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  1. #41
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    Quote Originally Posted by JST View Post
    Even my old "Amprobe" meters can be switched to be voltmeters. Having the functions as two separate meters definitely has the advantage that you can correlate voltage and current readings.

    I will give Jim R the point that you can read "VA" that way, but it takes a power meter to read the actual power in anything other than a heater or similar resistive circuit.
    Power meters are dear.

    Contact Thermistor "beads" are cheaper than "potting soil" AKA "dirt.

    "Resistive" ain't the only stuff as warms-up. The math ain't hard...

    If even yah can be bothered? I cannot. I doubt "Old Jim" wastes much time on it, either.



    .. given it is all mass-produced "commodity" goods, with nameplates, long history, and has all been "done to death"...

    ... to the point "yah just KNOW" what is likely going-on before yah even unwrap a 'scope or meter's leads.

    I ain't interested in getting down and dirty with the physics to "invent" any of this shite from a blank page. Some other teams of Pilgrims have BTDTGTTS arredy.

    I just USE such of it as I can buy cheaply.

    "Too complicated?" Cheapest solution is never to "re-engineer it".
    Scrap it or onpass it, rather.

    As I did with the three Vee Effing Dee's, and the older one of my two Phase-Perfect.

    Then try again:

    Also "cheaply".

    "Life is too DAMNED short..." etc.


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    Quote Originally Posted by jim rozen View Post
    Sorry, I was replying to somebody somehwere when he said:

    "Take DC resistance of the windings. Multiply by current squared. Whatever happens inductively, that heat is getting dumped into the stator windings. It's wire with amps going through it. Heat happens...."

    I am actually aware that rotary converters run on ac and the currents and voltages are hardly ever in phase with those, no matter what an amp-clamp meter reading
    might be indavertently be interpreted as, erroniously.
    The point is that that doesn't matter what the phase angle is for the I^2R losses in the windings.

    E.g. if you have 100A going through some cables to a PFC capacitor bank, that current is entirely reactive. Almost no active current. But those cables will get just as hot as if they were feeding 100A of resistive heating.

    The same goes for the stator windings in the motor, except that the induced voltage is actually distributed *throughout* the cable rather than at one point.

    Rotor losses and magnetic losses? Those change with load, and are fairly substantial causes of heating.

    Motor currents can increase substantially under load. Unloaded can be <0.5x rated current, even running on mains. Motor currents are a pretty good way of telling how loaded a motor is, and that's why we use thermal overloads.

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  4. #43
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    Quote Originally Posted by SomeoneSomewhere View Post
    Motor currents are a pretty good way of telling how loaded a motor is, .
    Except when they're not. Case in point, my idler. Actually power draw is about 200 watts, when you accout for the low
    power factor. If you calculated based on the inductive amp clamp meter, it would be more like 75 percent of the power
    of a 5 hp motor.

    The motor heats like it was dissipating 200 watts.

    Final take-away from this discussion: this question will come up again here in the not too distant future. How come my motor
    is drawing so much current, is there something wrong with it. The answers will be:

    1) needs new bearings.
    2) check it with a megger.
    3) buy a new motor, and old one is no good.
    4) its the wrong kind of motor for you

    At some point somebody (else, I'm done being that guy whos all about amp-clamp meters) will say, hmm, exactly how
    are you measuring that current?

    Standing bet, it will be 'with my amp clamp meter.' 20 bucks? I lose, I pay JST, right?

    (and I win, themite has to pay me....)

  5. #44
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    Jim, the data you get from ANY test has to be used according to what it is.

    Most motor questions of the type you mention are simply due to not knowing how motors work.

    The meter does not lie, it measures exactly what it is designed to measure, and gives you that result. Unless you know how to interpret a current reading, a voltage reading, a temperature reading, etc, you risk running off on wild goose chases.

    To bring it back to the machine shop directly, how many times does a newbie with a dial indicator get worked up because "the spindle on this lathe is bent"? Yep , its a regular feature of hobby forums. If I ran one (which I have no desire to do), I'd put up a sticky message for each of the most common issues that come up. Problem is, there are so many.

    In the "bent spindle" case, the person has likely not figured out what he is measuring, what else could affect it, and is jumping to a conclusion. This happens with electrical things more often simply because electricity is invisible in most cases, and is not easy to think about.

    How hot should a motor get? Almost surely hotter than you think, if you have no specific knowledge. Similar for current, voltage, and any other parameters you can think of.

    People do not want to hear this, but there is still absolutely no substitute for knowing what the heck you are doing.

  6. #45
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    Regarding a 5hp motor with 2/3rds of its windings passing 75% rated current, suppose it is running at no load, and consuming 200 watts.

