Marine Engineering Learning Hub

Ship’s Control Systems

The field of automation and control system is vast; there are many different ways of achieving the required effects by the operator of marine systems. It is impossible to cover all the aspects of the control used in ship. This article will give you an overview of automatic controls used in ships.

Introduction

Within the environment of a marine plant there are many parameters which need to be controlled or monitored including: temperatures, pressure, level, viscosity, flow control, speed, torque control, voltage, current, machinery status (on/ off), and equipment status (open/ closed).

In olden times it was the role of the watch keeping engineers to monitor and control the machinery plant. This was achieved by periodically taking rounds around the engine room and manually inspecting the condition of the running machinery. Often the engineer was totally dependent on his natural senses, frequently supported by only the minimum of widely distributed simple monitoring devices.

The demand to reduce manning level led to the development of automatic control arrangements for the engine room plant which enabled unattended operation of machinery spaces. With vessels capable of safe operation for any period of time in this mode, those ships were qualified as UMS (Unattended Machinery Space) ships.

In these types of ships, all the control systems and monitoring facilities are grouped together in an Engine Control Room and the ship machinery can also be controlled from this station.

Initially the control room was located in the engine room, with extended monitoring and alarm systems to the bridge and in the accommodation while the ship is running in UMS. But as of recent development, ships are built with the engine control room adjacent to the bridge.

In the latest trend, the control room uses the total integrated systems for all aspects of ship operation which includes engine room operation, cargo operation, navigation, and general administration.

Bridge Control of Main Engine

 

Bridge control of Main Engines:

Automatic control of the starting of the main engine can be done from the bridge as well as in Engine control room. The automatic controls employed in starting the engine is by the following sequence:

  1. Automatic control used in correctly positioning of the cam shaft.
  2. Admitting starting air
  3. Admitting fuel
  4. Starting air shut off
  5. Speed adjustment to the value required

Thus the engine is started and the various parameters like temperature, pressure, flow and tank level have to measured at every watch to make sure that the engine is running safely.

Temperatures of lube oil, JCW, exhaust gas, etc. are measured.

Pressures of lube oil, JCW, fuel oil, and starting air etc. are measured.

Flow of fuel oil is measured while running.

Tank levels of Heavy fuel oil, diesel oil, and lube oil are measured.

For measuring the parameters we make use of the transducers, in turn sending the input signal to the automatic control system.

Boiler Control System

Boiler control systems:

Automatic control of boiler is also done from the engine control room. The automatic systems employed in controlling the following:

  1. Boiler system (open / closed feed)
  2. Steam pressure
  3. Steam temperature
  4. Water level
  5. Feed water level
  6. Feed pump
  7. Feed water temperature
  8. Fuel oil system
  9. Air heater
  10. Smoke density
  11. Force draught fan

Thus if any of the above automatic system fails it give alarm and trips the boiler for a safe operation.

Turbine and Reduction Gear

In case of the steam propulsion ship the steam from the boiler enters in to the turbine and in turn it is coupled with reduction gear so some of the automatic controls used are.

  1. Controlling the speed of the rotation of the turbine in turn it controls the speed of the ship.
  2. Bleeder valve control
  3. Lube oil temperature control.
  4. Over speed trip control.
  5. Condensate system.

Diesel Generator

This is employed in automatic starting and stopping of the auxiliary generator from the engine control room. So of the automatic control used are:

  1. Starting and stopping of the engine according to the load demand.
  2. Synchronization of incoming generator.
  3. Closing the circuit breaker
  4. Load sharing between the alternators
  5. Maintaining the supply frequency and voltage.
  6. Engine and alternator fault protection
  7. Preferential tripping of non-essential loads.

Turbo Generator

In case if the steam propulsion ships they use turbo Generators for generating the power. In this type the small Steam turbines are coupled with the alternators and the automatic controls used are

  1. Reset governor trip level.
  2. Reset emergency stop valve.
  3. Staring auxiliary lube oil pump
  4. Staring the circulating pump
  5. Apply gland steam
  6. Starting extraction pump
  7. Staring the air ejector
  8. Opening the steam valve.

