Electrical Engineering ⇒ Topic : Split Phase Type

Split Phase Type

Split phase type is also called Ferraris type meter. It consists of a laminated magnet with the pairs of poles at right angles to each other as shown in Figure (a). Coils are wound on the poles, the opposite poles being connected in series. The coils on the two pairs of poles are connected in parallel. But one set of coils is connected through an inductance and the other with a high resistance to create a phase difference of 90°. The input to both the pairs of coils is the current to be measured. An aluminium drum is placed in the centre of the yoke and coil. Inside the drum, there is a cylindrical iron laminated core to strengthen the magnetic field.

                                                                                      figure (a)

Eddy currents are induced in the drum by the rotating magnetic field and a torque is produced by the reaction of magnetic field and current. This torque deflects the needle attached to the drum which is connected to the damper.

The torque produced is given by

                                                                                                         ................ (1)

            For a voltmeter

                                                                                                            ................. (2)

             For an ammeter

                                                                                                                 ................... (3)

where I_R is current in resistor, I_L is current in inductor, f is frequency of supply, Z is impedance of eddy current path, θ is phase angle between voltage and current in resistor, and a is phase angle between current resistor and inductor.

Since θ is almost zero and R is very much higher than reactive part of eddy current paths

α is made as high as possible to obtain high torque.