    Then at full load it will dissipate 538 watts of copper losses.

    This is consistent with a 87% efficient motor.

    Small motors are mostly copper losses. The iron losses will be constant regardless of the load. If you subtract 50 watts of iron losees from 200, copper losses increase to 400, total losses to 450, and efficiency is 89%

    If you were to wire up a lot of run caps to the point your generated leg is too high, amps on the input legs might go down, while total watts consumed goes up due to iron losses due to saturation.


    There is no free lunch and amps flowing equals copper losses. The extra heat from a saturated motor is an unknown not diagnosible from a single amps meter

  7. #46
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    Quote Originally Posted by jim rozen View Post
    Except when they're not. Case in point, my idler. Actually power draw is about 200 watts, when you account for the low
    power factor. If you calculated based on the inductive amp clamp meter, it would be more like 75 percent of the power
    of a 5 hp motor.

    The motor heats like it was dissipating 200 watts.

    Final take-away from this discussion: this question will come up again here in the not too distant future. How come my motor
    is drawing so much current, is there something wrong with it. The answers will be:

    1) needs new bearings.
    2) check it with a megger.
    3) buy a new motor, and old one is no good.
    4) its the wrong kind of motor for you

    At some point somebody (else, I'm done being that guy whos all about amp-clamp meters) will say, hmm, exactly how
    are you measuring that current?

    Standing bet, it will be 'with my amp clamp meter.' 20 bucks? I lose, I pay JST, right?

    (and I win, themite has to pay me....)
    You say it dissipates (draws) 200 watts. Is that a guess, estimate or actual measurement with a watt meter?

    Tom

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    Quote Originally Posted by TDegenhart View Post
    You say it dissipates (draws) 200 watts. Is that a guess, estimate or actual measurement with a watt meter?
    Tom
    Measurement, done two different ways. Power factor measured and then used to multiply the amp clamp number. Other way was by
    multiplying the voltage and current waveforms taken on an oscilloscope. Power factor BTW was obtained, again, by looking at the phase
    angle between current and voltage waveforms on the same scope.

    Current waveform was the voltage across a one ohm non-inductive resistor, in series with the input to the converter. Photos on request.

    (the difference between the power draw number (one was 200 watts, the other was around 240 watts) was probably from the non-sinusoidal
    waveforms)

    These days I probably could locate an honest-to-god wattmeter. I had at the time considered shutting off all the power to the house, save the converter,
    for a while, and integrate with the house watthour meter. Given wife and at that time, teenage daughter, I felt this approach would shorten
    my lifespan considerably.

  9. #48
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    I rather thought that was what you did. I have tried that with a scope using the zero crossing but because the current wave near zero is rather flat it was difficult to get a good value. Better to use a true watt meter.

    Tom

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    Quote Originally Posted by jim rozen View Post
    Except when they're not. Case in point, my idler. Actually power draw is about 200 watts, when you accout for the low
    power factor. If you calculated based on the inductive amp clamp meter, it would be more like 75 percent of the power
    of a 5 hp motor.

    The motor heats like it was dissipating 200 watts.

    Final take-away from this discussion: this question will come up again here in the not too distant future. How come my motor
    is drawing so much current, is there something wrong with it. The answers will be:

    1) needs new bearings.
    2) check it with a megger.
    3) buy a new motor, and old one is no good.
    4) its the wrong kind of motor for you

    At some point somebody (else, I'm done being that guy whos all about amp-clamp meters) will say, hmm, exactly how
    are you measuring that current?

    Standing bet, it will be 'with my amp clamp meter.' 20 bucks? I lose, I pay JST, right?

    (and I win, themite has to pay me....)
    Well, no. Because step 1 will be "how much current is it drawing", and step 2 will be "that's about normal for a motor of that size, unloaded". I'm sure that, somewhere in the first hour of any entry-level course about induction motors, you will hear 'power factor and efficiency increases with increased load.

    Line current is, as I said, a perfectly good way to approximate motor load. It's why motor overloads rely on it. You just have to be aware that 0% load is around 50% current, depending on the motor size. It's fairly linear-ish up to and past full load from there, and for the main purpose of asking "is this motor overloaded", it gives perfectly accurate answers.

    And as previously mentioned, if you try to size your branch circuits on the basis that most of your motors are running unloaded and therefore drawing little real current... reactive current can still trip a breaker. For determining if the circuit is overloaded, current is what you look at, not power.

    And if you wired it through a multimeter's 10A current shunt... same value. It's nothing to do with the clamp meter.

