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90 degree phase shift for capacitor heating

Discussion in 'Electronic Design' started by Industrial Electrician, Apr 14, 2006.

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  1. I am an electrician doing qualification testing of a 10KW power amplifier.
    Internally it has a single phase 220VAC input transformer driving a full-
    wave bridge rectifier and then large filter capacitors on the DC line.
    The end-use will be 50/60Hz ballast testing.

    I need to set up a worst-case test for capacitor heating. Please correct me
    if I am mistaken, but I think that the worst case is when the output feeds
    a resistive load bank with a sine wave that is phase shifted 90 degrees from
    the AC power input.

    The amplifier input has a 1 MegOhm input impedence and requires a 10V peak-
    to-peak input to reach the rated output voltage.

    I also have a 25KW load bank on the output that can be set to draw whatever
    current I want it to. I can get a capacitor load bank if needs be but it
    would be a hassle.

    If I remember correctly from my electronics 101 class, I can use a resistor
    and a capacitor to generate a 10VPP (7.07VAC RMS) sine wave with close to a
    90 degree phase shift from my 220VAC input.

    I am not quite sure how to calculate the resistance and capacitance to get a
    7VAC 90 degree phase shifed output from my 220VAC input. What values should
    I use? Does the capacitor or the resistor go from the 220VAC to the amp
    input? Does the capacitor or the resistor go from the the amp input to
    common?

    I am connecting at the service entrance where 220VAC GND and 220VAC NEUTRAL
    are connected together, and I am seeing less than 1/20th of a volt AC between
    them at the amplifier AC power input.

    Here is what I am going to test next week unless someone tells me differently:

    220VAC GND -> AMPLIFIER CHASSIS
    220VAC NEUTRAL -> AMPLIFIER AC POWER INPUT 1
    220VAC HOT -> AMPLIFIER AC POWER INPUT 2
    220VAC HOT -> 10 Microfarad 300V AC-Rated Capacitor 1
    10 Microfarad 300V AC-Rated Capacitor 2 -> 3300 Ohm 25W Resistor 1
    3300 Ohm 25W Resistor 2 -> 100 Ohm 10W Rheostat (Potentiometer) CW
    100 Ohm 10W Rheostat (Potentiometer) CCW -> 220VAC NEUTRAL
    100 Ohm 10W Rheostat (Potentiometer) WIPER -> AMPLIFIER SIGNAL INPUT HIGH
    100 Ohm 10W Rheostat (Potentiometer) CCW -> AMPLIFIER SIGNAL INPUT LOW

    Will this give me a 7VAC sine wave with close to a 90 degree phase shift
    between 220V and Amplifier input?

    I also have some 12V Zener diodes (current rating unknown). Should I put two
    of them connected back-to-back parallele across Rheostat CW and Rheostat CCW?
     
  2. colin

    colin Guest


    I would stronlgy suggest adding a safety isolation step down transformer to
    derive the signal. (for safetys sake obviously)

    Colin =^.^=
     
  3. Phil Allison

    Phil Allison Guest

    "Industrial Electrician"

    ** I take it you mean the DC filter caps inside the amplifier?


    ** Sounds very likely to be true. Charging peaks coincide with amplifier
    output zero crossings and load current peaks will be half way between
    charging peaks - creating worst case ripple current in the cap.


    ** Bet the amp would be unhappy too.


    ** Please DO NOT DO THIS !!!!



    ** For safety sake you MUST use an *isolation transformer* between the AC
    supply and the amplifier input !!!

    The simplest way to get a 90 degree phase shift relative to the AC supply is
    to use a transformer with a secondary that has a centre tap and connect that
    centre tap to ground. Then you will have two, safe low voltage AC supplies
    to use. One will be in-phase with the AC supply and the other in exact anti-
    phase.

    ( I suggest you get a small ( ie 20 or 30VA) *toroidal* transformer with a
    15 volt secondary that is centre tapped OR has two 7.5 volt secondaries
    connected in series for your testing )

    THEN, connect one end of 3.3 kohms resistor to one 7.5 volt supply and one
    end of a 1uF ( plastic film ) capacitor to the other, join the remaining
    cap and resistor leads, this point creates a 7.5 volt supply that is 90
    degrees shifted.

    Slight adjustments to the resistor or cap value will get the result spot on
    at 50 or 60 Hz.

