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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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  4. #24
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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.
    PLEASE - no friends requests or private emails, I simply don't have time to reply to them all.
    It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.



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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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  8. #28
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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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    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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  10. #30
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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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