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What is a Synchroscope?

Paul Scott
Paul Scott

A synchroscope is an instrument used to establish phase angle and frequency synchronization between alternating current (AC) power supplies. This is a critical safety measure when AC power networks or generator outputs are merged or connected together. Synchroscopes are available in two basic formats: electro-mechanical and electronic. Electro-mechanical instruments utilize a set of opposed static windings and a rotor to give a dial type indication. Electronic synchroscopes use a microprocessor to calculate and indicate input status via a digital or light emitting diode (LED) display.

The voltage of an alternating current power supply consists of a smooth sinusoidal wave form alternating between positive and negative peak values. The peak values are known as the amplitude of the supply; the number of times the cycle occurs per second is known as the frequency. When two AC power supplies of similar amplitude or voltage are compared, they are considered to be “in phase” when both reach the positive and negative peak points at the same time. When combining AC power supplies, it is of critical importance to ensure both are the same voltage, frequency, and in perfect phase with each other. Connecting AC supplies when they are not balanced in this way may lead to severe damage to the network.


The voltage of AC power supplies are generally fairly stable and easy to indicate with conventional instrumentation. The phase angle and frequency relationship between supplies, however, is seldom balanced without some adjustment. When generator or network supplies are combined, a synchroscope indicates any differences between the two. If there are discrepancies, generator speeds can be adjusted until the synchroscope indicates a perfect balance. At that point, the supplies may be safely combined.

There are two basic types of synchroscope. The first is the electro-mechanical type which indicates phase and frequency relationships with a pointer and dial indicator. This instrument is similar in construction to an electric motor. It has a set of static stator windings opposed at 90 degrees, a polarizing coil, and a rotor element which drives the indicator needle. One supply is connected to the stator windings and the other to the polarizing coil. If the phase and frequency of the two supplies vary, the rotor will turn and give an indication of the difference.

The second type of instrument is the electronic synchroscope. This type utilizes a microprocessor designed to sample and compare two AC power supplies for phase and frequency characteristics. Any differences between the two are then calculated and indicated on either a digital liquid crystal display (LCD) or by means of LEDs. The latter type synchroscope typically has a circle of LEDs on its front panel which illuminate in a set pattern to indicate the presence and magnitude of phase and frequency differences.

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