    The problem with using the AC supply as an input signal is the waveform is
    not a perfect sine wave - however it will likely be close enough or your
    purpose.

    Good luck testing the temp rise of the filter caps - make sure to monitor
    the air ambient near the caps as well as the actual cap body temp.




    ........ Phil
     
  4. Not a bit. AC power conversion amplifiers can handle any power factor,
    including 100% capacitive, 100% inductive, and nonlinear (cheap) switching
    power supplies.
    I know how to stay safe. This is in a controlled lab condition.
    We do things that we would b=never do in a real-world installation.

    In addition, the amplifier input has an input transformer with a
    15KV isolation rating.
    Anti-phase is 180 degrees, not 90 degrees.
    Ah. That is where you are getting the 90 degree shift?
    So you are saying I got the component values wrong? That is the part I am
    unsure about, but I calculated 10 microfarads, not 1. I believe that the
    -3db point (con=rner frequency) has to be at least 1/10 of my 50/60Hz.
    72 degree controlled climate, huge fan blowing air through the amp,
    capacitor near the inlet. But still, big caps don't dissapate well.
     
  5. Phil Allison

    Phil Allison Guest

    "Industrial Electrician"


    ** Like hell.

    What you describe is a fatal shock hazard to any operator.


    ** Asinine bullshit.


    ** You completely missed the safety issue.


    ** I see you have no patience at all.

    Bad thing in a electrician if he wants to stay alive.



    ** Yep, with anti-phase inputs a simple RC network that would otherwise
    produce a *45 degree* phase shift at the equal value frequency jumps to 90
    degrees.



    ** You got the electrical circuit theory wrong.

    Yours is the mad sparkys' damn dangerous way to do a very simple job.



    ** With forced air cooling like that, the DC filter caps surface temp will
    barely rise.



    ........ Phil
     
  6. Putting the capacitor to the hot line will give you a nearly 90 degree
    phase lead, but will also differentiate any noise spikes on the line
    and pass them to the resistor with little attenuation. Connecting the
    capacitor at the neutral side will integrate the line voltage, causing
    an almost 90 degree phase lag, but will low pass filter any noise,
    giving you a much cleaner sine wave signal.

    A 10 uF capacitor has about 265 ohms impedance at 60 Hz. As long as
    the series resistor has a much higher resistance than that, the phase
    shift (across the resistor) will approach 90 degrees.

    In order to have about 10 volts appear across the capacitor impedance,
    you need about .038 amperes to pass through it. A resistor of about
    6000 ohms rated for at least 10 watts would provide about that current.

    Then you could connect a 100k pot across the capacitor, and use the
    wiper to adjust the output voltage to exactly whatever value you
    wanted that any fraction of what is across the capacitor. That high
    resistance load will alter the phase shift almost not at all, but will
    provide a low enough impedance to drive the 1 meg input impedance with
    little sag (measure the final voltage while connected to the input, or
    substitute a 1 meg load across the pot to set the voltage).

    It would be a very good idea to have an isolation transformer between
    the line and this circuit, so the output low side has no voltage with
    respect to the amplifier common.
     
  7. I only had time for a quick test before we closed down Friday, but
    I did have time to try it both ways, and the noise is much, much
    lower doing it the way you suggested. Thanks! My oscilloscope trigger
    circuit thanks you as well. ;=)
    That makes sense. Just to see if I understand, the above relates to
    what I read about the -3DB point because the bigger resistor moves
    that frequency down, and if I then lowered the signal frequency I
    would not only moving closer to the -3DB point but also lowering the
    impedance of the capacitor. Am I getting all of this right?
    I see how that works. Just to see that I understand, that also means that
    the resistor/capacitor phase shift has a voltage loss, and that someone
    who needs the phase shift at the original voltage would need to use another
    technique such as an operational amplifier based circuit. Am I getting
    this right?
    That's a great idea. All of my high wattage pots are much smaller in
    resistance but using a parallel pot like you suggest beans that I can
    use a regular pot. That's very helpful.
    This is a temporary setup on a wooden bench, and I have separate
    3 foot lengths of 8 AWG wire connecting chassis, line neutral, and
    amplifier common to an earth ground that directly connects to a 12
    foot ground rod in moist soil, which in turn connects through a
    short length of grounding braid to the rebar in the concrete
    foundation and connects through another short length of grounding
    braid to the building water pipes. I have tested this earth ground
    with 250 amps. I seriously doubt that there will be a noticeable
    voltage at anything that I have connected to earth ground.