These are some of the automation commonly used in marine environment

In this article let us discuss the magnetic effect of electricity, how it is formed, and the theory supporting it.

Magnetic Effect Of Electricity:

In 1820, Danish physicist, Hans Christian Oersted observed that current through a wire caused a deflection in a nearby magnetic needle. This indicates that magnetic field is associated with a current carrying conductor.

Magnetic field around a straight conductor carrying current:

A smooth cardboard with iron filings spread over it is fixed in a horizontal plane with the help of a clamp. A straight wire passes through a hole made at the center of the cardboard.

A current is passed through the wire by connecting its ends to a battery. When the cardboard is gently tapped it is found that the iron filings arrange themselves along concentric circles. This clearly shows that magnetic field is developed around a current carrying conductor.

To find the direction of magnetic field, let us imagine a straight wire passes through the plane of the paper and perpendicular to it when a compass needle is placed it comes to rest in such a way that its axis is always tangential to a circular magnetic field around the conductor.

When the current is inwards the direction of the magnetic field around the conductor looks clockwise.

When the current direction is reversed that is outwards the direction of magnetic pole of the compass needle also changes showing the reversal of the direction of magnetic field. Now, it is anticlockwise around the conductor. This proves that the direction of magnetic field also depends on the direction of the current in conductor. This is given by Maxwell’s rule.

Magnetic Field around a Straight Conductor Carrying Current:

 

Right Hand cock Screw Rule

 

Law Supporting the Magnetic Effect of Electricity.

Maxwells’s Right Hand Cork Screw Rule:

If a right hand cock screw is rotated to advance along the direction of current through a conductor, then the direction of rotation of the screw gives the direction of the magnetic lines of force around the conductor.

Then comes the Biot-Savart Law:

Biot and savart conducted many experiments to determine the factors on which the magnetic field due to current in the conductor depends.

The results of the experiments are summarized as Biot-Savart Law.

Let us consider the current carrying conductor XY carrying current I. consider a small element in the conductor AB=dl. P is a point at a distance r from the midpoint Q of AB.

 

According to the Biot and Savart the Magnetic Induction dB at a point P due to the element of length dl is:

  1. Directly proportional to the current (I)
  2. Directly proportional to the length of the element (dl)
  3. Directly proportional to the sine of the angle between dl and the line joining element dl and the point p (sin Θ)
  4. Inversely proportional to the square of the distance of the point from the element (1/r2).

 

Magnetic Induction : dB ( directly proportional ) = ( I × dl × sinΘ ) / r2

Introducing the constant for Proportionality:

k = μ/ 4π

k = constant of proportionality.

So the magnetic induction produced by the small element dl is given by:

dB=μ/ 4π × ( I × dl × sinΘ ) / r2

This is how the amount of magnetic induction produced due to the flow of current through the conductor can be measured.

Let us discuss about an amplifier which works on the principle of magnetic saturation

Introduction

We have various amplifiers used for the amplification of current and voltage by means of electronic devices.

Magnetic amplifier is an electrical device which uses the principle of transformer along with the concept of magnetic saturation of the core. The amplification of the power is effective in these types of amplifiers.

It is a device use for controlling the flow of power to a Load by means of Saturating the Magnetic Core. These are widely used in recent years for High and Low Power application.

Construction of Magnetic Amplifier:

The Magnetic Materials which are specially suited for this purpose have a Rectangular Hysteresis Loop with Sharp Saturation and Low Hysteresis losses.

The Excitation level is adjusted such that the flux swing in the Core takes in between the knees of the B-H Curve.

It has the following parts :

  • R1-Load Resistance.
  • G-Gate winding.
  • C-Control winding.
  • Ec-Excitation D.C Source.
  • es-Input A.C Source
  • Is-Amplified current.(A.C)
  • IC-Control Current (D.C)

WORKING OF MAGNETIC AMPLIFIER:

When D.C is applied to the Second Winding / Control Winding which is called the Control Winding, the core will saturate during the part of half cycle due to additional Magnetization. The core will Saturate during the half cycle aided by D.C Magnetization.

