Saint Louis University
SCHOOL OF ENGINEERING AND ARCHITECTURE
LABORATORY MANUAL
Experiment No: 3
WATT, VAR, VOLT-AMPERE, AND POWER FACTOR
At the end of this experiment, the student should be able to:
TLO 3:
I.
Measure and differentiate apparent, active and reactive power.
INTRODUCTION:
We now know the following facts:
a) Apparent power supplied to a load is the simple product of voltage and
current
b) Real power supplied to a load is measured by a wattmeter.
When reactive power is involved, the apparent power is larger than the
real power. Reactive power may be inductive or capacitive. In most
electromechanical devices the reactive power will be inductive due to the
inductance presented by coils. Reactive power can be calculated by the
equation:
Reactive power =
√(𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 𝑃𝑜𝑤𝑒𝑟 2 ) − (𝑅𝑒𝑎𝑙 𝑃𝑜𝑤𝑒𝑟 2 )
(1)
If the phase angle between the voltage and current is known, the real
power can be found by the equation:
Real Power =
E x I x cos Ø = Apparent Power x cos Ø
(2)
The ratio of real power to apparent power is called the power factor of an
ac circuit. Power factor can be found by the equation:
PF = P/EI = Real Power / Apparent Power
EE DEPARTMENT
(3)
Saint Louis University
SCHOOL OF ENGINEERING AND ARCHITECTURE
LABORATORY MANUAL
The value of the power factor depends on how much the current and
voltage are out of phase. When the current and voltage are in phase, the real
power is equal to I x E, or in other words, the power factor is unity. When current
and voltage are out of phase by 90, as in purely capacitive or inductive circuit,
the power factor is zero, resulting in a zero value of actual power. In circuits
containing both resistance and reactance, the value of the power factor is some
value between one and zero. If the phase angle Ø between the voltage and
current is known the power factor can be found by the equation:
PF = cos Ø
(4)
Alternating current motors draw reactive power from the supply line to
create the magnetic field which they require. In addition, such motors also absorb
real power, most of which is converted into mechanical power, while the rest is
dissipated in the form of heat.
The reactive power travels back and forth between the motor and the ac
supply. Reactive power does no useful work, except in so far as it creates placed
in parallel with the motor, and the reactive power drawn by the capacitor is
exactly equal (but of opposite sign) to that drawn by the motor, then one reactive
power will neutralize the other. The result being that the power transmission line
need no longer carry any reactive power at all. This can result in a very
substantial reduction of current in the transmission line, which improves line
regulation and reduces need for large diameter transmission wires.
Prior to the addition of the capacitor, the power factor of the motor is quite
low. Once the capacitor is in place, the combination (motor + capacitor) improves
the power factor. By proper choice of capacitance, the power factor can be made
close to unity.
NOTE: In this Laboratory Experiment, the Split-Phase/Capacitor Start Motor
will be fully explained to you in later Laboratory Experiments.
EE DEPARTMENT
Saint Louis University
SCHOOL OF ENGINEERING AND ARCHITECTURE
LABORATORY MANUAL
II.
EQUIPMENT/ MATERIALS NEEDED:
Power Supply Module (120Vac)
Split-Phase/Capacitor Start Motor Module
AC Metering Module (2.5A/8A)
AC Metering Module (250V)
Capacitance Module (2)
Single-Phase Wattmeter Module (750W)
Connection Leads
III.
EMS 8821
EMS 8251
EMS 8425
EMS 8426
EMS 8331
EMS 8431
EMS 8941
PROCEDURES:
TLO 3: Measure and differentiate apparent, active and reactive power.
Caution: High voltages are present in this Laboratory Experiment! Do not
make any connections with the power on! The power should be turned off
after completing each individual measurement!
1. Using your EMS Split-Phase/Capacitor Start Motor, AC Metering,
Wattmeter and Power Supply Modules, connect the circuit shown in
Fig. 3-1. Use short leads to tie terminals 1 to 3, 2 to 6 and 4 to 7 on your
Motor Module. (The motor is now connected for split-phase operation).
DO NOT APPLY POWER AT THIS TIME!
Fig 3-1
2. Have your connected circuit checked by your instructor.
3. Turn on the power supply and adjust for 120Vac as indicated by the ac
voltmeter connected across the motor. (Your motor should be running).
EE DEPARTMENT
Saint Louis University
SCHOOL OF ENGINEERING AND ARCHITECTURE
LABORATORY MANUAL
a) Measure and record the line current.
IL = ___________ Aac
b) Measure and record the real power.
P = ___________ W
4. a) Return the voltage to zero and turn off the power supply.
b) Calculate the apparent power.
PA = ___________ VA
c) Calculate the power factor.
PF = ___________
d) Calculate the reactive power.
PR = ___________ var
5. Connect two Capacitance Modules in parallel with your motor as
shown in Fig. 3-2. Connect all six of the capacitance sections in parallel.
Open (down position) all of the associated capacitor toggle switches.
6. a) Turn on the power supply and adjust for 120Vac as Procedure 3.
(Your motor should be running).
b) Start to add capacitance to your circuit by closing the switches one
at a time. Note that the line current diminishes as capacitance is
added. At some point, as you keep adding more capacitance the line
current will start to increase. (The line current has gone through a
minimum value).
c) Adjust your capacitance for minimum line current.
d) Measure and record the line current.
IL = ___________ Aac
e) Measure and record the real power.
EE DEPARTMENT
Saint Louis University
SCHOOL OF ENGINEERING AND ARCHITECTURE
LABORATORY MANUAL
P = ___________ W
7. a) Return the voltage to zero and turn off the power supply.
b) Calculate the apparent power.
PA = ___________ VA
c) Calculate the power factor.
PF = ___________
d) Calculate the reactive power.
PR = ___________ var
Fig. 3-2
8. Compare the results of Procedure 3 and 4 with the results of Procedure
6 and 7.
a) Has there been a significant reduction in line current by adding
capacitance? _________
b) Has the motor operation been affected by adding capacitance?
__________
c) Is the real power approximately the same whether the capacitance
is added or not? _________
Explain.
___________________________________________________________
EE DEPARTMENT
Saint Louis University
SCHOOL OF ENGINEERING AND ARCHITECTURE
LABORATORY MANUAL
___________________________________________________________
___________________________________________________________
________________________________________________________________
9. Turn on the power supply and adjust for 120Vac as before.
a) Close all of the capacitance switches and measure the line current.
IL = ___________ Aac
b) Carefully adjust the switches for minimum line current, while
maintaining exactly 120Vac across the motor.
What reactance value gives the lowest line current?
XC = ___________ Ω
c) Return the voltage to zero and turn off the power supply.
IV.
REFERENCES:
LabVolt Manual
EE DEPARTMENT