ELECTRICAL THINGS

  Applications of DC Generators Separately-excited DC generator are rarely used in practical applications because of the additional expenses to be limit for the separate excitation supply. DC shunt generator can be used for general purpose lighting and low-voltage DC supply system. DC series generators do not find much application because of their rising voltage characteristics at higher loads, except these are sometimes used as boosters to compensate excessive voltage drops that can take place in a long DC feeder. Flat compound DC generators are most commonly used for low-voltage DC distribution systems. Flat compound generators can also be used for charging of batteries since they can give fairly constant output terminal voltage, irrespective of load current. Over-compound type DC generators can be used for lighting and general power supply applications because over-compounding can compensate for voltage drops in the distribution lines and thus voltage at the consumer e...

Hopkinson's Method  In this method, two identical DC machines are both mechanically and electrical coupled, and are tested simultaneously. One of the machines is run as a motor, whereas the other as a generator. the connection diagram of Hopkinson's test is given below:- For performing the test, machine I is started as a DC shunt motor and brought to rated speed with switch 'S' open. Both machines run at same speed as both are mechanically coupled. The field current of the generator (machine II) is so adjusted that its output terminal voltage changes and becomes equal to that supplied to the motor (machine I) terminals.   At this time voltmeter V 2     reads zero voltage. the switch 'S' is closed at that instant. Under this condition, the generator will neither taking nor giving current to the supply. After this state is achieved, any desired load can then be put on the generator by controlling the induced EMF of the machines. I f2  > I f1 , machine II...

  Swinburne's Method One of the easiest ways of measuring no-load losses of a machine is by Swinburne's method. Since this is a no-load test, it cannot be performed on DC Series Motors. This method is applicable for Shunt or Flat-Compound DC generators and motors.  In this test, the machine, whether it is a generator or a motor, is run as a motor with no-load at rated speed and excitation. If machine under test is generator, then the voltage across the armature = rated terminal voltage + the voltage drop in the armature circuit resistance at rated armature current. If machine under test is motor, then the voltage across the armature = rated terminal voltage - the voltage drop in the armature circuit resistance at rated armature current. Swinburne's method for measuring no-load losses of a DC Motor I ao = No-Load Armature Current I f = Field Current V t = Terminal Voltage →  Power input to the machine = V t ( I ao +   I f )  Power taken by the armature = V t I ao...

  Cumulatively Compound Motor cumulatively compound motor Torque-Armature Current Characteristics The flux in a cumulatively compound machine will be function of sum of field flux and armature flux. T = K Ⲫ I a = K I a f ( N se  I a + N sh  I f )  ,         N sh = N f = number of turns in shunt field winding  N se = number of turns in series field winding    f = Frequency There can be two cases depending upon whether Series Flux dominates or Shunt Flux dominates. ( for strong field flux ) ( for weak field flux ) For Strong Shunt Field, the machine behaves as a Shunt Machine but at higher values of Armature Current the series flux dominates and characteristics become similar to Series Excited Machine. For Weak Shunt Field, series flux dominates and machine behaves as a Series Excited DC Motor. Speed-Armature Current Characteristics E a = V s  - I a ( R a + R se  ) K Ⲫ N = V s - I a (  R a  + R se  ) Sp...

 The following characteristics are plotted in case of DC Motors: Speed- Armature current characteristics ( N or Ꞷ  vs  I a ) Torque- Armature current characteristics ( T  vs  I a ) Speed- Torque characteristics ( N or Ꞷ  vs  T ) NOTE:- For Shunt Excited DC Motor all characteristics are same as that of Separately Excited DC Motor. Shunt and Separately excited DC Motor The set-up of a Separately Excited motor is shown in the figure below: Shunt Excited DC Motor Separately Excited DC Motor Torque- Armature current characteristics Developed Torque in a DC Machine depends on Armature Current as shown below. We assume that field excitation is kept constant and hence flux is constant. T = K Ⲫ I a    ∝  I a      ,     if  Ⲫ = Constant      Speed- Armature current characteristics The performance equation of a separately excited DC Motor is given by, E b = (Vs ...

  Important Points When a differentially compounded motor is started, the starting current may be high enough to reverse the flux polarity and thus motor would start in reverse direction. However, since reverse field is weak, the acceleration may be low and thus starting current would remain high for long time and can thus damage the machine. So, series field winding is shorted while starting and when motor acquires normal speed, shorting may be removed. Sometimes, due to sudden overload in uncompensated shunt machine, the demagnetizing effect may be dominant and can cause sudden runaway. This is prevented by providing a few series turns to neutralize armature reaction (AR) and hence make it cumulatively compounded. Starting of DC Motors At Starting, the speed of DC Motor is zero and hence the induced emf which is proportional to speed of Motor is also zero. Due to which, V t = I a R a      for Shunt Motor and Separately Excited Motor V t = I a ...

Regenerative Braking In this form of braking, an attempts is made to return the stored Kinetic Energy of rotor back to source as much as possible. Braking can be achieved by keeping E a > V either by increasing excitation or by reducing terminal voltage. This would result in reversal of armature current and reversal of direction of torque and sends the machine to generator mode but machine cannot keep running as there is no Prime Mover to supply mechanical power. But armature current must be kept within limits and as soon as I a = 0 , machine must be switched off. Plugging In this technique, supply to armature terminals is reversed while field is left undisturbed. The current reverses resulting in to negative torque that brings the rotor quickly to rest and the supply in the motor should then be switched off to prevents its rotation in reverse direction. During plugging armature current should be maintained within safe limits by inserting an external resistance i...

Cumulatively Compound Generator are more common as compared to Differentially Compound Generator as they have almost constant voltage from No-Load to Full Load. 1. No-Load Characteristics For long shunt, due to lesser number of turns in series field winding, series MMF does not impact the no-load characteristics. For short shunt, series field winding does not carry any current and this no-load characteristics is same as that of shunt generator. Hence, in both cases no-load characteristics are same as that of a Shunt Generator. 2. External Characteristics In cumulatively compounded generator, series flux adds shunt flux and depending upon number of series field turns, It is classified as Under-compounded: V t   falls with load Flat-compounded: V t  is practically constant Over-compounded: V t  rises with load In differentially compounded generator, series flux opposes shunt flux and thus V t falls more rapidly with load. If we plot the character...