    Isolation transformers are very useful if you are poking around inside
    a TV, but when working with high current and high voltage systems, you
    are far safer by enclosing everything in an earth-grounded case, having
    neutral connected to earth ground instead of floating, and never
    energizing the circuit without having everything closed up inside earth-
    grounded finger-proof grounded conduits and boxes. If you or "Phil
    Allison" disagree, please describe, in detail, the exact path that you
    think current will take through the human body from a system using the
    industry-standard grounding system I am describing. I am open to being
    shown that I am wrong, but not without a good explanation showing why I
    and everyone else in my industry is wrong.
     
  8. I only had time for a quick test before we closed down Friday, but
    I did have time to try it both ways, and the noise is much, much
    lower doing it the way you suggested. Thanks! My oscilloscope trigger
    circuit thanks you as well. ;=)
    That makes sense. Just to see if I understand, the above relates to
    what I read about the -3DB point because the bigger resistor moves
    that frequency down, and if I then lowered the signal frequency I
    would not only moving closer to the -3DB point but also lowering the
    impedance of the capacitor. Am I getting all of this right?
    I see how that works. Just to see that I understand, that also means that
    the resistor/capacitor phase shift has a voltage loss, and that someone
    who needs the phase shift at the original voltage would need to use another
    technique such as an operational amplifier based circuit. Am I getting
    this right?
    That's a great idea. All of my high wattage pots are much smaller in
    resistance but using a parallel pot like you suggest beans that I can
    use a regular pot. That's very helpful.
    This is a temporary setup on a wooden bench, and I have separate
    3 foot lengths of 8 AWG wire connecting chassis, line neutral, and
    amplifier common to an earth ground that directly connects to a 12
    foot ground rod in moist soil, which in turn connects through a
    short length of grounding braid to the rebar in the concrete
    foundation and connects through another short length of grounding
    braid to the building water pipes. I have tested this earth ground
    with 250 amps. I seriously doubt that there will be a noticeable
    voltage at anything that I have connected to earth ground.

    Isolation transformers are very useful if you are poking around inside
    a TV, but when working with high current and high voltage systems, you
    are far safer by enclosing everything in an earth-grounded case, having
    neutral connected to earth ground instead of floating, and never
    energizing the circuit without having everything closed up inside earth-
    grounded finger-proof grounded conduits and boxes. If you or "Phil
    Allison" disagree, please describe, in detail, the exact path that you
    think current will take through the human body from a system using the
    industry-standard grounding system I am describing. I am open to being
    shown that I am wrong, but not without a good explanation showing why I
    and everyone else in my industry is wrong.
     
  9. I only had time for a quick test before we closed down Friday, but
    I did have time to try it both ways, and the noise is much, much
    lower doing it the way you suggested. Thanks! My oscilloscope trigger
    circuit thanks you as well. ;=)
    That makes sense. Just to see if I understand, the above relates to
    what I read about the -3DB point because the bigger resistor moves
    that frequency down, and if I then lowered the signal frequency I
    would not only moving closer to the -3DB point but also lowering the
    impedance of the capacitor. Am I getting all of this right?
    I see how that works. Just to see that I understand, that also means that
    the resistor/capacitor phase shift has a voltage loss, and that someone
    who needs the phase shift at the original voltage would need to use another
    technique such as an operational amplifier based circuit. Am I getting
    this right?
    That's a great idea. All of my high wattage pots are much smaller in
    resistance but using a parallel pot like you suggest beans that I can
    use a regular pot. That's very helpful.
    This is a temporary setup on a wooden bench, and I have separate
    3 foot lengths of 8 AWG wire connecting chassis, line neutral, and
    amplifier common to an earth ground that directly connects to a 12
    foot ground rod in moist soil, which in turn connects through a
    short length of grounding braid to the rebar in the concrete
    foundation and connects through another short length of grounding
    braid to the building water pipes. I have tested this earth ground
    with 250 amps. I seriously doubt that there will be a noticeable
    voltage at anything that I have connected to earth ground.

