R.N.G.P.I.T, Bardoli �Electrical Engineering Department

Subject: EMMI

​

​

Prepared By:

Dr. Shaikh Mohammed Suhel

Prepared By:

• Name: Dr. Shaikh Mohammed Suhel

(Assistant Professor RNGPIT, Surat)

• Former Assistant Professor in SCET, SNPIT&RC, VIT
• Qualification: PhD (Power- Electronics & Drives, NIT, Surat), M.Tech (Industrial Electronics, NIT-Surat), Gate, B.E. (Electrical Engineering., VNSGU-Surat).
• Experience: 13 Years.

​

​

​

​

CH: Analog Instrumentation

• This Lecture contain
• Introduction to analog instrumentation
• Details discussion on PMMC instrument

• Classification of analog instrumentation:
• (a)Absolute instrument
• (b)Secondary instrument
• Absolute instrument: give the value of electrical quantity in terms of constant K deflection.
• No comparison with other instrument required
• Only used in standard laboratories.
• Ex: tangent galvanometer gives the value of current in terms of tangent of deflection product (tanθ), the radius (r), number of turns (n), and horizontal components of earth field (H)

• Secondary instrument:
• (a)Indicating instrument
• (b)Integrating instrument
• (c)Recorded Type

​

• Indicating Instrument: Directly indicate the value of the electrical quantity being measured at the instant of its measurement.
• Example: Ammeter, voltmeter, wattmeter

​

• Secondary instrument:

​

• Integrating Instrument: They measured the total quantity of electricity (Amp-hours) or energy (watt-hours) in a given time.

​

​

• Example: Energy meter.

​

• Secondary instrument:

​

• Recording Instrument: Gives a continuous record of the quantity being measured over a specific period.

​

​

• Example: Recording voltmeter are used in generating station to record the voltage of supply mains during the day.

​

• Essentials of indicating instrument:

​

• (a) Deflecting (or operating )torque.
• (b) Controlling (or restoring) torque.
• (C) Damping torque

​

• Deflecting Torque: Cause the moving system to move from zero position to indicate the value of electrical quantity being measured
• If deflecting torque is acting along, the pointer would continue to move maximum deflecting position irrespective of magnitude of input quantity to be measured.

​

• Essentials of indicating instrument:

​

• Controlling Torque (Tc): Must oppose the deflecting torque (Td) and should increase with deflection of pointer.
• The pointer will come to rest at position when

Tc=Td.

1. To produce torque equal and opposite to the deflecting torque so that the final position of the pointer on the scale will be according to the magnitude of input quantity.
2. To bring the moving system back to zero position when deflecting torque is removed

(a)Spring control (b) gravity control

• Essentials of indicating instrument:

​

• Damping Torque :

​

• (a)Air damping
• (b)Fluid damping
• (c)Eddy current damping

• Eddy Current damping:

​

• Permanent magnet moving coil type instrument:
• This is the most accurate instrument for measurement of voltage and current
• Principle :
• The basic working principle is same as that of D’arsonal type galvanometer
• When a current carrying conductor is placed in a magnetic field, it experiences a force which tends to move out of the field.

• Permanent magnet moving coil type instrument:
• To get uniform flux density concentric cylindrical shape pole should be used. Flux pass radially and perpendicular to motion at every point

​

• Let,
• B=Flux density in the air gap (wb/m²)
• I= current in the coil (A)
• l= effective axial length of the coil (m).
• b= bredth of the coil
• N=number of turns in the coil

• Deflecting Torque:

​

• Deflecting Torque:

​

• Where,
• A=Face area of the coil (m²)
• K_d=B·A·n (Constant)

​

• Controlling Torque:

​

• Linear Scale

​

• Damping Torque:
• Damping Torque is provided by eddy current induced in the aluminum former

​

• Construction Details:

​

• Construction details:
• Core: Provide low reluctance path for magnetic field
• Former: Provide support to the coil and damping torque
• Pole shoe (soft iron): To provide radial field.
• Soft Iron Yoke: provide return path of flux and shielding from stray external fields.
• Coil: Number of turns of thin insulated copper coil is wound on aluminum former.
• Control springs: Helical spring used to provide control torque and also lead the current in & Out of the moving coil.

• Advantages:
• Low Power consumption.
• Very accurate & Reliable
• The scales are uniform
• High torque/weight ratio
• Very effective built in damping
• No effect of stray magnetic field.
• Disadvantages:
• Can not be used in direct AC current measurement.
• Costlier than the moving iron instrument.
• The coil being very fine, cannot withstand prolong overloading.

​

​

End