Electric Motors

March 7, 2014

• The drive power for compressor, pumps and fans aboard ship comes from electric motor.
• The most common type of motor is the 3-Φ, cage-rotor induction motor. It is simple, tough and requires very little attention.
• Another advantage is in starting and stopping these motors that can be done with simple, reliable direct-on-line contactor starters.
• Induction motors are usually supplied at 440V, 60Hz, but 3.3 kV, 60Hz is sometimes used for large drives such as bow thrusters and cargo pumps.
• DC commutator motors are often used for driving deck machinery where speed control is important.
• Single phase (Φ ) ac motors are used in low power drives such as galley equipment.

• Inductions motor has two main parts: stator and rotor.
• The stator carries the 3-Φ winding in slots cut into a laminated steel magnetic core.
• The ends of the stator windings are terminated into the stator terminal box.
• The rotor has an uninsulated cage winding which consists of copper or aluminum conductor bars which are connected together at the ends by end rings.
• The conductor bars are set in a laminated steel magnetic core.
• The essential reliability of the induction motor comes from having this type of simple, tough rotor which has no insulation and does not have any troublesome current collection gear like brushes, commutator or slip rings.
• Motor Enclosures
• Enclosure protection for electrical equipment is defined in terms of its opposition to the ingress of solid particles and liquids against mechanical impact.
• The enclosure protection is defined by the Ingress Protection (IP) Code.
• Drip proof open ventilated motors are used where the risk of dripping liquids from overhead pipes and valves may be a problem.
• Air is drawn into the machine by an internal fan to provide cooling.
• The ventilation ducts are fitted with mesh screens to prevent any objects from entering the motor and causing damage.
• These screens must always be kept clean and free from dust otherwise the motor will overheat due to inadequate ventilation.
• Air is drawn into the machine by an internal fan to provide cooling.
• The ventilation ducts are fitted with mesh screens to prevent any objects from entering the motor and causing damage.
• These screens must always be kept clean and free from dust otherwise the motor will overheat due to inadequate ventilation.
• When a greater degree of protection is required the enclosure is made Totally Enclosed Fan Ventilated (TEFV) and jet proof.
• No external air is allowed inside the motor.
To improve heat transfer the motor casing is finned to increase the surface area, and airflow across the fins is achieved by means of an external fan and cowl arrangement
• Motors located outside on weatherdecks have deck watertight enclosures but the external fan is omitted because of the possibility of ice formation.
• Deck watertight motors have sealed bearings and a watertight terminal box. They can be completely immersed in shallow water for short periods.
• Sealing washers are fitted under all screws and a coat of special corrosion resisting paint is generally applied to all external and internal surfaces.
• Deck motors for tankers must have a flameproof (Exd) enclosure if they are within 3m (4.5m for some ships) of an oil tank outlet.
• The Effect of Shaft Load
• If, while running normally, the load on the motor shaft is increased, the rotor will tend to slow down.
• This allows the motor to take more supply current to meet the increased power demand.
• Losses occur during the energy conversion which results in the production of heat in the motor.
• These losses increase when the load on the motor increases because the motor takes more current from the supply.

The life of the insulating materials used on motor windings depends on the temperature at which it is operated.
• Insulating materials are selected to have an adequate life-span on the assumption that the temperature limit associated with a particular insulation class is not exceeded.
• Max. Ambient Temperature 40C
• Rated Current – maximum value of current that the motor can continuously take from the supply without exceeding the temperature limit for the insulating materials used.
• Rated Voltage – if the rated voltage is not applied, overheating, stalling and burnout can result.
• Rated Frequency – motor speed is directly affected by the supply frequency, so are the motor losses. If the motor is operated at other than the rated frequency overheating can occur.
• Power Rating – is the shaft power output of the motor when it is connected to rated voltage and frequency and drawing rated current from the supply
• Rated Speed – is the full load speed of the motor when connected to rated voltage and frequency.
• IP Number – indicates the degree of protection given by the motor enclosure.
• The rating details are shown on the motor nameplate.