When no D.C is applied to Control winding, the core flux variation is with in saturation limit. Under this condition, the flux swings is very large and so the Gate Winding OFFERS VERY LARGE INDUCTANCE and the current drawn from the A.C source is only the Magnetizing current.

Suppose from the D.C side a current is passed through the Control winding. The Operating point is suddenly shifted. Now the core gets saturated in Positive Half Cycle. Under this condition the change of flux is very small compared to the case when there is no control current. Hence the Inductance of the Gate winding reduced to a very small value and now the entire supply voltage appear across the Load Resistance R1.

With higher D.C control current the core saturates earlier in the positive half cycle, thus delivering more current to the load. Thus the saturable reactor works like a gas filled Thyratron, the grid voltage shifts the Firing Angle and once the tube fires, and the grid loses its control. The plate current is controlled by the load resistance.

Similarly in the magnetic amplifier if the control current is such that the core can be saturated when the Magnetic Amplifier will Fire at an Angle α and once it fires, the load current is determined by the Load Resistance alone.

By varying the Control Current the Firing Angle α can be varied and hence the Load current can be controlled.Thus with an Expense of Very Small Power in the Control Side Large power in A .C side is controlled in the Saturable Reactor.

Major Drawbacks:

  • EMF is induced in the control winding by Transformer Action.
  • Output is delivered in One Half Cycle only .

Read on about the comparison between different types of control mechanisms such as electronic and pneumatic systems

Introduction

Control system is a collection of so many physical components connected together to achieve certain objective. We have studied about different types of controls systems which are commonly deployed in various processes in our last article, and saw controls such as manual control, automatic control and so forth.

What is a system?

A system is a collection of many objectives with definite relationship between attribute and objective.

Scientific way to represent control system :

  • INPUT → CONTROL SYSTEM → OUTPUT
  • Manipulated variable → Control system → Controlled variable

The above sequence shows the flow of a control system wherein a variable is controlled using the system to keep the output within a specified range or value, depending on the exact scenario or situation. We shall now see how pneumatic and electronic control system designs compare with each other. This will be done by a comparative study of both, listing the drawbacks and advantages of both. After going through both of these, the reader would have a good idea about the relative usefulness and application of both these types of systems.

Pneumatic Control System

ADVANTAGES :

  1. Simplicity of the components and no complex structure
  2. Easy maintainability
  3. Useful in flame -proof applications
  4. Low cost of installation
  5. Good reliablity and reproducibility
  6. Speed of response is relatively slow but steady
  7. Limited power capacity for large mass transfer

LIMITATIONS:

  1. Great distance lag takes place in this system
  2. Slow response
  3. Difficult to operate in sub-normal temperatures
  4. Copper piping is vulnerable to damage
  5. Pipe-couplings can give rise to leaks in certain ambient conditions

Electronic Control System

ADVANTAGES :

  1. No time lag or transmission delay
  2. Linear and quick response and good accuracy
  3. No entrainment or contamination in control medium
  4. Integrated control system and easier interconnections
  5. Low power requirements
  6. Speed of response is substantially instantaneous

DISADVANTAGES :

  1. Complex networks
  2. Difficult maintenance and test kit requirements
  3. Not possible for flame-proof areas
  4. Skilled maintenance regulations
  5. Expensive layout
  6. Good and safe wiring layout required
  7. Susceptible to line fluctuations of voltage and frequency

How Automatic Control Works ?

What ever may be the type of control system, the automatic process will be of the closed loop type; that is to say, it includes both the measuring means as well as the controlling means within the process itself, thereby completing the loop.

These are the steps followed in the automatic control system :

Step 1 :

A detection (measuring) means to sense the value of the process variable known as “primary element “.

Step 2 :

A transmission means of the measured signal to the measuring element .

