Study of Electrical

Design of submersible motor Submersible pumps are immersed in liquid. A motor is coupled to the pump. Since this assembly (pump+mortor) is immersed in liquid, it should be tightly sealed so that liquid should not enter inside the motor.  In conventional pumps, the pump-motor unit is installed above the liquid or water surface but this pump suffers from the drawback of cavity formation since airgap is formed between liquid or water level and the pump impeller. It is often termed as air is trapped. So the pump can not be started unless this trapped air is released by pouring water inside the suction head pipe.  But in case of submersible pump, there is no issue of cavity formation. Applications of submersible pumps:- Bore well for water pumping Sewage handling/drainage systems Slurry pumping Oil wells Design considerations:- Since the submersible pump is immersed in liquid or water, the motors used must is specially designed and manufactured to prev...

Design consideration of high voltage 3 - phase induction motor.  1. stator:- Suitable radial air ducts are provided to ensure uniform and efficient cooling. The stator is vacuum pressure impregnated and cured with a specially formulated epoxy resin. This ensures a closed and homogeneous insulation system, which eliminates low dielectric loss, high dielectric strength, excellent heat transfer and serious internal partial discharge. Motors are designed to stay within temperature class F (temperature limit 155°). The windings basic impulse level is kept high and the windings are designed to cope with the highest mechanical stresses. 2. Rotor:-  Minimum vibration is the key to the rotor's long operating life.  This is achieved through strong construction and a delicate balance.  Selecting the shaft material according to the demands of the application and the ambient conditions in which the motor is to operate. Squirre...

Design of Rotor slots Number of rotor slots: This is the first step in the design of rotor. In this number of rotor slots are selected in consideration with stator slots. It is to be noted that selection of number of rotor slots in squirrel cage induction motor is very important because with certain combination of number of poles, stator slots and rotor slots induction motor may behave abnormally, like refusal to start (cogging) or may rotate at some sub synchronous speed (crawling) and sometimes severe Vibrations and excessive noise is observed. Normally these effects are produced by the harmonic fields which are created due to  (i)windings       (ii) slotting  (iii) saturaaon and (iv) air gap irregularities. These harmonic Fields induce emfs in rotor windings and circulate harmonic current, winich poduce hamaonic torque. It is to be noted that hamaonic field has more number of poles (np where n is order of harnaonic e. g. If ...

Length of Air Gap ( Factors affecting selection of Air Gap of 3 phase induction motor)  Length of air gap is very important in the induction motor as it controls important characteristics of induction motor such as power factor, over load capacity etc.  Following points should be considered while selecting length of air gap: (1) Power factor:- In the performance characteristics of motor this is very important factor. The air gap consumes maximm amount of mmf out of various parts of magnetic circuit of induction mtor. Hence, larger the air gap, more wall be the mmf required to pass the flux through. it can be shown that mmf required to send the flux through air gap is proportional to the product of flux density and length of air gap.  Thus if length of air gap is increased then mmf required I. e. In turm magnetizing current is  increased, which reduces the power factor. Fig  shows the phasor diagram of inductor motor. As lengt...

Design of Stator 1). Stator winding:- Generally two types of winding are commonly used for stator of three phase induction motor.    (a) Single layer much winding is used for small motors with small number of slots. It is housed in semienclosed slots.     (B) Double layer lap winding with diamond shaped coils is used for medium size motors. It is housed in open slots. As per the temperature class (class E, B, F etc.) impregnated insulaaon is used. The three phases of stator windings can be connected in either star or delta depending upon the starting method employed. The squirrel cage induction motors are usually started by star delta starter and hence the stator is designed for delta connection, with six leads for connection to stator.  For slip ring induction motor I. e. wound rotor construction motors, starting method used is resistance in rotor circuit, so stator can be designed either for star or delta as per require...

Methods of Improving Starting Torque: Starting torque of induction motor can be increased by increasing the rotor resistance.This can be done by reducing cross sectional area of rotor bars and by changing the conductor material. However, This will reduce the efficiency of motor. This problem can be overcomed by using different design for rotor which gives high starting torque without compromising the efficiency. Following two types of rotor design's are used.  1). Double cage rotors.  2). Deep bar rotors.  1). Double cage rotors : In this type of rotor construction, rotor winding has two cages. As shown in the above Fig, the outer cage uses bars of small cross section, thus it gives high resistance. As this cage is near to the air gap, its leakage reactance is small. The inner cage has bars of larger cross section so it offers less resistance, but as its is away from the air gap it offers high leakage reactance. In this type of motors, d...

Effect of Skewing the Rotor Slots in Squirrel cage Motor: Skewing : Many problems in induction motor like noise, vibrations, cogging etc can be reduced or eliminated by skewing either the stator or rotor. Generally the rotor bars are skewed by an angle just sufficient to cause induction of two equal and opposite harmonic emf in each bar.The bars must be skewed such that they should lie under alternate hamonic poles of same polarity so they are skewed through 2 pole pitches.      Angle of skew  =  2 × 360°/np      Where,       n = Number of harmonic (I. e. 3, 5, 7 etc.)      P = Number of poles     QsK = 720°/np  (mechanical)      and          Qsk = 720°/np × p/2 = 360°/n  (Electrical) 

       Working of 3 phase induction motor When the 3-phase stator winding is replaced by a 3-phase A.C. Fixed size magnetic flow but rotates at a synchronous speed when feeding from the supply unit. The magnetic flux cuts the rotor conductors which are stationary. Due to the relative sped between the rotating flux and the stationary conductors an emf is induced in the conductors. The fiequency of this induced emf is same as the supply frequency, its magnitude is proportional to the relative velocity and its direction is given by Fleming's right-hand rule. Since the conductors form a complete closed circuit, the rotor currents are produced.And according to Lenz's law, this current tries to oppose the relative speed. Hence the rotor starts rotating in the same direction as that of the flux and tries to catch up with it. The same is explained as under: In fig, shows the stator magnetic field at one instant and it is acting downwards and also ro...

Constitution of 3-phase induction motor • Construction of 3-Phase Induction Motor: There are two main parts of a 3-phase induction motor 1.Stator 2.Rotor. 1) Stator: As the name suggests,it is a stationary part of the motor.This part is made or silicon steel stampings(i.e.thin sheets).The stampings are slotted. When complete stator is assembled,the through slots are formed on the inner side of the stator.The slots may be open type,semi-open type or closed type.In these slots,a            3-phase winding (made of super enamcled copper)is accommodated.This winding may be star or delta connected. Three ends of this coil are brought to the terminal box where a three-phase supply can be delivered. The stator windings, stator and the A.C. supply connected to the stator winding is shown in the fig. 2) Rotor: Here two types of induction motors. That are depend on the construction of the rotor. A)  S...

General Information of a 3-phase induction motor A. C. System is universally used for generation of electricity and so also for transmission and distribution on account of it's various advantages.  General Information of a 3-phase induction motor As a electrical drive,A.C.3-phase and single phase motors are therefore find the wide applications in various inctustries.Out of the so many types of 3-phase A.C.motors,the 3-phase inductiton motor is very popular in the industrics due to its following merits: Compact and very simple construction. Shape and weight is small in comparison with other types. Cheaper in cost. Very easy for repairs and rewinding. Maintenance cost is very low. In operation it requires less attention. Efficiency is hetter. Frictional losses are lesser. It is self-starting. On higher loads it operates at a reasonably better power-factor. Starters required for controlling the initial current are of very simpl...