Question { L&T, 69177 }

what are different types of alternator and discuss about the
methods of cooling

Answer

Alternator as the name itself says, it is a machine that converts mechanical energy to AC electric power at particular current and voltage. It is also called as synchronous generator.
Past of Alternator
Hippolyte Pixxi and Michale Faraday provided the first concept of alternator. They designed a rectangular rotating conductor within a magnetic field for producing alternating current in the outer static circuit. J.E.H. Gordon in the year 1886, designed and produced first prototype of useful model. A model of 100 to 300 Hz synchronous generator was designed by Kelvin and Sebastin Ferranti. In the year 1891, Nikola Tesla created a commercially useful 15 Khz generator. Poly Phase alternators after this year came into picture which can deliver multiple phase currents.
Alternator Uses
Modern Vehicles and electrical system power gives from alternator. In the olden days, DC generators or dynamos were utilized for a reason after alternator development, more robust and light weight alternators are used as a replacement for the DC dynamos. There is a general requirement of motor vehicles with direct current and is still an alternator along with rectifier diode rather than a DC generator with better choice as there is an absence of complicated communication.
Diesel electric locomotive is also another use of alternator. Alternator driven by diesel engine is the engine of the locomotive. DC is converted from AC produced by this generator and thereby integrated into silicon diode rectifiers for feeding all the DC traction motors. Similar to diesel electric locomotive it is used in marine. Designed with appropriate adaptations the synchronous generator used in marine for facing the salt-water environment. 12 or 24 volt is the output level of marine alternator. For providing large power more than one units are used for large marine. The power generated in the marine system by the alternator is first rectified by charging the engine battery starter and battery supply is auxiliary for marine.
Types of Alternator
Synchronous or alternator generators can be divided in many ways relying on their application and design. These machines are classified according to the following applications-
1. Automotive Type- Modern Automobile uses it
2. Diesel electric locomotive type- Diesel electric multiple unit uses it.
3. Marine Type- Marine environment uses it.
4. Brush less type- Electric power generation plant as main source of power uses it.
5. Radio alternators- Low brand radio frequency transmission uses it.
The two main types of alternator classified according to their design are:
1) Salient Pole Type
Medium and low speed alternator is used over here. A large number of poles projection have their cores dovetailed or bolted on a heavy magnetic wheel of iron cast or steel of good quality magnet. Large diameters and short axial lengths are some of the classifications of such generators. A big wheel like generator are mainly utilize for low turbine speed like hydel power plant.
2) Smooth Cylindrical type
Steam turbine driven alternator uses this type. It has a high speed rotor generator. Smooth solid forged cylinder is present is present in the rotor and is having lot of slots during periods from the outer periphery for staying of field coils.
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Question { L&T, 60289 }

what is the difference between power and distribution
transformer?

Answer

Power transformers
For transmitting networks of higher voltages for step-up and step-down application (400 kV, 200kV, 110kV, 33kV) and are generally rated above 200MVA in power transformers.
For lower voltage distribution networks as a means for connecting end users among 11 kV, 3.3kV, 440V, 230V is basically rated less than 200MVA in distribution transformers.
Transformer Size/ Insulation Level:
Power transformer is for heavy load transformation load, it has 100% efficiency and high voltage greater than 33 kV. Distribution transformer is lesser in size when compared to this transformer and the main uses of this device can be found in generating station and Transmission sub station and higher level insulation.
The distribution transformer is used for the electrical energy distribution at low voltage less than 33kV for works related to industry and 440V-220V for household purposes. A lower efficiency of 50-70% size is enough , lower magnetic losses, easier installation, without full load.
Iron Losses and Copper Losses
Transmission network uses Power transformer as they do not directly connect with the users, there is very less load fluctuation. Throughout the day for 24 hrs they are loaded and the specific weight that is the division of iron weight by copper weight is very less.
In distribution network a power transformer is used and so they are connected to the consumer which leads to higher load fluctuations. The iron losses takes place 24 hrs a day and copper losses depends on the load cycle as it is not loaded fully and the specific weight is more as average loads are about 75% of full load and these are made in such a way that efficiency occurs at its maximum of 75% of full load. All day efficiency is defined in order to calculate the efficiency with power basis.
The main use of power transformers are in step up devices for I2r loss minimizing a given power flow. For utilizing the core to maximum these transformers are designed and it will operate with more clarity near the B-H curve.
The above kind of design cannot be applied for distribution transformers. While designing it the efficiency throughout the day comes into the picture. Typical load cycle for supply depends on its supply. For taking care of the peak load the core design will be done along with all-day-efficiency.
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Question { 4062 }

The insulation resistance of a cable can be measured by

Answer

A very significant maintenance procedure for checking out the working condition of an electrical system is called IR test or Insulation resistance. It can be said that ht system or equipment is fine when insulation resistance is high.
Megger is an insulation resistance measurement tools for finding the equipment’s insulation resistance. Let us see how is the IR calculated?
Define Insulation resistance
There is an application of direct current (DC) and not alternating current (AC) and this is the first thing while doing the Insulation resistance test. for more :- http://crbtech.in/Electrical-Training/index.php/2017/07/31/define-insulation-resistance-and-its-calculation/

Question { Reliance, 4153 }

what is the acceptable insulation resistance for motor/transformer?

Answer

A very significant maintenance procedure for checking out the working condition of an electrical system is called IR test or Insulation resistance. It can be said that ht system or equipment is fine when insulation resistance is high.
Megger is an insulation resistance measurement tools for finding the equipment’s insulation resistance. Let us see how is the IR calculated? for more : http://crbtech.in/Electrical-Training/index.php/2017/07/31/define-insulation-resistance-and-its-calculation/
Define Insulation resistance
There is an application of direct current (DC) and not alternating current (AC) and this is the first thing while doing the Insulation resistance test.