Electrical Machines Demystified
Whenever you are starting Electrical Machines, you need to be well aware of the pre-requisite concepts which is the basic of Electromagnetic Theory and some basics of Electrical Material. These concepts will help you understand the working of each machine in detail and you will also be able to understand why some particular material is used in the construction of Electrical Machines. Though this may not directly come in handy in GATE exam it will give you an edge in the interview.
This is the beginning of Electrical Machines course and is a good platform to get acquainted with the basics of machines. This chapter will start with the basics of Transformers such as Construction and Working Principle. In construction, you should be clear about the differences in Core Type and Shell Type Transformers and where they are used. Then you can move onto the concept of Ideal Transformer and one by one removing the idealities we can realize a practical transformer. Then by removing each non-ideality, you should understand how the phasor diagram of the Transformer is modified like by introducing the core loss, magnetizing current, leakage flux etc. based on these phasor diagrams you can draw the equivalent circuit of Transformer.
Also, one of the most commonly asked concepts is the core losses which are Hysteresis and Eddy Current Losses so practice few problems on that where we are given total core losses and we need to calculate individual Hysteresis and Eddy Current Losses.
In case of three phase transformers, you have to understand how to draw the phasor diagram and calculate the phase difference between the line voltages on both sides. If you check out videos on our channel you will find a concept regarding calculation of currents in three phase transformers by the use Energy Conservation Concept and it can help you a lot. The questions nowadays are very common on this topic and are mostly based on connections and phase shift. You have to cover mainly for connections which are: Star-Star, Star-Delta, Delta-Star, Delta-Delta.
The next concept is the auto-transformer which I believe is the easiest as the entire question just depends on the ratio of low voltage to the high voltage rating of auto-transformer and we can scale each and every quantity of two winding transformer based on that. Generally, it would give you the parameters of two-winding transformers and will ask you corresponding parameters of Auto-transformer and when an Auto-Transformer is realized using the two-winding transformer.
The last concept would be Parallel Operation of Transformers in which you have to understand how to find the load sharing between the two transformers when a load is given and how to find the maximum load that can be supplied by the parallel connection. the concepts of Harmonics is good for understanding but is not required from GATE point of view.
This chapter starts with the construction of DC machines such as the construction of Core as well as the construction of Armature winding which is divided into two categories lap winding and Wave Winding. If you remember the features of each of the windings then you need not go into the details of construction of each type of winding. The only theoretical concepts that are important from GATE point of view as well as interview point of view is the concept of Armature reaction and Commutation. these concepts given an insight in the problems faced by DC machine in its operation.
From, the numericals point of view, this chapter totally revolves around 4 equations which are two performance equations one each for generator and motor and the expressions for Torque and EMF. The most commonly asked questions are based on speed control of DC Motor and we must always remember that whenever we want to calculate speed we need to calculate the back emf and then take the ratio of two emfs to compute the ratio of two speeds.
Like any other Electrical Machine, this chapter again starts from the study about the construction and Working principle of Synchronous Machines. In Construction you have to study about the nature of Slots on the stator and this concept is further carried over to Induction Machines as well. In Rotor Construction you have to study about the two types of Rotors i.e. Cylindrical Rotors and Salient Pole Rotor. For the working principle, you have to study about the concept of Rotating Magnetic Field and then understand how the motor and generator will work.
Also, we must remember the concept of Armature Reaction that whether this effect is magnetizing, de-magnetizing or cross-magnetizing based on the power factor at which the machine operates. The next concept is that of Synchronous Impedance that comes from Open Circuit Characteristics and Short Circuit Characteristics. The other characteristics are not much important such as Zero Power factor Characteristics used in Potier Triangle Method and Armature Characteristics.
Other than that this chapter is mostly based on two equations which are Power-Angle Equation and the emf-terminal voltage relation which comes from the equivalent circuit of the machine and you need to solve these two equation to calculate any parameter that is asked.
Also since, the physical calculator is not allowed it is better that you understand the scalar equations that can be used to find the emf rather than the phasor equations as that will save a lot of calculations. Also, remember how the equations change when the machine changes from Motor and Generator. Also, check carefully before starting any problem whether the machine is a motor or a generator.
This is the only machine whose functioning you need to understand well as mostly the questions are based upon the concept of slip and for that, you need to understand various things like speed of rotor and speed of rotor magnetic field and stuff like that. The concept that is now a days appearing and is non-trivial is the concept of Inverted Induction Motor where supply is connected to the rotor and stator terminals are connected to load there also you need to understand the speeds of all field and rotor speed also. So, if you understand the speed at which rotor and the magnetic fields rotate, you can find any relative speed.
Equivalent Circuit is not much important here. Probably the most important concept in this chapter is the torque-slip characteristics and conditions for the maximum toque and starting torque and question may be based on the ratio of these. The relation between torque at any slip and the maximum torque is very important. Usually, Stator Impedance is neglected and only Rotor Impedance is considered. Also, remember the formulas for Air-Gap Power and rotor copper loss and based on these powers you should be able to compute the efficiency of the machine.
The next concept is the starting of Induction Motor by Auto-Transformer or Star-Delta Starter which you need to study. Then you need to study about the Speed Control of Induction Motor in which the most important method is V/f control. Braking of Induction Motor is not very much important.