Step 3:

A measuring element to reckon the actual process variable and display it called as “secondary element “.

Step 4:

A comparison means to detect error with reference to the desired set value .

Step 5:

A computation means to convert the error signal to a proportionate controller output (in phase and magnitude).

Step 6 :

A corrective (regulating) means by which the controller output signal is transmitted by the control medium to the final control element.

 

 

This article deals with the construction and working of a single phase induction motors. How to make the single phase induction motor self-starting and various methods of self-starting the single phase induction motor are also discussed.

Introduction

In our day-to-day life, we come across many electrical devices like pumps, fans, blowers, etc. All this equipment needs a prime mover, which is an electric motor. In this article we are going to discuss how an electric motor works (single phase and three phase induction motor). An electric motor may be either AC (alternating current) or DC (direct current), but the AC motors have more applications as compared to the DC motors.

What is an Electric Motor?

An electrical motor is a machine that converts the electric power supplied to it into the mechanical work of rotating a motor shaft. This is accomplished by the interaction of the magnetic field produced and the current carrying conductor or windings. The electric power supplied may be single phase or three phase.

Construction and Working of Electric Motor

Before going to the working of the induction motor it is very important to know the constructional details of the electric motor. In general the electric motor has two important components. They are

1) Stator

2) Rotor

The stator of the induction motor has a solid laminated steel magnetic core. These laminated cores have slots in the inner surface. The phase windings are placed in the slots of the magnetic core and are separately insulated by placing the insulation sheets or dipping it in tank full of varnish and heating it. The ends of the phase windings are taken out to the terminal box. The terminal box has the incoming wire from the three phase supply.

The rotor of the induction motor consists of the laminated core which is mounted on the shaft. This laminated core have slots it the periphery where the conductor bars are placed. It is to be noted that slots are only for the conductor bars and not for the windings. This conductor bars are usually made of copper or aluminum. This bars are short circuited at their ends by the short circuiting ring.

When an AC voltage (single phase or three phase) is applied across the stator winding of the induction motor, current flows through the stator winding and produce a magnetic flux.

In case of three phase induction motor, the magnetic flux produced is a rotating magnetic flux. This rotating flux will rotate at the synchronous speed which will depend up on the number of poles and frequency of supply given to the motor.

synchronous speed Ns = (120 f ) / p

where

  • f is the frequency of the supply.
  • p is the number of poles.

This stator rotating magnetic flux cuts through the rotor conductor and induce an alternating EMF. The EMF induced will set up the current to flow through the rotor conductor and produce the magnetic flux. As a result there are two fluxes created (one is the rotating stator flux and the other is the rotor flux). The interaction between these two magnetic fluxes will produce a torque on the rotor, and the rotor rotates in the direction of the rotating magnetic flux. It is to be noted that the rate of cutting flux is directly proportional to the speed of the rotor.

Torque on rotor = Φ × IR × cosφ

where

  • Φ is the stator flux.
  • Iis the rotor current.
  • φ is the phase difference between the stator flux and rotor current.

Three Phase Induction Motor and its Rotating Magnetic Field

In the case of a single phase induction motor, the flux produced due to the single phase voltage is only the alternating flux. The alternating flux acting on the stationary rotor (may be a slip ring or squirrel cage rotor) cannot produce rotation on the rotor, and hence the single phase induction motor is not a self-starting motor whereas the three phase induction motor is self-starting motor. To overcome this drawback of the single phase induction motor and makes the motor self staring, the stator of a single phase motor is provided with two windings taking supply from the same phase.

1) Starting winding

2) main winding or running winding

These two windings are placed 90 degree electrically apart and are connected in parallel to the supply voltage. The current flow through these windings are varied by some means so that the magnitude of flux produced in the stator windings and running windings are different and hence there is some phase difference between these two magnetic flux.

This phase difference creates torque on the rotor for starting. Once the motor is started and attains the rated speed, the supply voltage to the starting windings can be stopped by having the centrifugal switch. As discussed above the current through the two windings are varied by having a high resistance starting winding and a low resistance running winding, or by connecting a capacitor in series with the starting winding.

