Low Frequency (1 Hz to 60 Hz) ammeters, voltmeters and CT's - Any suppliers / OEM's?

[edison123]

Any suppliers for Low Frequency to Power Frequency (1 Hz to 60 Hz) ammeters (5 A), voltmeters (0 to 750 V) and CT's (0 to 2000 Amp/5 A) ?

The standard meters & CT's work only from 45 to 65 Hz. I am looking for meters and CT's that work from 1 Hz onwards. Is it possible?

 

[jraef]

CTs, you probably can’t use, but you can get transducers that use Rogowski Coils as the sensing elements that will work at low frequencies. They work down to <1Hz and up to I think 400 Hz.

Voltage measurements at low frequencies like 1Hz is an entirely different situation. I have never seen any transducers that go lower than 25Hz. Inside of VFDs, they just sense the DC bus voltage and calculate the RMS AC output. If you are trying to ACCURATELY measure the output voltage of a VFD at 1Hz, you are likely not going to get there. An old style d’Arsonville type iron vane voltmeter is still going to be surprisingly good at it though, which is why I have kept my old Simpson and Triplet meters hanging around (although I rarely do field work any more). I just don’t know how accurate it is at super low frequencies like that, mostly because I don’t really care enough to consider it.

 

[edison123]

Thanks jeff. Yes, I understand the challenge of low Hz volt and amp measurements.

I do have the good old d’Arsonville analog voltmeter somewhere. Let me check.

 

[waross]

An old style d’Arsonville type iron vane voltmeter

Jeff, my respected friend, in the almost 20 years that we have shared on this site, the number of times that I have disagreed with you can probably be counted on one hand.
I think that you have just moved me to the ring finger.
Two completely different types of meter movement.
The D'Arsonval, or galvanometer is a DC meter and responds to the average of a varying DC current.
For voltage measurements a series resistor is used, for current measurements a shunt is used.
With the addition of diodes, AC voltages or currents may be indicated.
Unfortunately the value of interest for AC circuits is the RMS value, and the D'Arsonval movement responds to average values.
For use on power AC circuits, a form factor is applied to the meter scale calibration so that while average values are measured, RMS values are displayed.
As I recall, the form factor for a sine wave is 1.1:1, however the exact value is not as important the fact that the form factor changes with changes in the shape of waveform under test.
Short anecdote, long version on request.
A 277:480 Volt power transformer bank to supply a flood water pumping station:
When the voltages were checked on the unloaded transformer bank, the ratio between line to line versus line to neutral were noticeably NOT root three.
This was checked with three different meters.
Why?
The voltage waveforms of an unloaded wye transformer connection are distorted and not true sine waves.
This was before the days of digital meters and all of the meters were D'Arsonval based meters.
We were not looking at an error in voltages, we were looking at an error in measurement due to inappropriate form factors for the distorted waveforms.
By the way, once a load was placed on the transformer bank, the waveform distortion was swamped out and the measured voltages showed the proper ratio.

What does this mean here? A D'Arsonval based instrument may only be used for accurate AC measurments when the shape and form factor of the measured waveform are known.

What is the solution? An often use description of an RMS current is an equivalent heating effect to a DC current.
Will a hot wire or thermo-couple based instrument solve your measurement issue?

 

[waross]

I do have the good old d’Arsonville analog voltmeter somewhere. Let me check.

As the current through the meter is rectified, the meter should be relatively insensitive to the frequency, but will depend on the waveform.
If you can live with that, go with it.
I don't know if the effective form factor of a Pusle Width Modulated waveform will change with changes in the modulation.
I suspect that the ratio of off-to-on will change as the effective voltage is changed and whether this will change the form factor significantly.I don't know.
I value your comments on this Jeff.

 

[waross]

PS: The moving iron vane meter is an AC movement and does not do well on DC due to residual magnetism.
I suspect that the moving iron vane movement may respond to average, but not sure.

 

[edison123]

Bill - This is not VFD, it's good old MG set. No harmonics.

I do have some moving coil reciter type meters somewhere in our inventory. Time to root them out and see if they work at low Hz.

 

[waross]

They may be a slight bit off due to imperfect sine waves from many generators, but close enough.
I have seen the form factor errors on transformer banks, but I have taken many generator voltage readings and never worried about the form factor.
Please share with us your findings.
Any old analogue multi-meter should work.
You may be able to use the voltage drop on a supply cable from the generator to the load as a shunt for current measurements.
I trust your experience and ingenuity to find a way to calibrate the cable as a shunt.
Possibly take a voltage drop measurement with a known load and scale and calibrate from that.

 

[waross]

Just for fun:
If you have a wye connection on your generator output, try comparing the line to line voltages with the line to neutral voltages with no load on the generator.
See if you get the same form factor based errors that I saw on more than one unloaded transformer bank.