Motors pulling 75% FLA unloaded
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    Default Motors pulling 75% FLA unloaded

    Hello All,

    I've got two machines powered through phase-o-matic static phase converts and both pull 75%FLA unloaded (idle); ones a 3hp (mill) the other is a 7.5hp(lathe) motor. I pulled the 3hp motor out of the mill and replaced the bearings and carefully removed the old grease; i haven't torn into the 7.5hp motor. The 3hp still pulls 6amps or more (8fla) unloaded and out of the machine; it could be the original Allis-Chalmers motor from the 50s. I would expect 4/4.5 amps unloaded, but not 6+; no shorts in the winding to the case. Could this be a byproduct of running off 2-legs or running through a static phase converter or could it more likely be junk motors? There's a shop in town where I can have them tested, but I thought I would ask the community before I went that route.

    Thank you!

    Pinck

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    What instrument are you measuring the current with?

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    Quote Originally Posted by jim rozen View Post
    What instrument are you measuring the current with?
    I got an oldie but goodie.

    img_7251.jpg

    I orignally had the issue with the 3hp, then after I changed the bearings and got the same results I tried it on the 7.5hp motor. Both turn over easily, are smooth and no unusual sounds or vibrations.

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    Here is a case where either a wattmeter or power factor meter is needed in addition to the ammeter. You can be drawing mega amps but at zero power factor and not have any heating. Power, that which drives the machine is, volts x amps x power factor.

    The current being drawn is most likely just the magnetizing current which reactive (low power factor).

    Tom

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    Quote Originally Posted by TDegenhart View Post
    Here is a case where either a wattmeter or power factor meter is needed in addition to the ammeter. You can be drawing mega amps but at zero power factor and not have any heating. Power, that which drives the machine is, volts x amps x power factor.

    The current being drawn is most likely just the magnetizing current which reactive (low power factor).

    Tom
    Very interesting, I did not know it worked that way... Is there any way to measure this without inserting a test instrument in between the machine/motor and the supply?

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    Inductive amp-clamp meters: creating confusion where ever they may be.

    The meter is measuring both real and imaginary current - because the power factor is really low here, it is mostly imaginary, that is, out of phase current.

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    Again to avoid confusion, a clamp ammeter just measures current, the same as a current shunt. The ammeter has no knowledge as to the nature of the current, whether real or imaginary. It's just current.

    The meters themselves do not create confusion. Its the user that doesn't know what the reading means.

    Before people get too confused, real current is the current in phase with voltage i.e., 1.0 power factor. Imaginary current is current that 90 degrees out of phase with the voltage i.e., zero power factor. It's called imaginary not because it doesn't exist, but because in mathematics vectors are described as real and imaginary, the real part is the part that is in phase with resistive component and the imaginary is 90 degrees(+-) to the resistive axis. Also referred to as the i and j vectors, i is real, j is imaginary. Just EE talk.

    Tom

    Edit:- Another problem with instrumention is that most moving pointer volt/amp meters are calibrated for a sine wave. The better digital meters are true RMS. Why does this matter? For voltage not some much. Current in iron magnetic devices such as motors, contactors and such, the wave shape is almost anything but sine wave. It almost looks like pulse than a sine wave.

    T:-

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    Thank you Tom and Jim. This is good to know!
    Okay, I have a thought. Let me know if it has any weight: I have an amp meter that provides a value. When there isn't a load the value is 6 and when there is a load the value is 8 to 9. If i run the mill at 1/2 speed 5-10 minutes the starter trips. If i run the mill at full speed for a shorter period of time without cutting it will trip and if I am milling it will trip almost immediately (3/8" 4-flute, 0.02 DOC @ 1.5 ipm). The value before the starter and before the motor are nearly the same. Do I have enough information to determine if the motor needs further evaluation or should I be reviewing other parts of the circuit? Do I take the time to put the motor back in the machine and run the same operating tests since the only instruments I currently owm is a volt and amp meter? Or is this a case where I need instruments to destinguish between real and imaginary and therefore take the motor in to a professional with the proper tooling?

    Thank you!

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    Static phase convertors are often used just to get the motor turning and then they disconnect. You can verify that. Assume this is true, then the motor is only running on two of the three sets of windings, this is called open delta. The power out of an open delta motor is about 50% of rating.

    Something trips. Is this the OL relay attached to the starter or is it a Klixon type OL in the motor? Where is the starter in the circuit, before or after the phase convertor? Is the starter a two pole or three pole and if three pole, how is the third leg wired? Is the OL relay a thermal relay as in bimetals or melting solder pots or is it electronic. Do you have the proper heaters in the OL or the dial set properly for the electronic.

    Did this problem just start or has it been this way all along. Sounds to me like your problem is more the OL relay than the motor.

    Tom

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    Get a True RMS Fluke meter.

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    Quote Originally Posted by jim rozen View Post
    Inductive amp-clamp meters: creating confusion where ever they may be.

    The meter is measuring both real and imaginary current - because the power factor is really low here, it is mostly imaginary, that is, out of phase current.
    Beans......

    Motors draw magnetizing current, which is inductive current, perfectly real and measurable current, just not in phase with voltage, so it contributes no significant power.

    And, the motor ALWAYS draws that current. When you load the motor, it ALSO draws "power current" which is "in-phase" with voltage. The power factor increases with the "in-phase" current draw.

    But all that current is part of "FLA", or full load motor current. And, of course, part of any lower current as well.

