Power Factor

KW is Working Power (also called Actual Power or Active Power or Real Power).

It is the power that actually powers the equipment and performs useful

work.

KVAR is Reactive Power.

It is the power that magnetic equipment (transformer, motor and relay) needs to produce the magnetizing flux.

KVA is Apparent Power.

It is the vectorial summation of KVAR and KW.

Power factor is an important measurement in electrical AC systems because

• an overall power factor less than 1 indicates that the electricity supplier need to provide more generating capacity than actually required.
• the current waveform distortion that contributes to reduced power factor is caused by voltage waveform distortion and overheating in the neutral cables due to yhe generation of harmonics in  three-phase systems.

Reactive power (KVAR) required by inductive loads increases the amount of apparent power (KVA) in our distribution system . This increase in reactive and apparent power results in a larger angle (measured between KW and KVA). We know that, as angle  increases, cosine of that angle (or power factor) decreases.

 So, inductive loads (with large KVAR) result in low power factor.

 

Effect Of Loads:

• With a purely resistive load current and voltage changes polarity in step and the power factor will be 1. Electrical energy flows in a single direction across the network in each cycle.
• Inductive loads - transformers, motors and wound coils - consumes reactive power with current waveform lagging the voltage.
• Capacitive loads - capacitor banks or buried cables - generates reactive power with current phase leading the voltage.

Inductive and capacitive loads stores energy in magnetic or electric fields in the devices during parts of the AC cycles. The energy is returned back to the power source during the rest of the cycles.

DISADVANTAGE OF  LOW PF.:

Increased Power Supply:

We know that inductive loads, which require reactive power, caused our low power factor. This increase in required reactive power (KVAR) causes an increase in required apparent power (KVA), which is what the power plants are supplying. So, low power factor causes the power plants to have to increase its generation and transmission capacity in order to handle this extra demand. By lowering our power factor,we use less KVAR. This results in less KW, which increases savings of power generation.

Other harmful effects are:

      Increases heating losses in the transformers and distribution equipments.

      Reduce plant life.

      Unstabilise voltage levels.

      Increase  power losses.

      Decrease energy efficiency.

Marx impulse generator

 

                                                           

The Marx impulse generator can simulate a lightning strike. As most lightning discharges are in the order 1 to 100 kilo-amps the electric charge associated is usually 5 to 50 Coulombs. At a voltage of 50 to 500 million volts this transforms to a considerable amount of discharged energy. Even though this is a destructive event, the current rise is not as fast as the man-made impulse generator. The output of the Marx is under 100 kilo-amps but its current rise is faster than a natural lightning bolt. The rise time of the discharge is usually 1 to 10 microseconds with a fall time of 20 to 50 microseconds. (The reason for the relatively slow rise is due to the atmosphere not approaching an ideal spark gap switch.)

Materials testing such as cables transformers and power systems can be verified for integrity against lightning. Flash x-rays and the simulating of electro magnetic pulse (EMP) from nuclear detonations are possible using this device.

Marx generators are basically a stack of capacitors which are charged in a parallel configuration to a voltage "E" and then discharged in series with a voltage of "nE" where "n" in the number of capacitors charged. Selection of the capacitors will determine the peak current and rate of current rise during the discharge cycle. Not only the value of capacity and voltage is selected, but the discharge loop inductance and peak current handling is also considered. These capacitors once charged must be discharged in a series configuration through special precisely-spaced spark gap switches for each capacitor. Open air tungsten or molybdenum electrodes with Bruce or Rogowski discharge surfaces now used. Switching times are fast but can be improved in a nitrogen atmosphere or doping the electrodes with a radioactive isotope such as cesium 137 or nickel 63.

The initial charging of the capacitors is best done via a controlled current source by  the use of a current limited transformer operating at line frequency or a standard high voltage transformer with a current controlled reactor in series with the primary for higher power units. The use of semiconductors usually requires special circuit precautions and shielding from the EMP generated by most of the discharges. 

Usually charging resistance Rs is chosen to limit the charging current to about 50 to 100 mA while the generator capacitance C is chosen such that the product CRs is about to 10s to 1 minute. The discharge time constant CR1/n (for n stages) will be too small (microseconds), compared to the charging time constant CRs which will be few seconds.

Impulse generators are nominally rated by the total voltage (nominal), the number of stages and the gross energy stored. The nominal output voltage is the number of stages multiplied by the charging voltage [5]. The nominal energy stored is given by:

E=C1V²/2;

where;

V = nominal maximum voltage (n times charging voltage)

C1= discharge capacitance

The discharge capacitance, C1 given by:

C1=C/N;

where;

C = capacitance of the generator

n = number of stage

Back emf in Motors

Lenz’s Law can be used to explain an interesting effect in electric motors.  In an electric motor, a current supplied to a coil sitting in a magnetic field causes it to turn.  However, while the coil of the motor is rotating, it experiences a change in magnetic flux with time and by Faraday’s Law an emf is induced in the coil.  By Lenz’s Law this induced emf must oppose the supplied emf driving the coil.  Thus, the induced emf is called a back emf.  As the coil rotates faster, the back emf increases and the difference between the constant supplied emf and the back emf gets smaller.  Clearly, this difference between the two emf’s is equal to the potential difference across the motor coil and hence determines the actual current in the coil.