Single Phase Induction Motor

 

Normally a dry cell is referred to a Carbon-Zinc Leclanche type. It is an easily portable, compact, and modified form of Leclanche cell capable of producing an EMF of 1.5 V with a very small internal resistance in the order of 0.1 ohm.

Dry Cell History and Advances

The first dry cell was invented in the late 19th century. It used zinc as an anode, manganese dioxide as an “earthode,” and a gelled, moist mixture of ammonium chloride and zinc chloride as electrolyte.

Later they created a dry cell made up of carbon as a cathode, zinc as an anode, and sal ammoniac paste as an electrolyte. This type of dry cell is commonly known as a carbon zinc Leclanche cell. Even today, most of the dry cells manufactured are of this kind due to its lower manufacturing cost and its being suitable for all applications requiring intermittent current such as used in flashlights and transistor receivers.

These cells have few drawbacks such as low energy density and limited shelf life. In later years a large number of new types of dry cells were developed for new and different applications.

Modification in Leclanche cell to become a Dry Cell

The glass in Leclanche cell is replaced by a zinc container, and the ammonium chloride solution is replaced by a moist sal-ammoniac paste.

The Dry Cell is a Primary Cell

The cells from which electric energy is derived by irreversible chemical action are called primary cells. The primary cell is capable of providing an EMF when its constituent’s two electrodes and a suitable electrolyte are assembled together. The three main primary cells namely are the Daniel cell, the Leclanche cell, and the dry cell. None of these cells can be recharged electrically.

Dry cell

 

Chemical Energy is converted into Electrical Energy in Cells

Chemical effect of current:

Conversion of electric energy into chemical energy:

The passage of an electric current through a liquid causes chemical changes in a process called electrolysis. Conduction is possible only in liquids wherein charged ions can be dissociated in opposite directions. Such liquids are called electrolytes, and the plates through which current enters and leaves an electrolyte are known as electrodes.

The electrode towards which positive ions travel is called the cathode, and the electrode towards which negative ions travel is called the anode. The positive ions are called cations and negative ions are called anions.

Effect of Chemicals in Battries:

Conversion of chemical energy into electrical energy:

In this case, the reverse process takes place due to the chemical reaction between two electrodes in the presence of an electrolyte and an electric current is produced.

Faraday’s Law of Electrolysis:

First law:

The mass of a substance liberated at an electrode is directly proportional to the charge passing through electrode.

Second law:

The mass of a substance liberated at an electrode by a given amount of charges is proportional to the chemical equivalent of the substance.

 

Working of a Dry Cell:

Parts:

Anode (Negative Terminal) : Zinc

Cathode (Positive Terminal) : Carbon surrounded by MnO2.

Electrolyte: Mixture of plaster of ParisAmmonium Chloride and Zinc Chloride Called as Sal ammoniac paste.

Dry cells contain a Zinc container which itself acts as a negative electrode. The moist paste is made from a mixture of plaster of Paris, Ammonium Chloride, and Zinc Chloride called sal ammoniac paste. This forms the electrolyte of the cell. Zinc Chloride is hygroscopic in nature and helps to maintain the moistness of the paste. It is wrapped in a canvas sheet.

  • Anode reaction: Zinc oxidized to give two electrons.

Zn(soild) → Zn2 + (aqueous) + 2 (e-)

The carbon rod forms the positive electrode. This surrounded by MnO2 and powdered carbon. The powdered carbon reduces the internal resistance of the cell. The top of the cell contains a layer of sawdust. This acts as the base for the top layer of bitumen used for sealing purposes.

