AC OR DC ? Servo help

Discussion in 'General Electronics Chat' started by GARYN, Sep 28, 2009.

  1. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Hello, I am an Engineer in a production factory who tackles alot of the electrical problems....
    I would appreciate it if somebody can clear some confustion up for me regarding servo drives.....
    Not many state wheather they are AC and DC or are they a standard type of drive and dont need to. I was working on a machine today and noticed the servo motor said it was a BALDOR BRUSHLESS AC MOTOR but the drive card by ADVANCED MOTION CONTROLS just said it was a BRUSHLESS PWM SERVO AMP but needed a 170VDC supply.
    So is the drive AC or DC, if i put my meter on the card output do i put it on AC or DC to check voltageto motor.
    I am quite good on standard motors but this is one area that has always confused me, as it says PWM on the card does this automatically make it AC or DC or can it be both.
    The following pots were on the card including LOOP GAIN, CURRENT LIMIT and REF GAIN i dont know if this is helpfull also there were hall effect inputs from the motor.
    I assumed Hall effect sensors made it DC but when i metered out the motor voltage i could not get anything but a pulsed 17VAC when set to AC which confused me even more.. or does this type of output not show up on normal meters.
    I anyone can give me an easy electronic explanation between the differance between the 2, how to identify them and possible test techniques to see if the motor is getting the correct power/signal to aid faultfinding it would be a great help..
    Thanks in advance...:confused:
     
  2. GARYN

    Thread Starter Member

    Sep 4, 2009
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    Do i need to supply more info or is not a relevant topic for this site?
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
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    Hi,

    a typical 'AC' servo system consists of a power supply which provides a DC bus voltage, then one or more servo drives which typically have a three phase PWM power output output to the motor, plus a feedback cable from the motor to the drive.

    Some small systems have the PSU and servo in a single unit, but machine tool types are often modular multi-axis setups.

    The motor itself should have a low resistance between the three power terminals but they be isolated from earth / motor ground, very similar to a conventional three phase induction motor.

    The motors typically have a permanent magnet rotor, and the 'three phase' drive signal is adapted to the actual rotor position as read by the feedback system, and controlled by the torque or speed demand signal to the drive.

    In some motors the feedback is by hall effect sensors, other type have synchro resolvers.

    I believe there are also AC servos that use a squirrel cage type rotor plus a resolver for position feedback, but I've no experience of those.

    The power feed to the motor will probably consist of high frequency pulses, you may need a 'scope to see anything sensible, though I expect the voltage you see between phases with a meter should vary with the tourque the motor is providing.

    That system is one of the lower voltage types, many use what is effectively rectified three phase mains power, around 500 - 700V on the DC bus.

    Hope this is some help.
     
  4. GARYN

    Thread Starter Member

    Sep 4, 2009
    47
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    Thanks for the above response.....

    Just one thing because the PWM supply to the motor is from the DC Bus, is the voltage AC or DC because you were describing an AC servo.

    And how does this differ from a DC servo i.e. Is it just the mains supply to the amp thats differant.
     
  5. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
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    The output on each phase can vary in voltage between around 0V and the DC bus voltage.

    As any phase can be switched between 0V and V+ independently, it's AC as far as the motor is concerned - any of the three phases can be + or - in respect to any other.

    A typical high frequency (PWM) DC servo would be similar, just using two of three output stages. Comparing the other way, the AC version is just like an H-Bridge, but with three sections rather than two.

    The big difference is that the DC version uses a DC (brush + commutator) motor and reverses the ouput polarity for direction control, where the AC one uses the phase rotation between the three outputs to set direction.
     
    Last edited: Sep 29, 2009
  6. BillB3857

    Senior Member

    Feb 28, 2009
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    In addition to the above, in a true positioning servo system, the motor should have "holding" power. In that situation, the current balance between the three phases remains static, thereby holding the shaft in a defined position. Imagine a plot of three phases and a vertical line drawn from top to bottom. The DC equivalent of the individual phases at the intersection of the vertical line would be represented by PWM signals to the windings and define one or more specific positions of the rotor. The number of poles in the winding would determine how many positions..
     
  7. BillB3857

    Senior Member

    Feb 28, 2009
    2,400
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    For some interesting reading on the subject of Brushless DC motors, check out this site.

    http://www.mitchell-electronics.com/technicalinfo.html

    They manufacture test equipment specifically targeting repair and test of this type motor. I do not work for this company but do use their equipment.
     
  8. GARYN

    Thread Starter Member

    Sep 4, 2009
    47
    0
    Thanks eveyone for the above information......very helpful.
     
  9. alfred moyo

    New Member

    Apr 26, 2008
    2
    0
    hie garyn

    i liked your qestion on servo motors though now its been some time you posted .my self i work in the printing as an an electrician ive been having a lot of qestions on that subject.any where i i need your help also, im working at this printing machine originally it used a siemens drive which i replaced with a urotherm drive after it packed .initially all was well after i installed the drive though the urotherm was not as compatible as the siemens .nw its working well (smoth) on slow speeds ,but as you increase the speed it stars flactuation ,what do you think might be problem?

    regards

    Alfred
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,509
    2,369
    If you are using the same motor it would appear then to be in the tuning, my guess it may be going into current limit on accel/decel.
    Many of these μp based drives use a form of PC based software for tuning.
    Max.
     
    Last edited: Mar 3, 2014
  11. MrChips

    Moderator

    Oct 2, 2009
    12,421
    3,356
    I have move this thread from Projects Forum to General Electronics Chat.
     
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