Function 5

CHENNAI CENTRE

Internal: Mr. ShirishExternal: Mr. KrishnanAttempt : 1Fn4

(1) 1. Brushless alternator full explanation with drawing 

Answer:-The working principle of the brushless alternator is to provide excitation to the excitation winding of the exciter on the stator. The rotor armature winding of the generator will emit alternating current, and after rotating rectifier, it will supply power to the excitation winding of the main generator in the rotor.

Construction A brushless alternator is composed of two alternators built end-to-end on one shaft. Smaller brushless alternators may look like one unit but the two parts are readily identifiable on the large versions. The larger of the two sections is the main alternator and the smaller one is the exciter. The exciter has stationary field coils and a rotating armature (power coils). The main alternator uses the opposite configuration with a rotating field and stationary armature.

Exciter

The exciter field coils are on the stator and its armature is on the rotor. The AC output from the exciter armature is fed through a set of diodes that are also mounted on the rotor to produce a DC voltage. This is fed directly to the field coils of the main alternator, which are also located on the rotor. With this arrangement, brushes and slip rings are not required to feed current to the rotating field coils. This can be contrasted with a simple automotive alternator where brushes and slip rings are used to supply current to the rotating field.

Main Alternator

The main alternator has a rotating field as described above and a stationary armature (power generation windings). This is the part that can be confusing so take note that in this case, the armature is the stator, not the rotor. With the armature in the stationary portion of the alternator, the high current output does not have to go through brushes and slip rings. Although the electrical design is more complex, it results in a very reliable

alternator because the only parts subject to wear are the bearings.Control System

Varying the amount of current through the stationary exciter field coils controls the strength of the magnetic field in the exciter. This in turn controls the output from the exciter. The exciter output is fed into the rotating field of the main alternator to supply the magnetic field for it. The strength of the magnetic field in the main alternator then controls its output. The result of all this is that a small current, in the field of the exciter indirectly controls the output of the main alternator and none of it has to go through brushes and slip-rings.

AVR controls the pilot exciter

2. Alternator maintenance..  How to do diode test?  

1, Alternator is taken out of service.1. From the stator side, check the Insulation resistance with

the help of megger, phase to phase, phase to earth, (remove star point). Ensure cleanliness of the terminal box.

2. Clean the air filters (felt filters in the aft)3. Check the air gap with the help of a plastic feeler gaugew4. Pedestal bearing oil level to check5. Space heater to check whether is in good operating

condition6. If it is brushless alternator, pilot exciter rotor to be checked

for IR7. Both pilot exciter and main alternator field winding to be

checked for continuity.

8. Conditions of the diode bridge rectifier to check. (the heat sinks provided for diodes to be checked, and ensure the connections are tight)

Diodes can be checked after dismantling them from the bridge assembly,

With the help of multimeter (keep it in ohms range), when positive lead is connected to anode and negative lead is connected to cathode, diode will show conduction or needle will move showing continuity. If your reverse the leads, there will be no indication, the pointer or needle will not move.

2nd method, is connect a 12V lamp in series with the diode and apply 12V (provided the diode is rated for 12V), in one direction the light will burn and in the reversed connection the light will not burn.

3. Purpose of airgap?  How much clearance? Answer:-Purpose is to make the rotor turn freely without touching the stator. The air gap varies for different alternators. It depends on the rating. The air gap can very from 2mm to 3 mm

4. Busbar material? Thickness.. Safety provided from alternator cables to busbar?Answer:-Busbars are produced in a variety of shapes, such as flat strips, solid bars, or rods, and are typically composed of copper, brass, or aluminium as solid or hollow tubes.

Copper and Aluminium are the most common bus bar materials. These can also be supplied plated with Tin or Silver, primarily for corrosion resistance.

The cross section varies depending on the voltage applied and current carried. Cannot be exactly specified

Safety for a fault in the cables between alternator and bus bar is the differential Current relay protection.

