Construction of Electrodynamometer Type Instrument

Construction of Electrodynamometer Type Instrument 3 Figure.1 -Dynamometer Type Instrument 4 Wiring diagram of Dynamometer Type Instrument Figure.2-Dynamometer Type Instrument 5 Parts of Dynamometer Type Instrument With reference to fig 3 • Fixed coils • Moving Coil • Control Springs • Damping equipment • Shielding equipment • Scale • Case Figure 3 6 Salient features of Fixed coil • They produce the magnetic field required for the Instrument. • Fixed coil is divided into two sections. • To produce more uniform magnetic field at center. • To allow the passage of the instrument shaft. 7 • They are wound with fine wire for using as voltmeter, while for ammeters and watt meters it is wound with heavy wire • They are varnished and baked to form a solid assembly. • Solid assembly makes the construction rigid. • They are mounted on ceramic supports to reduce eddy current loss. 8 Salient Features of Moving Coil • It is an air cored coil. • Wound on a non metallic former to reduce the eddy current loss. • It is located in between the two fixed coils • Spindle is attached to this coil • Spindle is supported in a jeweled bearings 9 Control • Control Torque is produced by two control springs. • Springs also acts as leads to the moving system. 10 Moving System Contains • A moving coil mounted on an aluminium spindle. • Counter weights. • A pointer. • Control springs 11 Damping • Damping is provided by Air Friction. • Two aluminium vanes are attached to the spindle at the bottom. • These vanes are moving in a sector shaped chamber. 12 Shielding • The field produced by the fixed coils is very weak. • It is nearly 0.005 to 0.006 wb/m2. • Shielding is necessary to protect the instrument from stray magnetic fields. • Shielding is provided by high permeability alloy casing. 13 Scales • The scales are hand drawn using machine sub-dividing equipment. • Most of high precision instruments have a 300mm scale with 100,120 or 150 divisions. 14 Working Principle of Electrodynamometer Type Instrument 15 Principle • The interaction of the flux produced by the fixed coils and moving coil will produce the deflection torque. 16 Working of Dm Meter • The instrument is connected across the load to measure the power. • The current is divided in to two parts • One part I1 flows through the current coil (fixed coils) • Second part I2 flows through the pressure coil (moving coil) • Both fixed and moving coils produce fluxes in between the fixed coils 17 Working of Dm Meter cont.. • The interaction of these two fluxes produces force on the moving coil. • As a result the moving coil rotates (as it is free to rotate) • The pointer shows the corresponding power value on the graduated scale • The deflection torque or force on moving system depends on the current strengths • The current in the pressure coil is proportional to the voltage impressed across it 18 • Thus the deflection is decided by the product of r.m.s. values of two currents, cosine of the phase angle and rate of change of mutual inductance . • So the Dynamometer type instrument can be used for both AC &DC measurements. 19 Summary • We have discussed about the construction and working principle of Dynamometer type instrument. • Dynamometer type instruments are used for both AC & DC measurements. 20 Quiz 1.Number of fixed coils in a dynamometer type instrument are: A) 1 B) 2 C) 3 D) 4 21 Quiz 2. Dynamometer Type Instruments are used for __________ (a) AC only (b) DC only (c) Both AC and DC (d) None 22 Frequently Asked Questions 1. 2. 3. Explain the construction of Dynamometer type wattmeter with a neat sketch. (oct / nov-2006) Explain the working of the dynamometer type instrument. List the various parts of dynamometer type instrument. 23