  • Cathode reaction:

2MnO2(soild) + H2(gas)→ Mn2O3(s) + H2O(liquid)

  • Electrolyte reaction: Hydrogen from Ammonium chloride

2NH4 + (aqueous ) + 2 (e-) → H2(g) + 2NH3(aqueous)

Over all reaction in dry cell:

Zn(s) + 2MnO2(s) + 2NH4(+)(aqueous) → Mn2O3(solid) + Zn(NH3)2 (2+)(aqueous) + H2O(liquid)

A vent is provided in this layer to allow the gases formed in the chemical reaction to escape. Irrespective of the size of the dry cell, the EMF is 1.5 V because the zinc and carbon rods used as electrodes specified a chemical equivalent. The chemical equivalent changes from metal to metal and, depending up of type of combination used, the EMF differs.

 

 

Learn about various types of controls such as automatic control, remote control and so forth

Types of Controls:

Control and automation form a vital part of the engineering industry, be it any field of engineering. Needless to say, the controls are very important even in Electrical Engineering and help to ease the job of the concerned personnel by providing more control over the equipment or process under consideration.

There are various types of control systems which can be incorporated based on the requirements and other considerations. In this article, we will take a look at some of the control circuit types and the devices which are popularly used and most of us are perhaps familiar with one or more of these types from our everyday life and experience.

1. MANUAL CONTROL :

The link between the measuring element and the regulating unit is the human operator. In this type of control no automatic controls are used in the system. This type of system might be very simple to implement but the only drawback being that such a system needs constant human monitoring and vigilance.

2. SEMI-AUTOMATIC CONTROLS :

The human operator starts off a sequence of operations which are then carried out automatically in some predetermined manner. For example starting up an electric motor by pressing the start button or in start up a process in which the valves are operated in a definite sequence at fixed time interval by a timer.

3. AUTOMATIC CONTROL :

There is no human link between the measuring unit and the regulating unit. Hence the operator is replaced by the controller. This action is continuously variable and remote.

Automatic control system is one in which the actual value of the controlled parameter (such as pressure, temperature, flow, level etc) is compared with a desired value and corrective action is taken depending upon the deviation between the two values without the inclusion of a human element.

It includes both the measuring means as well as the controlling means and both are done automatically by the system itself, hence the nomenclature.

4. LOCAL CONTROL :

The regulating units is altered by means of a lever, hand wheel or other attachments fixed on the unit itself.

5. REMOTE CONTROL :

Some means of power transmission is used to connect the regulating unit to an actuating device mounted some distance away. The power transmission may be either through mechanical linkages, fluid linkages or electrical linkages.

6. ON/OFF CONTROL :

The regulating unit occupies only one of the available two extreme positions (as in case of electrical relay or switch). A very common example of this is the normal manual on-off switch that we use in our houses to turn on the lights and turn them off.

7. STEP- by -STEP CONTROL :

The regulating unit can occupy more than two positions but as the name implies, the action is not continuous and occurs in jerks or steps as in the case of notches on a speed regulator or starter of an electrical motor.

8. CONTINUOUSLY VARIABLE CONTROL :

The regulating unit can be at rest in any position between two definite limits as in slider of potentiometer slide wire to convert the vertical, horizontal and angular motion in to the voltage difference to check the set value and operate accordingly.

 

 

 

Know about the different types of controllers and their working principles

Automatic Control Systems

Understanding the functions of different controllers is necessary for control engineer to find the problem in the automatic controls. An automatic controller is a device which measures the value of a variable quantity or condition and operates to correct or limit the deviation of this measured value from a selected reference. It includes both the measuring means and the controlling means.

As a control engineer we should know about different controllers used and the function of different controllers and the action taken by them. We have already studied about control systems and focussed our attention on Pneumatic and Electronic Control Mechanisms. Now we will be discussing about Integral, Derivative and Proportional Controls in this article as follows:

On-Off Controller

This is the simplest, cheapest and most reliable of controls. It is found to be most competent in handling variables associated with a batch process.

Examples:

  • Space temperature control in dryers and in air conditioning
  • Level limits in tanks and reservoir etc

Such processes respond gradually and smoothly to the full on-off action of the controller and are defined as a “bath-tub ” processes, because of large demand in contrast to supply.

