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how to control AC Synchronous Motor

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Raady
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2015/02/05 04:55:54 (permalink)
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how to control AC Synchronous Motor

Hi,
 
I have an AC synchronous motor with 3 wires common, forward, and backward. How can I controll it, please suggest a method its cheap and efficient way! I was thinking of BT139 triac, does this make any sense ?
 
Regards,
Raady.
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    Nikolay_Po
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    Re: how to control AC Synchronous Motor 2015/02/05 06:06:14 (permalink)
    +3 (3)
    Depends on what is the motor? What is the power supply (DC or AC, AC 1 phase or 3 phases)? Is there speed control required? What is the power on motor shaft required?
    At your place where you can estimate nor the volume nor the complexity of the task I'd suggest to buy ready solution: the motor and it's controller as a bundle.
    What is the motor datasheet recommending?
    #2
    roundrocktom
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    Re: how to control AC Synchronous Motor 2015/02/05 06:51:02 (permalink)
    +1 (1)
    A single phase AC motor has a "starter circuit".  Centrifugal switch that can power up a separate winding to get the motor started. (larger motor will have an external start capacitor). 
     
    If you see two power leads typically they are "L1"  and "L2"  (connected to AC mains).  The Ground wire is just for safety (no current should flow through it during normal operation, but is the winding shorts to the case it trips a breaker).
     
    With the "L1" and "L2"  there typically is a little box on the motor with four leads from the motor.  Two for the main power, two for the start power.    To change rotation  you swap the two connections to the main power.
     
    I'm confused with the "Forward and Reverse and common" for a AC motor.  It's just not something I've seen.   If there is a 'data plate' on the motor take a photo and post it up. 
     
     
     
    post edited by roundrocktom - 2015/02/05 06:52:30
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    Raady
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    Re: how to control AC Synchronous Motor 2015/02/05 21:47:11 (permalink)
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    motor looks like this 

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    David
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    Re: how to control AC Synchronous Motor 2015/02/05 22:03:41 (permalink)
    +2 (2)
    Triac and a relay ?

    David
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    Raady
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    Re: how to control AC Synchronous Motor 2015/02/05 23:21:59 (permalink)
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    I am using two BT139's.
    one triac for forward direction and other for reverse direction.
    Triac gate is driven by an transistor and microcontroller. 
    Commons grounded. 
     
    Do we have any better circuit than this ? 



    post edited by Raady - 2015/02/05 23:24:24

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    Raady
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    Re: how to control AC Synchronous Motor 2015/02/06 02:41:39 (permalink)
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    how to switch triac without using Zero detection opto-isolators ?
     
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    Ian.M
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    Re: how to control AC Synchronous Motor 2015/02/06 04:33:28 (permalink)
    +2 (2)
    I would be concerned about out of phase induced voltage in the winding that isn't active over-stressing the TRIAC that's switched off.   You are in Europe so it is likely the supply voltage is 230V RMS (+10%, -6%).  The high end of that range gives a peak voltage of nearly 358V.  If the induced voltage is phase shifted relative to the supply by more than 90 deg, it will add to the voltage across the TRIAC.   It should be noted that the phase of the induced voltage will shift significantly during startup and reversing so having an adequate margin during steady state running is not sufficient.
     
    A BT139-600 is likely to be seriously overstressed and will probably fail catastrophically.  A  BT139-800 will be within its ratings, but may not have much margin so the supply will need to be well filtered with overvoltage clamping to eliminate spikes and transients.   If one of the TRIACs does fail, it will probably cause a cascading failure of the other TRIAC and unless the motor has over-temperature protection, it will burn out the motor.
     
    Why would you even want to eliminate ZC optotriacs?  With the consequences of false triggering being as serious as they are, I would be adding hardware interlocks to prevent both TRIACs being on simultaneously, or even changing the design to use a SPDT mechanical relay for reversing and the TRIAC merely for ON/OFF (so the relay can switch with no voltage across it or current through it for maximum contact life), rather than Muntzing the design.

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    David
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    Re: how to control AC Synchronous Motor 2015/02/06 05:01:57 (permalink)
    0 (2)
    Raady
    I am using two BT139's.
    one triac for forward direction and other for reverse direction.



    when they both turn on what happens ?

    David
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    Raady
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    Re: how to control AC Synchronous Motor 2015/02/06 23:55:44 (permalink)
    +1 (1)
    David
    when they both turn on what happens ?



    As Ian suggested the motor gets burned out. 

    I would be adding hardware interlocks to prevent both TRIACs being on simultaneously, 

    If I need to add interlocks how can I do that ? any references please !
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    David
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    Re: how to control AC Synchronous Motor 2015/02/07 02:07:42 (permalink)
    +2 (2)
    relay :)

    David
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    vini_i
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    Re: how to control AC Synchronous Motor 2015/02/07 06:12:46 (permalink)
    +1 (1)
    i haven't seen mentioned if you want speed control. 
     
    if no speed control then it's pretty easy. set up a passive triac control with a diac, capacitor and resistor. then place an optocopler on the control control line to the triac. by enabling and disabling the opto the motor will turn on and off. 
     
    if you want speed control then it gets difficult. you will need a zero crossing detection circuit. the hook up of the opto changes. finally your software must trigger the triac every half cycle. 
     
    the last thing that i don't see mentioned is a triac snubber. when using a regular triac it will need a snubber circuit. don't use a snubberless triac they are only designed to drive light loads and resistive loads. they are not designed to drive heavy inductive loads (motors)
    #12
    Ian.M
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    Re: how to control AC Synchronous Motor 2015/02/07 06:30:48 (permalink)
    +1 (1)
    Synchronous motors are generally *NOT* compatible with  phase angle speed control.  You can only use a simple TRIAC phase angle control with a universal motor.  Speed control of a synchronous motor usually requires a VFD.

