VIGNAN’S UNIVERSITY :: VADLAMUDI
EE216 - Electrical Measurements & Instrumentation (EM&I)
UNIT – III
Measurement of Power and Energy
MID-II
1Mark Questions
1.
Dynamometer type wattmeter used to measure--------------------a. Ac power only
2.
b. Dc power only
b. uniform
c. crowded in the beginning
d. both b & c
In dynamometer type wattmeter, the moving coil is connected----------------------a. In series with the fixed coil
c. In series with the load
4.
d .All of these
Scale of dynamometer type wattmeter is --------------------------a. non uniform
3.
c. both Ac and DC power
b. Across the supply
d. Across the load
The pressure coil of induction energy meter is ---------------------a. highly resistive
b. highly inductive c. highly capacitive
d. highly inductive
& capacitive
5.
If an induction type energy meter runs fast, It can be slow down by adjusting---------a. disc
6.
b. Braking magnet
d. series magnet
Damping provided in dynamometer type instrument is ----------------a. Eddy current
7.
c. Shunt magnet
b. Fluid friction
c. Air friction d. none of the above
In the two wattmeter method of measuring 3-phase power, The two wattmeter's
indicate equal and opposite readings when the load power factor angle is ---------------lagging
a.60 deg
b.0 deg
c.90 deg d.30deg
8.
Creeping in a single phase indution type energy meter may be due to --------------------
9.
Trivector meter measures-----------
10.
The power in a 3-phase circuit is-----------------------
11.
In a 3-phase power measurement by two wattmeter method, both the watt meters had
identical readings. The power factor of the load was
a. unity
b.0.8 lagging
c 0.8 leading
d. zero
12.
What is active power ?
13.
What is meant by reactive power ?
14.
What is meant by balanced system ?
15.
What is meant by unbalanced system?
16.
Power factor cosΦ for 3-phase wattmeter is-------------------------
17.
What is the other name for phantom loading-------------------------
18.
Draw the circuit diagram for measuring reactive power in a 3 phase circuit using
single wattmeter.
19.
Draw the circuit diagram for measuring reactive power in a 3 phase circuit using two
wattmeters.
20.
Driving torque in electro dynamo meter type wattmeter is___________
2 Mark Questions
1.
What is a meter constant ?
2.
What is a creeping ?
3.
What are the errors presented in the energy meters
4.
What are the errors in dynamometer type instruments ?
5.
What is the phase error?
6.
What is the average maximum demand formula ?
7.
Which type of the breaking system is used in induction type energy meters?
8.
What is the compensation used for temperature error?
9.
What is the registering system?
10.
What is the working principle of dynamometer type wattmeter?
11.
Mention some advantages of the dynamometer type wattmeter?
12.
Mention some disadvantages of dynamometer type wattmeter?
5 Mark Questions
1.
Describe in detail the working of a trivector meter?
2.
In a dynamometer wattmeter the moving coil has 500 turns of mean diameter 30
mm. Estimate the torque if the axis of the field and the moving coils are at: i)
600 ii) 900 when the flux density produced by field coils is 15 × 10-3 wb/m2, the
current in moving coil is 0.05A and the power factor is 0.866.
3.
What is phantom loading? Explain with an example how it is more advantages
than testing with direct loading.
4.
Explain how power can be measured in a 3 phase circuit with the help of two
wattmeter for a balanced star connected load?
5.
Explain the working of electrodynamometer type wattmeter and also derive the
expression for deflecting torque.
6.
A wattmeter has a current coil of 0.03 Ω resistance and a pressure coil of 6000Ω
resistance. Calculate the %error if the watt meter is so connected that
i) The current coil is on the load side,
ii) The pressure coil is on the load side
a) If the load takes 20 A at a voltage of 220 V and 0.6 pf in each case
b) What load current would give equal errors with the two conditions.
10 Mark Questions
1. a .Explain the construction and working principle of single phase induction type energy
meter.
b. An energy meter is designed to make 100 revolutions of disc for one unit of energy.
Calculate the number of revolutions made by it when connected to load carrying 40 A at
230V and 0.4 power factor for an hour. If it actually makes 360 revolutions, find the
percentage error.
2. a. Explain LPF and UPF Wattmeters
b. In a balanced 3-Φ system power is measured by Two-wattmeter method and the
ratio of two wattmeter readings is 2:1.Determine the power factor of the system.
3. a. Why ordinary electrodynamometer wattmeter is not suitable for the measurement
of power in circuit with low power factor?
What special features are to be
incorporated so as to make it suitable for the same?
b. Explain the following adjustments that are made in a single phase induction type
energy meter.
i) Lag adjustment
ii) Overload compensation
iii) Creeping
iv) Temperature compensation
4. a. Explain the driving system, moving system and braking system in a 1-Ø induction
type energy meter.
b. A 230V single phase watt hour meter has a constant load of 4A passing through it
for 6hrs at unity power factor. If the meter disc makes 2208 revolutions during this
period, what is the meter constant in revolutions per kilowatt hour? Calculate the
power factor if no. of revolutions made by the meter are 1472 when operating at
230v and 5A for 4hrs.
5. a Explain how the measurement of power with wattmeter in conjunction with
instrument transformers can be used in single- phase ac circuits.
b. A 3- phase, 440V motor load has a power factor of 0.6. Two wattmeters connected
to measure the power show the input to be 25 KW. Find the reading on each
instrument.
6. a. A 50 A, 230 V energy meter in a full load test with unity power factor makes 61
revolutions in 37 seconds. If the normal speed of the disc is 520 revolutions/KWH,
Compute the percentage error
b. Explain the measurement of 3-phase power with single phase watt meters.
7. a. Why maximum demand meters are used? Explain with neat sketch.
b. What are the different errors occurred in energy meters? How they are
compensated.
8. a) Explain how a dynamometer – type wattmeter can be used to measure power in a
circuit having low power factor (LPF)?
b) Explain the design considerations in case of a LPF wattmeter and write short
notes on phantom loading.
9 a) Explain the different errors in electrodynamometer watt meters
b) The power to a 3-Ph Induction Motor was measured by Two-wattmeter method
and the readings are 3400W & -1200W.calculate the power & power factor.
10.
Explain the different errors and compensation in energy meter.