    Moving on to the excess current issue......

    A motor operating on a "static phase converter" is pulling all motor current (and power) from TWO incoming wires. Normally that motor would pull the same power from THREE wires.

    So, why would it be surprising that if you pull the same power from two wires instead of three, that it means more current on those two than the motor would normally pull? You have added half-again more power to that set of wires, so current goes up.

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    Jerry says it so much more eloquently than I do.

    Tom

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    Quote Originally Posted by JST View Post
    Motors draw magnetizing current, which is inductive current, perfectly real and measurable current, just not in phase with voltage, so it contributes no significant power.
    Yep. No power. The inductive clamp on meter says the motor should be hotter than a pistol. But it's *not*. Why isn't it getting hot? Oh. No signifcant power.
    But the current, the meter says it should be getting hot. But it isn't. Round and round we go. Not a thread about phase converters. Just a thread about
    'why is the motor drawing so much current, is it a short, is it the bearings, do I need a new motor???'

    Nope. The meter is giving readings that confuse the issue unless they are understood properly. One of the single biggest issues on this sub-forum.

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    Quote Originally Posted by jim rozen View Post
    Yep. No power. The inductive clamp on meter says the motor should be hotter than a pistol. But it's *not*. Why isn't it getting hot? Oh. No signifcant power.
    But the current, the meter says it should be getting hot. But it isn't. Round and round we go. Not a thread about phase converters. Just a thread about
    'why is the motor drawing so much current, is it a short, is it the bearings, do I need a new motor???'

    Nope. The meter is giving readings that confuse the issue unless they are understood properly. One of the single biggest issues on this sub-forum.

    To borrow an old saying, "it's a poor technician who blames the meters".

    The "meter" does NOT give "readings that confuse the issue".

    You "connected" a meter to read current, and the meter READS current. Then you assume that with that current and given the voltage across the motor there should be a lot of power. The issue is that the current is not in phase with the voltage. If you wanted "power" you should use a power meter.

    You may just as well say the voltmeter gives "readings that confuse the issue".

    You connected a meter to read voltage, and the meter READS voltage. Then you assume that with that voltage and given the current through the motor there should be a lot of power. The issue is that the voltage is not in phase with the current. If you wanted "power" you should use a power meter.

    The problem is not with the instruments, the instruments generally tell the truth unless they are defective or seriously out of calibration. The problem is with the user understanding what is being measured.

    In the case of motors, they run on current. So current is a very good way of checking operation, it really is the "go to" measurement of motor operation. Voltage is what drives current, but you look at voltage mostly as a "sanity check" to make sure there is no large fault.

    You get voltage even if the circuit is open just after the measuring point. But that fault shows up immediately if you check current.

    IMO, trying to work with motors without a clamp-on meter is just like fighting blindfolded.

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    Quote Originally Posted by jim rozen View Post
    Yep. No power. The inductive clamp on meter says the motor should be hotter than a pistol. But it's *not*. Why isn't it getting hot? Oh. No signifcant power.
    But the current, the meter says it should be getting hot. But it isn't. Round and round we go. Not a thread about phase converters. Just a thread about
    'why is the motor drawing so much current, is it a short, is it the bearings, do I need a new motor???'

    Nope. The meter is giving readings that confuse the issue unless they are understood properly. One of the single biggest issues on this sub-forum.




    What would you suggest as a change/addition? Should we have a short course of EE fundamentals covering magnetic theory and measurement techniques?

    I have thought about something that but how many would have the background or interest in diving in deep, particularly since all this is covered in the internet.

    Tom

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    Quote Originally Posted by TDegenhart View Post
    [/COLOR]

    What would you suggest as a change/addition?
    A couple of run caps on that motor.....

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    Quote Originally Posted by TDegenhart View Post
    [/COLOR]
    What would you suggest as a change/addition?
    Good question, not sure how to make any good answer. But note how often this comes up - nobody has a current meter that
    reads power factor but everyone has an amp-clamp. I suggest that posters with this issue might not read stickies but who knows.
    In the meantime I just point out it's not staightforward to interpret those current readings.

    Complex plane, who invented that nonsense?

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    Measuring current with an ammeter is an entirely valid approach. The current it measures is the same current that is heating the motor windings, heating the supply cable, heating the thermal trips in the overload and circuit breaker, causing volt drop in your circuits etc.

    The only things it will not tell you is whether the power company is billing you for it, and how much of that current is performing useful work.

    And it's nothing to do with being an inductive meter either - a meter with a current shunt (like a standard multimeter) will give you exactly the same readings.

    For the sake of argument, I have a very nice AEMC 607 that will tell you the active, apparent, and reactive power. You have to hook up the voltage leads too.

    Remember also that reactive + active power does not equal apparent power. It's vector maths.

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    Quote Originally Posted by SomeoneSomewhere View Post
    Measuring current with an ammeter is an entirely valid approach. The current it measures is the same current that is heating the motor windings, ...
    Ah, actually not. Power factor is near to zero.

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    Quote Originally Posted by jim rozen View Post
    Ah, actually not. Power factor is near to zero.
    Not so. If the current flows in a resistive element, there will heat generated. That includes the leads to the motor, the windings and ac losses in the iron due to eddy currents and hysteresis losses. Don't forget the effective resistance of the rotor reflected to the stator windings. These are idling power losses. However to separate this current from the magnetizing current will require the power factor.

    Tom


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