Derive the relation for deflection torque in PMMC instrument. 

1 Answer

Answer :

Ans: The length of coil be l meter and width of coil be d meter. Assume I is the current flowing in the ciol having N turns. Assume B as the flux density in the air gap then F=BIL(N) 

image

The equation for the developed torque, derived from the basic

law for electromagnetic torque is

Ʈ= B*A*I*N

 Where r= torque Newton. Meter

 B = flux density in the air gap. Wb/m2

 A = effective coil area (m)

 N= number of turns of wire of the coil

 I = current in the movable coil (amperes)

The equation shows that the developed torque is proportional to the flux density of the field in which the coil rotates, the current coil constants (area and number of turns). Since both flux density and coil constants are fixed for a given instrument, the developed torque is a direct indication of the current in the coil. The pointer deflection can therefore be used to measure current.

Related questions

Description : Draw the diagram of PMMC instrument and state the deflection torque equation

Last Answer : Equation Ʈ= B*A*I*N Where r= torque Newton. Meter B = flux density in the air gap. Wb/m2 A = effective coil area (m) N= number of turns of wire of the coil

Description : List two advantages of PMMC instrument.

Last Answer : 1. It has uniform scale. 2. Power consumption is low 3. It can be obtained in wide ranges. 4. High sensitivity & accuracy 5. It is unaffected by external magnetic field. 6. Additional damping device not required. 7. Hysteresis problem is not there.

Description : Draw the construction and explain working principle of PMMC instruments

Last Answer : Working: When current passes through the coil a deflecting torque is produced. This deflecting torque is produced due to interaction between magnetic field produced by permanent magnet ... equilibrium; this is possible only when the controlling torque becomes equal to the deflecting torque 

Description : Draw PMMC meter movement and describe it.

Last Answer : Working principle of PMMC: 1. The working principle of PMMC is based on basic meter movement known as D'Arsonval principle stated as when current passes through the coil ... controlling torque becomes equal to deflecting torque then pointer attached with scale become stable at equilibrium. 

Description : Explain D’Arsonal PMMC movement in detail. 

Last Answer : OR Working: When current passes through the coil a deflecting torque is produced. This deflecting torque is produced due to interaction between magnetic field produced by permanent ... equilibrium; this is possible only when the controlling torque becomes equal to the deflecting torque.

Description : Sketch the circuit of basic DC ammeter , derive equation for shunt resistance.

Last Answer : Derivation The basic movement of dc ammeter circuit consists of D' Arsonval galvanometer. When large current is to be measured then some extra modification is required. For measurement of large current by using ... same. Vsh =Vm m IshRsh=ImRm Rsh=ImRm/Ish Rsh=ImRm/(I-Im) 

Description : Draw and explain block diagram of Horizontal deflection system. 

Last Answer :  Trigger circuit: A trigger circuit is used to convert the incoming signal into trigger pulses, so that the input signal and the sweep frequency can be synchronized. The trigger circuit is ... before it is applied to the horizontal section. The block consists of a push- pull amplifier  

Description : Draw and describe horizontal deflection system in CRO 

Last Answer : Block Diagram: Explanation: Input from vertical amplifier is given to Horizontal system. Trigger circuit: a. A trigger circuit is used to convert the incoming signal into trigger ... : Horizontal amplifier amplifies the signal & passes the signal to horizontal plates of CRT. 

Description : Explain vertical deflection subsystem with neat diagram.

Last Answer : vertical deflection subsystem The main function of the vertical deflection system is to provide an amplified signal of prope r level toderive the vertical deflection plates without any ... suppression, reduced no nlinear effects because none of the plates are at ground potential. 

Description : Sketch Block diagram of vertical deflection system used in CRO.

Last Answer : Block diagram of vertical deflection system used in CRO

Description : Which of the following factors limit the deflection of the pointer of a PMMC instrument of about 90o?  1. Its damping mechanism  2. Linearity of magnetic field in which the coil moves  3. Control spring arrangement  4. ... Only 1 and 3 (b) Only 2 and 4 (c) Only 2 and 3 (d) Only 1 and 4 

Last Answer : Which of the following factors limit the deflection of the pointer of a PMMC instrument of about 90o?  1. Its damping mechanism  2. Linearity of magnetic field in which the coil moves  3. Control spring arrangement  4. ... Only 1 and 3 (b) Only 2 and 4 (c) Only 2 and 3 (d) Only 1 and 4 

Description : State the advantages of Digital instrument.

Last Answer : 1. They are having high input impedance, so there is no loading effect 2. They are having higher accuracy 3. An unambiguous reading is obtained 4. The output can be interfaced with external equipment 5. They are available in smaller size

Description : Define: (i) Absolute Instrument (ii) Secondary Instrument 

Last Answer : i)Absolute Instrument:An instrument whose calibration can be determined by means of physical measurements on the instrument.  ii)Secondary Instrument:Secondary instruments are those, in which the ... instruments, only when they have been pre-calibrated by comparison with an absolute instrument.

Description : Suggest instrument to measure unknown frequency above 5 MHz and store result. Justify it.

