Classical Electromechanical Instrument Chapter Three Classical Electromechanical Instrument

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}

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

Deflection Instruments Fundamentals

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

Deflection Instruments Fundamentals

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

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

Deflection Instruments Fundamentals Controlling force Stops the pointer at its exact - final position. It Returns the pointer to its zero position . Its magnitude is proportional to the angle of deflection Φ

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

Deflection Instruments Fundamentals

Deflection Instruments Fundamentals Magnitude of the damping force is proportional to the pointer acceleration The direction of the eddy current damping force opposes the motion of the coil.

Methods Of Supporting The Moving System Of Deflection Instrument

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

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

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

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

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

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

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

Construction PMMC Instrument -A permanent magnet with two soft-iron pole shoes - A cylindrical soft-iron core is positioned between the shoes

Construction PMMC 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.

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

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

Construction PMMC Instrument - It cannot be used directly to measure alternating current Without rectifiers, it is purely a dc instrument

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

Torque Equation & Scale

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 = Cdef I

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

Torque Equation & Scale As Tcon α Φ 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

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

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 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).

Example: A PMMC inst. with a 100-turn coil 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 = 12x103 degree/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.

Solution: Tdef=0.2x1x10-3 Ax1.5x10-2mx100x1x10-2m =3x10-6 Nm. Tdef =BILND [Nm] Tdef=0.2x1x10-3 Ax1.5x10-2mx100x1x10-2m =3x10-6 Nm. Tdef = Cdef I 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

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

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

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

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

AC Operation Of Electrodynamics' Instruments

AC Operation Of Electrodynamics' Instruments

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

Advantages Of The Electro-dynamic Instruments Non polarized

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

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.

Solution Tdef= Cdef I2 = 5x103x(1x10-3)2 = 5x10-3 Nm K = Cdef / Ccon Ccon = 5x103 / 5x105 =1x10-2 Nm/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

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).

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.

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

Galvanometer Instrument A Galvanometer is a PMMC instrument designed to be : Very sensitive to very low currents Null detectors and its zero deflection is in mid-scale.

Galvanometer Instrument In order to achieve these requirements, Taut band suspension is used to increase its sensitivity. light beam pointer is used. using shunt damping resistor which controls the level of eddy currents.

Galvanometer Instrument Critical damping resistance value which gives just sufficient damping to allow the pointer to settle down quickly with a very small short lived oscillation. Rcd = SV / SI [Ω, μV /mm / μA /mm]

Galvanometer Instrument Current Sensitivity (SI) [μA /mm] It is amount of current (μA ) flowing through the instrument to give deflection (mm). .

Galvanometer Instrument Voltage Sensitivity (SV) [μV /mm] It is expressed for a given value of a damping resistance as follows: . SV = Rcd SI [μV /mm, Ω μA /mm]

Galvanometer Instrument Megohm sensitivity (SMΩ) [MΩ] It is the resistance connected in series with the instrument to restrict the deflection to one scale division for 1 V potential difference between its terminals . SV = Rcd SI [μV /mm, Ω μA /mm]

Protection Of The Galvanometer When the galvanometer is used as a null detector (e.g. across the diagonal of a Wheatstone bridge), initially high current may passes across it due to the unbalance of the bridge.

Protection Of The Galvanometer :

Example A galvanometer (a) has a current sensitivity of 1 μA/mm and a critical damping resistance of 1KΩ. Calculate its voltage sensitivity and megohm sensitivity. Another galvanometer (b) deflects by 2 cm when its current is 10 μA, find its current sensitivity, megohm sensitivity, and critical damping resistance if its voltage sensitivity is 2mV/mm.

Solution (a) SV = Rcd SI = 1KΩ x 1 μA/mm = 1 mV /mm SMΩ = 1 V/mm / SI = 1 V/mm / 1 μA/mm = 1 MΩ (b) Si = current (μA) / deflection in mm = 10 μA / 20 mm = 0.5 μA/mm SMΩ = 1 V/mm / SI = 1 V/mm / 0.5 μA/mm = 2 MΩ Rcd = SV / SI = 2 mV/mm / 0.5 μA/mm = 4 KΩ