what is dynamometer type instrument?
dynamometer type instrument is the modified version of the PMMC instrument which is used to measure the value of current and voltage.
this instrument is used in both of dc and ac source.we can be used this instrument as ammeter wattmeter and voltmeter but it generally used as the wattmeter.
principal of dynamometer instrument
the principal of the dynamometer type instrument is the current carrying conductor exists the mechanical force.
construction :
the dynamometer type instrument consists of fixed coil and moving coil. at first fixed coil is divided in two equal parts and it is parallel to each other. the moving coil is fixed between the two fixed coils. there is one resistor is connected in series with the moving coil which limits the high current coming from the source and the moving coil is placed on the non metallic material in some cases it gives the separate excertation to the two coils is depend on for which work or where we use this instrument. the pointer is attached to the moving coil which moves freely with the moving coil and it indicate the measured value on the calibrated scale located at the top of the instrument. In this instrument we use spring for the controlling torque and as a damping torque here we use the air damping for the steady deflection of the pointer.
working :
in the circuit when we flow the current along the fixed coil here we see the magnetic field generated and the same current is flows from the moving coil then the some torque acts on a moving coil because the moving coil is placed between the magnetic field of the fixed coil therefore the force acts on the moving coil is at tangentially and it is determined by the flamings left hand rule. the tangential force act on the moving coil is directly proportional to the value of current flows through the fixed coils. when the moving coil moves because of the magnetic field of fixed coil the the pointer will also moves because the pointer is attached to the moving coil and it indicates the deflection or the measured value on a calibrated scale by this process can measure the any electrical quantity
deflecting torque in dynamometer type instrument
the magnetic field strength around the fixed and moving coil is directly proportional to the input current.
the force applied on a moving coil is proportional to the input current and crested magnetic field strength.
therefore as above explanation
Td = If . Im
where , If = current in fixed coil Im = current in moving coil
advantages of dynamometer type instrument
- this instrument can operate both A.C. and D.C. currents
- this instrument is free from different losses like eddy current loss and hysteresis loss.
- they have more accuracy
- its measuring accuracy and reliability is high
- dynamometer type voltmeters are very useful where accurate RMS values of voltage, irrespective of waveforms,are required.
disadvantage of dynamometer type instrument
- it has not uniform scale
- this instrument have low sensitivity because of low torque.
- its torque and the weight ratio is low
- they are more expensive than M.I. and PMMC type instrument.
- they are expensive they have to handle with great care.
- operating current of these instrument is large because of low magnetic field
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