11. Current in a pressure coil of the Dynamometer type wattmeter ________
Lags the applied voltage
Leads the applied voltage
Is in phase with the applied voltage
There is a phase difference of 90 degrees
Answer.1. Lags the applied voltage
Explanation:
In Dynamometer type wattmeter the pressure coil circuit is purely resistive. In reality, however, the pressure coil will have certain inductance along with resistance.
Due to the inherent inductance of the pressure coil circuit, the current and voltage in the pressure coil will no longer be in phase, rather the current through the pressure coil will lag the voltage across it by a certain angle.
12. The circuit in fig is used to measure the power consumed by the load. The current coil and the voltage coil of the wattmeter have 0.02 Ω and 1000 Ω resistance respectively. The measured power compared to the load power will be.
0.4 % less
0.2 % less
0.2 % more
0.4 % more
Answer.3. 0.2 % more
Explanation:
True power = 200 × 20 × 1 = 4000 W
Measured power = 4000 + I2 R = 4008 W
Error = (Measured Value – True value)/ True Value
= (4008 – 4000)/4000 = 1/500
= 0.2 % more
13. Wattmeters are compensated for errors due to inductance by _________
Using a series capacitor
Using a parallel capacitor
Using a series resistance
Using a parallel resistance
Answer.4. Using a parallel resistance
Explanation:
The inductance of the potential coil is liable to cause an error in the reading of the wattmeter. Because of this error, the wattmeter gives a high reading on lagging power factors and low reading on the leading power factors. This error can be reduced or compensated by connecting a capacitor in parallel with a portion of the multiplier.
14. A 300 V, 20 A dynamometer instrument is used as a wattmeter. Its current coil has 0.2 Ω resistance and the pressure coil has 20 kΩ resistance with negligible inductance. What is the error in the instrument if it is used to measure the power in a circuit with a supply voltage of 300 V, load current of 16 A at 0.4 pf. Assume that the pressure coil is connected across the load side.
2.59%
0.72%
2.67%
0.23%
Answer.4. 0.23%
Explanation:
Given that, Voltage (V) = 300 V
Current (I) = 16 A
Current coil resistance (RC = 0.2 Ω
Pressure coil resistance (RP = 20 kΩ
Power factor = cos ϕ = 0.4
Power (P) = VI cos ϕ = 300 × 16 × 0.4 = 1.92 kW
As the pressure coil is connected across load side, the error is due to pressure coil resistance.
Error = V2/RPC
= (300)2/20 × 103 = 4.5 kW
Percentage error = 4.5/(1.92 × 103)
= 0.23%
15. Wattmeters are compensated for the power loss in a potential coil by _________
Using a series capacitor or compensating coil
Using a parallel capacitor
Using a series resistance
Using a parallel resistance
Answer.1. Using a series capacitor or compensating coil
Explanation:
Effect due to power loss in potential coil or current coil: Another possible error in the indicated power may be due to some voltage drop in the current coil or the current taken by the potential coil. This defect can be overcome by having an additional compensating winding that is connected in series with the potential coil but is so placed that it produces a field in opposite direction to that of the current coils
16. _______ may affect the accuracy of the wattmeter by altering the coil resistances.
Power
Temperature
Both Power and Temperature
Eddy current
Answer.2. Temperature
Explanation:
Temperature changes may affect the accuracy of the wattmeter by altering the coil resistances. The temperature may change due to changes in room temperature or even due to heating effects in conductors with the flow of current. Change in temperature also affects the spring stiffness, thereby introducing error in the deflection process.
High-precision instruments are fitted with temperature compensating resistors that tend to neutralize the effects of temperature variation.
17. The errors due to vibrations in the wattmeter are eliminated by designing ________
Twice the supply voltage frequency
Natural Frequency
Thrice the supply Frequency
Any of the above
Answer.2. Natural Frequency
Explanation:
The instantaneous torque on the moving system varies cyclically at twice the frequency of the voltage and current.
If some part of the moving system, say pointer or spring, possesses a natural frequency in approximation with the torque pulsation frequency, it vibrates with a significant magnitude.
Due to these vibrations, the readings become difficult to observe as well as the mean position of the pointer gets displaced from the respective power reading and points to the average power reading.
To eliminate such errors, the instruments are so designed that the natural frequency of the moving system is very much away from twice the frequency of the system on which the instrument is supposed to work.
18. _______ error is induced in metallic parts of the instrument.
Stray magnetic Error
Eddy current error
Vibration Error
Temperature error
Answer.2. Eddy current error
Explanation:
Error due to eddy currents: The alternating field of the current coil induces eddy currents in the solid metal parts that set up their own magnetic field. This alters the magnitude and phase of the magnetic field causing deflection. Thus, the error is introduced in the instrument reading. To reduce this error, the solid metal parts are removed as far away from the current coil as possible.
19. In wattmeter If the inductive and capacitive reactances are equal resulting in ______ error.
Low Error
High Error
Zero Error
Any of the above
Answer.3. Zero Error
Explanation:
In addition to the Inductance of the pressure coil, there may be capacitance due to interterm capacitance of the series resistance. The effect of capacitance is exactly the opposite of that of Inductance. Thus the wattmeter reads low on lagging power factors.
The phase angle between pressure coil current and applied voltage depends upon the reactance of the pressure coil circuit. The inductive reactance is normally greater than capacitive reactance, thus the phase angle increases with an increase in frequency.
If the capacity reactance of the pressure coil circuit is equal to its inductive reactance, there will be no error due to these effects since the two errors will neutralize each other.
20. In wattmeter the stray magnetic field error has _________
Strong magnetic field
Intermediate magnetic field
Weak magnetic field
No magnetic field
Answer.3. Weak magnetic field
Explanation:
Stray Magnetic Field Errors in wattmeter: The operating field in electrodynamometer-type instruments is weak, special care must be taken to protect these instruments from external magnetic fields. Hence, these instruments should be shielded against the effects of stray magnetic fields. Laminated iron shields are used in portable laboratory instruments, while steel casings are provided as shields in switchboard-mounted wattmeters.