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