What are absolute & secondary instruments ? Why is damping required in a measuring instrument ?
Absolute instrument: These instruments give the magnitude if the quantity under measurement terms of physical constants of the instrument.
Secondary instrument: These instruments are calibrated by the comparison with absolute instruments which have already been calibrated.
Damping force or torque is also necessary to avoid oscillations of the moving system about its final detected position owing to the inertia of the moving parts and to bring the moving system to rest in its final deflected position quickly.
In the absence of damping, the moving system of an instrument would oscillate about the position at which the deflecting and restoring torques are equal. The function of damping is to absorb energy from the oscillating system and to bring it to rest promptly in its equilibrium position so that its indication may be observed.
In a certain current transformer, the following data is obtained. Nominal ratio = 25/5A, Turn ratio = 3, primary terns = 40, secondary turns = 120, secondary resistance = 0⋅16z, secondary reactance = 0⋅195Ω, secondary burden = 15 VA, Burden power factor = 0⋅7, secondary terminal voltage = 3V.
Find ratio & phase angle errors. The magnetising and loss ampere turns corresponding to an emf of 4⋅26V are13 & 10⋅1 respectively.
Draw the circuit diagram of DC potentiometer & explain how it works.
Explain how a low resistance is measured by a potentiometer
Deduce the expression of torgue of electrodynamometer type instrument. Why multiplier is used with PMMC instrument? What do you mean by sensitivity of PMMC instrument? Why sensitivity of electrodynamometer type instrument is low? Why the scale of moving iron instrument is cramped at lower end?
A 20 Ω resistor, a choke coil having some inductance and some resistance and a capacitor are connected in series across a 25 V variable frequency source. When frequency is 400 Hz, the current is maximum and its value is 0.5 Amp and the potential difference across the capacitor is 150 V. Calculate the resistance and the inductance of the chokes coil and the capacitance of the capacitor.