The electromagnetic interaction between two currents produces
motor torque T_m = K I₁ I₂

If the motor shaft has a load torque TLoad < Tmotor, the motor
would accelerate to a speed at which TLoad = Tmotor, where it would stop accelerating and run at steady speed. The motor armature produces back voltage, or its equivalent, and draws current from the source, which is given by

It is important to understand that the armature draws just enough current that is required to develop torque to meet the load torque at the steady running speed.

Rated load on ship electrical motor

A 100-hp-rated motor does not always deliver full 100 hp regardless of the shaft load.
The motor delivers what is needed to drive the load and draws power from the source equal to what it delivers to the load plus the internal losses. Thus, the power drawn from the source may be less or more than the rated load, depending on the mechanically coupled load on the shaft.

Overload of ship electric motor

However, if continuously overloaded without added cooling, the motor would heat up and burn.
The shaft horsepower, torque, speed, and kW power delivered by the motor are related as follows:

Breakdown of motor types and their energy usage in various horsepower ratings. It shows that about 98% of all motors are induction motors that use
about 93% of the electrical energy used by all motors rated 5 hp and higher. Smaller motors do not use much energy because their use is intermittent, often less than an hour in a day.

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A typical excitation system has the corresponding current and voltage ratings, with the capability of varying the voltage Ef over a wide range of 1 to 3, or even more, without undue saturation in the magnetic circuit. Most excitation systems operate at 200 to 1,000 Vdc.

The excitation power to overcome the rotor winding I²R loss ranges from ½% to 1% of the generator rating.

All types of excitation on ship generators

For a large ship utility generator, four types of excitation system – dc, ac, static, and brushless.

DC exciter on ship generator

DC exciter: A suitably designed dc generator supplies the main field winding excitation through conventional slip rings and brushes. Due to low reliability and a high maintenance requirement, the conventional dc exciter is seldom used in modern ac generators of large ratings.

AC exciter on ship generator

AC exciter: It consists of a permanent magnet pilot exciter that excites the main exciter. The ac output of the pilot exciter is converted into dc by a floor-standing rectifier and supplied to the main exciter through slip rings. The main exciter’s ac output is converted into dc by means of a phase-controlled rectifier whose firing angle is changed in response to the terminal voltage variations.
After filtering the ripples, the dc is fed to the main generator field winding.

Static excitier on ship generator

Static exciter: It has no moving parts, as opposed to the rotating exciters
described. In the static exciter scheme, the controlled dc voltage is obtained from a suitable stationary ac source rectified and filtered. The dc voltage
is then fed to the main field winding through slip rings. This excitation
scheme has a fast dynamic response and is more reliable because it has no
rotating exciter with mechanical inertia.

Brushless exciter on ship generator system

Brushless exciter: Most modern synchronous generators of large ratings use
the brushless scheme of excitation to eliminate the need for slip rings and
brushes. The brushless exciter is placed on the same shaft as the main generator. The ac voltage induced in the exciter is rectified by rotating diodes on the rotor and filtered into pure dc. The dc is then fed directly into the rotor field coil.

The excitation control system modeling for analytical studies must be carefully done as it forms multiple feedback control loops that can become unstable. The IEEE has developed an industry standard for modeling the excitation systems. The model must account for any nonlinearity due to magnetic saturation that may be present in practical designs. The control system stability can be improved by supplementing the main control signal by auxiliary signals, such as speed and power, as required by the feedback control system stability.

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IMPORTANT
The category number of a DMM is more important than its voltage rating when determining the degree of protection that it provides. In other words, a CAT III, 600 V meter offers better protection against high-energy transients than a CAT II, 1000 V meter.

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1. Resistance thermometers operate on the principle that the resistance of a metal varies
in direct proportion to its temperature. They are normally used for temperatures up to approximately 1500°F.

2. Thermocouples operate on the principle that a difference in temperature in different metals generates a voltage, and are used for measuring temperatures up to about 3000°F.

3. Radiation pyrometers and optical pyrometers are generally used for temperatures above 3000°F. They combine the principle of the thermocouple with the effect of radiation of
heat and light.

k type thermocouple Temperature Sensors 0-1300C HIGH Temperature Explosion Proof temperature sensor

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Item 1 – Multimeter

It is essential that all test instruments be regularly checked for safety, and any connection leads, probes or clips checked to ensure that they are suitable for the voltage levels being tested. Never attempt to test a “LIVE” generator unless there is another competent person present who can switch off the power supply or shut down the engine in an emergency. The Multimeter is a comprehensive test instrument for measuring voltage, current and resistance.

Never expose “LIVE” connections unless you have created a safe working area around you. Make sure you have made all other persons in the immediate area fully aware of what you are doing.

Do not attempt to carry out tests on medium or high voltage generators without using specialised instruments and probes, with suitable protection equipment and procedures for grounding (earthing) the output terminals.

