Metraji Dîwarên Cîhêrên Dawer (PMMC)

Pîv Deyar Pêşkên Bi Mînayê Yekê (PMMC)

Pîv deyar pêşkên bi mînayê yekê (PMMC) - an jî hewceyek da ku di navê D’Arsonval an galvanometr bêne - instrument e ku te hewceya ku hûn bigihînin ku amperajê di navê pêşkên de bi serdandina pêşkên di mezuna mînayê yekê de.

Pîv deyar pêşkên bi mînayê yekê (PMMC) pêşkên têlê (yani birikdar) dide di navên du mînayê yekan de ku mezuna mînayê yekan werin çêkirin. Di navê Faraday’s Laws of electromagnetic induction de, birikdar yek ê ku amperaj li ser wekhev dikin di mezuna mînayê de, hewceyek ji bo serdandina wê di navê Fleming’s left hand rule de hatiye.

Girra (zorî) ya vê hewceyê berbar be amperaj li ser têlê. Birikdar yek ê ji bo serdandina pêşkên hatiye û li ser şeviya ekirin. Heta ku torqyan berbar ne, pêşka cihazê dikeve û serdandina pêşkê bi şeviya ekirin. Ji bo ku mînaya yekan uniform û spring linear be, serdandina pêşkê an jî linear e. Li vir, hûn dikarin vê girra berbar bikin bi amperaj li ser têlê.

Cihazên PMMC (yani D’Arsonval meters) tenha ji bo pîvan amperajê Direct Current (DC) bikaranîn. Ji bo ku hûn Alternating Current (AC) bikaranîn, serokê amperaj li ser negatîf half cycle de vegere, û serokê torque an jî vegere. Vê li vir, nîşanê torque zero ye - ji bo ku net movement against the scale tune.

Berî vê, cihazên PMMC dikarin amperajê DC bixweber pîvin.

Çêkirina Cihazê PMMC

Cihazê PMMC (yani D’Arsonval meters) ji 5 komponentên sernavî çêkirin:

• Bîrên Stasionar û Sisteman Mînayê Yekan
  

• Pêşka Çalak
  

• Sisteman Kontrol
  

• Sisteman Damping
  

• Meter
  

Bîrên Stasionar û Sisteman Mînayê Yekan

Di dema niha de, mînayên bi zorîya mezun û zorîya coercive yekan bikaranîn, ji bo ku mînayên U-shaped yekan bikaranîn ku pole pieces soft iron ne. Mînayên ku niha bikaranîn di navên malperên wek alcomax û alnico de ne, ku zorîya mezun derbas dandin.

Pêşka Çalak

Pêşka çalak dibe ku bi serdandina her du mînayê yekan deket. Pêşka ji girtên zi û zêde têlê kurdî û li ser aluminium rectangular pivoted e ku jeweled bearings e.

Sisteman Kontrol

Spring generally acts as control system for cîhazên PMMC. Spring also serves another important function by providing the path to lead current in and out of the coil.

Sisteman Damping

Hewceyek damping û torque ji ser serdandina former aluminium di mezuna mînayê yekan de ku ji bo mînayên yekan çêkirin.

Meter

Meter of these instruments consists of light weight pointer to have free movement and scale which is linear or uniform and varies with angle.

Equation Torque PMMC

Let us derive a general expression for torque in permanent magnet moving coil instruments or PMMC instruments. We know that in moving coil instruments the deflecting torque is given by the expression:

• Td = NBldI where N is number of turns,

• B is density flux mînayê di navên air gap,

• l is the length of moving coil,

• d is the width of the moving coil,

• I is the electric current.

Now for a moving coil instrument the deflecting torque should be proportional to current, mathematically we can write Td = GI. Thus on comparing we say G = NBIdl. At steady state we have both the controlling and deflecting torques are equal. Tc is controlling torque, on equating controlling torque with deflection torque we have

GI = K.x where x is deflection thus current is given by

Since the deflection is directly proportional to the current therefore we need a uniform scale on the meter for measurement of current.

Now we are going to discuss about the basic circuit diagram of the ammeter. Let us consider a circuit as shown below:

The current I is shown which breaks into two components at the point A. The two components are Is and Im. Before I comment on the magnitude values of these currents, let us know more about the construction of shunt resistance. The basic properties of shunt resistance are written below,

The resistance elektrik of these shunts should not differ at higher temperature, it they should posses very low value of temperature coefficient. Also the resistance should be time independent. Last and the most important property they should posses is that they should be able to carry high value of current without much rise in temperature. Usually manganin is used for making DC resistance. Thus we can say that the value of Is much greater than the value of Im as resistance of shunt is low. From the we have,

Where, Rs is resistance of shunt and Rm is the electrical resistance of the coil.

From the above two equations we can write,