Electronics > Projects, Designs, and Technical Stuff
how to measure the direction of a.c. power flow
soldar:
I did a test. I took an AC current of about 2 A and measured it directly on the scope on one channel and sensing through a current transformer on the other and the original appears 0.7 mS later on the scope. The CT leads by that amount of time which is about 1/25 of a cycle. Not huge but significant enough to degrade true measurements.
It was a quick and dirty test. I did not do other measurements to see if the phase difference changed significantly with the current. If it is pretty constant then it should not be difficult to correct it either with a capacitor at the CT or maybe just correct the voltage phase so it is in phase with the current reading. Should not be difficult.
But if you want true power reading you need to do it because otherwise You can have a purely resistive load and it looks like it is somewhat reactive and returning some energy when it is not.
The advantage of a CT is the isolation but if you want to have really good phase reference a shunt is much better.
As a side note: I have discovered that the second channel of my 'scope is defective and needs repair. It has a big DC offset upwards and I cannot get 0V into the screen. Oh well. That'll be a different thread.
Circlotron:
Was that CT phase shift test using the proper burden resistor? I expect that too high a resistance will give phase shift problems.
coppice:
--- Quote from: soldar on May 16, 2019, 06:45:52 pm ---I did a test. I took an AC current of about 2 A and measured it directly on the scope on one channel and sensing through a current transformer on the other and the original appears 0.7 mS later on the scope. The CT leads by that amount of time which is about 1/25 of a cycle. Not huge but significant enough to degrade true measurements.
It was a quick and dirty test. I did not do other measurements to see if the phase difference changed significantly with the current. If it is pretty constant then it should not be difficult to correct it either with a capacitor at the CT or maybe just correct the voltage phase so it is in phase with the current reading. Should not be difficult.
But if you want true power reading you need to do it because otherwise You can have a purely resistive load and it looks like it is somewhat reactive and returning some energy when it is not.
The advantage of a CT is the isolation but if you want to have really good phase reference a shunt is much better.
As a side note: I have discovered that the second channel of my 'scope is defective and needs repair. It has a big DC offset upwards and I cannot get 0V into the screen. Oh well. That'll be a different thread.
--- End quote ---
The delay through a CT, and its character, varies enormously with the core type. For some cores its approximately a time delay. For other cores is approximately a phase delay. 0.7ms is a huge delay for a CT. Is it made by VAC? They have some models with huge delays. Most CTs give a delay far smaller, and some give a delay so small that can be ignored for most purposes. For example, most large multiplier CTs are designed to introduce such a small delay that they have little effect on the calibration of the measurement unit whose range they are multiplying for.
A serious problem using CTs with a big delay is that the delay will vary a lot with frequency. Compensating for the delay at the fundamental may be straight forward, but getting the delay right for all the harmonics can be a nightmare.
soldar:
I tested with a lower resistor and the phase difference diminished just a bit. I could not measure it with precision because I am having problems with my scope.
Still, it should not be difficult to correct the voltage phase a bit so that it is in line with the current phase.
Frequency is not really a concern because it is for mains.
The purpose is not to precisely measure power as a meter might do but to determine in which direction power is flowing.
David Hess:
--- Quote from: David Hess on May 16, 2019, 03:05:50 pm ---
--- Quote from: soldar on May 16, 2019, 08:57:32 am ---Will a kill-o-watt indicate negative power? I have never tested that. It would be interesting to connect the load to the prongs and connect the power input to the receptacle and see what happens.
--- End quote ---
I am not sure either. Next time I visit Home Depot, I will get the parts to make a set of cords to hook one up backwards.
--- End quote ---
I figured this would be a good simple experiment for this Memorial Day weekend.
I made a pair of reversed extension cords and hooked the Kill-O-Watt that I have up backwards. It still read positive power flow. Bummer.
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