Electrical and Electronics Measurements MCQ Quiz - Objective Question with Answer for Electrical and Electronics Measurements - Download Free PDF

The instrument used to generate a signal of various frequency contents is an Arbitrary waveform synthesizer.

Explanation:

• Arbitrary waveform synthesizer: This instrument can generate virtually any waveform, which inherently means it can create signals with various and precisely controlled frequency components. It can synthesize complex waveforms by combining different frequencies, amplitudes, and phases, as specified by the user.
• Spectrum Analyzer: This instrument is used to measure and display the frequency spectrum of an electrical signal, showing its component frequencies and their amplitudes. It does not generate signals.
• Logic Analyzer: This instrument is used to capture and display digital signals and their timing relationships, primarily for debugging digital circuits. It does not generate analog signals with various frequency content.
• Frequency Counter: This instrument is used to measure the frequency of a periodic electronic signal. It does not generate signals.

Concept:

When a Lissajous pattern is formed on a Cathode Ray Oscilloscope (CRO), the ratio of frequencies of the vertical and horizontal signals can be determined from the pattern.

The general formula is:

Given:

Reference frequency (horizontal)

Lissajous pattern shows 2 vertical loops and 1 horizontal loop ⇒

Calculation:

Answer:

Option 3) 20 kHz

Explanation:

Ripple Factor Calculation:

The ripple factor is a measure of the residual AC component present in the output of a rectifier circuit in relation to the DC component. It quantifies the effectiveness of the rectifier in converting AC to DC. The formula for the ripple factor (ρ) is given by:

Ripple Factor (ρ) = (RMS value of the AC component) / (DC value)

To express the ripple factor in percentage:

Ripple Factor (%) = [(RMS value of the AC component) / (DC value)] × 100

From the problem statement:

• DC voltage (V_DC) = 25 V
• RMS voltage of the AC component (V_AC) = 1.5 V

Substituting the given values into the formula:

Ripple Factor (%) = [(1.5) / (25)] × 100

Ripple Factor (%) = (0.06) × 100

Ripple Factor (%) = 6%

Thus, the ripple factor is 6%, which matches Option 4.

Important Information:

The ripple factor is a crucial parameter in rectifier design and operation. It provides insights into the quality of the DC output and the extent of filtering required in the circuit. A lower ripple factor indicates a smoother and more stable DC output.

Analysis of Other Options

To further understand the analysis, let’s evaluate the other options:

• Option 1 (0.17): This value is significantly lower than the calculated ripple factor of 6%. It may have resulted from an incorrect calculation or misunderstanding of the formula.
• Option 2 (0.34): This value is also incorrect and does not match the calculated ripple factor. It suggests a calculation error or incorrect substitution of values.
• Option 3 (0.25): This value is closer to 25% and does not correspond to the calculated ripple factor of 6%. Again, it might result from an incorrect computation.
• Option 4 (0.06): This value is correct. When expressed as a percentage (multiplied by 100), it gives 6%, which aligns with the accurate calculation.

Conclusion:

The ripple factor is a key parameter for evaluating the performance of a rectifier circuit. For the given voltmeter readings, the ripple factor was calculated to be 6%, corresponding to Option 4. This value indicates the quality of the rectified output and helps in designing the appropriate filtering mechanism for further smoothing of the DC signal.

Ammeter:

• It is used to measure the current.
• An ideal ammeter has zero internal resistance and thus it provides the path for maximum current.
• It is always connected in series as it measures current.
• The range of ammeter can be extended by using a low shunt resistance.

Voltmeter:

• It is used to measure the voltage.
• An ideal voltmeter has infinite resistance and thus it provides the path for minimum current.
• It is always connected in parallel as it measures voltage.
• The range of voltmeter can be extended by using a high series resistance.

Concept:Absolute Error:

 The deviation of the measured value from the true value (or) actual value is called error. It is also known as a static error.

Static error (E) = Am – At

Am = Measured value

At = True value

Relative Static Error: The ratio of absolute error to the true value is called relative static error.

Limiting Error:

The maximum allowable error in the measurement is specified in terms of true value, is known as limiting error. It will give a range of errors. It is always with respect to true value, so it is a variable error.

Guaranteed Accuracy Error:

The allowable error in measurement is specified in terms of full-scale value is known as a guaranteed accuracy error. It is a variable error seen by the instrument since it is with respect to full-scale value.

