The central freshwater cooling system with single-stage scavenges air­cooler and a separate HT circuit.

As freshwater is the standard cooling medium of the scavenge air cooler(s), this involves using a central freshwater cooling system.

The central freshwater cooling system comprises “low-temperature” (LT) and “high-temperature” (HT) circuits. Freshwater-cooling systems reduce the amount of seawater pipework and its attendant problems and provide for improved cooling control. Opttmizrng central freshwater- cooling results in lower overall running costs compared to the conventional seawater cooling system.

Cooling Water Treatment

The correct treatment of cooling fresh water- is essential for safe engine operation. Only totally demineralized water or condensate must be used. In the event of an emergency, tap water may be used for a limited period, but afterward, the entire cylinder cooling water system is to be drained off flushed, and recharged with demineralized water.

In addition, the water used must be treated with a suitable corrosion inhibitor to prevent a corrosive attack, sludge formation, and scale deposits. Monitoring the corrosion inhibitor level and water softness level is essential to prevent downtimes due to component failures resulting from corrosion or impaired heat transfer. No internally galvanized steel pipes should be used in connection with treated freshwater because most corrosion inhibitors have a nitrite base. Nitrites attack the zinc lining of galvanized piping and create sludge.

Pre-Heating

To prevent corrosive liner wear when not in service or during short stays m the port, it is important that the main engine is kept warm. Warming through can be provided by a dedicated heater, using boiler-raised steam or hot water from the diesel auxiliaries, or through direct circulation from the diesel auxiliaries.

If the requirement for warming up is from the cooling water systems of the diesel auxiliaries, it is essential that the amount of heat available at the normal load is sufficient to warm the main engine If the main and auxiliary engines have a cooling water system that can be cross-connected. it has to be ensured that when the cross-connection is made, any pressure drop across the mam engine will not affect the cooling water pressure required by the auxiliaries. If the cooling water systems are apart, then a dedicated heat exchanger is required to transfer the heat to the mam cylinder water system.

If the mam cylinder water pump is to be used to circulate water through the engine during pre-heating, the heater is to be arranged parallel with the cylinder water system, and on/off control is to be provided by a dedicated temperature sensor at the cylinder water outlet of the engine The flow through the heater is set by throttling discs but not by valves, to assure flow through the heater.

If the requirement is for a separate pre-heating pump, a small unit with 10% of tire mam pump capacity and an additional non-return valve between the cylinder cooling water pump and the heater are to be installed. In addition, the pimps are to be electrically interlocked to prevent two pumps from running at the same time.

The recommended temperature for stalling and operating the engine is 60 C at the cylinder cooling water outlet. If the engine has to be started below’ the recommended temperature, the engine power should not exceed 80% of CMCR until the water- temperature has reached 60 C.

The ambient engine room temperature and warm-up tune are the key parameters for estimating the heater-power capacity required to achieve the target temperature of 60 C.

The figure shows the warm-up tune needed in relation to the ambient engine room temperature to arrive at the heat amount required per cylinder. The graph covers tire warming up of the engine components per cylinder, also taking the radiation heat into account.

The readable figure is then multiplied by the number of cylinders to show the heater- capacity required for the engine. All the figures are related to the requirements of the engine and should be used only for the first rough layout of the heater capacity. During pre-heater- selection, however, the shipyard or ship designer must also consider other aspects, such as the heat losses in the external piping system, the water volume inside the system, the pipe lengths, and the volume of the ancillary equipment.

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To reduce the disturbance on ship switchinng elements, coils can be connected to suppressor modules, also called surge protectors, which are connected across the coils and dissipate the energy from the collapsing magnetic field when switched off.

Suppressor modules for relays and contactor coils
include the following:

1. RC links – a resistor in series with a capacitor used with AC operated coils a)
2. diode elements – provide excellent voltage limitation and are used for DC operated coils b)
3. varistor elements – used for AC and DC operated contactors c) and achieve virtually unchanged switching times.

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Being a solid state there are no moving parts to wear out, no sparking on contact, and no contact corrosion and they can be switched on and off quicker than mechanical relays for massive ship electrical system sound great. However, they are expensive to build for very high current ratings where electromechanical contactors dominate.

Solid state relays will only open the AC circuit at a point of zero current as their inherent hysteresis (memory) maintains current continuity after the LED is de-energised, until It falls below the holding current’. Known as ‘zero-crossover switching’, this is the advantage of uninterrupted operation during the sine wave when using AC power and avoids large voltage spikes.
A problem associated with SCRs is that they are more likely to fail shorted (or closed) rather than open, as is the case of EM relays. As a ‘fail-open’ condition is deemed to be safer, EM relays are often preferred for certain applications.

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To ensure cables are connected correctly they are given unique numbers.

