Nietz001
Member
- Location
- St Paul MN
Can harmonics cause a low voltage reading? Somewhere around 107 volts AC on a 120 volt branch Ckt.
Harmonics
- Thread starter Nietz001
- Start date
- Status
Re: Harmonics
induction can cause false meter readings many circuits. especially if you are using a low impedance meter.
I once had 300 feet of underground wire read 90 volts without being hooked up wire to wire.
two problems were there. one was the meter was not true rms. two the meter was low impedance.
this is sometimes called a ghost.
induction can cause false meter readings many circuits. especially if you are using a low impedance meter.
I once had 300 feet of underground wire read 90 volts without being hooked up wire to wire.
two problems were there. one was the meter was not true rms. two the meter was low impedance.
this is sometimes called a ghost.
charlie tuna
Senior Member
- Location
- Florida
Re: Harmonics
harmonics is current--has no effect on voltage
harmonics is current--has no effect on voltage
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Harmonics
More current causes more voltage drop and lower voltage at the load. A load that has 10 amps of pure 60 hertz current will have a higher voltage at its terminals than a load with 10 amps of 60 hertz plus some harmonic currents added in.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Harmonics
I think that is the right set of questions to be asking first, as opposed to asking about harmonics. 107 volts is 11% low, on a 120 volt system. That is far too severe of a voltage drop. But did you measure the voltage at the source? Perhaps it was low, and the VD along the wires might not be the whole problem.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Harmonics
I don't think it's that simple. The original signal can be modeled as a sum of sine waves of increasing harmonic order. The RMS of anything starts with squaring the function. Then you take the mean value. Then you take the square root. But when you square a function that has a number of terms, you end up with each term being multiplied by each other term. It's too messy for me to deal with.
But still you might be right. That messy set of terms might wind up simplifying itself to the equation you showed, if the "mean value" of many of its terms end up as zeros. I'll leave that to someone else, as a "homework assignment."
I don't think it's that simple. The original signal can be modeled as a sum of sine waves of increasing harmonic order. The RMS of anything starts with squaring the function. Then you take the mean value. Then you take the square root. But when you square a function that has a number of terms, you end up with each term being multiplied by each other term. It's too messy for me to deal with.
But still you might be right. That messy set of terms might wind up simplifying itself to the equation you showed, if the "mean value" of many of its terms end up as zeros. I'll leave that to someone else, as a "homework assignment."
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Harmonics
Nietz001: Does that completely answer your question, or do you need additional information (I'm channeling my wife, the librarian )?
OK. If we stay on topic, the answer is "yes."
Nietz001: Does that completely answer your question, or do you need additional information (I'm channeling my wife, the librarian
)?ramsy
NoFixNoPay Electric
- Location
- LA basin, CA
- Occupation
- Service Electrician 2020 NEC
Re: Harmonics
Cl/amp meters show neutral-triplen harmonics in greater magnitudes than distorted volts do.
My 1996 ShureTest (Quick Start) guide states, "..3d harmonic computer loads will drop the peak voltage faster than the RMS voltage. This measurement is a quick guage of the harmonic distorion effects of a computer load. Heavy 3d harmonic loads cause the voltage sine wave to flat-top, affecting the ride-through capability of less expensive switch mode power supplies, increasing the chance of a reboot or lockup during a low voltage event..."
Unfortunately, while indicating 3rd order voltage harmonics less than 0.7% and THD voltage below 3.0% my ShureTest Peak-voltage function always shows ~7.0vac lower than its (RMS * 1.414) verified with my DMM. So, my ShureTest ST-1THD either drifts Peak-volt calibration, or this miniscule 3rd harmonic flat tops voltage peaks, from all service distributions in my region.
A scope or graphic-wave analyzer would best show flat-top or distorted wave forms, and solve my ShureTest calibration mystery, but no such animal exists without investing a fortune in the Los Angeles area. The work I get would not justify the cost of calibrating some of these power quality analyzers. But, I could really use one to verify these cheezy ShureTest values and the CMV proficiencies.
I found some of this information in my ShureTest ST-1THD manuals, which indicate when circuits arn't near overload, nor contain an unbalanced load between three phases, a common mode voltage G-N > 2.0 RMS likely indicates harmonic currents.
The caveat is that CMV varies widely with shared neutrals, changes in line noise, loads, and panel proximity. Further, with no such animals above in my aging residence their exists one duplex @ 2.2 vac G-N, with no load, which is likely from a hacked-in EGC, landed on warm rather than cold water pipes.
So, RMS cl/amp meters, THD analyzers, and scopes seem more dependable than CMV's > 2vac as a harmonic indicator. CMV may show relative differences in neutral harmonics at several places in a distribution system. If its not following panel proximity or shared neutrals, perhaps CMV could point to neutral THD increases with proximity to noisy equipment. But, that effort is fruitless where limits on device THD are specified according to phase conductor and require sampling with cl/amp meters anyway.
Except for ShureTest claims, I could'nt find any test evidence of using CMV as a harmonic indicator anywhere. Someone needs to compare this thing along side a real scope, and check this calibration-drift issue.
Cl/amp meters show neutral-triplen harmonics in greater magnitudes than distorted volts do.
My 1996 ShureTest (Quick Start) guide states, "..3d harmonic computer loads will drop the peak voltage faster than the RMS voltage. This measurement is a quick guage of the harmonic distorion effects of a computer load. Heavy 3d harmonic loads cause the voltage sine wave to flat-top, affecting the ride-through capability of less expensive switch mode power supplies, increasing the chance of a reboot or lockup during a low voltage event..."
Unfortunately, while indicating 3rd order voltage harmonics less than 0.7% and THD voltage below 3.0% my ShureTest Peak-voltage function always shows ~7.0vac lower than its (RMS * 1.414) verified with my DMM. So, my ShureTest ST-1THD either drifts Peak-volt calibration, or this miniscule 3rd harmonic flat tops voltage peaks, from all service distributions in my region.
A scope or graphic-wave analyzer would best show flat-top or distorted wave forms, and solve my ShureTest calibration mystery, but no such animal exists without investing a fortune in the Los Angeles area. The work I get would not justify the cost of calibrating some of these power quality analyzers. But, I could really use one to verify these cheezy ShureTest values and the CMV proficiencies.
I found some of this information in my ShureTest ST-1THD manuals, which indicate when circuits arn't near overload, nor contain an unbalanced load between three phases, a common mode voltage G-N > 2.0 RMS likely indicates harmonic currents.
The caveat is that CMV varies widely with shared neutrals, changes in line noise, loads, and panel proximity. Further, with no such animals above in my aging residence their exists one duplex @ 2.2 vac G-N, with no load, which is likely from a hacked-in EGC, landed on warm rather than cold water pipes.
So, RMS cl/amp meters, THD analyzers, and scopes seem more dependable than CMV's > 2vac as a harmonic indicator. CMV may show relative differences in neutral harmonics at several places in a distribution system. If its not following panel proximity or shared neutrals, perhaps CMV could point to neutral THD increases with proximity to noisy equipment. But, that effort is fruitless where limits on device THD are specified according to phase conductor and require sampling with cl/amp meters anyway.
Except for ShureTest claims, I could'nt find any test evidence of using CMV as a harmonic indicator anywhere. Someone needs to compare this thing along side a real scope, and check this calibration-drift issue.
- Status