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Deflection Instruments Fundamentals

This chapter provides an overview of deflection instruments fundamentals, including the principles of deflection, controlling, and damping forces. Examples such as Permanent Magnet Moving Coil (PMMC) instruments, electro-dynamic instruments, and moving iron instruments are discussed.

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Deflection Instruments Fundamentals

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  1. Chapter Three Classical Electromechanical Instrument

  2. Deflection Instruments Fundamentals • They have a pointer deflects over its scale to indicate the quantity to be measured. • Three forces are operating inside the instrument.{Deflecting, Controlling, and Damping Forces}

  3. Deflection Instruments Fundamentals Examples • Permanent Magnet Moving Coil (PMMC) instruments, Electro-dynamic instruments, Moving iron instruments

  4. Deflection Instruments Fundamentals

  5. Deflection Instruments Fundamentals • Deflecting force deflectsthe pointer to a deflecting angle proportional to the input quantity to be measured. • Its direction is towards the full scale deflection angle. • Itsmagnitude is proportional to the input quantity to be measured.

  6. Deflection Instruments Fundamentals

  7. Deflection Instruments Fundamentals Controlling force is generated due to two Spiral control springs in case of Jewel bearing suspension.

  8. Deflection Instruments Fundamentals Where as the taut band control force is generated in case of taut band suspension

  9. Deflection Instruments Fundamentals • Controlling force Stopsthe pointer at its exact - finalposition. • It Returnsthe pointer to its zero position . • Its magnitude is proportional to the angle of deflection Φ

  10. Deflection Instruments Fundamentals The correct damping have a fast and zero oscillation of the pointer movement.

  11. Deflection Instruments Fundamentals

  12. Deflection Instruments Fundamentals • Magnitudeof the damping forceis proportional to the pointer acceleration • The direction of the eddy current damping forceopposes the motion of the coil.

  13. Methods Of Supporting The Moving System Of Deflection Instrument

  14. Suspension The pointed ends of pivots fastened to the coil are inserted into cone-shaped cuts in jewel bearings

  15. Suspension • Some jewel bearings are spring supported to absorb such shocks more easily

  16. Suspension - The most sensitive jeweled-bearing instruments give full scale deflection (FSD) with a coil current of 25 µA

  17. Suspension Two flat metal ribbons (phosphor bronze or platinum alloy) are held under tension by springs to support the coil

  18. Suspension The ribbons also exert a controlling force as they twist, and they can be used as electrical connections to the moving coil

  19. Suspension With taut-band suspension instruments give FSD with a coil current may be little as 2 µA .

  20. Permanent Magnet Moving Coil (PMMC) Instruments The PMMC Inst. Is The Most Common Used As Deflection Type Instrument.

  21. ConstructionPMMC Instrument -A permanent magnet with two soft-iron pole shoes - A cylindrical soft-iron core is positioned between the shoes

  22. ConstructionPMMC Instrument • 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.

  23. ConstructionPMMC Instrument -The current in the coil must flow in one direction to cause the pointer to move from the zero position over the scale.

  24. ConstructionPMMC Instrument - The terminals (+) and (–) indicate the correct polarity for connection, and the instrument is said to be polarized

  25. ConstructionPMMC Instrument -Itcannot be used directly to measure alternating current Without rectifiers, it is purely a dc instrument

  26. Permanent Magnet Moving Coil Instrument • The mirror is placed below the pointer to get the accurate reading by removing the parallax.

  27. Torque Equation & Scale

  28. Torque Equation & Scale The force F affecting on both sides of the Coil ( N turns)┴ to B. F = BILN They produce a deflecting torque Tdef Tdef = BILND Tdef = BINA = CdefI

  29. Torque Equation & Scale Where A is the area of one turn of the coil [m2], Cdef = BNA is the deflection constant …......[Nm/Ampere]

  30. Torque Equation & Scale As Tcon αΦ Tcon = Ccon Φ Where Ccon is the control constant [Nm/degree]. At final position of the pointer: (Tdef) = (Tcon) Then Φ = KI Where K = Cdef / Ccon

  31. Torque Equation & Scale • Conclusion: • The pointer deflection is linearlyproportional to I . • The PMMC scale is linear (equally spaced). • If the current changes its direction(-ve current), the pointer will deflect off the scale.

