Digital Frequency Meter
Ohmmeter Working Principle of Ohmmeter
Potentiometer Working Principle of Potentiometer
Induction Type Meters
What is Low Power Factor Wattmeter ?
Energy Meter with Lag Adjustment Devices
Electrodynamometer Type Wattmeter
Measurement of Three Phase Power
Varmeter | Single Phase and Polyphase Varmeter
Megger | Working Principle Types History Uses of Megger
Weston Type Frequency Meter
Power Factor Meters | Electrodynamometer Type Power Factor Meter
Phase Sequence Indicator
Ammeter Working Principle and Types of Ammeter
Insulation Resistance Test and Polarization Index Test
Tan Delta Test |Loss Angle Test | Dissipation Factor Test
Electronic DC Voltmeter
Working Principle of Voltmeter and Types of Voltmeter
Digital Voltmeters Working Principle of Digital Voltmeter
Measurement of Electrical Energy
Energy Meter Testing
Construction of AC Energy Meter
Electrical Measuring Instruments | Types Accuracy Precision Resolution Speed
Errors in Measurement | Classification of Errors
Permanent Magnet Moving Coil Instrument or PMMC Instrument
Moving Iron Instrument
Electrostatic Type Instruments Construction Principle Torque Equation
Rectifier Type Instrument | Construction Principle of Operation
Thermocouple type Instruments Construction Principle of Operation
Wheatstone Bridge Circuit Theory and Principle
Kelvin Bridge Circuit | Kelvin Double Bridge
Maxwell Bridge Inductance Capacitance Bridge
Anderson′s Bridge | Advantages Disadvantages of Anderson′s Bridge
Hay′s Bridge Circuit Theory Phasor Diagram Advantages Applications
Owens Bridge Circuit and Advantages
Schering Bridge Measurement of Capacitance using Schering Bridge
De Sauty Bridge
Heaviside Bridge Circuit
Blavier Test | Murray Loop Test | Varley Loop Test | Fisher Loop Test
Transducer | Types of Transducer
Radiation Pyrometer | Types Working Principle
Resistance Temperature Detector or RTD | Construction and Working Principle
What is an Oscillator?
Temperature Sensor Temperature Measurement | Types of Temperature Sensor
Sensor | Types of Sensor
Optical Pyrometer | Construction and Working Principle
Bimetallic Strip Thermometer
Thermistor Thermometer | Thermistor Temperature Sensor | Construction and Principle
Thermocouple Temperature Measurement
Linear Variable Differential Transformer LVDT
Thermistor Definition Properties Construction Characteristics and Applications
Cathode Ray Oscilloscope | CRO
Lissajous Patterns of CRO or Cathode Ray Oscilloscope
Frequency Limitation of an Oscilloscope
Measurement of Voltage Current and Frequency by Oscilloscope
Digital Storage Oscilloscope
Double Beam Oscilloscope
Electrodynamometer Type Wattmeter
Construction and Working Principle of Electrodynamometer Type WattmeterNow let us look at constructional details of electrodynamometer. It consists of following parts There are two types of coils present in the electrodynamometer. They are :
Moving Coil Moving coil moves the pointer with the help of spring control instrument. A limited amount of current flows through the moving coil so as to avoid heating. So in order to limit the current we have connect the high value resistor in series with the moving coil. The moving is air cored and is mounted on a pivoted spindle and can moves freely. In electrodynamometer type wattmeter, moving coil works as pressure coil. Hence moving coil is connected across the voltage and thus the current flowing through this coil is always proportional to the voltage. Fixed Coil The fixed coil is divided into two equal parts and these are connected in series with the load, therefore the load current will flow through these coils. Now the reason is very obvious of using two fixed coils instead of one, so that it can be constructed to carry considerable amount of electric current. These coils are called the current coils of electrodynamometer type wattmeter. Earlier these fixed coils are designed to carry the current of about 100 amperes but now the modern wattmeter are designed to carry current of about 20 amperes in order to save power. Control System Out of two controlling systems i.e.
- Gravity control
- Spring control, only spring controlled systems are used in these types of wattmeter. Gravity controlled system cannot be employed because they will appreciable amount of errors.
Now let us derive the expressions for the controlling torque and deflecting torques. In order to derive these expressions let us consider the circuit diagram given below: We know that instantaneous torque in electrodynamic type instruments is directly proportional to product of instantaneous values of currents flowing through both the coils and the rate of change of flux linked with the circuit. Let I1 and I2 be the instantaneous values of currents in pressure and >current coils respectively. So the expression for the torque can be written as:
Advantages of Electrodynamometer Type WattmeterFollowing are the advantages of electrodynamometer type wattmeters and they are written as follows:
- Scale is uniform upto certain limit.
- They can be used for both to measure ac as well dc quantities as scale is calibrated for both.
Errors in Electrodynamometer Type WattmeterFollowing are the errors in the electrodynamometer type wattmeters:
- Errors in the pressure coil inductance.
- Errors may be due to pressure coil capacitance.
- Errors may be due to mutual inductance effects.
- Errors may be due connections.(i.e. pressure coil is connected after current coil)
- Error due to Eddy currents.
- Errors caused by vibration of moving system.
- Temperature error.
- Errors due to stray magnetic field.