ELECTRICAL EASY

The concept of stability of Induction Motor is very important and vital for interview point of view or from selection of a particular Induction Motor for a given load. In all Motors, the speed drops if we increase the load torque as the power of Induction Motor is constant (Power = TorquexSpeed). For getting a operating point for a given load torque, the point of intersection of load torque characteristics and Motor speed Torque characteristics determines the point of stable operation of Motor. But this does not guarantee that the point of intersection will be stable one. Suppose the Motor characteristics is such that as the load torque increases the speed of Motor increases then obviously this Motor and load combination will not result into stable operation. Thus it is very important for stable operation of the system consisting of Motor and Load that as the Load Torque increases Motor speed decreases and vice versa. Normally an Induction Motor is designed to opera...

This post gives is a quick derivation of the formula for calculating the steady state reactive power absorbed by a capacitor when excited by a sinusoidal voltage source. Given a capacitor with a capacitance value of C in Farads, excited by a voltage source V in volts, it will draw a current i amps into its positive terminal. If  V  is a steady state sinusoidal source with frequency of  ω=2πf , then we may use a Laplace Transform to solve the circuit in the frequency domain where  V  is the phasor rms voltage,  I  is the phasor rms current, and  Zc  is the capacitive reactance in ohms of the capacitor as follows: Note that the negative sign means that the capacitor is absorbing negative reactive power VARs which is equivalent to stating that the capacitor is supplying reactive power to the external circuit or system. For a three-phase system, multiply Q by 3 to get the total reactive power supplied by the Capacitor. Tha...

Carefully observe the curve below to have an insight into the situation. The peak is the Pull-Out torque in the curve or slip torque characteristics. To the right of the peak is the Stable region of operation and to the left is the unstable region of operation i.e. the motor cannot operate in unstable region. The slant lines are the load torque lines taken for demonstration purpose only, they are different for different types of loads. Under such a scenario, wherever the Motor torque curve and the load torque line meet, in the STABLE REGION, that is the point of operation of the motor i.e. the speed at which the motor operates for a particular value of load. Now, if we go on increasing the load, as we can see from the curve above, after the point of maximum torque, the load torque and the motor torque characteristics do not meet. This shows that there exists no point of operation for the motor. Thus, the motor decelerates and stops. For this reason, it is calle...

It can easily be seen from the slip torque characteristics of an induction motor that, there is some finite torque when the slip s=1 i.e. speed is zero. This simply means that Induction Motor is a self-starting motor and begins to rotate on its own when connected to a 3 phase supply. At the instant of starting, a three phase Induction Motor behaves like a Transformer with its secondary winding shorted. Therefore, Induction Motor during starting takes a high current from the supply mains. To limit this high starting current of Induction Motor, different starting methods are used. In this post we will have a look at the Auto-Transformer Starting Method of Induction Motor. The main philosophy of starting any Induction Motor is to start it at a reduced voltage and as soon as the motor reaches its rated speed, full supply voltage is applied to the terminals of Induction Motor. A schematic diagram for Auto-Transformer Starting of an Induction Motor is given below. ...

Mathematics is an important subject for securing a good rank in GATE exam. If you see the statistics, you will note that every year around five questions are asked form Mathematics. Marks wise it will stand around 12. So once can secure his/her 12 marks by just preparing Mathematics. There are three sections where math is asked. One in Aptitude Section (1 Question Carrying one marks and 1 Question of two marks), Section-1 (carrying one marks) and Section-2 (Carrying two marks). The math question coming in Aptitude question is quite simple and can be solved by having a good knowledge of mathematics up to class 12. It does not need any special preparation to solve mathematics question of Aptitude Section. Please have a look at some of the mathematics Questions from previous year in Aptitude Section. Year 2015: 1 Marks: 2 Marks: Year 2014: 1 Marks Question: 2 Marks Questions: It is clear that the above two questions are s...

Losses in the transformer are of the order of 1% of its full load kW rating. There are mainly two types of losses in a Transformer i.e. Core Loss and Ohmic Loss. Because of these losses the winding of transformer as well as Core gets heat up and we need to provide sufficient cooling to remove heat and maintain a constant winding and Core temperature. Various types of cooling methodologies are used for Transformer but before going to each methodology, we will have a look at the General Method of designating a particular cooling method i.e. class. In the nameplate of Transformer, four letter represents the cooling method of the Transformer. The above table is self explanatory, therefore if you see ONAN on the Name Plate of Transformer then it means that Oil with flash point less than 300° is used as cooling media and Natural convection is used for removing heat. In secondary side of cooling mechanism, Air with Natural convection is used by employing Radiator. ...

Current Transformers are classified as Dead Tank Type and Live Tank Type CTs based on their construction. The term Dead and Live has their own meaning from the view point of potential of Secondary Windings and Core. Dead Tank itself means that the potential of the Tank of CT is at earth potential i.e. zero as shown in the figure below. The classification of CT i.e. whether Dead Tank or Live Tank comes from whether the Core and Secondary Winding of CT is located in Dead Tank or Live Tank of CT. If the Core and Secondary Winding is located in Dead Tank, then it is Dead Tank type CT while in case the Core and Secondary Winding is located in Live Tank portion then it is Live Tank type CT. Figure below depicts the Dead Tank type and Live Tank type CT. Dead Tank type CT: Observe that the Core and Secondary Winding is located in Dead Tank portion and hence it is Dead Tank type CT. Live Tank type CT: Carefully observe that Core and Secondary ...