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    AC motor brake system

    I have this single phase AC motor 110 V/750 Watt/10 Amps. Installed in a meat grinder. Safety regulations required motor stop within 2sec or less once you press emergency button of open the lid. I tried brake circuit using relay reversing but it is not really functioning. Please can anyone help with a suggestion ? I am not much into design but more into building a circuit. Is there a thyristor powered circuit that can do the brake ?

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    Re: AC motor brake system

    Apply DC.
    From the same rectified AC power. Half wave rectification is fine with a single diode.

    Just make sure it is a temporary application, otherwise the motor will burn up.
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    Re: AC motor brake system

    If I understood you well, I will need DC but same power 750 Watt, > So I can get a heavy rectifier and capacitor to give 750 Watt DC but I can not guarantee the temporary application of DC because when someone opens the feed lid, it will stay open untill they close the lid., So I probably have to come up with an electronic circuit that takes just a pulse from the mircoswitch. Or modifiy microswitch position so that it just presses on momentarily when feed lid is opened.
    Quote Originally Posted by schmitt trigger View Post
    Apply DC.
    From the same rectified AC power. Half wave rectification is fine with a single diode.

    Just make sure it is a temporary application, otherwise the motor will burn up.



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    Re: AC motor brake system

    When you run mains AC through a series diode and capacitor, it provides DC waveforms to the load. These rapidly diminish as the capacitor charges.
    By choosing the right value of capacitor you might get sufficient motor braking.

    Click image for larger version. 

Name:	open switch series diode-cap sends a few posi mains waveforms at 10A for 2 seconds.png 
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    The switch was opened a tenth of a second into the run. Notice automatic operation of the diode-capacitor pair (while the switch remains open). After 2 seconds the load gets little current.

    You'll need to discharge the capacitor after each use.



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    Re: AC motor brake system

    A capacitor circuit won't be able to provide sufficient braking time, e.g. several 100 ms up to a 2 seconds. An electromechanical or electronic sequence control for the DC brake relays seems to be the only practical solution.

    Before sticking to a specific solution, you may check if the drive inertia allows a stop time by DC braking within 2 seconds. If not, a mechanical brake may be necessary.



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    Re: AC motor brake system

    FvM brings a good point: braking a motor without breaking anything due to inertia.
    In that case, a series resistor with the applied DC, which would reduce the braking current, would do the trick. You'll have to find the resistor value empirically, but I would start with a value similar to the SC motor's winding resistance.

    VERY IMPORTANT NOTE: this braking trick only works if the motor is an induction motor. If it is an universal motor, applying DC will not stop at all!
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    Re: AC motor brake system

    Click image for larger version. 

Name:	AC Motor Brake System.jpg 
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    I was going to suggest this circuit which is similar to brads. After playing with it a bit yesterday i realized what FvM already stated. The capacitance needed to stop the motor would likely be extremely large.



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    Re: AC motor brake system

    A 1500 uf cap wouldn't quite completely stop a .25hp Psc motor. Motor would turn about 3-5 revolutions after the initial discharge. A 2400 uf would stop it dead in it's tracks. But that's a smaller motor with no gear box attached.



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    Re: AC motor brake system

    Thanks alot all of you guys. Based on all the points, Ive drawn this sketch, please tell me what you think.
    The concept is when switch from operation to Brake, a timer will allow the bridge rectifier to feed the motor with 110 Volt DC for 5 seconds only which is powerful enough to stop it.
    After 5 seconds the DC will be cutoff from motor to prevent it from burning.
    If you guys agree, then another question may pop, will this brake several times a day shorten the life of the motor ?
    Click image for larger version. 

Name:	brake1.JPG 
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ID:	150406



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    Re: AC motor brake system

    Quote Originally Posted by Goldenshuttle View Post
    Thanks alot all of you guys. Based on all the points, Ive drawn this sketch, please tell me what you think.
    The concept is when switch from operation to Brake, a timer will allow the bridge rectifier to feed the motor with 110 Volt DC for 5 seconds only which is powerful enough to stop it.
    After 5 seconds the DC will be cutoff from motor to prevent it from burning.
    If you guys agree, then another question may pop, will this brake several times a day shorten the life of the motor ?
    Click image for larger version. 

Name:	brake1.JPG 
Views:	6 
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ID:	150406
    You said you need the motor stop in 2 seconds previously. Have your requirements changed.



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    Re: AC motor brake system

    A 1500 uf cap wouldn't quite completely stop a .25hp Psc motor. Motor would turn about 3-5 revolutions after the initial discharge. A 2400 uf would stop it dead in it's tracks. But that's a smaller motor with no gear box attached.
    The motor is 750W or about 1hp. Let us assume that 1/2 of total energy of the motor is taken up by the RC circuit (and the rest is dissipated within the load).

    750W is 750J/s and in 2 sec the energy will be 1500J and 1/2 of that will be 750J.

    You can guess the size and value of the RC circuit will need.

    Plugging the motor (via a heavy duty resistor = equal to the winding resistance of the motor) will be better but some details are missing.

    If the motor is shaded pole or capacitor start? Most of the grinder motors (in common use) are universal (brushed) motors and plugging will work well.

    - - - Updated - - -

    Quote Originally Posted by Goldenshuttle View Post
    Thanks alot all of you guys. Based on all the points, Ive drawn this sketch, please tell me what you think.
    The concept is when switch from operation to Brake, a timer will allow the bridge rectifier to feed the motor with 110 Volt DC for 5 seconds only which is powerful enough to stop it.
    After 5 seconds the DC will be cutoff from motor to prevent it from burning.
    If you guys agree, then another question may pop, will this brake several times a day shorten the life of the motor ?
    Click image for larger version. 

