Electrodynamometer (Electrodynamics) Type Instruments




•    The necessity for the a.c. calibration of moving iron instruments as well as other types of instruments, which cannot be correctly calibrated, requires the use of a transfer type of instrument.

•    A transfer instrument is one that may be calibrated with a d.c. Source and then used without modification to measure a.c.

•    This requires the transfer type instrument to have same accuracy for both d.c. and a.c., which the electrodynamometer instruments have.

•    These standards are precision resistors and the Weston standard cell (which is a d.c. cell).

•     It is obvious, therefore, that it would be impossible to calibrate an a.c. instrument directly against the fundamental standards.

•    The calibration of an a.c. instrument may be performed as follows.

•     The transfer instrument is first calibrated on d.c.

•    This calibration is then transferred to the a.c. instrument on alternating current, using operating conditions under which the latter operates properly.

•    Electrodynamics instruments are capable of service as transfer instruments.

•    Indeed, their principal use as ammeters and voltmeters in laboratory and measurement work is for the transfer calibration of working instruments and as standards for calibration of other instruments as their accuracy is very high.

•    Electrodynamometer types of instruments are used as a.c. voltmeters and ammeters both in the range of power frequencies and lower part of the audio power frequency range. They are used as wattmeters, voltmeters and with some modification as power factor meters and frequency meters.







Operating Principle

•    It would have a torque in one direction during one half of the cycle and an equal effect in the opposite direction during the other half of the cycle.

•    If the frequency were very low, the pointer would swing back and forth around the zero point.

•     However, for an ordinary meter, the inertia is so great that on power frequencies the pointer does not go very far in either direction but merely stays (vibrates slightly) around zero.

•     If, however, we were to reverse the direction of the flux each time the current through the movable coil reverses, a unidirectional torque would be produced for both positive and negative halves of the cycle.

•     In electrodynamometer instruments the field can be made to reverse simultaneously with the current in the movable coil if the field (fixed) coil is connected in series with the movable coil.


Construction

Fixed Coils

•    The field is produced by a fixed coil.
•     This coil is divided into two sections to give a more uniform field near the centre and to allow passage of the instrument shaft.

Moving Coil

•    A single element instrument has one moving coil.
•    The moving coil is wound either as a self-sustaining coil or else on a non-metallic former.
•    A metallic former cannot be used as eddy current would be induced in it by the alternating field.
•    Light but rigid construction is used for the moving coil.
•     It should be noted that both fixed and moving coils are air cored.

Control

•    The controlling torque is provided by two control springs.
•    These springs act as leads to the moving coil.

Moving System

•    The moving coil is mounted on an aluminum spindle.
•    The moving system also carries the counter weights and truss type pointer.
•    Sometimes a suspension may be used in case a high sensitivity is desired.

Damping

•    Air friction damping is employed for these instruments and is provided by a pair of aluminum vanes, attached to the spindle at the bottom.
•    These vanes move in sector shaped chambers.
•    Eddy current damping cannot be used in these instruments as the operating field is very weak (on account of the fact that the coils are air cored) and any introduction of a permanent magnet required for eddy current damping would distort the operating magnetic field of the instrument.

Shielding

•    The field produced by the fixed coils is somewhat weaker than in other types of instruments
•    It is nearly 0.005 to 0.006 Wb/m
•    In D.C. Measurements even the earth magnetic field may affect the readings.
•    Thus it is necessary to shield an electrodynamometer type instrument from the effect of stray magnetic fields.
•    Air cored electrodynamometer type instruments are protected against external magnetic fields by enclosing them in a casing of high permeability alloy.
•    This shunts external magnetic fields around the instrument mechanism and minimizes their effects on the indication.

Cases and Scales


•    Laboratory standard instruments are usually contained in highly polished wooden cases.
•    These cases are so constructed as to remain dimensionally stable over long periods of time.
•    The glass is coated with some conducting material to completely remove the electrostatic effects.
•    Adjustable leveling screws support the case.
•    A spirit level is also provided to ensure proper leveling.
•    The scales are hand drawn, using machine sub-dividing equipment.
•    Diagonal lines for fine sub-division are usually drawn for main markings on the scale.
•    Most of the high-precision instruments have a 300 mr scale with 100, 120 or 150 divisions.



