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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The salient-pole construction provides more physical space for the rotor winding and also produces additional torque (called reluctance torque) that increases the machine power output at a small power angle δ.
The flux in the main magnetic axis (called direct axis or d-axis) has a low-reluctance path (less air gap) than the flux in the quadrature axis (called q-axis).

For this reason, the protruding salient poles have a natural tendency to align with the stator field axis under ferromagnetic attraction even in the absence of rotor current. This requires additional torque from the prime mover, twice in one electrical cycle, to drive the rotor away from said natural alignment.

The total torque input and hence the power output of the generator has a sin(2δ) component superimposed on the sinδ component.

The power output analysis of the salient-pole generator requires employing the two-axis theory using Park transformation.

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ΔT˚C = K × (watts)^0.8

Overloading ship electrical equipment above the rated temperature limit reduces life to about ½ for every 10°C rise above the design temperature.

This 10°C rule for half-life is based on typical properties of ship insulation’s normally used in electrical machines. Experience on other ship electrical components, such as ship power electronics devices, suggest a different rule, such as every 7°C rise for reducing the life to one-half.

Ship cable overload problem

This a significant reduction of life, that continuous overload, even by 10%–15%, is not economical. That is why 15% overload on electrical
equipment is limited to an hour or two, and that also only on occasion.

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The industry-standard ambient air temperature is 40°C. It can be higher, up to 65°C, in special places such as the ship boiler room uptake. The maximum permissible operating temperature depends on the class of insulation used in the equipment.

For example, a temperature rise of 80°C is permitted in equipment’s with class B insulation. In 40°C standard ambient air, it would have an operating temperature of 80°C + 40°C = 120°C. For some insulation’s, such as NomexTM, the allowable operating temperature can be up to 200°C.
The temperature rise is determined by the cooling medium and the surface area available for dissipating the internal power loss on ship system that heats the equipment body.

Common Aspects of Power Equipment

Air-cooled ship equipment dissipates the internal heat mostly by convection and radiation, and negligibly by conduction. For electrical equipment on ship normally operating around 100°C, the temperature rise above the ambient air is approximately given by:

ΔT˚C = K × (watts)0.8 (3.23)

where watts = power loss to be dissipated and K = constant for a given body.

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Optimized design and compromises can only be achieved with a common concept language and mutual understanding of the different subjects.
The objective of this section is to give an introduction to electro-technology in general, and put special emphasis on installations for electric propulsion.

It is the aim to give engineers with marine competence and background
the necessary understanding of the most important electro-technical subjects used in design and configuration of ships with electric propulsion.

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When ships Electrical Engineer ETO need find and diagnose intermittent faults, where a short, open, or other problem occurs only temporarily, or only under certain conditions, is always a difficult troubleshooting problem. Two features found on most DMMs can help with identifying intermittent faults.

Continuity capture mode

This feature is useful for finding intermittent connections with small gauge wires and wiring bundles, and even intermittent ships relay contact. To check for intermittent opens, place the leads across the normally closed
or shorted connection and select Continuity Capture mode on the DMM. Wiggle the wire(s) and heat the connection with a heat gun, or cool it with circuit cooler to make the intermittent open appear. When the open is captured (as short as 250 μs), the display shows a transition from open to a short.
Intermittent shorts can be found the same way, by connecting to a normally open circuit and using the wiggling and heating/cooling techniques to capture the short. The only difference is that the transition
lines will go from the bottom of the display to the top.

Recording mode

Sometimes intermittent faults cannot be successfully induced while observing the DMM display. Some higher-end units have a recording mode with a date and time stamp. This type of DMM can be left connected to a circuit or piece of electrical equipment for an extended period of time to record the occurrence of an intermittent fault. The date and time of occurrence may provide clues that allow the electrician or technician to trace the cause of the fault.

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Availability of fast plug connections and seamless load transfer without blackout allow the full range of in-port functions to continue during the switchover. It is becoming an important part of the clean
port regulations that major ports around the world have rigorously started to implement.

For example, all major container, liquid bulk, and cruise ship terminals
at the Port of Los Angeles will have shore-side electrical power within five
years, and all container and one crude oil terminal at the Port of Long Beach will have it within five to ten years.

This will allow shutting down dirty diesel- powered auxiliary engines when at port, and taking advantage of relatively cleaner, although more expensive, shore power for reducing ship emissions around the port.

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An electro technical officer handles several responsibilities on vessel mainly related to electrical systems. Like most jobs at sea, as a ship officer handling electronics on a vessel, you are most likely to work under a chief engineer who supervises everything on a ship. And while under a chief engineer, your ship jobs would include the following things.

• Maintenance and working of electrical equipments on board

• Working of electrical parts like engine rooms, radio communications, electronic navigation needs like echo sounders, Gyro compass, Weather Fax, auto pilot, RADAR system, Broadcast and internal aerial system, telephone and talk back system and satellite communications

• Look into basic electric needs like refrigeration, bridge systems and control rooms

• Maintenance of emergency systems including emergency switches, fire alarms and detectors

• Vessel’s electrical components like navigational lights, battery backups and electrically operated propelling machinery

• Assist chief officer in handling routine works especially related to electrical handling

• Coordinate work with on shore technicians

• During emergencies, an electrical officer plays important role like every other ship officer. You would be required to handle the emergency situations and ensure all the equipment needed for safety is always ready

• An electro technical officer looks into electrical and technical aspects on a vessel. Hence working of computer controlled machinery would be supervised by an ETO.

• An electrical officer is immensely important on cruise ships and passenger vessels which have huge electrical requirements and unmanned technology handling them. Everything from refrigeration to air conditioning on such vessels would come under your supervision as an electro technical officer. 

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