Electrical Engineering ⇒ Topic : Motor Meters
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Sonali
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Motor Meters These are the most widely used meters. They can be used both in AC and DC. It has a small motor whose instantaneous speed is proportional to the circuit current for an ampere-hour meter and power for watt-hour meter. The current or power consumed is proportional to the speed of rotation or number of revolutions by the disk or rotor. The three types of motor meters are: (i) Mercury motor meters, (ii) Commutator motor meters, and (iii) Induction motor meters. In commercial establishments and households, the induction motor meters are widely used.An induction motor meter consists of a current coil in series with the line and load and the voltage coil is across the line. The coils are wound on a metal frame of spherical design. A light aluminium disk is suspended in the magnetic field of the current coil and eddy currents are induced in it. The flux created by the eddy current reacts with the flux of the voltage coil and the disk rotates under a torque at a high speed. In order to brake the high speed into a steady rotating speed, the disk rotates between two small permanent magnets located opposite to each other at the of the disk. The rotating disk cuts magnetic lines of force inducing eddy currents and flux opposing the motion of the disk, thereby creating braking action. The torque developed is proportional to the electric field of the voltage coil and the eddy currents induced in the disk. The shaft of the aluminium disk is connected by the gear arrangement to a digital readout in kWh. Calibration of the watthour meter is done at the full load and 10% of full load. In a polyphase meter, each phase has its own disk but all the disks are mounted on a common shaft. The torque developed by each shaft is mechanically summed. The total three-phase energy consumed is proportional to the total number of revolutions per minute of the common shaft. Suitable positioning and screening of the parts are done to prevent interaction between the elements of the three individual phases. | |
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