Power Factor Meter - Working Principle - Construction - Types - Formula - Working - Advantages - Disadvantages
Power Factor Meter
A power factor meter is an electric instrument which is used
to measure the power factor of various electrical machines like DC Generator,
AC Motor, Transformer etc and for measuring the power factor of various
transmission and distribution lines of various electric power supplies. These
power supplies may be from Grid Stations, Substations or from Power Houses.
Power factor must be 0.8 to unity if power factor decreases from 0.8 then the
voltage and the current will imbalanced and the power is disturbed. So, in grid
stations, Substations the power factor must be between 0.8 to unity (1)
leading. The Power Factor is the cosine of angle φ between the voltage and the
current of the transmission lines and electrical machines.
Power Factor Meters are mostly used for the observation of
electric power to maintain the power factor for the balancing of voltage and
current. The power factor of transmission and the distribution lines is unity
(1) leading because of the losses occurred in these lines and the reason of the
losses is the length of the conductor or wire. If we increase the length of a
wire and the power we provide on its first end constant then on the other end
of the wire we will get less power as compared to the first end because of the
large length of the conductor.
Formula of Power Factor
The formula to measure the power factor in power factor
meters is given below. In simple words, the following formula is used in power
factor meters to measure the power factor of the transmission/distribution
lines and electrical machines:
For single phase transmission/distribution lines
For three phase transmission/distribution lines
For single phase electrical machines
For three phase electrical machines
The power factor meters are divided into many types and these
types are discussed in detail below:
Types of Power Factor Meter
Power Factor Meters consist of the following types and all
these types are explained in detail below:
Moving-iron Power Factor Meter
- Nalder Lipman Moving-iron Power Factor Meter
Electrodynamic
Power Factor Meter
- Single Phase Power Factor Meter
- Three Phase Power Factor Meter
Moving-iron Power Factor Meter
Moving Iron Power Factor meters are the three phase power
factor meters. These power factor meters consist of a moving iron
pointer. It consists of a three fixed coils R, Y and B. These coils are connected
to three phase supply on which it is to be used.
Construction
These power factor meters have three fixed coils as mentioned
above and it has three terminals of R, Y and B with axes and these fixed coils
are connected mutually at 120 degrees. These coils intersect on the center line
of the instrument. A fixed coil C is located at the center of the three fixed
coils. This coil C is connected in series with high resistance across
one of the lines. The coil B is threaded by the spindle. It carries an iron
cylinder and the sector shaped iron vanes are fixed on it. The spindle has
no control springs and it carries a pointer and damping vanes.
Working
The coil C produces alternating flux with the
three current coils and it acts on a moving system for the
deflection of a pointer. The angular deflection of the iron vanes from the line
M, N is equal to the phase angle φ. The rotating field of the coils of R, Y and
B turns the moving iron continuously in a direction of a rotating flux. The
high resistance for the moving iron is used to reduce eddy current in it.
Advantages
These meters are strong and cheap.
Their scales are up to 360 degrees.
There are no electrical connections in the moving parts.
Disadvantages
These instruments have so many errors.
These instruments are not accurate.
These instruments have hysteresis losses and eddy current losses
in their iron parts.
Nalder Lipman Moving-iron Power Factor Meter
Nalder Lipman Moving-iron Power Factor Meter are more
efficient in working as compared to electrodynamic power factor meters.
These power factor meters are used on three phase supply. Its
construction and working are very simple. So, the construction and working of
this power factor meter is written below:
Construction
Its moving system holds three iron elements.
These three elements carries iron cylinder. These elements are mounted on a
shaft and these are separated with one another with non-magnetic
distance pieces. The shaft carries the damping vanes and
a pointer. These three elements are at 120 degrees to each other.
Each iron element is magnetized with their individual voltage coils named
as B1, B2 and B3. These are connected in a star connection with the supply
lines. The two parallel F1 and F2 of a single current coil is connected on a
line of supply.
Working
The whole system is free to move between the two parallel halves.
These meters read the power factor directly from the supply.
The phase angle φ is produced in a moving system. The
deflection Θ is equal to phase angle and the pointer deflects
on a scale.
Advantages
This instrument is not affected by the variations in voltage and
in wave forms in power supply.
Rotating magnetic field is not produced so the
moving system deflects properly.
Electrodynamic Power Factor Meter
Before we start to study about Electrodynamic power factor
meters, we should have the knowledge about what is the Power
Factor?
The answer is that a Power Factor is
a unit of measurement of various electric drives like
induction motors, Synchronous motors and Generators etc. Power Factor is the
ratio between the real power and the apparent power.
Now the question rises that what is a real power and apparent power? So,
the real power reacts like a resistance in an electric drive.
It may be called as power consumed in an electric drive or a term “the total
load drawn on the electric drive” can be used for the real power. Real power is
denoted by Power (P). It is measured in watts (W).
While the apparent power is the product of
an electric current and voltage. It is measured in
KVAR. It is denoted by S.
Electrodynamic / Electrodynamometer Type Power Factor meters are
used to measure the power factor of AC Drives and these meters are of its two
types according to its phases, which are mentioned below:
- Single
Phase Power Factor Meter
- Three Phase Power Factor Meter
Single Phase Power Factor
Meter
Construction of Single-Phase Power Factor Meter
These single-phase power factor meters are used to measure
the power factor of single-phase electric drives and single-phase
electric supplies etc. These power factor meters consist of a stationary
coil and it is divided into two parts named as SC1 and
SC2. These coils produce uniform field which is proportional to the
line current. The two moving coils are located in this field which are attached
to each other named as C1 and C2. These are mounted on a spindle. These moving
coils are the voltage coils, C1 consists
of series resistance and C2 consists of series
inductor. Both moving coils have equal turns on it and these are exactly
equal to same ampere-turns. These two moving coils cannot produce controlling
torque in it because of the current passes through
the ligaments in it, which cannot use control torque.
Working of Single-Phase Power Factor Meter
The current is in phase with the voltage when the load power
factor is unity. The current I1 is
in phase with the current I and the current I2 lags
behind by 90 degrees. The torque acts on coil C1 and
sets its plane perpendicular to the magnetic axes of the stationary coils SC1 and
SC2. The torque does not act on the moving coil C2. When
the power factor is zero, the current lags with voltage by 90 degrees. The
current I2 is in phase with the current and the current I1 will be
90 degree out of phase and there is no torque on the moving coil C1 but
it acts on C2. It will bring its plane perpendicular to the common magnetic
axis of SC1 and SC2.
This instrument must be calibrated at the frequency of the supply
on which it is operated for accurate readings. If the frequency of the supply
changes its value continuously then this power factor meter will cause serious
errors.
Three Phase Power Factor
Meter
Construction of Three-Phase Power Factor Meter
This instrument is not affected by the variations in frequency.
These power factor meters have the same construction as the single-phase
power factor meter has. The main difference of construction between
the single-phase and three-phase power factor is that the moving coils C1 and
C2 are at 120 degrees to each other and these moving coils C1 and
C2 are connected across the two various phases of the supply.
The stationary coils SC1 and SC2 are
connected in series with third phase. The current I1 and
I2 are not determined by the phase splitting circuit.
Working of Three Phase Power Factor Meter
Advantages
These meters are more reliable.
These three-phase power factor meter
provides actual readings.