Torque Equation

Let,
    i1 = instantaneous value of current in the fixed coils: A.

    i2   = instantaneous value of current in the moving coil: A.

    L1 = self-inductance of fixed coils: H.

    L2 = self-inductance of moving coils H,

    M = mutual inductance between fixed and moving coils:

Flux linkages of coil 1, ψ1 = L1 i1 + Mi2
 
Flux linkages f coil 2, ψ2  = L2 i2 + Mi1
 
Electrical input energy     = e1i1dt+e2i2dt




Errors in Electrodynamometer Instruments

i)    Frequency error

ii)    Eddy current error

iii)    External magnetic field

iv)    Temperature changes

Advantages
i)    These instruments can be used on both a.c & d.c

ii)    Accurate rms value

Disadvantages

(i) They have a low torque/weight ratio and hence have a low sensitivity.

(ii) Low torque/weight ratio gives increased frictional losses.

(iii) They are more expensive than either the PMMC or the moving iron type instruments.

(iv) These instruments are sensitive to overloads and mechanical impacts. Therefore, they must be handled with great care.

(v) The operating current of these instruments is large owing to the fact that they have weak magnetic field. The flux density is about 0.006 Wb/m as against 0.1 to 0.5 Wb/m in PMCC instruments

(vi) They have a non-uniform scale.

9 comments:

  1. Really useful and appreciable work....that too in simplified way...awesome

    ReplyDelete
  2. Replies
    1. Thank you! We appreciate your feedback!

      Delete
  3. sir what would happen if current in fixed coil and moving coil are both ac source ?