It's the back EMF that sets the speed for a particular voltage, which is why DC motors can easily be speed controlled by varying the supply voltage.
That will keep happening (more torque loading = less speed & more current) all the way down until the motor stalls and it is taking the full current allowed by the resistance of the circuit with zero back EMF.

When the motor is first turned on and the coil begins to rotate, the back-emf is very small, since the rate of cutting flux is small.  This means that the current passing through the coil in the forward direction is very large and could possibly burn out the motor.  To ensure that this does not happen, adjustable starting resistors in series with the motor are often used, especially with large motors.  Once the motor has reached its normal operating speed, these starting resistors can be switched out, since by then the back emf has reached a maximum and has thereby minimised the current in the coil.

If the load on the motor is increased at some time, the motor will slow down, reducing the back-emf and allowing a larger current to flow in the coil.  Since torque is proportional to current, an automatic increase in torque will follow an increase in load on the motor.

Magnetic Tape Recording

Magnetic recording techniques are one of the most common way of recording signals. The system relies on the imposition of a magnetic field, derived from an electrical signal, on a magnetically susceptible medium, which becomes magnetized. The magnetic medium employed is magnetic tape: a thin plastic ribbon with randomly oriented microscopic magnetic particles glued to the surface. The record head magnetic field alters the polarization (not the physical orientation) of the tiny particles so that they align their magnetic domains with the imposed field: the stronger the imposed field, the more particles align their orientations with the field, until all of the particles are magnetized. The retained pattern of magnetization stores the representation of the signal. When the magnetized medium is moved past a read head, an electrical signal is produced. Unfortunately, the process is very non-linear, so the resulting playback signal is different from the original signal. Much of the circuitry employed in an analog tape recorder is necessary to undo the non-linear distortion introduced by the physics of the system.

Recording 

 The record head converts an electrical signal into a magnetic field which can be used to create a

pattern of magnetization in the tiny magnetic particles of the tape. The head consists of a torroidal core with a small air gap. A coil of wire is wound around the core, which is made of a magnetically permeable metal. Much like a transformer, the record head converts an electrical signal into a changing magnetic field. As the tape moves away from the gap, the magnetic flux (the magnetic equivalent of current) decreases as the inverse square of distance. At some distance, it is no longer strong enough to change the magnetic particles on the tape and the magnetization pattern then present is retained. This means that the actual recording takes place at the so-called “trailing edge” of the gap, rather over the entire gap length. The process of recording information to magnetic polarizations involves the interaction of  imposed magnetic field with a magnetizable layer on the tape.

 As the field strength increases, it begins to magnetize some particles. For some amount of signal level increase, the magnetization left on the tape increases linearly. At high levels, there are fewer and fewer magnetic particles left to magnetize and the tape becomes saturated until no unmagnetized particles are left. This results in what is called hysteresis: tape magnetic domains are not linearly changed by the imposed signal. This results in distortion of the recorded signal.

 Reproduction

The reproduce process is conceptually the reverse of the recording process: as the magnetized

tape is moved past the reproduce head gap, it’s magnetic field induces a flux in the head. This flux then causes a current to flow in the coil of wire which is wrapped around the head core. Unlike the recording head, the length of the reproduce head gap is critically related to the ability of the head to reproduce the frequencies recorded on the tape. This is because the flux generated in the gap is created over the entire gap length rather than at the trailing edge as in the recording process.

Electrolyte Vs. Capacitor batteries

Rather than operating as a stand-alone energy storage device, supercapacitors work well as low-maintenance memory backup to bridge short power interruptions. The virtue of ultra-rapid charging and delivery of high current on demand makes the supercapacitor an ideal candidate as a peak-load enhancer for hybrid vehicles, as well as fuel cell applications.

The charge time of a supercapacitor is about 10 seconds. The charge characteristic is similar to an electrochemical battery and the charge current is, to a large extent, limited by the charger. The initial charge can be made very fast, and the topping charge will take extra time. Provision must be made to limit the initial current inrush when charging an empty supercapacitor. The supercapacitor cannot go into overcharge and does not require full-charge detection; the current simply stops flowing when the capacitor is full.

The supercapacitor can be charged and discharged virtually an unlimited number of times. Unlike the electrochemical battery, which has a defined cycle life, there is little wear and tear by cycling a supercapacitor. Nor does age affect the device, as it would a battery. Under normal conditions, a supercapacitor fades from the original 100 percent capacity to 80 percent in 10 years. Applying higher voltages than specified shortens the life. The supercapacitor functions well at hot and cold temperatures.

The self-discharge of a supercapacitor is substantially higher than that of an electrostatic capacitor and somewhat higher than the electrochemical battery. The organic electrolyte contributes to this. The stored energy of a supercapacitor decreases from 100 to 50 percent in 30 to 40 days. A nickel-based battery self-discharges 10 to 15 percent per month. Li-ion discharges only five percent per month.

Supercapacitors are expensive in terms of cost per watt. Some design engineers argue that the money for the supercapacitor would better be spent on a larger battery. We need to realize that the supercapacitor and chemical battery are not in competition; rather they are different products serving unique applications.