Electrodynamometer type instrument.pptx

Electrodynamometer type instrument

1. Electronics Measurement and Instrument Electrodynamometer type instrument By Prof. Chandra Prakash

2. Electrodynamometer type Instrument   An electrodynamic instrument is a moving-coil instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil. This instrument can be used either as an ammeter or a voltmeter but is generally used as a wattmeter.   It can be used for measurement of current, voltage & power for both AC and DC up to 125Hz and with some modifications as power factor & frequency meters. Accuracy is very high and hence can be used in laboratory for calibration of other instruments.

3. The  electrodynamometer  is a transfer-type instrument. A transfer-type instrument is one that may be calibrated with a dc source and then used without modification to measure AC. This requires the transfer type instruments to have the  same accuracy for both DC and AC . If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same time, and the turning force continues in the original direction. Since the reversing the current direction does not reverse the turning force, this type of instruments can be used to measure AC or DC current, voltage, or its major application as a wattmeter for power measurement.

4. Construction The construction of the Electrodynamometer includes fixed coil, moving coil, control, damping, scales, and pointer.

5. Fixed Coils: The field is produced by a fixed coil. This coil is divided into two sections to give a more uniform field near the centre and to allow passage of the instrument shaft. Fixed coils are usually wound with heavy wire carrying the main current in ammeters and watt meters. The wire is stranded where necessary to reduce eddy current losses in conductors. Moving Coil: A single element instrument has one moving coil (pressure or voltage coil). The moving coil is wound either as a self-sustaining coil or else on a non-metallic former. A metallic former cannot be used as eddy current would be induced in it by the alternating field. It should be noted that both fixed and moving coils are air cored.

6. Control: The controlling torque is provided by two control springs. These springs act as leads to the moving coil. Moving System: The moving coil is mounted on an aluminum spindle. The moving system also carries the counter weights and truss type pointer. Sometimes a suspension may be used in case a high sensitivity is desired. Damping: Air friction damping is employed for these instruments and is provided by a pair of aluminum vanes, attached to the spindle at the bottom. These vanes move in sector shaped chambers. Eddy current damping cannot be used in these instruments as the operating field is very weak (on account of the fact that the coils are air cored) and any introduction of a permanent magnet required for eddy current damping would distort the operating magnetic field of the instrument.

7. Shielding: The field produced by the fixed coils is somewhat weaker than in other types of instruments. It is nearly 0.005 to 0.006 Wb/m. In D.C. Measurements even the earth magnetic field may affect the readings. Thus it is necessary to shield an electrodynamometer type instrument from the effect of stray magnetic fields. Air cored electrodynamometer type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy. This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication.

8. Working Principle Electro-dynamometer type instruments are very similar to PMMC type instrument in which the operating field is produced, not by a permanent magnet but by another fixed coil (usually two fixed air cored coils are used). The PMMC instrument cannot be used on A.C currents or voltages. If A.C supply is given to these instruments, an alternating torque will be developed. Due to moment of inertia of the moving system, the pointer will not follow the rapidly changing alternating torque and will fail to show any reading. In order that the instrument should be able to read A.C quantities, the magnetic field in the air gap must change along with the change in current. This principle is used in the electro-dynamometer type instrument.

9. Torque Equation Let, i 1  = instantaneous value of current in the fixed coils, (A) i 2  = instantaneous value of current in the moving coils, (A) L 1  = self-inductance of fixed coils, (H) L 2  = self-inductance of moving coil, (H) M = mutual inductance between fixed and moving coils (H)

10. Flux linkage of Coil 1,  ψ 1  = L 1  i 1  + Mi 2 Flux linkage of Coil 2, ψ 2  = L 2  i 2  + Mi 1 Electrical input energy  = e 1  i 1  dt + e 2  i 2  dt = i 1  dψ 1  + i 2  dψ 2 As e 1  = d ψ 1 /dt and e 1  =  dψ 2 /dt Electrical input energy  = i 1  d( L 1  i 1  + Mi 2  )+ i 2  d(L 2  i 2  + Mi 1 ) = i 1 L 1 di 1  + i 1 2 dL 1  + i 1 i 2 dM + i 1 Mdi 2  + i 2 L 2 di 2  + i 2 2 dL 2  + i 1 i 2 dM + i 2 Mdi 1 Since L 1  and L 2  are constant, therefore dL 1  = 0 and dL 2  = 0 = i 1 L 1 di 1  + i 1 i 2 dM + i 1 Mdi 2  + i 2 L 2 di 2  + i 1 i 2 dM + i 2 Mdi 1  …………(1) Energy stored in the magnetic field  = ½ i 1 2 L 1  + ½ i 2 2 L 2  + i 1  i 2 M Change in energy stored = d( ½ i 1 2 L 1  + ½ i 2 2 L 2  + i 1  i 2 M) = i 1 L 1 di 1 + i 2 L 2 di 2 + i 1 Mdi 2  + i 2 Mdi 1 + i 1 i 2 dM+(i 1 2 /2)dL 1  + (i 2 2 /2)dL 2 But L 1  and L 2  are constant, therefore dL 1  = 0 and dL 2  = 0 = i 1 L 1 di 1 + i 2 L 2 di 2 + i 1 Mdi 2  + i 2 Mdi 1 + i 1 i 2 dM ……(2)

11. From the principle of conservation of energy, Total electrical input energy = Change in energy in energy stored + mechanical energy The mechanical energy can be obtained by subtracting above equations Therefore, mechanical energy = i 1  i 2 dM suppose  T d   is the deflecting torque and  dθ  is the change in deflection, then, Mechanical energy = work done =  T d   dθ Thus we have, T d   dθ = i 1  i 2 dM T d   = i 1  i 2  ( dM / dθ ) controlling torque  T c  = k θ   where k = spring constant At final steady position  T d  = T c I 1  I 2  ( dM /d θ) = k θ Deflection, θ = ( I 1  I 2 /k)( dM /d θ)

12. Advantages Can be used for both a.c. as well as d.c. measurements (transfer instrument) Accuracy is very high (can be used for calibration of other instrument). Free from hysteresis and eddy current loss (coils are air cored) As the instrument exhibit square law response the angular deflection of these instrument is directly in terms of RMS value of the a.c parameter under measurement.

13. Disadvantages Low torque to weight ratio hence have low sensitivity. The scale is non-uniform. Highly expensive as compared other instrument for ammeter, voltmeter. therefore used for precision measurement. Frictional losses are high. Higher power consumption as compared to PMMC instrument.

14. Sources of Errors Error due to low torque to weight ratio. Error due to temperature change Error due to eddy current. Error due to external magnetic field Error due to frequency