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
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.
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
Its working is same as the single-phase power factor meter
works. The main difference is that the current lags with 120 degrees in place
of 90 degrees. 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 120 degree. 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 120 degrees. The
current I2 is in phase with the current and the current I1
will be 120 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.