chapter 6 Flashcards
NO fundamentaldifference between voltmeter and ammeter. explain
both defliction is due to the magnetic effect of a current.
differnece between voltmeter and ammeter
ammeter it is the current to be measured and voltmeter it is the current proportional to the voltage.
volt and ammeters consist of the following
fixed field system permanent magnet or electromagnet
a pointerattached to a spindle
controlling system two hair springs fixed to spindle and wound in oppposite direction to compensate for heat.
damping system to prevent pointer from iscilating.
moving iron meter
as the coil through which the current flows is fixed , it can be wound with a few turns of thick wire and be used to measure relatively large currents directly as an ammeter
name two different kinds of moving iron meters
attraction type moving iron meter
repulsion type moving iron meter
define attraction type moving irron meter
current is flowing through the coil , the lat sofft iron armeture which is attached to thespindle will move towards and into the coil. the amount of deflection is dependant on the amount of current flowing through the coil.the larger the current , the stronger the pulling force is against the control springs and the further the pointer will move.
repulsion type moving iron meter
when the current flows through the coil the fixed and moving irons are both magnetised in the sam edirection. since like poles repulse each other the moving ironn will move away.the larger the current flowing through the coil , the larger the deflecting torque against the control springs will be and the further the pointer will move
advantages for moving iron meters
wowrk on ac and dc supplies
the are cheap
robust construction
disadvantages of moving iron type meters
readeings are efeected easily by magnetic fields
the scale is not unifrom
moving coil meter(permanent magnet type)
coil is wound around an alimunium frame and fixed spindle. it is free to ratate air gaps between poles and the soft core. the control springs provide control torque and they also lead current to and from the coil.
operation of a moving coil meter.
when current is flowing through the coil, magnetic field is set up around the the windings of the coil. force is exerted on the two sides of the coil in the same way as the force is exerted on the current carrying conductors in the magnetic field
advantages of moving coil meter
very accurate
almost uniform scale
well shielded from stray magnetic fields
disadvantages of moving coil meter
only do dc supplies
they are expensive
