest VDC that works.
    On the other option, I am thinking to use arduino, feeding PWM to a high power Triac which should ramp output from 0-110 within 2 seconds. The only problem here is Arduino takes few seconds to function as it takes time to roll the code..it does not function immediately...keeping it powered all the time may overcome this...but i am not sure if I keep it powered all the time will be a safe option.



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    Re: AC motor brake system

    Thanks for reporting. Are you sure that the 50V supply is maintaining it's output voltage when driving the motor? Similar DC voltage levels work well with VFD built-in DC brake.



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    Re: AC motor brake system

    I do not have an adjustable power supply, but tried one with fixed output of 50VDC, when applied to that monster motor, it did not brake took few seconds to stop...
    That is not the expected result.

    If your line voltage is 110V/60Hz, then a single rectifier (half wave rectification) should be able to do the trick. The current will depend on the total energy of the rotating mass. For a typical case, you may consider 1/2 of the power to be present in the loaded rotor (when the rotor is overloaded, it moves very slow and the energy stored in the rotating mass is less. Similarly, when there is no load, the rotor is moving very fast but it has a low mass and low energy storage).

    Do you have some idea about the current it took (when connected to 50VDC?)



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  14. #34
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    Re: AC motor brake system

    The bridge rectifier that delivers the DC fried out. although it was rated 25A/400V. On a 2nd thought, these bridge althu branded are made in China, I doubt the work as rated, and I don t have enough lab to test their actual capacity.



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    Re: AC motor brake system

    Hi,

    Quote Originally Posted by Goldenshuttle View Post
    The bridge rectifier that delivers the DC fried out. although it was rated 25A/400V. On a 2nd thought, these bridge althu branded are made in China, I doubt the work as rated, and I don t have enough lab to test their actual capacity.
    As the bridge rectifier has a datasheet, worth comparing maximum rated PD to calculated actual PD.



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    Re: AC motor brake system

    As the bridge rectifier has a datasheet, worth comparing maximum rated PD to calculated actual PD.
    Curiously, basic parameters like motor DC resistance have been never mentioned anywhere in this lengthy thread. I would expect a value below 1 ohm and respectively > 100 A breaking current. Frying the rectifier with it isn't that surprising at all.



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    Re: AC motor brake system

    @TvM, probably right..motor is 110V/10 Amps but i did not measure its resistance...I thought bridge of 25A/600V can survive..it did the job 2 or three brakes, then fried..i am gona letgo here coz financing 4 this project was pulled. I had already built a good alternative in my head, as mentioned before, a ucontroller feeds PWM into a dimmer circuit, within 2 seconds ramps output from zero to full. I am almost positive it will work.



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    Re: AC motor brake system

    Hi,

    as mentioned before, a ucontroller feeds PWM into a dimmer circuit, within 2 seconds ramps output from zero to full. I am almost positive it will work.
    ...and as mentioned before, too:
    * a dimmer doesn´t work with PWM
    * independent of bridge rectifier or triac ... the current will be the same and thus I assume you will fry the next part. Hopefully not the motor...


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