  32. 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.

  33. Advantages Of The PMMC Instruments • Linear scale . • Simple and cheap. • Can be constructed with very high sensitivity (specially if taut band suspension is used).

  34. Disadvantages Of The PMMC Instruments • Polarized • 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).

  35. Example: A PMMC inst. with a 100-turncoil has a magnetic flux density in its air gaps of B= 0.2T. The coil dimensions are D=1 cm and L=1.5 cm. calculate the torque Tdef on the coil for a current of 1mA and its deflection constant (Cdef). If the device constant K = 12x103degree/A, find: The spring (control) constant Ccon. Find the angle of deflection (Φ) for the input currents: 1,2,4, and 8mA. Conclude your results.

  36. Solution: Tdef =BILND [Nm] Tdef=0.2x1x10-3 Ax1.5x10-2mx100x1x10-2m =3x10-6 Nm. Tdef = CdefI Cdef = Tdef / I =3x10-6 /1x10-3 Nm/ A K= Cdef / Ccon Ccon= (3x10-3 Nm/A)/ (12x103 0/A) Nm/degree. Φ = K I =12x103x1x10-3=120 = 12x103x2x10-3=240 = 12x103x4x10-3=480 =12x103x4x10-3=960

  37. Solution: Conclusion: If the current is doubled the angle of deflection is doubled (i.e. PMMC has a linear scale)

  38. Electrodynamics' Instruments Deflecting force is generated due to the interaction between a magnetic field generated due to a …………….

  39. Electrodynamics' Instruments Air vane chamber used to generate the air damping force (torque).

  40. Electrodynamics' Instruments As the pointer moves the air van moves in the closed chamber and an air damping torque is generated .

  41. AC Operation Of Electrodynamics' Instruments

  42. AC Operation Of Electrodynamics' Instruments

  43. Torque Equation And Scale TdefαI1I2 Tdef= CdefI1I2 where Cdef is the deflection constant [Nm/A2] As Tcon αΦ Tcon = Ccon Φ where Cconis the control constant [Nm/degree] But at final position of the pointer: Tdef = Tcon, then: Φ = KI1I2 Where K = Cdef / Ccon is the device constant [degree/A2] If I1 = I2 then Ф =K I2

  44. Advantages Of The Electro-dynamic Instruments • Non polarized

  45. Disadvantages Of The Electro-dynamic Instruments • Its scale is non linear. • Low frequency AC measurements. • Less sensitive than the PMMC instrument.

  46. Example For an electro-dynamic instrument used as an ammeter, calculate the deflection torque (Tdef) on its moving coil for a current 1mA if its deflection constant (Cdef) equals to 5x103 Nm/A2. if the device constant (K) equals to 5x105 o/A2, find: • The spring (control) constant (Ccon). • The angle of deflection (Ф) for the input currents: 1,2,4,8 mA. Conclude your results.

  47. Solution Tdef= Cdef I2= 5x103x(1x10-3)2 = 5x10-3 Nm K = Cdef / Ccon Ccon = 5x103 / 5x105 =1x10-2Nm/degree Ф =K I2 = 5x105x(1x10-3)2 = 0.5o = 5x105x(2x10-3)2 = 2o = 5x105x(4x10-3)2 = 8o = 5x105x(8x10-3)2 = 32o

  48. Solution Conclusion of the example: If the current is doubled, the angle is multiplied by 4. (i.e the electro-dynamic ammeter has a non linear scale).

  49. Moving Iron Instruments: Deflecting force is generated due to the repulsion force generated between a movable and a fixed iron pieces placed in a magnetic field which is generated due to passing current in stationary coil.

  50. Moving Iron Instruments: The deflecting force causes the rotation of the movable iron piece to which the pointer is connected.

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