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3.5 Other AC Meter Movement :
3.5 Other AC Meter Movement :
CONSTRUCTION
IRON-VANE MOVEMENT
If the current is reversed, the field polarity and the polarity of the moving coil reverse, and the force continues in the same direction. Due to this characteristic of the electrodynamometer movement, it can be used in both AC and DC systems to measure current.
WORKING PRINCIPLE
IRON-VANE MOVEMENT
The moving iron vane meter operates on the principle of magnetic repulsion between like poles.
The measured current flows through a field coi lwhich produces a magnetic field proportional to the magnitude of current.
RESPONSE
Has a square law response,but the fixed coil can be designed to provide a relatively linear response
DC APPLICATION
Although the iron-vane movement is responsive to direct current,the hysteresis,or magnetic lag in the iron vanes causes appreciable error. Therefore ,moving vane instruments for measuring direct current are rarely used except for very inexpensive indicator such as charge-discharge indicator on automobiles
AC APPLICATION
The magnetic lag present no problems.therefore, IVM are used extensively in industry for measuring alternating current when error on orders of 5% to 10% are acceptable.
IRON-VANE AS AMMETER
The magnetic field produced by the current flow magnetizes the two iron vanes with the same polarity regardless of the direction of current through the coil.
Since like poles repel one another, the moving iron vane pulls away from the fixed vane and moves the meter pointer.This motion exerts a force against a spring.
The distance the moving iron vane will travel against the spring depends on the strength of the magnetic field. The strength of the magnetic field depends on the magnitude of current flow.
IRON-VANE MOVEMENT
ELECTRODYNAMOMETER MOVEMENT
CONSTRUCTION
ELECTRODYNAMOMETER MOVEMENT
The moving coil and pointer, which are attached to the coil, are suspended between and connected in series with the two field coils (fix coil).
The two field coils and moving coil are connected in series such that the same current flows through each coil.
ELECTRODYNAMOMETER MOVEMENT
Current flow through the three coils in either direction causes a magnetic field to be produced between the field coils.
The same current flows through the moving coil cause it to act as a magnet exerting/applying a force against the spring
WORKING PRINCIPLE
CURRENT CAPACITY
The basic electrodynamometer is capable of handling more current than D’Arsonval movement. A 100mA is an approximately value of maximum current without resistor.
CHARACTERISTICS
If the current is reversed, the field polarity and the polarity of the moving coil reverse, and the force continues in the same direction. Due to this characteristic of the electrodynamometer movement, it can be used in both AC and DC systems to measure current.
SENSITIVITY
If the current is reversed, the field polarity and the polarity of the moving coil reverse, and the force continues in the same direction. Due to this characteristic of the electrodynamometer movement, it can be used in both AC and DC systems to measure current.
ELECTRODYNAMOMETER METER AS WATTMETERS
The most extensive application of the electrodynamometer meter is in watt meters either to measure dc or ac power.
The ac signal are not restricted to sinusoidal forms so that power developed by any ac waveform may be measured.
When used as a wattmeter, the fixed coils are in series with the load and therefore conduct the same current as the load(plus a small current through the moving coil).
The moving coil is connected as a voltmeter across the load where the resistor Rs is the multiplier for the voltage sensitive meter.
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