The single-phase energy meter in domestic use works on the
Answer: A
Two alternating fluxes induce eddy currents in an aluminium disc, and the interaction of each flux with the currents due to the other produces a steady torque. The disc therefore rotates continuously, which is what an integrating instrument needs.
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An energy meter differs from a wattmeter in that it
Answer: B
A wattmeter indicates the power at each instant; the meter's disc turns at a speed proportional to power, so the number of revolutions counted by the register is proportional to the energy. It is an integrating instrument rather than an indicating one.
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The shunt magnet of an induction energy meter carries a winding of
Answer: C
The shunt magnet is the pressure element and its winding is highly inductive, so its flux is nearly proportional to the voltage and lags it by close to ninety degrees. The series magnet carries the load current in a few thick turns.
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The braking magnet of an energy meter is
Answer: D
Eddy currents induced in the moving disc by the permanent magnet produce a torque opposing the motion and proportional to the speed. The disc therefore settles at a speed at which braking torque equals driving torque, so speed is proportional to power.
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The steady speed of the disc is proportional to the power because the braking torque is proportional to
Answer: A
Eddy current braking torque rises in direct proportion to speed. Equating it to a driving torque proportional to power gives a speed proportional to power, so the revolutions counted over a period are proportional to the energy consumed in it.
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The registering mechanism of an energy meter is driven from the spindle through
Answer: B
A worm on the spindle drives a train of gears carrying the pointers or cyclometer drums, and the ratio is chosen so that the register reads directly in kilowatt hours. The friction of this train is one of the reasons a light-load adjustment is needed.
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The disc of an induction energy meter is made of aluminium because it is
Answer: C
Low density keeps the inertia and the bearing friction small, and high conductivity allows large eddy currents for a given flux. Aluminium is not magnetic, and it does not need to be: the torque comes from currents induced in it, not from attraction.
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The driving torque of an induction energy meter is proportional to the product of the two fluxes and
Answer: D
The interaction of two alternating fluxes with the eddy currents each induces gives a mean torque proportional to \( \Phi_1 \Phi_2 \sin\alpha \). Making the angle between them the complement of the load phase angle turns the sine into the cosine of that angle and the torque into power.
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For the meter to register active power correctly, the flux of the shunt magnet must lag the applied voltage by
Answer: A
With that lag the angle between the two fluxes becomes ninety degrees minus the load angle, and the sine of it is the cosine of the load angle. The torque is then proportional to \( V I \cos\phi \), which is the active power.
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The lag or quadrature adjustment of an energy meter is provided by
Answer: B
The shunt magnet flux would otherwise lag the voltage by rather less than ninety degrees because of iron loss and winding resistance. A copper loop or shading band whose position can be shifted brings the lag to the required value, and the adjustment is made at unity power factor and again at 0.5.
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