# Dynamometer Wattmeter Errors MCQ || Errors in Dynamometer Wattmeter Questions and Answers

1. Pressure coil of a wattmeter has ________

1. Capacitance and inductance
2. Inductance and resistance
3. Resistance and capacitance
4. Only inductance

Explanation:

The wattmeter method of power measurement in a single-phase electrical circuit has the following errors:

• Voltage (or pressure) coil inductance error
• Voltage coil capacitance error
• Eddy current error

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.

The voltage coil capacitance error is caused due to inter-turn capacitance in the high-value series resistance of the voltage coil circuit, which may have capacitance as well as inductance.

If the capacitive reactance of the voltage coil circuit is equal to its inductive reactance, there will be no error due to these effects since the two individuals.

2. Find the self-inductance (in mH) of the pressure coil if a dynamometer wattmeter measures power in 50 Hz, single-phase circuit without error, at all power factors the resistance of voltage coil and its series resistance are 800 Ω and 18000 Ω. The series resistance has distributed self-capacitance equivalent to a shunt capacity of 25 PF.

1. 10.2 mH
2. 9.6 mH
3. 8.1 mH
4. 7.4 mH

Explanation:

We know that self-inductance of the pressure coil is given by

LP = CC × R2s

Self capacitance CC = 25 = 25 × 10-12

Series resistance = 18000 Ω

LP =  25 × 10-12 × (1800)2

LP = 8.1 mH

3. The effect of capacitance on wattmeter reading is?

1. Aiding the inductance
2. Opposite to that of inductance
3. Aiding the capacitance
4. Opposite to that of resistance

Answer.2. Opposite to that of inductance

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.

4. A dynamometer type of wattmeter is rated at 15 A and 180 V with a full-scale reading of 1500 W. The inductance of the voltage circuit is 10 mH and its resistance is 2 kΩ. If the voltage drop across the current coil of the wattmeter is negligible. The error in the wattmeter at the rated VA rating with 0.5 power factor is

1. 4.48 W
2. 8.64 W
3. 5.36 W
4. 3.67 W

Explanation:

Reactance of pressure coil = 2π × 50 × 10 × 10-3 = 3.14 Ω

Pressure coil phase angle β

⇒ tanβ = 3.14/2000 = 1.57 × 10−3 rad/sec

Error in wattmeter with zero power factor

= VI Sinϕ tanβ = 180 × 15 × 0.866 × 1.57 × 10-3

= 3.67 W

5. Wattmeter reading is affected by the change in ______

1. Connection
2. Temperature
3. Both connection and Temperature
4. None of the above

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.

6. The coils of a wattmeter have resistances of 0.01 Ω and 1000 Ω; their inductances may be neglected The wattmeter is connected as shown in the figure, to measure the power consumed by a load, which draws 25A at power factor 0.8. The voltage across the load terminals is 30 V. The percentage error on the wattmeter reading is –

1. 0.15%
2. 2.20%
3. 0.20%
4. 0.10%

Explanation:

Given,

Load consumes 30 V, 25 A, 0.8 power factor

True power consumed by the load

Ptrue = VI cos ϕ

Ptrue = 30 ×  25 × 0.8 = 600 W

The given connection is an LC connection, therefore the losses will be due to pressure coil

Given,

Pressure coil resistance = 1000 Ω

Pressure coil loss = V2/R = (30)2 / 1000 = 0.9 W

Measured power  = True Power + Pressure coil losses

Pmeas = 600 + 0.9 = 600.9 W

Relative Static Error RSE = |Am − At| × 100 /At

= |600.9 − 600| × 100 /600

RSE = 0.15%

7. Dynamometer type wattmeter has _________

1. Strong magnetic field
2. Intermediate magnetic field
3. Weak magnetic field
4. No magnetic field

Explanation:

The dynamometer-type wattmeter has a relatively weak operating field. Therefore, stray fields affect the reading of this instrument considerably and cause serious errors. Hence, this type of instrument must be shielded against stray magnetic fields by using iron cases or providing thin iron shields over the working parts.

8. The resistances of two coils of a wattmeter are 0.01 ohm and 1000 ohms respectively and both are non-inductive. The load current is 20 A and the voltage across the load is 30 V. If one of the ways of connecting the voltage coil (LC short connection), the error in the reading would be

1. 0.1% too high
2. 0.2% too high
3. 0.15% too high
4. Zero

Explanation:

The error in wattmeter due to pressure coil resistance = V2/RP

= 302/1000 = 0.9 W

The true power = VLIL = 30 × 20 = 600 W

The percentage error = 0.9 × 100/600 = 0.15%

9.  Wattmeter reading has errors induced by _________

1. Resistance
2. Self-capacitance
3. Self-inductance
4. Mutual inductance

Explanation:

The mutual inductance between pressure and current coils of the wattmeter results in errors. These errors are directly proportional to frequency. At power frequencies, these errors are very low but become prominent with an increase in frequency. These errors change the phase angles of wattmeters. When the pressure coil is connected on the load side, the phase angle increases, while it decreases when the current coil is connected on the load side.

10. The potential coil of a single-phase dynamometer wattmeter has 4840 Ω resistance. The voltage across the load is 220 V with the potential coil connected on the load side, the meter indicates 100 W. Approximate percentage error due to wattmeter connection is

1. 11%
2. 9%
3. 1.1%
4. 3%

Explanation:

Meter reading (Am) = 100 W

Power loss in potential coil

PL = V2/P = (220)2/4840 = 10 W

Actual value (At) = 100 – 10 = 90 W

Error = (Measured Value – True value)/ True Value

= (100 – 90)/90 = 1/9

% Error = 11.11%

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