I saw an old (single phase) ceb energy meter (a meter used to measure number of electrical units) from 1979 at a friend's place. And we were having a discussion about the meter - if it could be hacked by placing a magnet across it. :)
Image Source: Wikipedia
- Voltage coil, C2
- Current coil, C1
- Stator - concentrates and confines magnetic field
- Aluminum disc, D
- Brake magnets
- Gear mechamism
- Display dials
The disc cuts through the fluxes of these two magnets and therefore two circular eddy currents are generated in the disc. The rotating torque is produced by the interaction of these eddy currents and the fluxes induced by the two electromagnets.
M1 is connected in series and it is also known as the series magnet. It produces an alternating magnetic flux of Φ1, which is proportional to and in-phase with the line current, I.
M2, also known as the shunt magnet, is connected across the supply line and carries a current proportional to the supply voltage, V. Therefore the magnetic flux, Φ2, it generates is proportional to the supply voltage, V. However, Φ2 is not in-phase with the V. In fact, it is set to be 90 ° lagging to V. This is done by having two or three properly adjusted copper rings, C. Some meters use winding with a series connected lag adjusting resisteror.
The current through the M2 winding is V/ωL.
Tdrive = kdrive . ω . (V/ωL) . I sinα
Tdrive = kdrive V.I cosΦ
Tdrive = P
Where α is the phase angle between Φ1 and Φ2, kdrive is the proportionality constant, P is the electrical power consumed and Tdrive is the driving torque on the aluminum disc, D.
The breaking torque, is obtained by two permanent magnets mounted in opposite directions.
Where Tbreak is the breaking torque acting on the aluminum disc, Φbreak is the magnetic flux of the permanent magnets, N is the rotation speed of the aluminum disc, kbreak is the proportionality constant and Reddy is the resistance of the eddy current path.
⇒ N = k . P
The meter counter records the number of revolutions, which is the ∫N.dt over time, t.
In a human readable form, the energy meter measures the power by two electromagnets - one generating a magnetic field related to the Voltage and the other related to the Current. And we know that current and voltage gives power.
My friends have often asked if the meter reading can be altered by placing a strong magnet in front of the meter...
And my answer is no! :)
Why? because the aluminum disc would require a very strong magnetic field to generate higher eddy currents. And if you notice the breaking magnets are placed very close to the Aluminum discs. However there is another path to explore - by adding a alternating magnetic field which may interact with Φ1 and Φ2. Ceylon Electricity Board (CEB) owns the energy meter (kWh meter, electricity meter or whatever you want to call it)! And its illegal to tamper with the meter so be prepared to get screwed!
There was a video on youtube where this guy makes an apparatus and claims that it reduced his electricity bill. He has not provided analytical proof.
The other interesting thing is, these meters only measure the active power, VI cos Φ