    Isolation transformers are very useful if you are poking around inside
    a TV, but when working with high current and high voltage systems, you
    are far safer by enclosing everything in an earth-grounded case, having
    neutral connected to earth ground instead of floating, and never
    energizing the circuit without having everything closed up inside earth-
    grounded finger-proof grounded conduits and boxes. If you or "Phil
    Allison" disagree, please describe, in detail, the exact path that you
    think current will take through the human body from a system using the
    industry-standard grounding system I am describing. I am open to being
    shown that I am wrong, but not without a good explanation showing why I
    and everyone else in my industry is wrong.
     
  10. All but the capacitor part. The impedance of a capacitor foes up as
    frequency goes down. Xc=1/(2*pi*f*C)

    At the -3db frequency (where Xc=R) the phase shift is 45 degrees, at
    frequencies much higher than this, the phase shift approaches 90
    degrees. Operating where a capacitor has an impedance of 265 ohms, in
    series with a 6000 ohm resistor is far enough above the -3db frequency
    that the approach to 90 degrees is pretty close.
    Yes. this technique involves an essential voltage loss.
    (snip)

    Very well.
     
  11. Phil Allison

    Phil Allison Guest

    "Industrial Electrician"

    ** Your smug arrogance is only exceeded by your sparkies pig ignorance.

    Isolation transformers are included inside EVERY electronic and electrical
    device where human contact with external wiring is possible - as is the
    case with your 90 degree phase shifted "signal generator".


    ** Irrelevant crapology !!

    Using an isolation step-down transformer means you AVOID working with a
    dangerous high voltage system in the first place.



    ** Q. What happens if the link to neutral ever becomes disconnected ?

    A. The previous low voltage (ie 90 degree phase shifted supply) jumps to
    220 volts AC with plenty of current available to cause electrocution.

    NOW:

    Stop being such a conceited, bloody ass !!

    A small isolation transformer and one R and one C network method is far
    CHEAPER and well as enormously SAFER than your asinine scheme.





    ......... Phil
     
  12. Nothing of any importance. The system will stop working, but the fact
    that it is fully enclosed in earth-grounded conduit and boxes makes it
    impossible for the now-hot neutral to harm a human. Your rejection of
    the basic safety scheme used in all modern electrical wiring (earth
    grounded boxes and conduit enclosing a neutral that is earth-grounded
    only at the service entrance and a hot that is protected by a fuse or
    breaker) is based on ignorance. Your suggestion that an isolation
    transformer be added and the neutral be allowed to float is dangerous
    and illegal in most municipalities. Stay away from AC power, please;
    you are sure to hurt someone with your lack of knowledge and refusal
    to learn from those who know what they are talking about.
    There already exists a hot wire with 220VAC on it inside the grounded
    conduit and boxes. Your belief that an open neutral that creates
    another wire with 220VAC on it inside the grounded conduit and boxes
    is a safety issue is magical thinking; both hot and neutral are fully
    protected from contact with a human, as is the capacitor and resistor
    that comprises the phase shift network. Your inability to describe,
    in detail, the exact path that you think current will take through
    the human body says it all. Until you can describe that path, all your
    blustering and magical thinking will only lead you farther astray.
    Use of personal attacks to hide an obvious lack of technical rigor in
    your claims noted. Bozo Filter now engaged.

    Plonk.
     
  13. Phil Allison

    Phil Allison Guest

    "Industrial Electrician"

    ** WHAT A PIG ARROGANT ASSHOLE !!


    ** The previous 7 volt AC signal becomes 220 volts and lethal.


    ** Shame about those connections to the amplifier's input - they are now
    lethal for anyone to touch.

    Dumbfuck sparkie assholes like YOU have no training in or comprehension of
    basic electrical safety.



    ** Go shove your posturing opinions up your stupid arse.


    ** Only I did not make that suggestion.

    The use of an isolation step down *removes* the need for a connection to
    neutral to exist to guarantee safety - since loss of a neutral connection
    merely disables the transformer.

    Don't have the SLIGHTEST idea why electronic appliances have isolation
    trasnformers inside ?


    ** You are an ARROGANT MORON who ought to be taken out and shot through
    the head before YOU kill someone.

    Likely you will electrocute yourself real soon.

    I sincerely hope you do not kill a co-worker.




    ........ Phil
     
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