Watt hour meters and frequency.

[mbrooke]

Does frequency effect electro-mechanical watt hour meters? Ie, if I used a 50Hz meter on a 60Hz supply would it measure the same power or measure 1.2x more power (5/6 ratio speed)?

 

[gar]

170528-0938 EDT

Theoretically an electromechanical watt-hour meter takes the instantaneous product of voltage and current (instantaneous power) and integrates this (sums very small samples) to get a cumulative average over a time period from one reading to another. Thus, ideally it should not be frequency dependent.

In the practical world there are adjustments and modifiers to correct for some types of errors. Do an Internet search for watt-hour meter accuracy, and adjustment.

How much typical error there is going from 60 tyo 50 Hz I don't know.

If you use a integrated circuit like is used in the TED power and energy monitor, then accuracy can possibly be better than a mechanical meter. However, the TED system has accuracy problems because of their use of current transformers. Primarily relative to load power factor. Said integrated circuit if properly applied is not frequency dependent.

.

 

[mbrooke]

170528-0938 EDT

Theoretically an electromechanical watt-hour meter takes the instantaneous product of voltage and current (instantaneous power) and integrates this (sums very small samples) to get a cumulative average over a time period from one reading to another. Thus, ideally it should not be frequency dependent.

In the practical world there are adjustments and modifiers to correct for some types of errors. Do an Internet search for watt-hour meter accuracy, and adjustment.

How much typical error there is going from 60 tyo 50 Hz I don't know.

If you use a integrated circuit like is used in the TED power and energy monitor, then accuracy can possibly be better than a mechanical meter. However, the TED system has accuracy problems because of their use of current transformers. Primarily relative to load power factor. Said integrated circuit if properly applied is not frequency dependent.

.

But how does a meter work on instantaneous power when it relies on a sinewave to spin the disk?

 

[GoldDigger]

But how does a meter work on instantaneous power when it relies on a sinewave to spin the disk?

OK, that is a fair question.
The meter works on the mutual attraction between the magnetic field of one coil and the current induced in the metal disk by the other coil. That pretty much forces it to depend on the time varying nature of the driving voltage and current, since otherwise the eddy currents would simply decay to zero.

Whether the meter is frequency dependent or independent hinges on whether the induced eddy currents are (initially, before they start to decay) proportional to the line current or to the time rate of change of the line current.

My first take is that, like a current transformer, the eddy currents are directly proportional to the driving current as long as the frequency is high enough to allow us to ignore the current decay caused by the disk resistance. That is also consistent with the eddy current being in phase with the driving current and therefore able to properly deal with the power factor angle of the power it measures.

At some low end frequency limit the meter response will fall because of the disk resistivity. At some high end frequency limit other factors may interfere with the meter response.
In the region around the design frequency, big enough to include both 50Hz and 60Hz, the response should be good enough. At 400Hz, I am not so confident, and a slightly different design in terms of coil turns and spacing might be needed.

 

[Fulthrotl]

In the region around the design frequency, big enough to include both 50Hz and 60Hz, the response should be good enough. At 400Hz, I am not so confident, and a slightly different design in terms of coil turns and spacing might be needed.

before i sparkied for a living, i working manufacturing power capacitors.

i don't remember the particulars, it's been a long time, but power at 60 hz
versus 400 hz entailed significant changes in engineering and fabrication,
as well as testing. there were correction factors for dissipation factor, among
other things.

 

[mbrooke]

OK, that is a fair question.
The meter works on the mutual attraction between the magnetic field of one coil and the current induced in the metal disk by the other coil. That pretty much forces it to depend on the time varying nature of the driving voltage and current, since otherwise the eddy currents would simply decay to zero.

Whether the meter is frequency dependent or independent hinges on whether the induced eddy currents are (initially, before they start to decay) proportional to the line current or to the time rate of change of the line current.

My first take is that, like a current transformer, the eddy currents are directly proportional to the driving current as long as the frequency is high enough to allow us to ignore the current decay caused by the disk resistance. That is also consistent with the eddy current being in phase with the driving current and therefore able to properly deal with the power factor angle of the power it measures.

At some low end frequency limit the meter response will fall because of the disk resistivity. At some high end frequency limit other factors may interfere with the meter response.
In the region around the design frequency, big enough to include both 50Hz and 60Hz, the response should be good enough. At 400Hz, I am not so confident, and a slightly different design in terms of coil turns and spacing might be needed.

So to a degree the flux also depends on the voltage/frequency ratio- hence the slight deviation?

 

[kwired]

170528-0938 EDT



If you use a integrated circuit like is used in the TED power and energy monitor, then accuracy can possibly be better than a mechanical meter. However, the TED system has accuracy problems because of their use of current transformers. Primarily relative to load power factor. Said integrated circuit if properly applied is not frequency dependent.

.

FWIW, most metering of circuits that handle over 400 amps use current transformers.

 

[GoldDigger]

And part of the distinction would be the quality of the current transformers used and their linearity over a wide current range.

Sent from my XT1585 using Tapatalk

 

[GoldDigger]

So to a degree the flux also depends on the voltage/frequency ratio- hence the slight deviation?

I would say yes to that.
Approximating it crudely you have an LR circuit and, depending on the relative magnitude of the L and R impedances you could have a current which is weakly dependent on the frequency.

 

[mbrooke]

I would say yes to that.
Approximating it crudely you have an LR circuit and, depending on the relative magnitude of the L and R impedances you could have a current which is weakly dependent on the frequency.

Ok- that is nice to know- good starting point- I am starting to grasp this.

 

[gar]

170528-2248 EDT

kwired:

I was commenting on the current transformers used in the TED system in combination with the TED electronics.

I ran an experiment with a TED system to see what it would show about the effectiveness of a so called "Power Saver". It could be used to probably show that the power saver worked.

This results from inadequate correction for the current transformer phase shift, or in the case of the TED current transformer it may not be possible to correct the phase shift.

I also ran tests on TED with very high quality capacitors and TED did not perform well. On the other hand a low cost Kill-A-Watt EZ performs well on the capacitor test.

.