5. Soft starter full with diagram Answer:-

n By triggering a thyristor, it switches from ‘off’ to ‘on’ until the current stops flowing though it - which occurs every half cycle in an AC. supply.

n By controlling electronically the thyristor turn on point, it is possible to regulate the energy passing through it.

n By starting with a large delay angle and gradually reducing it , the motor terminal voltage is increased from a low value to full voltage, giving a smooth, stepless, start.

n By controlling the switch-on point of a thyristor relative to the voltage zero crossing in each half wave of an alternating current, it is possible to regulate the energy passing through the device. The closer the turn-on point is to the voltage zero crossing point, the longer the energy is allowed to flow during the half-cycle. Conversely, delaying the turn-on point reduces the time for the energy to flow. Putting two thyristors back-to-back (or anti-parallel) in each of the phase connections to a motor, and by precisely controlling their turn-on points, an electronic soft starter continuously adjusts the passage of energy from the supply so that it is just sufficient for the motor to perform satisfactorily.

6. Autotransformer principle..  Auto transformer starter diagram explain,  application Answer:-

An autotransformer is an electrical transformer with only one winding. In an autotransformer, portions of the same winding act as both the primary and secondary sides of the transformer. In contrast, an ordinary transformer has separate primary and secondary windings which are not electrically connected.The winding has at least three taps where electrical connections are made. Since part of the winding does "double duty", autotransformers have the advantages of often being smaller, lighter, and cheaper than typical dual-winding transformers, but the disadvantage of not providing electrical

isolation between primary and secondary circuits. Other advantages of autotransformers include lower leakage reactance, lower losses, lower excitation current, and increased VA rating for a given size and mass.

Theory Of Operation –

Autotransformer Starter The autotransformer reduced-voltage starter places the motor on the secondary of the autotransformer while starting. The taps on the autotransformer limit the voltage applied to the motor to 50%, 65% or 80% of the nominal voltage. With autotransformer starting, the line current is always less than the motor current during starting because the motor is on the secondary of a transformer during acceleration. If a motor is connected to the 50% tap of the autotransformer, the motor current would be reduced to 50% of the normal starting value, but the line current would be only 25% of the normal starting current. The difference between line and motor current is due to the transformer in the circuit. The lower line current is the reason the autotransformer starter is a very popular type of reduced-voltage starter. Since the motor starting current is greater than the line current with an autotransformer starter, the starter produces more torque-per-ampere of line current than any other type of reduced-voltage starter.

• Autotransformer starter:• During starting 1 & 3 closed, when motor is nearly up to

speed; those contacts opened & 2 closed• Note: as starting current reduced proportional to decrease

in voltage, starting torque decreased as square of applied voltage, therefore just a certain reduction possible if motor is to start with a shaft load attached

Another diagram for the same starter, (which ever is simpler that can be used)

7. IGBT working?  Construction details. ApplicationsAnswer:- 

The termIGBT is a short form of insulated gate bipolar transistor, it is a three-terminal semiconductor device with huge bipolar current-carrying capability. So, this device is designed to make use of the benefits of both BJT and MOSFET devices

The working principle of gate drive circuits for the IGBT are like a N-channel power MOSFET. The main difference is that the resistance offered by the conducting channel when current supplies through the device in its active state is very small in the IGBT.

The IGBT is used in medium- to high-powerapplications like switched-mode power supplies, traction motor control and induction heating.

The term IGBT is a semiconductor device and the acronym of the IGBT is insulated gate bipolar transistor. It consists of three terminals with a vast range of bipolar current carrying capacity.The designers of the IGBT think that it is a voltage controlled bipolar device with CMOS input and bipolar output. The design of the IGBT can be done using both devices such as BJT and MOSFET in monolithic form. It combines the best assets of both to achieve the optimal device characteristics. The applications of the insulated gate bipolar transistor include power circuits, pulse width modulation, power electronics, uninterruptible power supply and many more. This device is used to increase the performance, efficiency and reduces the audible noise level. It is also fixed in resonant-mode converter circuits. Optimized insulated gate bipolar transistor is accessible for both low conduction and switching loss.

