When synchronous motor is over excited it takes leading p.f. current. If synchronous motor is on no load, where load angle ? is very small and it is over excited (Eb > V) then power factor angle increases almost upto 90o. And motor runs with almost zero leading power factor condition. This is shown in the phasor diagram Fig. 1.
|Fig. 1 Synchronous condenser|
This characteristics is similar to a normal capacitor which takes
leading power factor current. Hence over excited synchronous motor
operating on no load condition is called as synchronous condenser or
synchronous capacitor. This is the property due to which synchronous
motor is used as a phase advancer or as power improvement device.
In various industries, many machines are of induction motor type. The
lighting and heating loads are supplied through transformers. The
induction motors and transformers draw lagging current from the supply.
Hence the overall power factor is very low and lagging in nature.
The supply voltage is constant and hence for supplying a fixed power P,
the current is inversely proportional to the p.f. cos?. Let P = KW is
to be supplied with a voltage of 230 V then,
Case ii) cos = 0.6,
Thus as p.f. decreases, becomes low, the current drawn from the supply
increases to supply same power to the load. But if p.f. maintained high,
the current drawn from supply is less.
The high current due to low p.f. has following disadvantages :
1. For higher current, conductor size required is more which increases the cost.
2. The p.f. is given by
cos? = Active power/ Apparent = (P in KW)/ (S i.e. KVA rating)
3. Large current means more copper losses and poor efficiency.
drops in transmission lines, alternators and other equipments. This
results into poor regulation. To compensate such drop extra equipments
is necessary which further increases the cost.
The low power factor increases the cost of generation, distribution and
transmission of the electrical energy. Hence such low power factor
needs to be corrected. Such power factor correction is possible by
connecting synchronous motor across the supply and operating it on no
load with over excitation.
The synchronous motor acting as a synchronous condenser is now
connected across the same supply. This draws a leading current of I2ph.
|Fig. 2 Power factor correction by synchronous condenser|
This is how the synchronous motor as a synchronous condenser is used to improve power factor of the combined load.