Application:

Given-

A_m = 125 V, A_t = 125.5 V

∴ Static error (E) = 125 - 125.5

E = 0.5 V

The essential features are possessed by an indicating instrument deflecting, controlling, and a damping device.

• Deflecting device: The deflection device produces deflecting torque which causes the moving system to move from its zero position.
• Controlling device: The controlling device produces the controlling torque (Tc) which opposes the deflecting torque and increases with the deflection of the moving system. It also brings the pointer back to zero when the deflecting torque is removed.
• Damping device: This device produces damping torque this torque is necessary to bring the pointer to rest quickly. This damping torque (Td)  is used to reduce the oscillation.

• A Galvanometer is used to detect the direction of the current.
• This device is used for detecting and measuring a small amount of electric current.
• Johann Schweigger invented the device in 1820.
   

Measuring Instrument	Quantity
Photometers	Light
Magnetometer	Magnetic induction
Sound Level Meter	sound

Moving iron meter has large magnetic reluctance as compared to PMMC meter. That’s why more power is required to operate the moving iron meter.

Advantages of moving iron:

• It is a universal instrument which can be used for the measurement of AC and DC quantities
• These instruments can withstand large loads and are not damaged even under severe overload conditions
• It is very cheap due to the simple construction

Disadvantages of moving iron:

• These instruments suffer from error due to hysteresis, frequency change and stray losses
• The reading of the instrument is affected by temperature variation

Note: In terms of accuracy PMMC meter has the highest accuracy. The order of accuracy is given below.

Induction

The correct answer is 1 KWh.

Concept:

Electric bulb: It is an electric device that converts electric energy into heat and light energy.

• Resistance of the bulb 
• If we consider the bulb as a resistor then, we can easily find the current and voltage drops.
• Power consumed ∝ Brightness

The rate of work done by the electric current is called as electric power.

The difference of potential between two points is called a potential difference.

• Electric energy (E) = electric power (P) × time (t)

Unit of electric energy = unit of electric power × unit of time = kilowatt × hour = kilowatt-hour

Key Points

• Given that
  • Power of the bulb = 100 W
  • Time = 10 hours

We know that,

Energy = Power × Time

⇒ Energy consume = 100 w × 10 hrs = 1000 watt - hr = 1 KWh.

In a two-wattmeter method,

The reading of first wattmeter (W₁) = V_L I_L cos (30 – ϕ)

The reading of second wattmeter (W₂) = V_L I_L cos (30 + ϕ)

Total power in the circuit (P) = W₁ + W₂ = √3 V­­_LI_L cos ϕ

Total reactive power in the circuit 

Power factor = cos ϕ

Energy meter:

Energy meter or Watt-hour meter is used to measure the energy in kWh.

It is an integrating type instrument.

Its working principle is similar to the transformer.

There are three essential mechanisms required in the energy meter named Driving torque, Braking torque, and registered mechanism.

Driving torque:

This torque is required to revolve the disc or rotate the disc.

It is obtained by the electromagnetic induction effect.

Braking torque:

It is required to rotate the disc at a constant speed.

It is obtained by using a permanent magnet placed inside the energy meter near the Aluminum disc.

Eddy currents have induced in the magnet due to the movement of the rotating disc through the magnetic field. This eddy current reacts with the flux and exerts a braking torque which opposes the motion of the disk. The speed of the disk can be controlled by changing flux.

Breaking torque of induction type single-phase energy meter is:

K = constant

ϕ = flux

N = speed in rpm

R = radius of the disc

R_e = resistance in path of current (i.e. disc)

The braking torque of induction type single-phase energy meter is directly proportional to the square of the flux.

Registered mechanism:

It registers the no. of rotations or revolutions of the disc which is proportional to the energy consumed in kWh.

Meter constant = (No. of revolutions / kWh)

Points to remember:

Creeping:

Sometimes the disc of the energy meter makes the slow but continuous rotation at no load i.e. when the potential coil is excited but with no current flowing in the load called creeping error

This error may be caused due to overcompensation for friction, excessive supply voltage, vibrations, stray magnetic fields, etc

It can be reduced by making two opposite holes on the disc.

Null type instrument: An instrument in which zero or null indication determines the magnitude of measured quantity, such type of instrument is called a null type instrument.

It uses a null detector which indicating the null condition when the measured quantity and the opposite quantity are same.