Last Answer : For measurement of frequency CRO, DSO SPECTRUM ANALYZER & FREQUENCY COUNTER can be used. In above specification we can used CRO & DSO for measurement, but the data has to be stored so we ... width or higher bandwidth can be used. Because DSO has measurement facility as well as storage facility.

Description : As the deflection of the moving system increases, the controlling torque in an indicating instrument.

Last Answer : increases

Description : Which of the following instruments have least torque/weight ratio: a) Dynamometer type b) PMMC c) Attraction type moving iron d) Repulsion type moving iron

Last Answer : Ans: A

Description : Controlling torque in PMMC

Last Answer : spring action

Last Answer : In the case of PMMC rectifier type instruments, the deflecting torque is proportional to Average value of AC current.

Description : Draw block diagram of digital frequency meter in time mode and describe its operation.

Last Answer : OR Operation: The input signal is amplified and converted to a square wave by a Schmitt Trigger circuit. In this diagram the square wave is differentiated and clipped to ... thereby closing it. The decimal counter and display unit output correspond to the number of input pulses. 

Description : Draw block diagram of DMM. State its advantages.

Last Answer : Advantages of DMM: 1. DMM provides numerical readouts that eliminate observational errors. Thus providing better readability. 2. DMM offers better accuracy and versatility as compared to ... memory devices for further computations. 5. The decreased size of DMM increases the portability

Description : What is LCR meter? Draw digital LCR –Q meter block diagram.

Last Answer : An LCR meter is a type of electronic test equipment used to measure the inductance (L), capacitance (C), and resistance (R) of an electronic component. Block diagram digital LCR –Q meter  

Description : Draw Q meter circuit of series connection and explain it.

Last Answer : Q meter circuit of series connection OR Explanation The Q meter works on series resonant. The resonance is the condition exists in the circuit when their inductance and capacitance reactance are ... . Hence E=Q Ec so ,the voltmeter is calibrated for finding the value of Q factor.  

Description : Draw block diagram of distortion factor meter and explain its operation.

Last Answer : Block Diagram Operation This meter consist following four sections 1. Impedance converter along with input circuit 2. Rejection amplifier 3. Metering circuit 4. Power circuit 1. ... of the total input voltage. 4. The power supply unit provides the supply for proper operation 

Description : Draw block diagram of spectrum analyser, State applications of spectrum analyser.

Last Answer : Block Diagram: Applications: 1. Spectrum analyzer is a very important item of test equipment for someone designing or repairing electronic equipment that uses radio frequency signals. 2. Its ... of the dominant frequency within a signal 8. Used to measure the properties of RF devices

Description : Define wave analyser and state its need, Draw the block diagram of it. 

Last Answer : Wave analyzer:- Wave analyzer is an instrument which is used to measure the magnitude of the various harmonics of a complex waveform. It is an instrument that is designed to measure the relative amplitudes ... distortion and the system signal to noise ratio. Block Diagram of Wave analyser

Description : Draw block diagram of RF signal generator and explain its operation.

Last Answer : RF signal generator contains three other main sections: 1.OSCILLATOR CIRCUIT, 2. MODULATOR, 3. OUTPUT CONTROL CIRCUIT. The modulator modulates the RF signal of the ... . Metal shielding surrounds the unit to prevent signals from the oscillator from affecting the circuit under test

Description : State and describe different triggering available in CRO

Last Answer : A trigger is an external signal or input signal condition that initiates one or more instrument functions. Trigger settings include digital edges, software functions, and analog levels. This tutorial ... the external trigger input. The following figure shows an entering window trigger.  

Description : Draw block diagram of DSO . State applications of DSO

Last Answer : Applications of DSO : 1. It can be used to measure both AC and DC voltages and currents. It can also calculate RMS value, peak value, peak to peak value etc. 2. It ... . 5. It can be used to determine the modulation characteristics and detect the standing waves of transmission lines.

Description : Draw and state how Aryton shunt type DC ammeter . State advantage of using Aryton shunt.

Last Answer : OR The Aryton shunt eliminates the possibility of having the meter in the circuit without a shunt. This advantage is gained at the price of slightly higher overall Aryton shunt ammeter. In this ... between shunts. 3. Another advantage is that it may be used as a wide range ammeter. 

Description : Explain sensitivity and loading effect in voltmeter.

Last Answer : Sensitivity: The sensitivity of a voltmeter is just its capability to detect voltage, usually meaning smaller and smaller voltages. When selecting a meter for a certain voltage measurement, it is ... called loading effect of the voltmeters. It is mainly caused due to low sensitivity instruments.

Description : What is video pattern generator? State its application.

Last Answer : A pattern generator provides video signals directly, and with RF modulation, on standard TV channels for alignment ,testing and servicing of TV receivers. The output signal designed to produce simple ... section checking 6. Trouble shooting video amplifier and using variable video output.

Description : Draw block diagram of function generator and list two specifications of it. 

Last Answer : Specifications: 1. Output- Square wave , sine wave, Triangular, TTL pulse 2. Frequency Ranges- 0.1 Hz to 11 MHz, up/down range switchable in eight decade steps 3. Dial Range-1 to 11 calibrated 0.1 to 1 uncalibrated 4. Pulse and Ramp Aspect Ratio-95:5

Description : Draw the basic block diagram of single trace CRO and describe the function of delay line.

