Frequency and Power Factor Measurement MCQ || Measurement of Frequency and Power Factor Questions and Answers

Answer.3. Sweep frequency plots of impedance and phase angle versus frequency

Explanation:

The Vector Impedance Meter is employed for measuring both the amplitude and phase angle of impedance (Z).

• Impedance can be obtained in polar form
• Its coverage can be obtained with a sweep frequency plot of impedance and phase angle versus frequency.

Answer.3. Measurement of power in resistive loads

Explanation:

The low power factor meter measures the low value of the power factor accurately known as the Low Power Factor Wattmeter (LPFW). The low power factor meter is used for measuring the power of the highly inductive circuit.

The ordinary Wattmeter used for measuring the low power factor gives an inaccurate result. This happens because of two reasons.

• In a low power factor meter, the magnitude of deflecting torque on the moving coil is small even after the full excitation of the pressure and current coil.
• The error occurs in the reading because of the pressure coil inductance.
• It is not suitable for the measurement of power in resistive loads.

Answer.1. Measuring flicker

Explanation:

• IEC 61000-4-15 gives a functional and design specification for flicker measuring apparatus intended to indicate the correct flicker perception level for all practical voltage fluctuation waveforms. Information is presented to enable such an instrument to be constructed.
• A method is given for the evaluation of flicker severity on the basis of the output of flicker meters complying with this standard.
• The flicker meter specifications in this part of IEC 61000 relate only to measurements of 120 V and 230 V, 50 Hz and 60 Hz inputs.

Answer.3. They have high torque to weight ratio.

Explanation:

Advantages of Electrodynamic Type Power Factor Meters

• Air cored coils are used, they are generally free from hysteresis and eddy current errors when used on AC circuits.
• These instruments can be used for both DC. and AC measurements.
• They have high torque to weight ratio.
• It has precision grade accuracy when used as a wattmeter.
• Less power consumption.

Disadvantages of Electrodynamic Type Power Factor Meters

1. Working forces are small as compared to moving iron-type instruments.
2. The scale is not extended over 360^o.
3. The calibration of electrodynamometer-type instruments is highly affected by changing the supply voltage frequency.
4. They are quite costly as compared to other instruments.

Answer.1. Converting sinusoidal waveforms into rectangle pulses

Explanation:

A digital frequency meter is a general-purpose instrument that displays the frequency of a periodic electrical signal to an accuracy of three decimal places.

Working principle:

• The unknown frequency signal is fed to the Schmitt trigger. This signal may be amplified before being applied to the Schmitt trigger.
• In a Schmitt trigger, the sinusoidal signal is converted into a square wave with very fast rise and fall times, then differentiated and clipped.
• As a result, the output from a Schmitt trigger is a train of pulses, one pulse, for each cycle of the signal.
• The output pulses from the Schmitt trigger are fed to a start/stop gate when this gate opens (start), the input pulses pass through this gate and are fed to an electronic counter which starts registering the input pulses.

Answer.4. Electrical resonance type

Explanation:

• The electrical resonance type frequency meter is an indicating type instrument.
• Its action depends upon the electrical resonance.
• It mainly consists of a fixed coil and a moving coil. There is a laminated iron core of varying cross-sections.
• This varying laminated core holds the fixed coil at its one end. Then we connect this fixed coil across the supply mains.

In the Electrical resonance type frequency meter, the frequency is found when the torque in the moving coil becomes zero.

Torque Equation:

Let us consider I₁ as the supply current of the fixed coil and I₂ as the induced current of the moving coil.

We have the phase angle between the supply current I₁ (current in the fixed coil) and the emf induced in the moving coil is 90°.

Again there is a phase difference between the induced emf and the induced current I₂ (current in the moving coil).

Let us consider the angle of this phase difference is α.

So, the actual phase difference between I₁ and I₂ will be (90° – α).

Therefore, we can write the expression of the torque (T) as,

T = I₁I₂ cos (90° – α)

From the above expression of the torque, we can see that the torque will be zero when α is zero.

