Electronics > Projects, Designs, and Technical Stuff
AC Household Power Meter
ruko:
Hello
I understand non electronic power meters, the ones with a spinning disk, can and do run backward if fed power from a solar array or some other home source. Since the current is alternating through the meter no matter the source, flowing first one way then the other, how does the meter differentiate between the mains and the home source and run backward?
Thanks
ruko
dmills:
They have two coils, and the torque driving the disk is the product of the two magnetic fields.
One coil in connected in series with the line to sense current and the other across the line to sense voltage...
If you think about it, with the power flowing one way voltage * current is positive on average and for power flowing the other way voltage * current is negative on average... It is a gloriously elegant bit of applied physics, that is actually surprisingly difficult to emulate well with a purely electronic solution.
Regards, Dan.
soldar:
--- Quote from: ruko on January 28, 2019, 11:09:22 pm --- Since the current is alternating through the meter no matter the source, flowing first one way then the other, how does the meter differentiate between the mains and the home source and run backward?
--- End quote ---
When the energy is flowing into the house the current is in phase with the voltage. When the power is flowing in the opposite direction then the current is in opposite phase. Logically the meter will spin in the opposite direction. Note though that most meters have a ratchet which prevents backwards spinning.
ruko:
Quote: "power flowing the other way" Isn't current flowing in both directions with AC? Still confused.
Thanks
IDEngineer:
I'll openly admit: I have a similar mental block regarding simple VSWR measuring circuits. They accomplish this in a low-precision way even in very inexpensive ham radios with a couple of diodes, yet I have yet to wrap my head around how it works since it's obviously AC that is being generated and sent to the antenna. I understand the RF coupling, and that each diode rectifies the inductively coupled current in its associated direction, but since everything is AC it would seem that (at least for a CW signal, to keep it simple) the two resulting signals would always be equal regardless of mismatch.
I suppose it could be a summation thing, where the reflected portion sums with the forward portion, but then performance would be phase sensitive depending upon coax length and propagation velocity. I've never seen any mention of such SWR circuits being frequency dependent, despite that coax runs can be anything from almost zero (handhelds and mobiles) to hundreds of feet (tower mounted antennas). Depending upon the frequency, such lengths can be significant modulo percentages of the wavelength.
The spinning disk power meter doesn't phase me at all (pun intended!). But I can't quite grok how SWR can be measured with two diodes on a signal that is zero-centered AC. Everyone has a blind spot, and this is mine.
Navigation
[0] Message Index
[#] Next page
Go to full version