This is vital for maintenance and troubleshooting in the event of equipment failure.

Conductors on all electrical drawings are represented by straight lines.

we can see a line break for the conductor 45, assigned with the tag identifier, this is a line break reference (=GA02/5.6) indicating where the conductor’s continuation can be found.

The reference points to a different diagram group, therefore it is preceded with a plant identification sign (=). The continuation of the conductor 45 can be located in diagram group =GA02, sheet 5, sector 6, where it is assigned with a reference =GA01/10.8.

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Ship electrical diagrams are composed of symbols representing electrical devices, lines representing conductors or wires for interconnections and can include:

• 
  Symbols that identify the electrical equipment, its component parts and connections
• Ship electrical equipment and terminal markings  
• References at or near the representations of equipment and circuit connectors which indicate where in the schematic diagram
• Corresponding part of the equipment or the continuation of the circuitry can be found, ie in which section, square or possibly which page.

These features are usually sufficient to explain the circuitry and its mode of operation and can be used to follow through the circuits when tracing a fault.

For the ship engine department, it is important to read and understand them and to use them as an aid while troubleshooting electrical Installations. 

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Carefully examine the stator core for signs of rubbing with the rotor, usually caused by u worn bearing. Even slight rubbing of the rotor against the stator will generate enough heat to destroy the stator insulation. Replace the bearings before putting the motor back into service.

Laminated steel core plates which have been badly scored may cause a local hot spot to be generated when the motor is running. This is because the Fe (iron) losses will increase in the damaged area. After the motor has been put back into service with new bearings, check the motor running temperature.

After a short period of service dismantle the motor and check for discolouration at the core damage which will indicate local heating. If you suspect core hot spots then the motor core will need to be dismantled for the laminations to be cleaned and re-insulated    definitely a shore job.

The insulation resistance reading is the best indication as to the presence of moisture in the motor windings. Break-downs due to insulation failure usually result in an earth fault, short-circuit turns in a phase or phase-to-phase faults.

Larger motors are usually six-terminal, which means that all six ends of the stator windings are brought out to the terminal block. Links between the terminals are used to stator delta connect the motor. Disconnect the supply leads and remove the links. Test between phases with an insulation resistance tester.

A problem can arise on small, three-terminal motors where the star or delta connection is made inside the motor. Only one end of each winding is available at the terminal block. Phase-to-phase insulation resistance cannot be checked. If a three terminal motor is to be rewound, ask the repairer to convert it to a six terminal arrangement.

Fluke F1587 FC 2-in-1 Insulation Tester Multimeter With FC Connect function

The Best Offer

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Electronic Thermal Electromagnetic

Electronic OCIT (overcurrent inverse time) relays have largely superseded
electromagnetic types as they have no moving parts (except for their output trip relay) and their very reliable tripping characteristics can be closely matched to the motor circuit.
Such relay is are robust, smaller and lighter than the equivalent electromagnetic type.

The block diagram of the electronic OCIT relay shows that the current and time settings can be adjusted over a limited range to match the motor FLC and run-up time. A self-test of the OCR performance can usually be applied with a fixed setting of, typically, 6 x FLC and the tripping-time can be measured and compared against the manufacturers current/time characteristics.

Although electromagnetic devices with time delays can give adequate protection against large, sustained overloads to motors which are operated well below their maximum output and temperature, they have been found to be inadequate for continuous maximum rated (CMR) motors.

Most ship LV motors

are protected by less expensive thermal OCRs. Inverse-time thermal OCRs usually work with bi-metal strips.
The strips are heated by the motor current and bend depending on the temperature. If the motor takes an overload current, the strips operate a normally-closed (NC) contact which trips out the line contactor to stop the motor.

The minimum tripping current of such a device can be adjusted over a small range.

This adjustment alters the distance the strips have to bend before operating the trip contact.
For larger motors, the heaters do not carry the full motor current. They are supplied from current transformers (CTs) which proportionally step-down the motor current so that smaller heater components may be used to operate correctly, induction motors must be connected to a three phase a.c. supply.

Once started they may continue to run even if one of the three supply lines becomes disconnected. This is called single-phasing and can result in motor burn-out.

Single-phasing is usually caused when one of the three back-up fuses blows or if one of the contactor contacts is open-circuited.
The effect of single-phasing is to increase the current in the two remaining lines and cause the motor to become very noisy due to the uneven torque produced in the rotor.

An increase in line current due to single-phasing will be detected by the
Protective OCR.
The three thermal elements of an OCR are arranged in such a way that unequal heating of the bi-metal strips causes a differential movement which operates the OCR switch contacts to trip out the motor contactor.

