The non -uniformity in the voltage distribution over a string of suspension insulators is expressed in terms of a parameter called “string efficiency“.
The voltage across the unit nearer to the conductor is more than the voltage in the unit nearer to the support. This unequal potential distribution over the string is expressed in terms of string efficiency.
The efficiency of a string is defined as the ratio of voltage across the whole string to the product of the number of discs and the voltage across the disc nearest to the line conductor. Mathematically, it can be expressed as,
Where n = Number of the unit in the string
The more the value of the string efficiency, the more uniform is the voltage distribution across the string. For the ideal case, the string efficiency is 100% and the voltage across each disc of the string is equal. Practically various methods are used to obtain string efficiency as high as possible.
In the capacitor start, single-phase induction motor the capacitor is connected in series with the starting auxiliary winding. In this manner, the current in the starting winding may be made to lead the line voltage. Since the running winding current lags the line voltage, the phase displacement between the two currents is made to approximately 90° on starting.
Placing the capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and the auxiliary windings. Due to greater phase difference capacitor Start motors have very high starting torque for a single-phase AC motor.
The synchronous machine can operate at lagging, leading, and unity power factors. In the synchronous machine, the total air gap flux is produced by the dc source. If dc excitation is decreased, lagging reactive power will be drawn from the ac source to aid magnetization and thus machine will operate at lagging power factor. If dc excitation is more, leading current drawn from ac source to compensate (oppose) the magnetization and the machine will operate a leading power factor.
Thus it can be concluded that an over-excited motor(Eb > V) draws a leading current (acts like a capacitive load) but an under-excited motor(Eb < V) draws a lagging current (acts as an inductive load).
Susceptance is symbolized by the capital letter B. It is the reciprocal of AC reactance. Susceptance, like reactance, can be either capacitive or inductive. In the case of a magnetic field, the susceptance is inductive. In the case of an electric field, the susceptance is capacitive. Capacitive susceptance is symbolized BC and inductive susceptance is symbolized BL. In the case of a magnetic field, the susceptance is inductive. In the case of an electric field, the susceptance is capacitive. Inductive susceptance is assigned negative imaginary number values, and capacitive susceptance is assigned positive imaginary number values.
The formula for inductive susceptance is
The unit of Inductive susceptance is Siemen or Mho.