Times Of Electric

 What is known as dynamic braking in DC motor?  When an electric motor rotates, a kinetic energy is stored in its rotating mass. If the motor is disconnected from the supply it continues to rotate for a period of time until the kinetic energy is totally dissipated in the form of rotational losses or frictional loss. The faster the dissipation of the kinetic energy, the more rapid is the braking.  During the dynamic braking the kinetic energy of the motor is converted into electrical energy. This energy is dissipated in resistive elements. The rate of dissipation can be increased by the design of the braking resistance. When the motor is connected to source, the power flows from source to motor. If it is connected from the source, and the ends are connected to braking resistance which is connected across the motor terminals. In braking condition, motor acts as generator and braking current flows motor to resistor, by which the kinetic energy is dissipated by heat. If we ar...

  What is plugging? T he process of suddenly stopping the running motor by electrical method is called Electrical braking. It is used to stop the motor at a specific position, preventing the motor from over speeding.   One of the type of electrical braking is called plugging. It is also known as counter current braking. Plugging is the process of braking the motor by changing the direction of the motor rotation. This direction of rotation of electric motor is dependent on several variables. For ac machines, the phase sequence of the stator winding are one of the variable and dc machines, the polarities of the field or armature voltage. In ac machines, the shaft of the machines rotates in the same direction as the magnetic field. This direction of rotation of the field in the air gap depends on the phase sequences connection of the stator windings.   When the three phase stator windings are connected in the RYB sequence, the air gap field rotates in one direction at nearer...

  What is electrical braking and its types?   Whenever an electric motor or drive is disconnected from the supply. The speed of the motor gradually decreases and becomes zero. Braking is a generic terms used to set a described operating conditions for electric drives. Such as rapid stopping of motor, holding the shaft to a specific position,maintaining the speed to a desired value preventing from over speeding. The process done by electrical equipment is called Electrical braking.   Electrical braking is used more than mechanical braking because of this reasons. High operating efficiency No heat emission produced due to friction in mechanical braking Less maintenance.   During electrical braking, the motor is made to work as a generator, it produces a negative slip and negative torque(braking torque). This is achieved by suitable changing the electrical connections of the motor.   The braking of the drive should be such as to stop the motor at the specified poin...

 What is the function of oil in transformer?   The oil used is purposed for two main reasons; 1.  Cooling 2.  Insulating   The oil used must possess the following properties. 1.  High dielectric strength 2.  Free from inorganic acid, alkali and corrosive sulphur to prevent injury to the conductor or insulation. 3.  Low viscosity to provide good heat transfer 4.  Free from sludging under normal operating conditions. 5.  Good resistance to emulsion so that the oil may throw down any moisture entering the tank.   Cooling The transformer windings dissipate waste heat, which needs to be removed. Transformer oil absorbs this heat from the windings and conducts it to the outside of the transformer, where it can be dissipated to the outside air. It is possible to build an air-cooled transformer. These are used where spillage of the insulating oil is an unacceptable hazzard. I.e., for indoor hazard.   Insulation All parts which might be...

  Why Transformer is rated in KVA?   Well there are different two reasons for this.   The first reason they normally use to explain for this is depending on the losses of the transformer. Consider a single phase transformer supplying a power VICosØ to the load. The two losses that occur in transformer are copper loss and core or iron loss. This copper loss depends on the current flowing through the conductor and iron loss depends on the voltage and not on the phase angle between voltage and current(cosØ). That is the losses are independent to the power factor of the load. Hence the total losses depends upon the volt-ampere(VA), the transformer is rated in VA or KVA or MVA.   The second reason is from the view point of manufacturer. The manufacturer doesn’t know the power factor of the load or the type of load connected at the consumer end. Since the power handled by the transformer depends on load pf., rated power(KW) of the transformer cannot be a satisfactory one. ...

Why we use stepped core in transformer?  Usually, rectangular core is used for small rated transformer while the coil can be either square or rectangular.  But for highly rated power transformer, we require circular coils only, so we have to use circular core(stepped core). Circular coil is preferred in power transformer due to its Superior mechanical characteristics.  On circular coils, the mechanical force acts radially, so the stress is less and shape of coil do not change, however if we use rectangular coils, then the mechanical forces acts perpendicular to the coil results in creating high stress which would deform the coil shape. This is the reason why circular coils are preferred over other coils for power transformer.  So the reason for circular core is that, if we use rectangular core, then the useful space is not utilized better resulting a air between the coil and core. This affects in increasing the length of mean-turn of the winding which further incr...

Why USA use 110v/60Hz instead of 230V/50hz ?  Look at this picture.. Well there is not any technical reason for this, its just historic. We all know Thomas alva Edison     invented practical incandescent bulb that glows on direct current more than 1500hrs  in 1880’s. Untill that bulbs were always connected to the  battery. So then he decides build a transmission system to implement his invention in all house hold and streets in America. In 1882, he build power plant in Newyork which is said to be the first commercial power plant. Edison intended to give 100V supply to consumers and build a 110V dc generating powerplant so that extra 10V (which is 10%) will compensate the loss that occurs due to the resistance of the transmission lines. But due to this low voltage, a high current flows through transmission lines and hence the I^2R losses increases resulting a   high voltage drop. Due to the drop, 110v was only capable for a transmiss...

Voltage or Current. Which is the real killer? Hey guys, This might be the most controversial   this or that question after the famous chicken or egg. Lets see it clearly, It is not the high current or high voltage that kills you.        Take  for eg., a 24V   battery can produce much higher current than the wall socket (in the range of 10-15 amps) in our home does.  Still it can’t electrocute us if we touch both the terminals.       So what it is? Is it the high voltage that kills us. No, it is not for sure.       To get clear about this take an example of Van de graaff generator what you might have heard in high schools or in youtube videos. So a large Van de graaff generator  can produce 500000 volts or 500kv , but we had seen a stranger touching the sphere with his bare hand. The reason here is the over all rated output current generated in Van de graaff is in th...