For large HV machines a separate device, called a negative phase sequence (NPS) relay, is used to measure the amount of unbalance in the motor current.
For star connected motor windings the phase and line currents are equal to the line connected OCR is correctly sensing the winding current.
If the overcurrent setting is exceeded during a single-phase fault the motor will be tripped off.
The situation is not so simple with a delta connected motor. Normally the line current divides phasorally between two phases of the motor windings.
The phase current is just over half the line current.

When one of the lines becomes open circuited a balanced here phase condition no longer exists. Now the sets of line and phage currents are no longer balanced.

Look at the condition where the motor is at 50% of full load when single-phasing occurs: the line currents are 102 % of the full-load value but the current in winding C is 131% of its full-load value. The 102 % line current will probably not activate a line connected OCR and the motor remains connected.
However, the local overheating in winding C of the motor will quickly result in damage.

Motors can he protected against this condition by using a differential type relay which trips out with unbalanced currents.
In fact, most modern thermal OCRs for motors have this protection against single-phasing incorporated as a normal feature.

If single-phasing occurs when in operation on light load, the motor keeps on running unless the protection trips the contactor.
If the motor is stopped, it will not restart.
When the contactor is closed, the motor will take a large starting current but develop no rotating torque. The OCR is set to allow the starting current to flow long enough for the motor, under normal conditions, to run up to speed.
With no ventilation on the stationary motor, this time delay will result in rapid and severe overheating. Worse still, if the operator makes several attempts to restart the motor, it will burn out.
If a motor fails to start after two attempts, you must investigate the cause.
Undervoltage protection is necessary in a distribution system that supplies motors.
If there is a total voltage loss or black-out, all the motors must be disconnected from the supply.
This is to prevent all the motors restarting together which would result in a huge current surge, tripping out the generator again.
Motors must be restarted in a controlled sequence after a supply failure.

Undervoltage (UV) protection for LV motors is simply provided by the spring-loaded motor contactor because it will drop out when the supply voltage is lost.
For large HV motor the UV protection function will be covered by a relay separate from the OCR function or it may be part of a special motor relay which incorporates all of the necessary protection functions.
When the supply voltage becomes available, the motor will not restart until its contactor coil is energised.
This will usually require the operator to press the stop/reset button before initiating the start sequence.
For essential loads, the restart may be performed automatically by a sequence restart system.
This system ensures that essential service are restarted automatically on restoration of supply following a blackout. Timer relays in the starters of essential motor circuits are set to initiate start-up in a controlled sequence.

The post Protection of ship electromotors appeared first on Electro-technical Officer (ETO).

Two main forms of speed change/control are available:

Pole-changing for induction motors to give two or more fixed speeds:
2-speed forced-draught fans
3-speed winches

Continuously variable speed control, e.g. smooth control of deck cranes, winches and electric ship propulsion using variable frequency.
Fixed set speeds can be obtained from a cage-rotor induction motor by using a dual wound stator winding, each winding being designed to create a different number of magnetic poles.
A 3 speed pole-change ship winch motor can be arranged by having two cage rotors mounted on the same drive shaft.

One stator winding (usually 2-pole) gives a low speed while the other is dual wound
to give medium speed (8-pole) and high speed (4-pole) outputs.

Speed control and drive direction are achieved by u set of switching and reversing contactors operated from the winch control pedestal.

Remember that to reverse the rotation of an induction motor it is necessary to switch over two of the supply lines to the stator winding.

An alternative method giving two fixed speeds in a 2:1 ratio from a cage-rotor induction motor is to use a single stator winding which has centre-tap connections available on each phase. This method uses a starter with a set of contactors to switch the phase windings into either single-star (low speed) or double-star (high speed).

Two of the supply lines are interchanged in the double-star connection – this is to maintain the same direction of rotation as in the low speed connection.

A continuously variable speed range of ship’s motor control involves more complication and expense than that required to obtain a couple of set speeds.
Various methods are available which include:

Electro-hydraulic drive.
Wound-rotor resistance control of induction motors
Ward-Leonard d.c. motor drive.
Variable-frequency induction or synchronous motor control.

The electro-hydraulic drive, often used for deck crane control, has a relatively simple electrical section. This is a constant single-speed induction motor supplied from a DOL or star-delta starter.
The motor runs continuously to maintain oil pressure to the variable-speed hydraulic motors.
A crude form of speed control is provided by the wound rotor induction motor.
The rotor has a 3-phasewinding (similar to its stator winding) which is connected to 3 slip rings mounted on the shaft.
An external 3-phase resistor bank is connected to brushes on the rotor slip rings. A set of contactors or a slide wiper (for small motors) varies the amount of resistance added to the rotor circuit.

Increasing the value of external resistance decreases the rotor speed.

