different types of errors of pmmc

errors of pmmc

the most common errors in a moving coil instrument are due to

1. ageing of permanent magnets

2. ageing of springs and

3. temperature variations

1. errors due to ageing of permanent magnets

due ageing of permanent magnet.the field produced by it is weakened.this reduces the deflections torque and hence, the reading or deflection obtained is lesser than that of actual one

2. errors due to ageing of springs

due to ageing.the stiffness of the spring reduces results in the reduction of control torque.hence,the deflection obtained in case is greater.the effect due to ageing of magnet and springs are opposite to each other(i.e.,the former results in lesser deflection whereas the latter results in greater deflection)

3. errors due to temperature variations

due to variations in temperature,the stiffness of the spring and flux density in the air gap between the poles varies in such a way to cancel each others effectresultingin slight variations in temperature ,the resistances moving coil of current through the coil.the temperature coefficient of copper is 0.004/'c i.e.. the deflection of the instrument is reduced by 0.045per 'crises temperature

working principal of pmmc instrument with a neat sketch

pmmc(permanent magnet moving coil)

pmmc instrument is used only for D.C measurements.the working of a pmmc instrument is similar to the working of a D'arsonal galvanometer but the difference is that direct reading can be made in pmmc instruments with the help of a pointer and scale.the schematic of pmmc is shown in the following figure

as it consists of a permanent magnet and coil which is moving. it is termed as permanent magnet moving coil(pmmc)

working

this principal of working is same as that of working of a d.c motor. whenever, a current carrying coil is kept in the magnetic field,it experiences a force,and it tends to move.when the current to be measured is made to flow through the coil,a deflecting torque proportional to the flux density and dimensions of the coil is produced which makes the coil to move.as the coil moves,the pointer shows deflection on the scale .the deflection torqueis proportional to the coils current and the magnitude ofthe current flowing through the coil is indicated by a pointer which moves over a graduated scale.when the deflecting torque is equal to the controlling torque,a balance is attained and the pointers shows zero deflection.here,as the deflection is directly proportional to the current in the coil,uniform scale is obtained