    ReplyDelete

Labels

PROJECTS 8086 PIN CONFIGURATION 80X86 PROCESSORS TRANSDUCERS 8086 – ARCHITECTURE Hall-Effect Transducers INTEL 8085 OPTICAL MATERIALS BIPOLAR TRANSISTORS INTEL 8255 Optoelectronic Devices Thermistors thevenin's theorem MAXIMUM MODE CONFIGURATION OF 8086 SYSTEM ASSEMBLY LANGUAGE PROGRAMME OF 80X86 PROCESSORS POWER PLANT ENGINEERING PRIME MOVERS 8279 with 8085 MINIMUM MODE CONFIGURATION OF 8086 SYSTEM MISCELLANEOUS DEVICES MODERN ENGINEERING MATERIALS 8085 Processor- Q and A-1 BASIC CONCEPTS OF FLUID MECHANICS OSCILLATORS 8085 Processor- Q and A-2 Features of 8086 PUMPS AND TURBINES 8031/8051 MICROCONTROLLER Chemfet Transducers DIODES FIRST LAW OF THERMODYNAMICS METHOD OF STATEMENTS 8279 with 8086 HIGH VOLTAGE ENGINEERING OVERVOLATGES AND INSULATION COORDINATION Thermocouples 8251A to 8086 ARCHITECTURE OF 8031/8051 Angle-Beam Transducers DATA TRANSFER INSTRUCTIONS IN 8051/8031 INSTRUCTION SET FOR 8051/8031 INTEL 8279 KEYBOARD AND DISPLAY INTERFACES USING 8279 LOGICAL INSTRUCTIONS FOR 8051/8031 Photonic Transducers TECHNOLOGICAL TIPS THREE POINT STARTER 8257 with 8085 ARITHMETIC INSTRUCTIONS IN 8051/8031 LIGHTNING PHENOMENA Photoelectric Detectors Physical Strain Gage Transducers 8259 PROCESSOR APPLICATIONS OF HALL EFFECT BRANCHING INSTRUCTIONS FOR 8051/8031 CPU OF 8031/8051 Capacitive Transducers DECODER Electromagnetic Transducer Hall voltage INTEL 8051 MICROCONTROLLER INTEL 8251A Insulation Resistance Test PINS AND SIGNALS OF 8031/8051 Physical Transducers Resistive Transducer STARTERS Thermocouple Vacuum Gages USART-INTEL 8251A APPLICATIONs OF 8085 MICROPROCESSOR CAPACITANCE Data Transfer Instructions In 8086 Processors EARTH FAULT RELAY ELECTRIC MOTORS ELECTRICAL AND ELECTRONIC INSTRUMENTS ELECTRICAL BREAKDOWN IN GASES FIELD EFFECT TRANSISTOR (FET) INTEL 8257 IONIZATION AND DECAY PROCESSES Inductive Transducers Microprocessor and Microcontroller OVER CURRENT RELAY OVER CURRENT RELAY TESTING METHODS PhotoConductive Detectors PhotoVoltaic Detectors Registers Of 8051/8031 Microcontroller Testing Methods ADC INTERFACE AMPLIFIERS APPLICATIONS OF 8259 EARTH ELECTRODE RESISTANCE MEASUREMENT TESTING METHODS EARTH FAULT RELAY TESTING METHODS Electricity Ferrodynamic Wattmeter Fiber-Optic Transducers INTERRUPTS Intravascular imaging transducer LIGHTNING ARRESTERS MEASUREMENT SYSTEM Mechanical imaging transducers Mesh Current-2 Millman's Theorem NEGATIVE FEEDBACK Norton's Polarity Test Potentiometric transducers Ratio Test SERIAL DATA COMMUNICATION SFR OF 8051/8031 SOLIDS AND LIQUIDS Speed Control System 8085 Stepper Motor Control System Winding Resistance Test 20 MVA A-to-D A/D ADC ADVANTAGES OF CORONA ALTERNATOR BY POTIER & ASA METHOD ANALOG TO DIGITAL CONVERTER AUXILIARY TRANSFORMER AUXILIARY TRANSFORMER TESTING AUXILIARY TRANSFORMER TESTING METHODS Analog Devices A–D BERNOULLI’S PRINCIPLE BUS BAR BUS BAR TESTING Basic measuring circuits Bernoulli's Equation Bit Manipulation Instruction Buchholz relay test CORONA POWER LOSS CURRENT TRANSFORMER CURRENT TRANSFORMER TESTING Contact resistance test Current to voltage converter DAC INTERFACE Digital Storage Oscilloscope ELPLUS NT-111 EPROM AND STATIC RAM Electrical Machines II- Exp NO.1 Energy Meters FACTORS AFFECTING CORONA FLIP FLOPS Fluid Dynamics and Bernoulli's Equation Fluorescence Chemical Transducers Foil Strain Gages HALL EFFECT HIGH VOLTAGE ENGG HV test HYSTERESIS MOTOR Hall co-efficient Hall voltage and Hall Co-efficient High Voltage Insulator Coating Hot-wire anemometer How to Read a Capacitor? INSTRUMENT TRANSFORMERS Importance of Hall Effect Insulation resistance check Insulator Coating Knee point Test LEDs Display Driver LM35 LPT LPT PORT LPT PORT EXPANDER Life Gone? METHOD OF STATEMENT FOR TRANSFORMER STABILITY TEST METHODS OF REDUCING CORONA EFFECT Mesh Current Mesh Current-1 Moving Iron Instruments Multiplexing Network Theorems Node Voltage Method On-No Load And On Load Condition POTIER & ASA METHOD POWER TRANSFORMER POWER TRANSFORMER TESTING POWER TRANSFORMER TESTING METHODS Parallel Port EXPANDER Paschen's law Piezoelectric Wave-Propagation Transducers Potential Transformer RADIO INTERFERENCE RECTIFIERS REGULATION OF ALTERNATOR REGULATION OF THREE PHASE ALTERNATOR Read a Capacitor SOLIDS AND LIQUIDS Classical gas laws Secondary effects Semiconductor strain gages Speaker Driver Strain Gages Streamer theory Superposition Superposition theorem Swinburne’s Test TMOD TRANSFORMER TESTING METHODS Tape Recorder Three-Phase Wattmeter Transformer Tap Changer Transformer Testing Vector group test Virus Activity Voltage Insulator Coating Voltage To Frequency Converter Voltage to current converter What is analog-to-digital conversion Windows work for Nokia capacitor labels excitation current test magnetic balance voltage to frequency converter wiki electronic frequency converter testing voltage with a multimeter 50 hz voltages voltmeter

Search More Posts

Followers