Generally, the starters of wound-rotor motors are interlocked to allow start-up only when maximum rotor resistance is in circuit.
This has the benefits of reducing the starting current surge while providing a high starting torque.
The wound-rotor arrangement is more expensive than an equivalent cage-rotor machine. It requires more maintenance on account of the slip rings and the external resistor bank which may require special cooling facilities.

Where continuously variable speed has to be combined with high torque, smooth acceleration, including inching control and regenerative braking, it is necessary to consider the merits of a d.c. motor drive.
Speed and torque control of a d.c. motor is basically simple requiring the variation of armature voltage and field current.
The problem is: where does the necessary d.c. power supply come from on a ship with an a.c. electrical system?

A traditional method for lifts, cranes and winches is found in the Ward-Leonard drive. Here a constant speed induction motor drives a d.c. generator which in turn supplies one or more d.c. motors.
The generator output voltage is controlled by adjusting its small excitation current via the speed regulator. The d.c. motor speed is directly controlled by the generator voltage.

Obviously the motor-generator (M-G) set requires space and maintenance.
An alternative is to replace the rotary M-G set with a static electronic thyristor controller which is supplied with constant a.c. voltage but delivers a variable d.c. output voltage to the drive motor.

Although the Ward-Leonard scheme provides an excellent power drive, practical commutators are limited to about 750 V d.c. maximum which also limits the upper power range.
The commutators on the d.c. machines also demand an increased maintenance requirement.
To eliminate these problems means returning to the simplicity of the cage-rotor induction motor. However, the only way to achieve a continuously variable speed output by electrical control is to vary the supply frequency to the motor.
A static electronic transistor or thyristor (high power) controller can be used to generate such a variable frequency output to directly control the speed of the motor.

In an electronic variable speed drive (VSD), the fixed a.c. input is rectified and smoothed by a capacitor to a steady d.c. link voltage (about 600V d.c. from a 440V rms a.c. supply).
The d.c. voltage is then chopped into variable-width, but constant level, voltage pulses in the computer controlled inverter section using IGBTs (insulated gate bipolar transistors). This process is called pulse width modulation or PWM.
By varying the pulse widths and polarity of the d.c. voltage it is possible to generate an averaged sinusoidal a.c. output over a wide range of frequencies.
Due to the smoothing effect of the motor inductance, the motor currents appear to be approximately sinusoidal in shape.
By directing the currents in sequence into the three stator winding a reversible rotating magnetic field is produced at a frequency set by the PWM modulator.

VSDs, being digitally controlled/ can be easily networked to other computer devices e.g. programmable logic controllers (PLCs) for the overall control of a complex process.
A disadvantage of chopping large currents with such a drive creates harmonic voltage back into the power supply network.
A harmonic voltage waveform is a distorted sinusoidal wave shape.
Such harmonic voltage disturbances caused by current switching can interfere with other equipment connected to the power system.
progressive insulation breakdown due to high voltage spikes
flickering of the lighting
malfunction of low current devices such as electronic computers and instrumentation/control circuits

The post All about speed control of ship electrical motors appeared first on Electro-technical Officer (ETO).

For large power motors on ship, the phase windings are automatically switched using contactors controlled by u timing relay.
A choice to time delay relays are available whose action is governed by thermal, pneumatic, mechanical or electronic control devices.

At the instant of starting when the supply has just been switched on and the motor has not yet started to rotate, there is no mechanical output from the motor. The only factors which determine the current taken by the motor are the supply voltage (V) and the impedance of the motor phase windings (Zph).

This shows that the starting current of a delta connected motor can be reduced to one third if the motor is star connected for starting.
The shaft torque is also reduced to one-third which reduces the shaft acceleration and increases the run-up time for the drive but this is not usually a problem.

Induction motor on ship electrical system

When an induction motor is running on load it is converting electrical energy input to mechanical energy output. The input current is now determined by the load on the motor shaft.
An induction motor will run at the same speed when it is star connected as when it is delta connected because the flux speed is the same in both cases being set by the supply frequency.
This means that the power output from the motor is the same when the motor is star connected as when the motor is delta connected, so the power inputs and line currents must be the same when running in either connection.

If the motor is designed to run in delta but is run as star connected, and on full load, then each stator phase winding will be carrying an overcurrent of 1.73 rated phase current.

This is because phase and line currents are equal in a star connection.

This will cause overheating and eventual burnout unless tripped by the overcurrent relay.
Remember that the motor copper losses are produced by the heating effect so the motor will run 3 times hotter if left to run in the star connection when designed for delta running.
This malfunction may occur if the control timing sequence is not completed or the star contactor remains closed while a mechanical interlock prevents the delta contactor from closing.
For correct overcurrent protection, the overcurrent relays must be fitted in the phase connections and not in the line connections.

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