That means there must not be any phase difference between the induced current and the induced emf in the moving coil.

Answer.1. Deflection

Explanation:

1. The Weston frequency meter is a moving iron instrument used for measuring the unknown frequency of a signal.
2. A Weston type frequency meter is based on the principle of deflection.
3. It consists of 2 coils, which are resistive coil and an inductive coil.
4. The current in the coil changes, whenever the signal frequency deviates from the standard frequency.
5. The deflection of the pointer will move towards a higher magnetic field.

Answer.2. 2

Explanation:

Multiplying factor is given by

M_F = VICosφ/Full scale Power

Voltage = 150 V

Current = 10 A

PF = 0.2

Full scale power = 150 W

M_F = (150 × 10 × 0.2)/150

M_F = 2

Answer.3. 2 kHz

Explanation:

Let

f_x = unknown frequency

f_c = crystal frequency

f_o = crossover frequency

Then, If f_x < f_o periodic mode

And f_x > f_o frequency mode

Now crossover frequency f_o = √fc

f_o = √4 × 10⁶ = 2 × 10³

f_o = 2 kHz

Answer.3. Schering bridge

Explanation:

Schering bridge is used to measure the relative permittivity, dielectric loss, and power factor of a capacitor.

Advantages of Schering Bridge

• Balance equations are free from frequency.
• The arrangement of the bridge is less costly compared to the other bridges.

Answer.1. Wien’s bridge

Explanation:

The Wien’s bridge is used in AC circuits for determining the value of unknown frequency. The bridge measures the frequencies from 100Hz to 100kHz.

The accuracy of the bridges lies between 0.1 to 0.5 percent. The bridge is used for various other applications like capacitance measurement, the harmonic distortion analyzer, and an HF frequency oscillator.

Answer.3. Both current and voltage coil

Explanation:

• The power factor meter measures the power factor of a transmission system.
• The power factor meter determines the types of load used on the line, and it also calculates the losses that occur on it.
• The meter has one fixed coil which acts as a current coil. This coil is split into two parts and carries the current under test.
• The meter has two identical pressure coils A and B. Both the coils are pivoted on the spindle.

• The pressure coil A has no inductive resistance connected in series with the circuit, and the coil B has a highly inductive coil connected in series with the circuit.
• The current in coil A is in phase with the circuit while the current in coil B lag by the voltage nearly equal to 90°.
• The meter has two deflecting torque one acting on coil A, and the other is on coil B. The windings are so arranged that they are opposite in directions. The pointer is in equilibrium when the torques are equal.

Answer.1. 

Explanation:

• Electrodynamic Power Factor Meter is also known as the Dynamometer phase angle meter and Dynamometer power factor meter.
• It measures the power factor or cosine of the phase angle between voltage and current.
• There are 2 stationary coils (SC) also called current coils that are connected in series to the load.
• The current coil produces a magnetic field proportional to the current.
• There are 2 moving coils (MC) also called voltage or pressure coils, that are connected parallel to the load.
• One moving coil is connected with a high resistor while another with a high inductor. These 2 coils make separation of 90° electrical.
• The coils are arranged in such a way that 2 equal and opposite torques are produced and the pointer shows the desired result.
• Therefore, there is no requirement for a controlling system.

Answer.3. Both potential coil and the current coils

Explanation:

• Three-phase power factor is an electric instrument, used to measure the power factor.
• Three current coils produce alternating flux and it acts on the pointer to deflect.
• The angular deflection of the pointer gives the phase angle ϕ.
• The rotating field of the coils of the 3-phase turns the moving pointer continuously in the direction of rotating flux.
• The moving pointer is a potential coil. The flux produced by the current coil is linked with both the current and potential coil.
• Moving potential coil is made up of high resistive material to reduce the effect of eddy current.

Answer.1. 0.5

Explanation:

The Power factor of saw mill is in the range of 0.45-0.60.