Proportional Control

This type of control mechanism is useful for controlling most variables which lack the stabilising effects of favourable storage capacity. Such “shower-bath” processes as they are more commonly known, require that the transfer of material (fluid flow rate, gas pressure, liquid level) or energy , temperature and pressure must be kept constinuously in step with the demand.

Proportional controllers are used in case of ” How much error exists?”. In this type of controller, it judges the error and acts accordingly.

Output = Gain x Error

Proportional contoller output is directly proportional to the error signal so that it aims is to correct the error and attain the set point. Hence it is also defined as a control which is proportional to the deviation from set-point of the measured value.

It is represented by:

proportional controller – proportional to “Θ”

Typical examples of these are self acting temperature controllers and conventional pressure reducing valves .

Integral Controller

Integral controllers are used in the case “How far is the error from the set value?”. This types of controller judges the position of the set value and acts accordingly.

It is defined as the control action which changes at a rate proportional to the deviation, the rate of change being proportional to the deviation.

In practice this means that with measured value and set point coincident, the final control element may be in any position within the range.

It is represented by:

Integral controller – proportional to – ∫ ∂Θ ⁄ ∂t.

Derivative Control

A derivative controller is used in case of “How much time it takes to bring the system to the set-point”?. This is used wherever transfer lag problems are involved.

It is represented by:

DERIVATIVE CONTROLLER – proportional to – ∂Θ ⁄ ∂t

The derivative controller cannot be used independently. Either it should be used along with proportional controller or with integral controller. The set valve is quickly attained by using the derivative controller.

Derivative action over-correction is good as an experienced operator will give manual over-correction before seeking a final average setting.

 

 

Brushless DC motors (BLDC) are used where there are limitations in the use of the brush-type DC motors. In this article we discuss how it is possible operate a DC motor with no brush arrangement and also about the back EMF in a brushless DC motor (BLDC).

Introduction

A DC motor is a one which operates on supply from a DC source. The DC source may be either DC generator or from a battery. DC motors may be classified as:

  • Series wound DC motor
  • Shunt wound DC motor
  • Compound wound DC motor
  • Separately wound DC motor

In all types of DC motors, the supply is given to both stators to make it as an electromagnet. This supply is necessary because the operation of a DC motor depends on the attraction and repulsion principles of magnetism.

In the stator, the supply voltage from a DC source is given directly, and in the rotor of DC motor it is supplied by means of a brush arrangement. But in case of brushless DC motors, this supply voltage to the rotor should be supplied without any brush arrangement. Brushless DC motors are more complicated than ordinary DC motor with brush arrangements, but certain applications needs this brushless DC motor, and hence it exists.

In a brushless DC motor (BLDC), we have an exciter rotor mounted on the same shaft of the rotor of a DC motor. This exciter stator induces an EMF when a small voltage is applied to the stator of this exciter. The voltage induced in the exciter rotor is an AC voltage and this is rectified to DC by means of a rotating rectifier diode arrangements mounted on the same shaft of the motor. The rectified DC voltage is applied to the rotor of DC motor, and there is no brush required so the DC motor with this type of complicated arrangement is called a brushless DC motor (BLDC).
 

Back EMF in Brushless DC Motor (BLDC):

According to Faradays law of electromagnetic induction, when a current carrying conductor is placed in a magnetic field that is if the conductor cuts the magnetic field), an EMF is induced or produced in a conductor and if a closed path is provided current flows through it.

When the same thing happens in a brushless DC motor (BLDC) as a result of motor torque, the EMF produced is known as “back EMF.” It is so called because this EMF that is induced in the motor opposes the EMF of the generator.

This back EMF that is induced in the brushless DC motor (BLDC) is directly proportional to the speed of the armature (rotor) and field strength of the motor, which means that if the speed of the motor or field strength is increased, the back EMF will be increased and if the speed of the motor or field strength is decreased, the back EMF is decreased.