The insulated gate bipolar transistor is a three terminal semiconductor device and these terminals are named as gate, emitter and collector. Emitter and collector terminals of the IGBT are associated with a conductance path & gate terminal is associated with its control. The calculation of amplification is attained by the IGBT is a radio b/n its i/p & o/p signal. For a conventional BJT, the sum of gain is nearly equivalent to the radio to the output current to the input current which is termed as beta. The insulated gate bipolar transistors are mainly used in amplifier circuits such as MOSFETS or BJTs.

Basic Construction of IGBT

The basic construction of the N-channel IGBT is given below. The structure of this device is plain and the Si section of the IGBT is nearly similar to that of a vertical power of a MOSFET excluding P+ injecting layer. It shares the equal structure of metal oxide semiconductor’s gate & P-wells through N+ source

regions. In the following construction the N+ layer consists of four layers and that are situated at the upper is called as the source and the lowest layer is called as a collector or drain.

There are two  kinds of IGBTS namely, non punch through IGBT (NPT IGBTS) and punch through IGBT (PT IGBTs). These two IGBTs are defined as, when the IGBT is designed with the N+ buffer layer then it is called as  PT IGBT, similarly when the IGBT is designed without a N+ buffer layer is called as NPT IGBT. The performance of the IGBT can be increased by existing the buffer layer. The operation of an IGBT is faster than the power BJT and power MOSFET.

Circuit Diagram of an IGBTBased on the basic construction of the insulated gate bipolar transistor, a simple IGBT driver circuit is designed using PNP and NPN Transistors, JFET, OSFET, that is given in the below figure. The JFET transistor is used to connect the collector of the NPN transistor to the base of the PNP transistor. These transistors indicate the parasitic thyristor to create a negative feedback loop.

The RB resistor indicates the BE terminals of the NPN transistor to confirm that the thyristor doesn’t latch up, that will lead to the IGBT latch up. The transistor denotes the structure of current among any two neighboring IGBT cells. It  lets the MOSFET and supports most of the voltage. The circuit symbol of the IGBT is shown below, that contains of three terminals namely emitter, gate and collector.

8. Variable speed drive?  Frequency?  Applications Answer:-

A Variable Frequency Drive (VFD) is a type of motor controller that drives an electric motor by varying the frequency and voltage supplied to the electric motor. Other names for a VFD are variable speed drive, adjustable speed drive, adjustable frequency drive, AC drive, microdrive, and inverter.

There are four types of VFDs1. Cyclo converter2. Synchro converter3. Current source inverter4. Voltage source inverter

All the four types work on the principle of pulse width modulation. Width of AC wave form can be reduced or increased to increase / decrease frequency. N = 120 f /P

 Frequency (or hertz) is directly related to the motor’s speed (RPMs). In other words, the faster the frequency, the faster the RPMs go. If an application does not require an electric motor to run at full speed, the VFD can be used to ramp down the frequency and voltage to meet the requirements of the electric motor’s load. As the application’s motor speed requirements change, the VFD can simply turn up or down the motor speed to meet the speed requirement.

The first stage of a Variable Frequency AC Drive, or VFD, is the Converter. The converter is comprised of six diodes, which are similar to check valves used in plumbing systems. They allow current to flow in only one direction; the direction shown by the arrow in the diode symbol. For example, whenever A-phase voltage (voltage is similar to pressure in plumbing systems) is more positive than B or C phase voltages, then that diode will open and allow current to flow. When B-phase becomes more positive than A-phase, then the B-phase diode will open and the A-phase diode will close. The same is true for the 3 diodes on the negative side of the bus. Thus, we get six current “pulses” as each diode opens and closes. This is called a “six-pulse VFD”, which is the standard configuration for current Variable Frequency Drives

 The diode bridge converter that converts AC-to-DC, is sometimes just referred to as a converter. The converter that converts the dc back to ac is also a converter, but to distinguish it from the diode converter, it is usually referred to as an “inverter”. It has become common in the industry to refer to any DC-to-AC converter as an inverter. 