Last Answer : Function of Delay line: This block is used to delay the signal for a period of time in the vertical section of CRT. The input signal is not applied directly to the vertical plates because the part ... when the delay time is not used. Therefore, the input signal is delayed by a period of time. 

Description : Define the following: 1) Speed of response 2) Lag 3) Fidelity 4) Dynamic Error. 

Last Answer : (1) Speed of Response: It is the rapidity with which an instrument responds to changes in the measured quantity.  (2) Lag : It is the retardation or delay in the response of an instrument ... with time and value indicated by instrument if no static error is assumed is called as dynamic error.

Description : Explain measurement of phase and frequency using Lissajous pattern. Write the formula for each one.

Last Answer : Phase measurement of Lissajous pattern: The phase measurement can be done by using Lissajous figures. The CRO is set to operate in the X- Y mode, then the display obtained on the screen of a ... or sub- multiple of the unknown frequency.  The Lissajous pattern appears stationary.     

Description : Explain the concept of single beam dual trace CRO with its block diagram.

Last Answer : Diagram:   Explanation: The single beam dual trace CRO consists of one electron gun, an electronic switch which switches the two signals to a single vertical amplifier and a single pair of ... Amplifier are identical & have the same delay time, accurate X-Y measurements can be taken. 

Description : Explain the measurement of voltage and frequency using CRO.

Last Answer : Voltage Measurement: CRO displays the voltage signal as a function of time on its screen. The amplitude of that voltage signal is constant, but we can vary the number of divisions that cover the voltage ... /division nv is the number of divisions that cover the signal in vertical direction.

Description : Draw the construction of CRT. Write two materials used for CRT display screen. 

Last Answer : Two materials used for CRT display screen: The role of this part is to display where the electrons are hitting the CRT. It is a screen coated with a material that emits light when struck by electrons. Zinc sulphide or Phosphorus are two commonly used materials. 

Description : Explain gross error, systematic error and random error.

Last Answer : 1) Gross Errors -These errors occur due to human mistakes while taking reading, handling instrument, incorrect setting or adjustment and improper used of instrument. The complete elimination of gross ... remain since the systematic and gross error are removed. Generally these errors are very small.

Description : How are instruments classified? Describe the different types.

Last Answer : The instruments are classified depending upon: a. Principle of operation: E.g.: Magnetic meter, induction meter. b. Permissible percentage error c. Depending upon application: E. ... measured by observing the output indicated by instrument. Example: Voltmeter, Thermometer, pressure guage.

Description : Define standards. State and explain classification of standards.

Last Answer : Standards:-Standard is a physical representation of a unit of measurement. A known accurate measure of physical quantity is termed as standard. These standards are used to determine ... used to check components and calibrating laboratory instruments to achieve good accuracy and better performance.

Description : State two uses of logic analyser. 

Last Answer : 1. Real time application 2. Troubleshooting and analysis of digital systems 3. Compatible with printer using RS 232 and IEEE 488 interface 4. Testing the IC s to detect defect before constructed. ... Industries. 7. In various software industries. 8. To find fault in Bio- medical instruments. 

Description : Define signal generator and state its need.

Last Answer : Signal Generator: A signal generator is an electronic device that generates repeating or Non-repeating electronic signals in either the analog or the digital domain. Need: The generation of ... of various electronic systems and for replacing missing signals in systems being analysed for repair. 

Description : State the difference between CRO and DSO.

Last Answer : CRO DSO Direcly reads analog voltage and displays it on screen. It reads the analog voltage and converts it into digital form before being displayed on the screen. ... DSP algorithms available and ported on microprocessor which can operate on stored samples of input voltage. 

Description : List different types of CRO probes

Last Answer : 1. Passive probes 2. Active probes 3. High voltage probes 4. Current probes 5. Differential probes

Description : State applications of digital frequency meter.

Last Answer : 1. For testing radio equipment 2. Measuring the temperature, pressure, and other physical values. 3. Measuring vibration, strain 4. Measuring transducers

Description : Define: i)Accuracy ii)Precision

Last Answer : Accuracy: The degree of closeness with which an instrument approaches the true value of the quantity being measured is known as accuracy.  Precision:The measure of the degree to which successive measurements differ from each other is known as precision 

Description : Draw block diagram of digital LCR –Q meter and explain its working.

Last Answer : Block diagram Working The output of LCR-Q meter is given to current to voltage converter, AC amplifier or DC amplifer depending upon position of switch Current to voltage ... /D converter. A/D converter converts analog signal into digital output andfinaaly displayed on display.

Description : Draw block diagram of Dual slope integrating voltmeter and also draw waveform for voltage V/s time. 

Last Answer : Block diagram

Description : Draw block diagram of successive approximation digital voltmeter. State its two advantages and two disadvantages

Last Answer : Block diagram Or Advantages: o It is economical method of analog to digital conversion. o The resolution is up to 5 important digits are obtained. Disadvantages: o ... depends on which type of switches are used. o The conversion time required for digital to analog converter.