Lec04_Part01 Flashcards
3 principles of operation of analog meters
(i) electromagnetic: the magnetic effect of electric circuits
(ii) electrostatic: the forces between electrically charged conductors
(iii) electro-thermal : the heating effects
electromechanical meter
electrical signal -> mechanical force/torque as an output -> voltmeter/ammeter
AC current or voltage when being applied the the analog instru
AC current ammeter and voltmeters must represent the RMS values of the current or voltage, respectively, when being applied to the instru
Analog instruments may be classified into two ways:
Absolute instruments (đo lường trực tiếp) Secondary instruments (đo lường thứ cấp/gián tiếp):
Absolute instruments (đo lường trực tiếp)
no comparison with another
instrument being required (the constants of the instruments and its deflection
=>no calibration of the instrument is required)
e.g. the tangent galvanometer, PMMC
Secondary instruments (đo lường thứ cấp/gián tiếp):
Secondary instruments (đo lường thứ cấp/gián tiếp): (being calibrated by
comparison with either an absolute instrument or one which has already been
calibrated), classified by:
1. Indicating instruments: to indicate the magnitude of a quantity being measured
e.g. VOM
- Recording instruments: to give a continuous record of the quantity being measured over a specified period.
e. g. Oscilloscope - Integrating instruments: to record the totalized events over a specified period of time
e. g. Ampere hour and watt hour (energy) meters
Principle of Operation of Analog meter
- Magnetic effect
- Heating effect (thermalcouple and hotwire instruments)
- Electrostatic effect
- Electromagnetic effect (e.g. Wattmeter)
- Hall effect
Operating Torques:
- Deflecting torque/force:
- Controlling torque/force
- Damping torque/force
Deflecting torque/force
depending upon the electrical signal to be measured
=> helps in rotating the instrument movement from its zero position.
Controlling torque/force:
spiral springs or gravity used to produce the controlling torque (from the controlling system)
Function of the controlling system:
- To produce a torque equal and opposite to the deflecting torque at the final steady position of the pointer in order to make the deflection of the pointer definite for a particular magnitude of current.
- To bring the moving system back to its zero position when the force causing the instrument moving system deflect is removed.
Damping torque/force
to bring the moving system to rest at the final
deflected position quickly (without oscillation or very small oscillation)
- Air friction, fluid friction, eddy currents provide the damping torque/force to act
Deflecting torque»_space; Controlling torque
-> underdamped
Deflecting torque = Controlling torque
Critically damped
Deflecting torque «_space;Controlling torque
Overdamped
Constructional Details
- Moving System
- Controlling System
- Damping System
Moving System
- Properties:
+ The moving parts should be light (proportional to the weight of the moving part)
+ The frictional force should be minimum (opposite to the movement) - Supports:
+ Suspension (hệ thống giảm xốc)
+ Taut Suspension (hế thống giảm xốc hai bên)
+ Pivot and Jewel bearing (chốt quay và ổ bi loại Jewel)
Controlling system
- Spring control
- Gravity control
Spring Control
- 2 control springs, graduated scale, spindle, balance weight.
- formula…
advantages of Gravity control
- cheaper
- independent of temperature
- does not deteriorate with time
Air-friction damping
A light aluminum piston is attached to the moving system and moves in an air
chamber closed at one en
Fluid-friction damping
A light vane attached to the spindle of the moving system, dips into a pot of
damping oil and should be completely submerged by oil. The frictional drag in the
disc is always in the direction opposing motion.
Eddy-current damping
- When a sheet of conducting material moves in the magnetic field -> cut through lines of force -> create eddy currents in the material -> induce the damping force against the motion of moving system.
- The damping force is proportional to the magnitude of current and the strength of
the field. - The damping force is proportional to the velocity of the moving system and is
ZERO in case of no movement of the system.
Eddy current damping, 2 types
- Eddy-current damping torque of metal former
- Eddy-current damping torque of metal disc
The moving coil in a PMMC instrument is supported by what ?
supported by
+ jeweled bearings
+ 2 flat metal ribbons held taut by springs
=> Taut band suspension is the toughest and the most sensitive of the two
the damping force in a PMMC instrument is provided by what ?
the damping force in a PMMC instrument is provided by eddy currents induced in the aluminum coil former as it moves thru the magnetic field
the damping force is usually provided by what ?
A deflection instrument requires a damping force to stop the pointer oscillating about the indicated reading.
The damping force is usually produced by eddy currents in a nonmagnetic coil former. These exist when the coil is in motion.
Control torque Tc of PMMC instrument is propotional to ?
to the deflection angle theta and to the current => the scale is linear or uniformly divided.
describe damping torque in in PMMC
- When the aluminum former moves with the coil in the field of permanent magnet, a voltage is induced, causing eddy current to flow in it.
-> these current exerts a force on the former
.=> by Lenz’s law, this force oppose the motion producing it.
Advantages of PMMC
- Sensitive to small current
- Very accurate and reliable
- Uniform scale up to 270 degree
- Very effective built in damping
- Low power consumption, varies from 25uW to 200uW
- Free from hysteresis and not effected by external fields because its permanent magnet shields the coil from external magnetic fields
- Easily adopted as a multirange instrument