This back EMF created acts as a resistance and we all know that any resistance in a line reduces and opposes the current flow so if the speed of the DC motor or field strength is increases, the back EMF increases which it turn increases the resistance to the current flow in windings and hence only less amount of current is delivered to the armature of DC motor. Also if the speed of Dc motor armature or field strength decreases, the back EMF decreases, which in turn reduces the resistance and hence more amount of current flow to the armature of DC motor.

When the DC motor is first started, there is no back EMF induced and as discussed above there is maximum current flow from the DC generator or distribution lines to the motor armature and as a result the motor toque will be maximum. In this case there is no resistance offered by back EMF. The only resistance available is the motor winding resistance.

During normal operation (rated speed) of DC motor, the back EMF induced will be maximum which will reduces the motor armature current to its minimum level and as a result the motor torque will also be reduced.

When the load on the motor is increased, the motor speed (RPM) is decreased and this will reducs the back EMF. This decreases in back EMF will automatically increase the motor torque thereby bringing the motor to its rated speed.

Starting of Generator Engine

Starting of an engine from “stop” state is something which needs to be done with care, especially if the interval of starting is sufficiently long. The following is a checklist of all the checks which ideally need to be carried out before starting the generator. In actual practice sometimes the engineers might take some of these for granted and skip, but it is advisable not to indulge in such a practice. In fact these checks are generic for any four stroke engine starting process

  1. Check the turbocharger sump oil level, governor, alternator, forward and aft lube oil levels, and diesel oil level in service tank
  2. Open the indicator cock
  3. Prime the lube oil to all parts by hand pump or by motor driven priming pump
  4. Ensure that all jacket cooler valves, lube oil cooler valves, air cooler valves should be in open position
  5. With use of the Turning bar turn the fly wheel and check for any resistance on the bottom end bearing and check any water / fuel coming out through indicator cocks
  6. While turning engine, check all visible lube oil points are lubricated
  7. Remove the turning bar from fly wheel and put in the place
  8. Drain the auxiliary air bottle

Blow through engine (i.e.: by turning engine with air). In order to ensure that no water is inside combustion chamber if it is present it may cause water hammering

  1. Close the indicator cocks and pull lever from stop to start
  2. When the needle in RPM indicator deflects to some value of (0-25 rpm) put the lever in run condition
  3. The engine will run on fuel oil once the generator picks up the rated speed
  4. Put generator on load by closing air circuit breaker
  5. For checking the alternator fore and aft bearing lube oil level by opening oil plug in the alternator and the ring bearing while rotating splash lube oil from the sump can be seen
  6. In order to synchronize the incoming generator with running generator syncroscope method/dark lamp method is used

Starting of generator

Checks to be made while running

Once the generator has actually started to run, there are several checks which must be performed before it is left on its own to continue running. These checks pertain to verifying various parameters related to lube oil levels, temperatures and so forth. Given below is a brief checklist related to the same.

Lube oil checks

  1. Sump lube oil level
  2. Governor lube oil level
  3. Rocker arm lube oil level
  4. Alternator forward and aft bearing lube oil level
  5. Lube oil in turbine & blower side of turbo charger

Temperature checks

  1. Exhaust gas temperature
  2. Turbocharger (inlet-outlet) temperature
  3. Booster air inlet temperature

Cooler temperatures

  1. Cooling sea water (inlet – out let) temperature in cooler
  2. Jacket cooling water (inlet – outlet) temperature
  3. Air cooler (inlet -outlet) temperature

Safety Devices

Once the above mentioned parameters have been checked and found within normal range, it is safe to continue running the generator. Yet a fault can develop even at a later stage, so for this very purpose various trips and alarms are situated on the generators. An alarm gives the signal of an impeding danger and requires quick action while a trip actually trips the generator immediately because of the nature of the fault.

The various trips and alarms are mentioned as follows

  1. Alternator bearing low oil level alarm & trip
  2. Alternator bearing high temperature lube oil alarm &trip
  3. Low sump oil level alarm and trip
  4. Lube low oil pressure alarm and trip
  5. Reverse current trip
  6. Over speed trip
  7. Over load trip
  8. High and low frequency trip
  9. Jacket cooling water low pressure alarm