Note that in a real VFD, the switches shown would actually beIGBTs (on the inverter side) When we close one of the top switches in the inverter, that phase of the motor is connected to the positive dc bus and the voltage on that phase becomes positive. When we close one of the bottom switches in the converter, that phase is connected to the negative dc bus and becomes negative. Thus, we can make any phase on the motor become positive or negative at will and can thus generate any frequency that we want. So, we can make any phase be positive, negative, or zero.  

The blue sine-wave is shown for comparison purposes only. The drive does not generate this sine wave. Notice that the output from the VFD is a “rectangular” wave form. VFD’s do not produce a sinusoidal output. This rectangular waveform would not be a good choice for a general purpose distribution system, but is perfectly adequate for a motor. If we want to reduce the motor frequency to 30 Hz, then we simply switch the inverter output transistors more slowly. But, if we reduce the frequency to 30Hz, then we must also reduce the voltage to 240V in order to maintain the V/Hz ratio (see the VFD Motor Theory presentation for more on this). How are we going to reduce the voltage if the only voltage we have is 650VDC? This is called Pulse Width Modulation or PWM. Imagine that we could control the pressure in a water line by turning the valve on and off at a high rate of speed. While this would not be practical for plumbing systems, it works very well for VFD’s. Notice that during the first half cycle, the voltage is ON half the time and OFF half the time. Thus, the average voltage is half of

480V or 240V. By pulsing the output, we can achieve any average voltage on the output of the VFD.

9. Electric propulsion 

Answer:-

The propeller shaft of the ship is connected to large motors, which can be D.C or A.C driven and are known as propulsion motors. Power for propulsion motor is supplied by the ship’s generator and prime mover assembly.

Credits: yanmar.comArrangement or operation

The generator can be direct or alternating current type with diesel or steam driven prime mover, depending upon the requirement or demand of the owner/ship.

In the electrical propulsion system, the direction of the rotation of propeller is governed by either the electrical control of the motor itself or by changing the electrical supply.

Normally variable speed electrical motor is used for fixed pitch propeller system and constant or variable can be used for variable pitch propeller or CPP.

Other type is an Azipod arrangement, wherein the motor is enclosed in the casing in the aft, the casing can turn 360°. No rudder is used The motor can be induction or synchronous motor depending on whether propeller is CPP or FPP.

In the Azipod unit, the electric motor is mounted inside the propulsion unit and the propeller is connected directly to the motor shaft Electric power for the propulsion motor is conducted through slip rings that let the Azipod unit rotate 360 degrees about the vertical axis. Because Azipod units utilize fixed-pitch propellers, power is always fed through a variable-frequency drive or cycloconverter that allows speed and direction control of the propulsion motors.The pod's propeller usually faces forward because in this pulling (or tractor) configuration the propeller is more efficient due to operation in undisturbed flow. Because it can rotate around its mount axis, the pod can apply its thrust in any direction. Azimuth thrusters allow ships to be more maneuverable and enable them to travel backward nearly as efficiently as they can travel forward. In another type the Power Take Off and Power Take in Type of electric propulsion is used, mainly on large container ships. In this main engine is driving a shaft generator, hence power is produced by the SG while sailing (PTO). When Main engine has broken down or any maintenance has to be done, the drive of SG can be declutched and clutched to drive mode, in which it acts as motor to drive the propeller (of course at reduced

speed) (PTI). Ship should have enough capacity of generators for this operation.

10.Battery onboard,  it's regulations,  battery supply items difference in cargo ships and passenger ships.. Everything about both Lead acid and nickel cadmium batteries and their maintenance.Answer:- 

Kindly refer SOLASch II-1. Part D regulations 40 – 45 for the requirements of Emergency source of supply.

This calls for a complete chapter of batteries which will consume more time to prepare.

Kindly refere text books or Websites relating to the various batteries.

Kindly remember that emergency batteries are different and engine starting batteries are different (LB eng and Em.Generator engine)