Item 2 – Tachometer or Frequency meter

This instrument is for measuring the shaft speed of the alternator and should be capable of measuring speeds between 0 and 5000 revolutions per minute, (RPM). An alternative to the tachometer is the frequency meter. However the alternator must be generating its normal output voltage for this instrument to be accurate.

Item 3 – Megger (Insulation test meter)

This instrument generates a voltage of 500V or 1000V, and is used to measure the resistance value of the insulation to earth (ground). It may be an electronic push button type, or a hand cranked generator type.

Item 4 – Clip-On Ammeter (Clampmeter)

Used to measure AC current, it consists of a pair of callipers, which are clamped around the conductor, and by means of a transformer action, gives an indication of the amperes flowing in the conductor. Useful ranges to have on this meter are
AC Amps 0-10-50-100-250-500-1000

Item 5 – Kelvin Bridge – low resistance meter

This instrument is used to measure resistance values below 1.0 ohm. They are bulky, and expensive, but are the only means of accurately measuring very low resistances, such as main stator and exciter rotor windings.
However, there are other methods of testing low resistance windings, and these are included in the various test procedures, i.e. Test Method A. This section will enable the main generator windings to be tested while running the generator at normal speed without load.

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These are important users that need not be in continuous operation;

however they are necessary to maintain propulsion and steering, including a minimum level of safety for dangerous cargoes to be carried.

Some of them are as follows:

1) Windlass.
2) Fuel oil transfer pumps and fuel oil treatment equipment.
3) Lubrication oil transfer pumps and lubrication oil treatment equipment
4) Pre-heaters for heavy fuel oil.
5) Starting air and control air compressors.
6) Bilge, ballast and heeling pumps.
7) Fire pumps and other fire extinguishing medium pumps.
8) Ventilating fans for engine and boiler rooms,
9) Services considered necessary to maintain dangerous spaces in a safe condition (inert gas system of an oil carrier, ventilation for Ro-Ro cargo spaces, etc.).
10) Navigation lights, aids and signals.
11) Internal communication equipment.
12) Lighting system.
13) Electrical equipment for watertight and fire-tight closing appliances.
14) Electric generators and associated power sources supplying secondary essential equipment.
15) Hydraulic pumps supplying secondary essential equipment.
16) Control, monitoring and safety systems for cargo containment systems.
17) Control, monitoring and safety devices/systems of equipment for secondary essential services.
18) Ambient temperature control equipment.
19) Other thrusters like boosters, etc.

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When converting a ship’s wiring diagram into a circuit diagram certain basic rules and conventions should be followed.

• Every sequence should be drawn from left to right and from top to bottom (where possible)

• Each stage should be in order of occurrence from left to right.

• All contacts and components which are in series should be drawn in a straight line (where possible) with the component they control.
• All contacts and components which are in parallel should be drawn side by side and at the same level to emphasise their parallel function.

• All major components operating at bus-bar voltage should be drawn at the same level (or aligned horizontally) to help identify the required components quickly.
• All contacts should be shown open or closed in their normal or de-energised condition.

There are other conventions but these cover the main points of good systematic diagrams on ship electrical system. Block, system, circuit and wiring diagrams are the main types in general use for electrical work on vessel. Other types of diagram are sometimes used to give information for which the basic types are unsuitable (e.g. a pictorial view of a component).

Need to study the ship’s electrical diagrams to gain an understanding of equipment operation prior to carrying out maintenance or fault finding.

Diagrams on ship should be regarded as an essential tool when carrying out work on ship’s electrical equipment.

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But in some cases, the transformer may continue trying to operate with excessive overheating due to the very large circulating current. This heat will often melt the insulation inside the transformer, which is easily detected.

Also, there will be no voltage output across the shorted winding and the secondary circuit supplied by that winding will be dead.

The short may be in the external secondary circuit or it may be in the transformer’s winding.

To determine its location, disconnect the secondary circuit from the winding and take a reading with a voltmeter. If the voltage is normal with the external circuit disconnected, then the problem is in the external circuit.

However, if the voltage reading is still zero across the secondary leads, the transformer is shorted and must be replaced.

Grounded Windings

Insulation breakdown is quite common in older transformers especially those that have been overloaded. At some point, insulation breaks or deteriorates and bare conductors become exposed. The exposed wire often comes into contact with the transformer housing and grounds the winding.

If a winding develops a ground, and a point in the external circuit connected to this winding is also grounded, part of the winding will be shorted out. The symptoms are overheating, usually detected by feel or smell, and a low voltage reading as indicated on a voltmeter scale. In most cases, transformers with this condition must be replaced.

A megohmmeter is used to test for this condition. Disconnect the leads from both the primary and secondary windings. Tests can then be performed on either winding by connecting the megger negative test lead to an associated ground and the positive test lead to the winding to be measured.

Insulation resistance should then be measured between the windings themselves, by connecting one test lead to the primary and the second test lead to the secondary.

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