The Weston type synchroscope, which we also refer to as electrodynamometer type synchroscope is a type of dynamometer type instrument.
Construction of Weston Type Synchroscope
It consists of a lamp transformer. Lamp transformer is a kind of transformers which has three limbs magnetic core. We connect the winding of one of the outer limbs between any two phases of the existing electrical system. Then we connect the winding of the other outer limbs across the same two phases of the incoming generator. Lastly, we connect an electric lamp with the winding of the central limb.
The flux of two outer limbs flows through the central limb. Therefore, the resultant flux in the central limb is the phasor sum of the flux of two outer limbs. Hence, this resultant flux induces an emf across the winding of the central limbs. As a result of the induced emf, the lamp glows. But the brightness of the glowing lamp depends on the strength of the resultant flux.
We turn the winding of the outer limbs in such direction, that, in normal working condition the resultant flux of the central limb is the additive of the flux of two outer limbs.
Working of Electrodynamometer Type Synchroscope
Suppose at a certain condition there is no phase difference between the existing electrical system and the incoming generator. So, at that time resultant flux will be maximum at the central limb. Therefore, the lamp glows with its maximum brightness.
If the phase difference between the existing electrical system and the generator is 180° (electrical), there will be zero flux in the central limb of the core of the Weston type synchroscope. Therefore, the lamp does not glow at all.
But at any intermediate phase difference between these two systems, the bulb will glow with a certain brightness.
If the frequency of those two systems differs, the bulb will flicker. The frequency of the flickering is exactly equal to the difference of the frequency of this existing electrical system and the incoming generator.
As the time of synchronism, the flickering of the lamp becomes very slow. The correct instant of switching on the incoming generator is when the brightness of the lamp becomes maximum.
Drawbacks of Weston Type Synchroscope
There is a major drawback in Weston type synchroscope. Because it does not tell us whether the incoming generator is running faster or slower.
We can overcome this difficulty by connecting s dynamometer type instrument along with the lamp transformer in the electrodynamometer type synchroscope.
This second instrument does have a coil divided into two equal parts. We connect this coil across the same two phases of the incoming generator along with resistance and inductance in series with it. Then we place the moving coil at the center of the space between two halves of the fixed coil. Then we connect the moving coil across the two same phases of the existing electrical system along with a capacitor in series with it. Here we use an inductance in series with the fixed coil and the capacitance in series with the moving coil to make the currents of those coils in quadrature when the voltages of two systems are in phase.
When there is no phase difference between the existing electrical system and the incoming generator, the currents in the moving coil and fixed coil are in quadrature. Therefore, there will be no torque acting on the moving coil of the electrodynamometer type instrument. Therefore, the pointer will remain stationary to indicate the synchronism condition. At the same time, the flux in the central limb of the lamp transformer is maximum. Because at that condition flux of the central limb is the simple sum of the flux of two outer limbs. So, the lamp glows with its full brightness. This is the perfect time of switching the incoming generator to the bus.
Although practically the pointer does not become fully stationary at synchronism position. Rather it slightly oscillates about the position. But we take it as the condition of synchronism.