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    Ah. Photos, found them:

    First is the setup for measuring power factor, don't try this at home:



    Gold item under the hammer handle is a one ohm non-inductive resistor in series with one line of the converter power. One scope trace is the
    voltage across that resistor. The red wire goes to the other side of the incoming line, so the second trace is the voltage across the converter.
    The amp-clamp meter there of course reads the sum of real and reactive currents going into the converter. (because I did not put that number
    into the text file for the photos, I had to go back and do it a second time, 8 amps is the reading. Again the converter is a 5 hp allis chalmers motor,
    running unloaded:

    Scope trace, red is voltage, green current:



    What it shows: The time difference between zero crossings is 3.94 ms as shown by the cursors values at the bottom of the screen.
    60 cycles means that 360 degrees of sine wave happens in 16.7 ms in round numbers. This means the phase angle between voltage
    and current is just about 85 degrees. The power factor is the cosine of that: 0.088

    So the motor draws real power: 240 volts X 8 amps X cos 85 = 170 wattts give or take. This small value shows up as heat in the motor.

    Some comments: a naive reading of the amp clamp number would ignore the power factor and show around 2KW going into the 5 hp idler motor.
    That would be around 50 percent of FLA for this motor. The entire converter has cartridge fuses (10 amps) in the knife switch, and they've never
    blown even under load.

    Another photo of the setup:



    And my attempt to show what happens when the converter is loaded with a machine, with mechanical load present,
    the power factor increases (phase delay between I and V reduces):


  13. #51
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    This is very good, I hope some of the non-EE's study this.

    The current here is vary close of sinusoidal. Not so much when there is little or no air gap.

    Tom

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    Quote Originally Posted by jim rozen View Post
    Ah. Photos, found them:

    First is the setup for measuring power factor, don't try this at home:



    Gold item under the hammer handle is a one ohm non-inductive resistor in series with one line of the converter power. One scope trace is the
    voltage across that resistor. The red wire goes to the other side of the incoming line, so the second trace is the voltage across the converter.
    The amp-clamp meter there of course reads the sum of real and reactive currents going into the converter. (because I did not put that number
    into the text file for the photos, I had to go back and do it a second time, 8 amps is the reading. Again the converter is a 5 hp allis chalmers motor,
    running unloaded:

    Scope trace, red is voltage, green current:



    What it shows: The time difference between zero crossings is 3.94 ms as shown by the cursors values at the bottom of the screen.
    60 cycles means that 360 degrees of sine wave happens in 16.7 ms in round numbers. This means the phase angle between voltage
    and current is just about 85 degrees. The power factor is the cosine of that: 0.088

    So the motor draws real power: 240 volts X 8 amps X cos 85 = 170 wattts give or take. This small value shows up as heat in the motor.

    Some comments: a naive reading of the amp clamp number would ignore the power factor and show around 2KW going into the 5 hp idler motor.
    That would be around 50 percent of FLA for this motor. The entire converter has cartridge fuses (10 amps) in the knife switch, and they've never
    blown even under load.

    Another photo of the setup:



    And my attempt to show what happens when the converter is loaded with a machine, with mechanical load present,
    the power factor increases (phase delay between I and V reduces):

    We did more or less this in sparky school, just with a power factor meter instead of a scope. For what it's worth, I don't think there was any difference between the moving-coil analogue meters and the clip-on ammeters. Both read the current perfectly accurately.

    But again, it would be a *very very* naive person, given that an unloaded motor is literally the textbook example of a load with poor power factor.

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    Inductive amp clamp meters can report NO phase information. They are only measuring ONE parameter. TWO are needed to
    measure power factor, and hence TWO parameters at least are needed to measure power delivered to a load, for ac circuits.

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    Quote Originally Posted by jim rozen View Post
    Inductive amp clamp meters can report NO phase information. They are only measuring ONE parameter. TWO are needed to
    measure power factor, and hence TWO parameters at least are needed to measure power delivered to a load, for ac circuits.

    A voltmeter is the same. It reports no more information than the clamp ammeter.

    "Phase" is always "with respect to" something else, it is a "phase relationship", so OF COURSE you need to sense that "something else" in order to even make sense speaking about phase.

    But that is not "about" the type of sensor, it is about how many you have of them. Two volt sensors, two inductive current sensors, one of each, etc, any of those can tell you phase relationship.

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    Quote Originally Posted by JST View Post
    A voltmeter is the same. It reports no more information than the clamp ammeter.

    But oddly that is how Fitch Williams suggested tuning up a phase converter: with a voltmeter. Smarter man than me, for certain.

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    My answer would be "both".

    Ammeter to get the torque producing current maximized (under load the power factor is not an issue in the same way)

    Voltmeter to make sure things do not get out of hand at low load/no load, which is what he was no doubt trying to assure.


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