Name:	brake1.JPG 
Views:	6 
Size:	119.1 KB 
ID:	150406
    Under normal operation, the power line is shorted. Unver brake, you need some current limit.


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    Re: AC motor brake system

    Hi,

    750W is 750J/s and in 2 sec the energy will be 1500J and 1/2 of that will be 750J.
    This calculation usues the continous motor power.

    But one needs to calculate the energy of the rotating mass.
    We do not know this, the caculation via motor power at least gives a raw estimation.

    The inertia of a disk saw (mainly because of the diameter) may be huge. Maybe it needs some seconds at start to get it´s RPM...and causing a lot of breaking energy.
    The inertia of a drilling machine may be less - even at the same motor power. .. causing less breaking energy.

    ****
    I´ve never build a three phase motor breaking system. But when you run DC through the coils the current may become huge, because the usual motor inductance does not count at DC.
    I agree: there should be a current limit. Maybe a series capacitor in front of the rectifier.

    Klaus
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    Re: AC motor brake system

    This calculation usues the continous motor power...
    I know my assumptions are crude, but they are a starting point.

    Most of the input energy (must motor losses) will be transferred to the load.

    Most of the time the load will work as a brake (in the meat grinder case; not for a locomotive!!)

    In absence of the load, the motor rotor (rotational kinetic energy) energy must be dissipated in the resistor.

    This will be the max power (that will be less if the meat is there in the grinder) that must be handled by the RC snubber.

    I just wanted to get and present a rough idea of the power involved.



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    Re: AC motor brake system

    Quote Originally Posted by Kajunbee View Post
    You said you need the motor stop in 2 seconds previously. Have your requirements changed.

    Good observation Kajun. My requirements did not change, I still need the brake for 2 seconds not more, is to protect the motor coil from burning by DC current. I just tested the circuit that I suggested on a bigger 1.75 hp motor, using a high power bridge diode., it did brake very well stopping the motor instantly. But I -manualy- switched it off withing 2 seconds. So it is probably, needed to add a timer that cuts off the DC within 2 seconds. what do you think ?



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    Re: AC motor brake system

    This motor power is 750 Watts, it weights like 3 kg.
    The circuit I sketched worked great brakes within a second..pushing the brake few times a minute caused the bridge rectifier to go warm despite that it has 600V/35 amp rating. BUT, as the motor is big, the vibration caused by the mass makes a need 2fix the motor to the base with 4 strong bolts.
    My question now is, if I come up with a circuit that feeds the DC gradually, and quickly from 0-to-110 VDC withing the few seconds, will this reduce the vibration and make brake more smooth ?
    if the answer is yes, anyone has a hint of how this circuit would look like ?



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    Re: AC motor brake system

    Hi,

    Where does the timer come from, what is it in hardware?

    What about small, fast pulses building up to a final large pulse to halt the motor (similar to braking rapidly in a car several times instead of slamming the brakes full on)?

    Not sure how you'd fit it all together but there could be e.g. a linear ramp generator or a staircase generator in there to increase the DC voltage incrementally, connected to a squarewave pulse.

    Oddly enough, I have recently modelled that sort of thing using a CD4017 (a ring counter), a lot of logic ICs, some resistors, and some diodes, feeding the base of one transistor; and a 555 to create the clock pulse for the two CD4017s. I do not recommend such a pointlessly large circuit. I wanted an analog, 1970's-style soft start and soft stop for a LED circuit that has a very low duty cycle output* to use less power. You may be able to do the same thing with a microcontroller, not sure.

    It's not a very beautiful stepped ramp and the steps are not very balanced, but it looks relatively effective for the purpose...
    * I changed the duty cycle and pulse speed in the simulation to make it take 2 minutes, otherwise it was taking 30 mins to complete the simulation and my life began to ebb away in the process of waiting.
    Ignore what's happening after ~25 seconds as that's no use to what you want:

    Click image for larger version. 

Name:	soft start stop output voltage and current waveforms.JPG 
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    A resistor + capacitor can be used to slowly turn on a BJT, low parts count. Again, not sure where that could fit into your circuit.



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    Re: AC motor brake system

    @d123, the timer is to cut the power within 2 seconds after the brake is pressed. This is to protect the motor coils from frying by long DC application. what you said I can do; making a ladder supply , but at low voltages..my need is how to make it in high power, like applying a gradual 0 - 110 Volt DC at 10 amps. This may brake motor gradually instead of the jump happening when I suddenly apply 100VDV from the bridge rectifier.



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    Re: AC motor brake system

    Hi,

    From the block diagram you have, the timer is powered by 100V DC?

    You don't even need to make the ladder or ramp in high power, that's just what I'd call "control circuitry". A low voltage circuit can drive a high voltage circuit, e.g. a MOSFET with an ON gate voltage which may range from 3V or less these days to 12V and whose drain-source can handle high voltages (100 - 600 V DC) can be used to level shift. Turning on the gate more and more via resistor dividers or a ramp (and there must be other and perhaps better ways of doing that) is one way. Alternatively, you could use an appropriately-sized SSR and incrementally turn on the LED with pulses, and that way you'd also have isolation between low power and high power stages.



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    Re: AC motor brake system

    Yes, smooth braking is very much desirable for the health of the motor. You must not slam on the brake, the current should build up linearly over 2s and then cut off. But should have a current limit somewhere in the design.



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    Re: AC motor brake system

    What I have now is a working brake system>>But it is like you said, slamming brake on a hi speed car..the motor jumps, needs to be bolted to a strong table. I now must come up with a circuit that builds up AD from 0 - 100 in 2 seconds linearily to prevent the jump. if anyone has a suggestion plz do...



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