The electromechanical watthour meter is basically a very specialized electric motor, consisting of
• A stator and a rotor that together produce torque
• A brake that creates a counter torque
• A register to count and display the revolutions of the rotor

Single Stator Electromechanical Meter
A two-wire single stator meter is the simplest electromechanical meter. The single stator consists of two electromagnets.

One electromagnet is the potential coil connected between the two circuit conductors. The other electromagnet is the current coil connected in series with the load current.

Figure 7.1 shows the major components of a single stator meter.

The electromagnetic fields of the current coil and the potential coil interact to generate torque on the rotor of the meter. This torque is proportional to the product of the source voltage, the line current, and the cosine of the phase angle between the two. Thus, the torque is also proportional to the power in the metered circuit.

The device described so far is incomplete. In measuring a steady power in a circuit, this meter would generate constant torque causing steady acceleration of the rotor. The rotor would spin faster and faster until the torque could no longer overcome friction and other forces acting on the rotor.

This ultimate speed would not represent the level of power present in the metered circuit. To address these problems, designers add a permanent magnet whose magnetic field acts on the rotor.

This field interacts with the rotor to cause a counter torque proportional to the speed of the rotor. Careful design and adjustment of the magnet strength yields a meter that rotates at a speed proportional to power.

This speed can be kept relatively slow. The product of the rotor speed and time is revolutions of the rotor. The revolutions are proportional to energy consumed in the metered circuit.

One revolution of the rotor represents a fixed number of watthours. The revolutions are easily converted via mechanical gearing or other methods into a display of watthours or, more commonly, kilowatthours.

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