L-4

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Damping Mechanisms

Two Mechanisms are used: Mechanical and Electromagnetic Induction 1. Mechanical damping: - Caused mainly by the motion of the coil through the air surrounding

it. - Independent of any electrical current throught the coil - Bearing friction and flexing of suspension springs also contribute to

mechanical damping effects - A simple damping mechanism is provided by an aluminum vane,

attached to the shaft of the moving coil. As the coil rotates, the vane

moves in an air chamber.

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2. Damping by EM Induction: - This kind of damping works on the principle of Lenzs law (a) In the metal coil frame: Caused by induction in the metal coil

frame. The rotation sets up eddy currents in the conductive metal

frame, causing a retarding torque that opposes the motion of the coil.

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(b) In the coil: When the coil rotates in the magnetic field, a voltage is generated in the coil. If an external resistance is connected across

the coil, this generated voltage circulates a current through the coil and provides damping action.

- For any galvanometer, a value for the external resistor can be

found that produces critical damping. This resistance is called the

Critical Damping Resistance External (CDRX). It is an

important galvanometer constant.

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Analog Meters: PMMC Meter

Permanent Magnet Moving Coil (PMMC) meter: Main difference with the DArsonval galvanometer: mirror assembly is replaced by a pointer-scale assembly

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PMMC Meter

Advantages: - uniform linear scale ( ) - High Torque-Weight ratio is possible with powerful magnet. - high sensitivity - power consumption is surprisingly small. Typical values range from 25 W 200 W - Accuracy is higher (2-5 % of the full scale reading) - Extension of instrument range is possible

Disadvantages: - Not suitable for AC (why?) - Aging of permanent magnet and springs introduces error - high cost due to permanent magnet

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Temperature Compensation

- Deflection, , depends on temperature-dependent parameters

- As temperature , both B and K , and Coil resistance

So, at increased temperature: * Reduced B and increased coil resistance tend to make the pointer read low *Reduced K, conversely, tends to make the pointer read high - The effect are not identical. So, an uncompensated meter tends to read low by approximately 0.2% per C rise in temperature

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Temperature Compensation

(a) Simple Compensation:

A swamping resistors in series with the moving coil. Swamping resistor

is made of manganin, which has a temperature co-efficient of practically

zero, combined with copper in the ratio of 20/1 to 30/1

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(b)

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Circuit Symbol of Basic PMMC Meter

Symbols of basic PMMC meter may NOT be unique Im Full-scale deflection current through the basic meter

Rm resistance of the basic meter

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DC PMMC Ammeter

Im can be only about 1 mA. For large current measurements, a shunt resistance Rsh must be connected to the basic meter.

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Multirange Ammeter

Various values of resistances can be used by means of suitable switching arrangement, to increase the number of current ranges. 1. Simple arrangement:

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Disadvantages:

1. During the changeover of switch, the shunt is momentarily removed from the meter. Then, the full current flows through the meter coil. This can damage the coil.

2. The contact resistance of the switch is in series with the shunt. It can be of variable nature. So, the ammeter indication may not be accurate.

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Multirange Ammeter

2. Ayrton Shunt/universal shunt Arrangement: - Eliminates the possibility of momentary disconnection of the

shunt. - The contact resistance of the switch is external to the the shunt

and meter.

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Ammeter Loading Effect

If the internal resistance of the meter is a large portion of the load

resistance of the circuit under measurement, an effect knonw as meter-loading occurs. - Meter loading occurs when the insertion of the meter into the

circuit changes the voltage/current values sof that circuit. We say that the meter is loading the circuit.

EX: Shown in the whiteboard

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DC PMMC Voltmeter

Let, Im = 1 mA, and Rm = 1000 So the basic meter can be connected across a maximum of 1 V only. -To increase the range, resistors (Rs) are conncected in series with the basic meter. Sometimes these resistors are called multipliers.

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Multirange Voltmeter

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EX:

A 1 mA meter movement with an internal resistance of 50 is to be used in a 0-1

V, 0-10 V, 0-50 V, and 0-100 V ranges in the arrangement shown above. Find the

value of the required multiplier resistances.

Ans: R1 = 950

R2 = 9 k

R3 = 40 k

R4 = 50 k

Voltmeter Sensitivity

- Sensitivity of a voltmeter is given in /V or A-1 -Obtained by dividing the sum of Rm and Rs by the full-scale reading in volts.

- In terms of full-scale deflection current,

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Voltmeter Loading Effect

- It is caused principally by low-S instruments.

- So, S is an important factor when selecting a meter for a particular

measurement

- For voltage measurement in high-resistance circuits, high S voltmeter should be used to prevent the loading effect

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Voltmeter Loading Effect

Solve the Problem

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Rectifier-based AC Meters

Half-wave rectifier AC Meter:

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