Synchroscope Construction and Working

Synchroscope is used to synchronize two alternators. Synchronize means that the machine of equal frequency and voltage are operating parallel to each other. We connect the alternator in parallel with one another with bus bar to shore the load. In electrical power system, a synchroscope is a device which indicates the degree to which two systems are synchronized with each other.

Synchroscope Synchronization

The Synchroscope determines the exact instant where the condition of synchronizing is satisfies. These conditions are:

  1. The alternators should have equal magnitude of voltage.
  2. They should have same frequency.
  3. They should have same phase sequence. The function of this instrument is to indicate any difference in phase or in frequency. The phase sequence however is verified by a “phase sequence indicator”. We can check the voltage with the help of two voltmeter.

Types of Synchroscopes

The synchroscopes are the special form of power factor meters and are of types

  • Electrodynamic type synchroscopes.
  • Moving iron type synchroscopes.



It consists of a three limb transformer and an electrodynamic instrument. The winding on one outer limb is connected with the bus bars and the winding on other outer limb is connected with the incoming alternator. The winding on the central limb of the transformer is connected to a lamp.

The windings or the outer limbs produce two fluxes. Two fluxes flows through the central limb, where the resultant flux is obtained which is the phasor sum of the two fluxes. This resultant flux induces an emf in the central limit winding.

Electrodynamic synchroscope
Electrodynamic synchroscope

The outer windings are so arranged that when the voltages of the bus bar and of the incoming alternator are in limit are added up and induced emf in the central limit is maximum as a result, the lamp glows with maximum brightness.


When the two voltages are 180 degree out of phase. Then the resultant flux is zero and no emf induced in the central limb winding. Thus the lamp does not glow at all.

If the frequency of the incoming alternator is different from the supply frequency of the bus bars, the lamp will flicker and the frequency of the flickering is equal to the difference of the two frequencies.

But the flickering of the lamp cannot indicate whether the incoming alternator is fast or slow. For this purpose an electrodynamic instrument is provided with the arrangement as shown in figure.

Coil Arrangement of Weston Type Synchroscope

The electrodynamic instrument consists of 2 fixed coils FF and a moving coil M. The fixed coils FF carry a small current and connected across any two bus bars through s resistance R. The moving coil M connected across the incoming alternator through a capacitor C. The moving coil carries a pointer which moves on a scale showing fast/slow (F/S).

Dail of Synchroscope
Dial of Synchroscope

Figure shows the dial of electrodynamic synchroscope. Dial of the synchroscope mark with two arrows which indicates the direction of rotation of the pointer. This arrow indicates the anticlockwise and clockwise direction of pointer. The anticlockwise show fast movement and clockwise shows slow movement of incoming machine.

If the incoming machine’s frequency is more than that of generator frequency , the pointer deflects toward fast mark and vice-versa. The correct instant of synchronizing is that where the pointer is visible at its central position and is moving very slowly.



It consists of a fixed coil in two parts. The fixed coils FF are designed for a small current and are connected across two phases of the bus bars. There are two iron cylinders C1 and C2. Two iron cylinders are mounted on a spindle and are separated by spacers. Each cylinder is provided with two iron vanes whose axes are 180 apart.

The cylinders are excited by two pressure coils P1 and P2. These are connected to two phases of the incoming alternator. One pressure F coil has a resistance R and other has an inductance L connected in series. It establishes a 90 phase difference between their currents. The pointer attached with the spindle moves over a dial marked fast and slow.

Working of Moving Iron Synchroscope

When the frequency of the incoming alternator is same as that of the bus bars. Then instrument behaves as a moving iron P.F meter. The movement of the pointer is equal to the phase difference between the two voltages. The pointer does not deflect at all. If there is no phase difference between the two voltages.

Moving iron type synchroscope
Moving Iron type synchroscope

When the two frequencies are different. Then the pointer moves at a speed corresponding to the difference in the frequencies. The direction of motion of the pointer shows whether the alternator is “fast” or “slow”. When the pointer is at zero, the synchronizing switch is closes automatically.

The M.I. synchronoscope are more common in use. They are cheaper and have long scale spread over 360.


So, the conclusion is that synchroscope is a helpful device for synchronization of two alternators.

Read also: Measuring Instrument Introduction
           PMMC Instruments
           Moving Iron Instruments