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

Subject: EMMI

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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.

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CH: Analog Instrumentation

• This Lecture contain
• Electrodynamics or Dynamometer type instrument, Power measurement

Introduction

• 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.

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• Dynamometer Type Instrument:

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• Can be used for measurement of current, voltage, power for both DC and AC up to 125Hz and with some modifications as power factor & frequency meter.
• They are transfer type instrument as they have same accuracy for d.c and a.c. upto 125Hz
• i.e. They may be calibrated with d.c. source and then can be used without modification to measure a.c.
• Accuracy is very high and hence can be used in laboratory for calibration of other instruments.

• Construction and operation:
• Consist of two coils
• 1) Fixed coil: To produce operating field. Split into 2 parts to have uniform distribution of flux around moving coil.
• 2) Moving coil: Produce deflecting torque (T_d).
• The coils are air cored to avoid hysteresis & eddy current errors
• Spiral hair springs are used to supply current to MC and also to produce controlling torque.

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• Construction and operation:
• Air friction damping is used
• The operating field is very weak (0.005 to 0.006 wb/m²), eddy current damping cannot be used.
• Proper shielding is provided against stray magnetic field using laminated steel shield.

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• Torque equation:

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• Let i₁= instantaneous value of current in the fixed coils (A).
• i₂= instantaneous value of current in the moving coils (A).
• L₁=self inductance of fixed coil (H).
• L₂=self inductance of moving coil (H).
• M=mutual inductance between fixed and moving coils (H)

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• Flux linkages of coil 1, ψ₁=L₁i₁+Mi₂ ; Flux linkages of coil 2, ψ₂=L₂i₂+Mi₁

Electrical energy input=e₁i₁dt+e₂i₂dt

• =i₁dψ₁+i₂dψ₂

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• Operation with DC:
• I₁=Current in Fixed Coil
• I₂=Current in Moving Coil

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• Operation with AC:
• I₁=I_m1·sinωt
• I₂=I_m2·sin(ωt-ϕ)
• As instantaneous current varies with time T_d also varies with time.
• But due to inertia, moving system cannot follow such rapid variation and respond only to average torque.

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• Operation with AC:
• If I₁=I₂=I then

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• If I₁=I∠-ϕ, and I₂=V/K₂∠0

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• If current is passed through fixed coil and voltage is applied across moving coil then instrument measure average power and scale is uniform

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• If I₁=I₂=V/K₁ then

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• Advantage of Moving Type Instrument:

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• Accuracy is very high.
• Can be used for both AC and DC.
• Free from hysteresis and eddy current loss

• Disadvantage:

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• Low torque to weight ratio and hence low sensitivity.
• Highly expensive compare to other instrument for voltmeter and ammeter.
• Scale is non uniform
• Frictional losses are high
• Higher power consumption compare to PMMC instrument.

End