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Synchronous motor have field poles on the stator winding or rotor and 3Φ armature on other member. (based on the size and capacity of machine). Field poles on rotor are preferred construction for high voltage machine otherwise high voltage leads have to be taken out through three slip rings placed on the rotating member, causing difficulties of insulation and conducting current through the contacts rubbing on the rotating slip rings. Further, a synchronous motor may have salient (projecting) poles or cylindrical poles with field winding distributed out, an arrangement which is preferred in high speed alternators run by steam turbine.
The speed and the frequency of synchronous motor are related as follows,
N = 120f/P
As this relationship holds under all operating conditions, the speed of synchronous motor is called the Synchronous Speed.
The induction motor has a stator which is 3Φ wound, similar to the synchronous motor. The rotor in an induction motor carries secondary winding which may be merely a set of copper bars shorted out at each end or a not properly wound rotor with three winding connections brought out through slip rings and shorted externally. These differences in construction leads to two types of induction motors- squirrel cage and slip ring types.
Squirrel Cage rotor of Induction Motor
An induction motor with its primary(stator) excited causes a rotating field in the air gap with consequent induction of currents in the shorted rotor conductors.
Torque is developed by the interaction of rotating field and he rotor current. The motor runs at subsynchronous speed which somewhat reduces with load as compared to a synchronous motor which runs at synchronous speed at all loads.
A transmission line has four constants R, L, C and shunt conductance. But generally, three constants R, L and C are considered and they are uniform along the whole length of line. The fourth constant shunt conductance between conductors or between conductor and ground and accounts for the leakage current at the insulators. It is very small in case of overhead lines and may be assumed zero. The capacitance existing between conductors for 1φ line or 3φ line forms a shunt path throughout the length of line. Therefore capacitance effects introduce complication in transmission line calculation. Depending upon the manner in which capacitance is taken into account, the overhead transmission line are classified as, Short transmission linesMedium transmission linesLong transmission lines
Short transmission linesA short transmission line is one in which the line voltage is comparatively low (< 20kV) and the length of an overhead transmission line is upto about 50km. Due to smaller length and …
Group DrivesIt consists of single machine which actuates several machines or mechanism by means of one or more line shaft. Hence this is also called "line shaft drive". This line shafts are connected to multi stepped pulleys and belts that connect this pulley and shaft of the driven machine, serves to vary their speed.
Group drive is economical in consideration for the cost of motor and control gear. A single motor of large capacity costs less than that of the total cost of number of small motors for same total capacity namely, a single motor of 100KW costs less than that of ten motors of 10KW each. Since all the motors may not operate on full load at the same time, the KW rating of motor of group drive is often less than the aggregate KW output rating of the individual motor and further cause reduction in cost. Individual DriveIn individual drive an electric motor used for transmitting motion to various parts or mechanism belonging to single equipment. For example, such drive…
An induction motor consists of two main parts StatorRotor
The stationary frame is called stator and the rotating armature is called rotor. The stator of induction motor is similar to that of the synchronous motor or generator. It is made of large number of stampings. These stampings are slotted in order to receive the stator windings. Figure below shows the construction of Induction motor.
The functions of various parts is as follows:-Frame:- Its function is to provide mechanical support to the entire construction. The frame also contains the stator winding of Induction motor.Air gap:- Air gap provides the space for the rotating magnetic field between the stator and rotor.Fan:- The fan rotates with the rotor. Its function is to cool down the motor.Slip rings:- The rotor winding terminals are permanently connected to the slip rings(in slip ring type induction motor). The slip rings are continuously in contact with the brushes which are pressed against slip rings. Externa…
An induction motor with two cage rotor is used for high starting torque. The slotting arrangement for double cage induction motor is as shown in above figure. As the name indicate the double cage induction motor has two winding in rotor. The outer bars consists of rotor bars having low reactance and high resistance. On the other hand, the inner cage consists of rotor bars having high reactance and low resistance.
At start the rotor frequency is high, the outer cage carries most of the current despite its high resistance. The inner cage has low reactance and is mostly ineffective. This gives high starting torque and low starting current. As the motor picks up the speed, the rotor frequency reduces and the inner cage carries most of the current. Under normal running condition, the outer cage and inner cage are in parallel giving low combined resistance and both the cages are active.
When the speed is normal frequency reduces and it is so small that the reactance of…
When the anode voltage voltage is made positive with respect to the cathode, the junctions J1 and J3 are forward biased but the middle junction J2 is reverse biased and only a small leakage current flows from anode to cathode due to the mobile charges. The junction J2, because of the presence of depletion layer does not allow any current to flow through the device. The leakage current is insufficient to make the device conduct. The depletion layer mostly of immovable charges does not constitute any flow of current. The SCR is then said to be in the forward blocking or OFF sate condition and the leakage current is known as OFF state current ID. When the cathode voltage is positive with respect to the anode, the middle junction J2 becomes forward biased but the two outer junctions J1 and J3 becomes reverse biased. This is like two series connected diodes with reverse voltage across them. The junction J1 and J3 do not allow any current to flow through the device. Only a very small leakage…
Crawling It has been found that induction motor practically this squirrel cage type ,sometimes exhibit,a tendency to run stable at speeds as low as 1/7th of their synchronous speed .This is known as crawling of induction motor. This action is due to ,the fact that the AC winding of the stator produces a flux wave which is not pure sine wave .It is a complex wave consisting of a fundamental wave which revolves synchronously and odd harmonics like 3rd,5th,7th etc. which rotate either in the forward or backward direction at Ns/3, Ns/5, Ns/7 speed respectively. As a result in addition to the fundamental torque ,harmonics torques also developed whose synchronous speed for fundamental torque. For Example Ns/n, where N is the order of harmonics torque. Since the 3rd harmonics current are absent in a balanced three phase system ,they produce no torque .Hence total motor torque has there components. The fundamental torque rotating with synchronous speed Fifth harmonics torque rotating at Ns/5…