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    DarioG
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    Re: how to control AC Synchronous Motor 2015/02/07 07:31:32 (permalink)
    +1 (1)
    By the way, anybody else did ever try the "count the sines" method?
    I.e. say every 10 sines (zero-crossing based) you turn the triac on for, say, 3 or 5 or 10 of them...
     
    I used it with a basic 200W motor and looked pretty fine.

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    tom maier
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    Re: how to control AC Synchronous Motor 2015/02/07 17:51:24 (permalink)
    0
     
    Hi Dario!
     
    Yes, you can modulate the power by cutting out cycles. Under heavy load the motor might start chattering or back-slipping. To make it work better, you can even out the on-off cycles, so don't just turn them off for 5 cycles and back on for 5 cycles, but create a spaced out pattern of the cycles to avoid having that long gap of no power. You will get less chatter.
     
    And for the prevention of accidentally turning both triacs on at the same time, you can use a "lock-out" made from logic gates.  When both A and B try to turn on, one or both are forced to turn off. I looked around my old files and can't find the circuit I usually use, but it is an exclusive OR gate arrangement. I usually use NAND gates.
     
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    DarioG
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    Re: how to control AC Synchronous Motor 2015/02/08 03:37:27 (permalink)
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    Yeah, a pattern may make it even better Smile
    I guess it also depends upon inertia. Mine was a fan, and more or less everything up from 0 to 100% in 10% steps.

    GENOVA :D :D ! GODO
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    Raady
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    Re: how to control AC Synchronous Motor 2015/02/09 00:17:38 (permalink)
    +1 (1)
    vini_i
    i haven't seen mentioned if you want speed control. 
     
    if no speed control then it's pretty easy. set up a passive triac control with a diac, capacitor and resistor. then place an optocopler on the control control line to the triac. by enabling and disabling the opto the motor will turn on and off. 
     
    if you want speed control then it gets difficult. you will need a zero crossing detection circuit. the hook up of the opto changes. finally your software must trigger the triac every half cycle. 
     
    the last thing that i don't see mentioned is a triac snubber. when using a regular triac it will need a snubber circuit. don't use a snubberless triac they are only designed to drive light loads and resistive loads. they are not designed to drive heavy inductive loads (motors)




     
    Hi Vini, 
     
    Is it possible to control the speed of motor with single triac ? 
    And we have been using two triacs (BT139) with snubber and MOC3083 to drive Triac, we were able to move motor in both forward and reverse direction. Can we control the speed of motor with precise ?
     
    Regards,
    Raady 
     
    #17
    Raady
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    Re: how to control AC Synchronous Motor 2015/02/09 00:20:52 (permalink)
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    tom maier 
    And for the prevention of accidentally turning both triacs on at the same time, you can use a "lock-out" made from logic gates.  When both A and B try to turn on, one or both are forced to turn off. I looked around my old files and can't find the circuit I usually use, but it is an exclusive OR gate arrangement. I usually use NAND gates.
     

    Could you please help to design Lock out circuit. I feel this will definitely add value to my design.
     
    #18
    Ian.M
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    Re: how to control AC Synchronous Motor 2015/02/09 02:31:42 (permalink)
    0
    For a simple command interlock you need to implement:
    • GF=Forward^¬Reverse
    • GR=Reverse^¬Forward
    It depends on the configuration of the optocoupler's LED drive circuit. At its simplest, with active high outputs from the PIC, and the LEDs wired cathode to ground, you need to AND each signal with the inverse of the other signal.  Using De Morgan duality, you substitute NOR gates for the AND gates and invert the main signal input.   Use a quad two input NOR, with two of its gates simply used as inverters.
     
    I would certainly include this circuit during software development although it may be possible to omit it from the final board once the firmware is fully debugged.
     
    However I'd still prefer a relay for reversing.  If you use a relay, the only possible failure modes result in the motor running constantly, possibly in th wrong direction or not running at all.  There is no way that both motor windings can get powered unless the relay contacts get crushed by external damage, so motor burn-out is extremely unlikely.
     
    If you are determined to avoid using a relay, consider adding a motor current sensor.   If you can sense if the motor is still drawing current with the control signals off, you have detected that one of the TRIACs has failed so can avoid switching on the other TRIAC and burning out the motor.  You can also detect the increased current of a locked rotor and shut off the TRIAC if the current doesn't fall to the normal operating level after the normal acceleration time.
     
    The motor should still either have a thermal trip, or be protected by a slow blow fuse carefully chosen to withstand the startup surge but blow if both windings are powered.
     

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    David
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    Re: how to control AC Synchronous Motor 2015/02/09 03:03:27 (permalink)
    +1 (1)
    just think if your designing  a conveyer into a furnace if it failed driving forward loading the furnace or backwards keeping stuff away from furnace, one way is safer. what does your motor do ?

    David
    I support http://picforum.ric323.com because this forum is often too broken to use!
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