Electrical Instruments & Measurements PDF

# Section 1 Electrical Instruments & Measurements ## Deflection-Type Method In deflection type instruments, the value of the quantity being measured is displayed in terms of the amount movement of a pointer. The pressure-measuring device shown is an example of a deflection type instrument. ## Defl...

# Section 1 Electrical Instruments & Measurements ## Deflection-Type Method In deflection type instruments, the value of the quantity being measured is displayed in terms of the amount movement of a pointer. The pressure-measuring device shown is an example of a deflection type instrument. ## Deflection Instruments Fundamentals ### Examples * Permanent Magnet Moving Coil (PMMC) instruments * Electro-dynamic instruments * Moving iron instruments ## Permanent Magnet Moving Coil (PMMC) Instruments The PMMC Inst is the most common used as deflection type instrument. ## PMMC Instrument Construction * A permanent magnet with two soft-iron pole shoes * A cylindrical soft-iron core is positioned between the shoes * One of the two controlling spiral springs is shown * One end of this spring is fastened to the pivoted coil, and the other end is connected to an adjustable zero-position control * The current in the coil must flow in one direction to cause the pointer to move from the zero position over the scale * It cannot be used directly to measure alternating current. Without rectifiers, it is purely a dc instrument ## Permanent Magnet Moving Coil Instrument A mirror is placed below the pointer to get the accurate reading by removing the parallax. ## PMMC Instrument Is Called A Polarized Instrument * Its deflection depends on the polarity of its input quantity. * It cannot be used to measure an (ac) directly, but a rectifier must be used firstly to convert (ac) quantity to (dc) quantity before applying it to instrument. ## Advantages Of The PMMC Instruments * Linear scale * Simple and cheap * Can be constructed with very high sensitivity (specially if taut band suspension is used) ## Disadvantages Of The PMMC Instruments * The moving coil instrument can only be used on D.C * External magnetic fields badly affect its operation. This can be avoided by using core magnet type PMMC construction. * Not very sensitive (to have sensitive device the taut band suspension must be used: which is expensive) ## DIFFERENT REASONS THAT CAUSE AN ERROR IN PMMC * Temperature effect: Error in the reading of the PMMC may cause due to change in the temperature which will affect the resistance of the moving coil * Spring material and age: The other factor which may lead to error in the PMMC reading is the quality and contortion of the spring. Old ageing spring will not allow the pointer to show the correct reading making an error * Ageing of Magnet: Along with the age, the effect of heat and vibration will reduce the magnetic effect of the permanent magnet which will produce an error in the reading ## CAN PMMC BE USED TO MEASURE AC? If the frequency is low enough, the PMMC with the addition to a rectifier can be used to measure AC which converts the measured quantity into a dc current typically less than 1mA. Add an appropriate scale and you have a meter. ## WHAT WILL HAPPEN IF I USE IT WITH HIGH-FREQUENCY AC? If the frequency of ac is high, the meter just vibrates in and around the zero value (preferable to verify with a centre zero meter), and finally stops responding to ac.