UPPCL JE 2016 Electrical question paper with Solution (Evening Shift)

Electrostatic instruments are the only instruments that are directly voltage-sensitive and do not depend on a current for their operation. Basically, electrostatic instruments are all voltmeters.

Practically such instruments may be used for the measurement of current and power but both types of measurements require measurement o the voltage across a known impedance. The main advantage of such instruments is the measurement of high voltages in both a.c. and d.c. circuits without any errors due to eddy current losses and hysteresis.

The operation of all the electrostatic instruments is based on the principle that there exists a force between the two plates with opposite charges. This force can be obtained using the principle that the mechanical work is done is equal to the stored energy if there is a relative motion of plates.

Advantages of Electrostatic Instrument

1. They can be manufactured with very high accuracy.
2. The instruments may be calibrated with d.c., and yet the calibration would be valid for a.c. also since the deflection is independent of the waveform of the applied voltage.
3. Since no iron is used for their construction, they are free from hysteresis, eddy current losses and temperature errors.
4. Once the discs are charged, no more current is drawn from the circuit and the instrument represents infinite impedance.

The hexagonal closest packed (hcp) has a coordination number of 12 and contains 6 atoms per unit cell.

The speed and torque of a single-phase series motor can be varied by applying variable voltage. Usually, the operating voltage of the motor is smaller than the supply voltage.

Various voltages for starting and speed control be obtained from a transformer (which is an integral part of the equipment) which is provided with a number of taps on the secondary side.

With this not only the starting resistances are avoided (Thus there being no loss of power during starting period) but also each control point (tapping) becomes a running point so that e number of economical speeds are available.

The crystal is usually made of quartz material and provides a high degree of frequency stability and accuracy or we can say that a crystal oscillator is basically a tuned oscillator.

It uses a piezoelectric crystal (when an ac voltage is applied across a crystal it starts vibrating at the frequency of supply voltage, the effect is known as the piezoelectric effect and the crystal which exhibits this effect is known as piezoelectric crystals, conversely, when these crystals are placed under mechanical strain to vibrate, they produce an ac voltage.

The stability of time and frequency standards is closely related to their quality factor or Q. The Q of an oscillator is its resonance frequency divided by its resonance width. A high-Q resonator will not oscillate at all unless it is near its resonance frequency.

Quartz oscillator achieves the highest Q of any mechanical type device.A typical Q for a quartz oscillator ranges from 10⁴ to 10⁶. The maximum Q for a high—stability quartz oscillator can be estimated as Q = 16 million/f, where f is the resonance frequency in MHZ.

• Starting torque is directly proportional to the rotor resistance of an induction motor.
• In SLIP RING induction motor the ends of the rotor windings are externally connected by a variable rheostat (resistance is varied in order to give it proper starting and running current).
• So more the resistance, more the torque. When we add resistance to the rotor the torque is high, the slip is high and the current is reduced.
• Therefore slip ring induction motor is used for providing high starting torque.
• Belted slip ring induction motor is almost invariably used as, very often, the motor has to start against a heavy load or a stuck crusher.

A Measuring Instrument is should measure the quantity it is supposed to measure without drawing any power from the system in which it measures.

However, in most of the analog measurements, the Instrument does draw some power without which it cannot measure the quantity.

This loading effect can be reduced by choosing instruments that have a very high impedance. Such an instrument imposes less load on the circuit and gives an indication much closer to the true value.

There are mainly two types of wind turbines based on the axis in which the turbine rotates. They are

1. Horizontal axis wind turbines (HAWT): Horizontal axis wind turbines: The rotor shaft and the generator are located at the top of the tower and face the wind.
2. Vertical axis wind turbines (VAWT): The rotor shaft is arranged vertically and the turbine need not point into the wind. These kinds of turbines are rarely used. Darrieus turbine is the only vertical axis turbine used commercially.

Frequency-shift keying (FSK) allows digital information to be transmitted by changes or shifts in the frequency of a carrier signal, most commonly an analog carrier sine wave.

There are two binary states in a signal, zero (0) and one (1), each of which is represented by an analog waveform. This binary data is converted by a modem into an FSK signal, which can be transmitted via telephone lines, fiber optics or wireless media.

Slip of an Induction motor is given as

$S = \frac{{{N_s} – {N_r}}}{{{N_s}}}$

Where Ns = Synchronous speed

Nr = Rotor speed

When the rotor is stationary (standstill) its speed is zero and s = 1 i.e 100%. As the rotor speed increases slip tends to decrease, when the rotor speed becomes equal to synchronous speed then the slip is 0%.

Class B Amplifier also is known as a push-pull amplifier configuration.

Class-B amplifiers use two or more transistors biased in such a way so that each transistor only conducts during the one-half cycle of the input waveform.