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measurement presentaion/ pmmc
Transcript of measurement presentaion/ pmmc
Shadi awwad Electromechanical indicating instruments History In 1820 Hans Christian Oersted announced his discovery that a compass needle deflected when hung over a wire carrying an electric current Early suspension galvanometer Also known as Thompson mirror galvanometer Suspension galvanometer
(PMMC) Use galvanometer to measure high current and voltage OHMMETER A coil of fined wire is suspended in a magnetic field (magnet). and according on what was mentioned before the coil will start rotating in the magnetic field when it carries a current. the elasticity of the filaments sets up a torque in the opposite direction. cause the ciol to stop rotating when the mechanical counter torque balances the electromagnetic torque. then the mirror deflects a beam of light causing light to move on a scale. T=B x A x I x N B= flux density
A=coil area same as thomson galvanometer principle but used springs instead of filaments and a pointer instead of light also called d'arsonval movement DYNAMIC BEHAVIOR The dynamic behavior can be observed by suddenly interrupting the applied current. it will be seen that as a result of inertia the pointer swings past the zero mark in the opposite direction.
the motion of a moving coil in a magnetic field is characterized by three quantities:
1- moment of inertia
2- opposing torque
3- damping constant one solution : using an aluminum vane V-jewel mechanism to measure high current V-shunt = V-movement Rs = Im Rm I-Im example:
A 1 mA meter with an internal resistance of 100 ohms is to be converted into a 0-100 mA ammeter. calculate the value of the shunt resistance required . Rs = Im Rm I-Im = 1 mA x 100 ohm/ 99 mA = 1.01 ohms multi-range ammeters Simple multi-range ammeter Universal or Ayrton shunt Dc voltmeter 1 5 Series-type ohmmeter R1=Rh - Rm x Rh x I(full scale) E R2 = I(full scale) x Rm I2 Thanks for listening main references : - Electrical Measurements And Measuring Instruments
By U.A.Bakshi, A.V.Bakshi
-electronic instrumentaion and measurement techniques
-http://en.wikipedia.org/wiki/Thermocouple for the rest of references please contact us Types of electrical indicating devices Analog Digital - These types of multimeters are now called analog multimeters due to the Evolution of digital multimeters
- The ammeter , the voltmeter, and the ohmmeter in the analog multimeter all use the d’Arsonval movement
- The difference between these instruments is the circuit in which the basic movement is used
- It can measure DC and AC current and voltage
2-calibration of a DC voltmeter
a rheostat is placed in the circuit to control the amount of current and therefore the drop across the resistor , R, so that several points on the voltmeter scale can be calibrated Resolution:
- The resolution of a multimeter is the smallest part of the scale which can be shown.
- Resolution of analog multimeters is limited by:
1-the width of the scale pointer
3-vibration of the pointer
4-errors due to non-horizontal use of the mechanical display Shunt-Type Ohmmeter Elements of a single- phase watthour meter
read pages 99, 100 - A rough calibration may be done by measuring a standard resistance and noting the reading of the ohmmeter
- Doing this for several points on the ohmmeter scale and on several ranges allows one to obtain an indication of the correct operation of the instrumnet 3- calibration of an ohmmeter 1- Calibration of a DC ammeter
A rheostat is placed in the circuit to control the current to any desired value, so that different points on the meter scale can be calibrated Calibration of DC instruments
The invention of the first multimeter is attributed to British Post Office engineer, Donald Macadie, in the early 1920s who became dissatisfied with having to carry many separate instruments required for the maintenance of the telecommunications circuits. A multimeter also known as a VOM (Volt-Ohm meter), is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter would include basic features such as the ability to measure voltage, current, and resistance Multimeter (VOM)
- Measurement of low-value resistors
- Not a commonly used instrument
- Found in laboratories
- The unknown resistance is connected in parallel
with the meter hence the name given to the instrument
- The intermediate markings can be done by connecting resistances of the known values across the terminals A-B - Electric Meter, or Watt-hour Meter, an instrument that measures the amount of electric energy used by a consumer
- The meter is calibrated in kilowatt-hours
- One kilowatt-hour is the amount of electric energy required to provide 1,000 watts of power for a period of one hour
Ten 100-watt light bulbs left on for one hour consume one kilowatt-hour of electric energy Watthour meter Thermocouple Instrument - Thermocouple : A thermocouple consists of two conductors of different materials (usually metal alloys) that produce a voltage in the point where the two conductors are in contact
- This voltage rises in proportion to the temperature of the junction
- The heat generated by the current is directly proportional to the current squared Thermoinstruments the number of rotations of the disk is proportional to the energy consumed by the load in a certain time interval and is measured in terms of KWh