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Electrodynamometer movement meter
Has the same basic operating principle as the D’Arsonval meter movement, except that the permanent magnet is replaced by fixed coils.
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 flows through each coil.
Current flow through the three coils in either direction causes a magnetic field to be produced between the field coils.
The same current flow through the moving coil causes it to act as a magnet exerting a force against the spring.
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
CHARACTERISTIC
If the current is reversed, the field polarity and the polarity of the moving cell reverse, and the force continues in the same direction
due to this characteristic of the electrodynamometer movement, it can be in both AC and DC systems to measure current
SENSITIVITY
The magnetic coupling between the fix coils and moving coils is across and air gap a weak magnetic field.
To be sufficient more current must flow through the coils, which mean that a large diameter wire must be used.
However, the large diameter has a less resistance than a smaller diameter.
This cause the electrodynamometer movement to have a very low sensitivity rating of approximately 20-100 Ω/v
ELECTRODYNAMOMETER AS WATTMETER
The most extensive application of the electrodynamometer meter is in watt meters either to 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 as the load(plus a small current through the moving coil)
The moving coil is connected as voltmeter across the load where the resistor Rs is the multiplier for the voltage sensitivity meter
The instrument whose working depends on the reaction between the magnetic field of moving and fixed coils is known as the Electrodynamometer Wattmeter. It uses for measuring the power of both AC and DC circuits.
The working principle of the principle of the Electrodynamometer Wattmeter is very simple and easy. Their working depends on the theory that the current carrying conductor placed in a magnetic field experiences a mechanical force.
ACCURACY
Because of the inherent accuracy of the electrodynamometer movement, it is used in standard instruments those used the calibration of other meters.
he term transfer instrument is applied to an instrument that may be calibrated with a DC source and then used without modification to measure alternating current.
This gives a direct means of equating AC and DC measurements of voltage and current.
Most electrodynamometer movement are accurate over the frequency range from 0-125Hz.
Video on Electrodynamometer Movement
Iron Vane meter
It can be used to measure both AC current and voltage. By changing the meter scale calibration, the movement can be used to measure DC current and voltage.
Working principle
The moving iron vane meter operates on the principle of magnetic repulsion between like poles.
The measured current flows through a field coil which produces a magnetic field proportional to the magnitude of current.
Construction
Suspended in this coil field are two iron vanes attached to a pointer.
The two iron vanes consist of one fixed and one moveable vane.
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 as voltmeter
This type of meter movement may also be used to measure voltage
When this type of movement is used to measure voltage, the field coil consists of many turns of fine wire used to generate a strong magnetic field with only a small current flow.
This basic current responding iron vane meter movement can be used to measure voltage by adding a multiplier resistor as with D’ Arsonval movement.
It is very sensitive to frequency change can be expected to provide accurate readings over a limited frequency range, approximately 25-125Hz.
R E S P O N S E
The basic iron vane movement has a square law response, but the fixed coil can be designed to provide a relatively linear response.
D C -A P P L I C A T I O N
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 in automobiles.
A C -A P P L I C A T I O N
For an AC application, the magnetic lag present no problems. Therefore, iron-vane meter movements are used extensively in industry for measuring alternating current when errors on order of 5% to 10% are acceptable.
Video on Iron-Vane Meter Movement
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