Question

Q2i) A PMMC voltmeter of range (0-550V) with a resistance of 10 ohms has a coil of effective depth of 3 mm and a width of 2.5 cm, the coil has about 100 turns. The flux density spread in the air gap is of 0.01Wb/m². The spring constant is considered to be 3.2*10* Calculate, Control torque produced by the coil, when it is connected to 37V to measure. [2.5] b. The angle of deflection that the pointer takes, if the field produces a flux [2.5] density of 50mWb/m' for input voltage of 75V. a. ii) Discuss the scheme of converting PMMC meter into ammeter and voltmeter and also mention the importance of connecting resistance in series with moving coil. [2]

Answer 1

02)) given PMMC Volmeter öt range C6-5500) with reçiçtance R = 10-2 depth I width - 3mm = 205 cm Area A = 3 x 10 3 x 2.5x16 เอ้ No. of turns (N): - 100 flua density 0.01 Wb l m 2 The Spring constant Kc = 3.2 x 10 4 a) control torgue Tc when voltmeter is connected to 37 v I - 37 3.7 4. 10 Debleclica Torque (Id ) = BINA where, dencity B = Magnetic field intensity CS Scanned with CamScanner

I - current throupb Moving coil turns. Сь Moving coil Area of coil. Taa 0.01 x 3.7 x 100 x 7,5 x lo 5 Ta= 27.75 5 X 10 N et steady State To = Tc Tc 27.7.5 xion or, O, 2775 MN. b ) ot, BE 50m wb/mos?, and voltage applied ve 750, os? then, Į 75 7.54 angle of detection, 10 Td BINA CS Scanned with CamScanner 50 x 103 x 7.5 100 * 7.5 x 105

Ta = 5 x 7,5 * 7.5 x los 281,25 x 105 5 n -At Steady state Te Td C Teao kao BINA Тc - ko BINA k c o angle angle of deflection kea spring constant 0 281025 x 105 = 8.78 radian 3.2 x tot CS Scanned with CamScanner

ii) Simplified diagram of pmme inftrument pmmc Rm 1 A 1 where Rm is moving coil recicroogee Cinternal resiętance) Pmme instrument into ammeter let us say Range of Ammo instrument is co-In).m mon with internal Reçiętance Rm This pmme inetreement Ammeter can be converted as Voll-meter by Connecting asheedt resistance croce pmme instrument. Network esuivalent circuit of Ammeter 1 where T is the I - Im Im š Full scale Rch Rm Jalue Of the ymmeter. I CS Scanned with CamScanner

Co- Imma PMmc instrlem ent converted into co-I) A Ammeter. by using circuit (I - Im) Rsh т om Rsh Rm (Im-) R Reh = T Im m so, to convert PMMC instreement into desired Range 06 Ammeter then, shunt recitance (Rs) of suitable value should be added. pm me instrument into u olt meter is co-Im) mA Range Of PMmc instrument let us say Scanned with CamScanner CS

with internal resistance Rol This Pmme instreement can be Conterte b V014 weken biry Adding series recictance the emme instrumout Network equivalent equivaleut ckt of Ucumeter: 1 co-v) voltç is where . v-um & Ree ] range of voltmeter. - From Network Vm R se Rm Rse Rm centim Rse = Rm [m 1] multiplication doctor. CS Scanned with CamScanner

- so to convert pmmic incarcement into voltmeter A series recictance (Rse) should be added. It we increaçe the value Rse, Range of Voltmeter increaces. CS Scanned with CamScanner