The ac wattmeter is based on an alternating rotating magnetic field with a phase difference of 9a0 between the current coil and the voltage coil, and is rotated by an axial aluminum disk rotor through the air gap in the magnetic field induced by eddy current. Due to both magnetic flux through the disc parallel to the drive shaft, the resulting torque M = KUIcos Φ, is proportional to the reactive power, under the action of magnetic brake, the disc rotation speed which stands for the size of the consumed power, electrical equipment which read electricity from a counter accumulative frequency, namely kilowatt hours.
Since the current coil is taken from the same power source, when the load is pure resistance, the magnetic field generated by the two will strictly maintain the phase Angle of 90°, and the disk rotor can always operate normally. However, if the load contains inductance or capacitance, the phase difference of the magnetic field will deviate by 90°(greater or less). The speed of the disk rotor slows or even reverses. If the phase difference is 0° or 180°, the rotational torque is zero.
In some single-phase or two-phase electrical equipment more occasions, its phase current is definitely extremely unbalanced. In this case, the current transformer of the tempering furnace watt-hour meter shall be connected to the watt-hour meter by a separate wire, and the Central Line shall not be used as a loop nearby. Because, in the work of many single (two) phase equipment, because the current between the three phase lines is extremely unbalanced, that is, there is a large current through the center line, resulting in a voltage drop. Between the points in the midline. The potential difference that exists. The device's main power supply is thus cut off. However, as a result of the operation of nearby equipment, the transformer will pick up the potential difference (U}) produced at the two points in the middle line and form a current, causing the meter to rotate.
It should be noted, in particular, that the current generated by this potential difference. Its phase is random, and it will constantly change with the rise and fall of the current in each phase of the neighboring equipment. Therefore, the speed of the meter turntable will reverse from time to time, which is very similar to the load component with inductance and capacitance, resulting in additional errors, which reduces the accuracy of measurement. Attention must be paid.
On 2018-08-08 11:45:46
