ELECTRICAL EASY

Rated Normal Current: The rated normal current of a circuit-breaker is the r.m.s. value of the current which the circuit breaker can carry continuously and with temperature rise of the various parts within specified limits. The design of contacts and other current carrying parts in the interrupter of the circuit breaker are generally based on the limits of temperature rise. For a given cross section of the conductor and a certain value of current, the temperature rise depends upon the conductivity of the material. Hence, high conductivity material is preferred for current carrying parts. The cross-section of the conductors should be increased for materials with lower conductivity. The use of magnetic materials in close circuits should be avoided to prevent heating due to hysteresis loss and eddy currents. The rated current of a circuit-breaker is verified by conducting temperature-rise tests. Rated Short Circuit Breaking Current: The rated short-circuit-breakin...

Before going into the calculation part of Stabilizing Resistor, I will first explain the purpose of Stabilizing Resistor in High Impedance Differential Protection. Stabilizing Resistor in High Impedance Differential Protection is used to prevent the operation of Relay in case of through fault. Through fault is a fault outside the zone of protection. Lets us assume that High Impedance Differential protection is used to protect a Bus bar as shown in figure. It shall be noted here that, in High Impedance Differential Protection, all the CTs are connected in parallel and then the four wires i.e. R, Y, B and N are connected with the Relay as shown in figure above. If there is any fault in the bus, the according to Kirchhoff’s current law, the summation of current will not be zero and a net current will flow through the Relay coil to operate it. In normal condition, the summation of current will be zero and hence no current will flow through the Relay coil and hence the Re...

The concept of Subtransient, Transient and Steady State arises in case of fault in an Alternator. Let us assume a sudden short circuit in three phase of alternator. The fault current will flow in all the three phases of alternator and its waveform will be as shown in figure below. When the alternator is short-circuited, the currents in all the three-phases rise rapidly to a high value of about 10 to 18 times of full load current, during the first quarter cycle. The flux crossing the air gap is large during a first couple of cycles. The reactance during these first two or three cycle is least and the short circuit current is high. This reactance is called subtransient reactance and is denoted by X". The first few cycles come under sub-transient state. After a first few cycles, the decrement in the r.m.s. value of short circuit current is less rapid than the decrements during the first few cycles. This state is called the Transient  State and the reactance in th...

We have already discussed about staring methods of Single Phase Induction Motor. If you miss that please read here, 1)      Revolving Field Theory of Single Phase Induction Motors 2)      Starting Methods of Single Phase Induction Motors The schematic diagram of Capacitor Split Phase Motor is shown in figure below. Like in resistor split phase motor , there are two windings, Main and Auxiliary winding but the basic difference between the two method is that in Capacitor Split Phase Motors a capacitor of suitable value is connected in series with the auxiliary winding.  Capacitor is connected in series with the auxiliary winding to obtain the desired time phase displacement between the auxiliary winding current Ia and main winding current Im. A centrifugal Switch is also provided the cut out auxiliary winding when the speed of Single Phase Induction Motor reaches 70 to 80% of synchronous speed. You may like to...

As discussed in earlier post  Revolving Field Theory of Single Phase Induction Motors , a single phase induction motor with main stator winding alone has no inherent starting torque as the main stator winding only produces stationary pulsating air gap flux wave. For the development of starting torque, rotating field at the starting must be produced. There are various methods of starting a single induction motor, which can be classified as below, 1)     Split phase starting 2)     Shaded pole starting 3)     Repulsion motor starting 4)     Reluctance starting Generally a single phase induction motor is known by the method employed for its starting. Basically the selection of a particular single phase induction motor and choice of its starting method depends upon the following factors: 1)     Torque speed characteristics of load from starting to normal operating speed 2)  ...

Revolving Field Theory is used for the analysis of behavior of Single Phase Induction Motor. Before going into the details of Revolving Field Theory, first we should know why do we need to study this theory? Well, a Single Phase Induction Motor with its main winding alone can be represented as shown in figure below. When a Single Phase Induction Motor is supplied with a voltage source to its stator, an alternating flux is developed by the stator in the air gap which is stationary but pulsating in nature. This flux links with the Rotor winding and produces a current in the rotor bars. For the assumed direction of stator flux Øs as shown in figure above, the current distribution along the periphery of the Squirrel cage Rotor bars will be as shown in figure. This distribution of current in the rotor produces a flux Ør. Notice that the direction of rotor produced flux Ør is in opposition with the stator produced flux Øs which is in accordance with the Lenz’s law. As...