Practical Marine Electrical Knowledge- By Hall
Siskind Charles , Electrical Machine
Preventive Maintenance of Electrical Machine by Hubert, Charles (2nd Edition)




This article is focused on definition of terms. It is better to have an illustration especially in construction with explanation so that we can visualize the mechanism of each part of the electric motor. This article may help us to understand some topics that we didn't understand on the book or some references and we easily find the terms related to electric motor.

An induction or asynchronous motor is an AC electric motor in which the electric current in the rotor needed to produce torque is induced by electromagnetic induction from the magnetic field of the stator winding. An induction motor therefore does not require mechanical commutation, separate-excitation or self-excitation for all or part of the energy transferred from stator to rotor, as in universal, DC and large synchronous motors. An induction motor's rotor can be either wound type or squirrel-cage type.

Based from i have just read. This article is discussing mainly about electric motors. Electric motors on board the ship gives the drive power for compressor, fans and pumps on board.For electric motors, we have cage-rotor induction motor, DC commutator motor and ac motors. then the article discussed the construction and some of the common problems face by these motors and how it is avoided based on its construction like why seals and fans are fitted in the motor. In the end of the article, the definition of terms related to induction motors is given which is very easy to understand

well, i think this is one way to freshen up the mind of readers. This makes the readers more interesting and be related to any of this topics being discussed. The terms here will prevent confusion and better understanding in the future articles to come. This is a good start for the whole lesson sir!

This is all about the introduction to electric motors. parts and functions were discussed here and the construction of motor as well. however, it is hard to understand due to lack of illustration, as mentioned on the previous comments.there are also important terms to be considered in the said lecture.

the topic were being discussed thoroughly...definition of terms were given which helps a lot in understanding electric motor, this will help as a guide in the remaining topics...

For the 1st part, it was a very fitting introduction to an otherwise complicated topic. Good job sir!!!

The article helps a lot as you go on the topic. It will help you not to be astray on the succeeding topics.

well as i read about the article sir the only thing that came to my mind is the concept of electric motor that- An electric motor is an electric machine that converts electrical energy into mechanical energy.And one thing sir, as far as i know that a dynamo can act as an electric motor when supplied with direct current from a battery or another dynamo, am i right sir? in my conclusion about the article , it help us in our studies and furthermore i hope you can also discuss it to us in class more briefly for better understanding. :)

As I read , it helps me to visualize the operation on how the Electric motors involve rotating coils of wire which are driven by the magnetic force exerted by a magnetic field on an electric current. muchísimas gracias!

Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings can be viewed as two-phase motors, since a capacitor is used to generate a second power phase 90° from the single-phase supply and feeds it to the second motor winding. Single-phase motors require some mechanism to produce a rotating field on startup.

The article really helps... Thanks a lot! Gonna read more.

This article is mostly focused on definition of terms. Placing pictures will provide more information as it will give the readers more idea on how each part or the mechanism works. \m/

in addition.. induction motors operation there is only only one supply, so it is really interesting to know that how it works. It is very simple, from the name itself we can understand that there is induction process occurred. Actually when we are giving the supply to the stator winding, flux will generate in the coil due to flow of current in the coil. Now the rotor winding is arranged in such a way that it becomes short circuited in the rotor itself. The flux from the stator will cut the coil in the rotor and since the rotor coils are short circuited, according to Faraday's law of electromagnetic induction, electric current will start flowing in the coil of the rotor. When the current will flow, another flux will get generated in the rotor. Now there will be two flux, one is stator flux and another is rotor flux and the rotor flux will be lagging to the stator flux. Due to this, the rotor will feel a torque which will make the rotor to rotate in the direction of rotating magnetic flux. So the speed of the rotor will be depending upon the ac supply and the speed can be controlled by varying the input supply. This is the working principle of an induction motor of either type. sir thank you for this article.. it really help us un studying more:)

This really helps us a lot regarding our shipboard training preparations.

this article will guide us on how to preserve and protect the electric motors.

pretty good...this can help us to know more about electric motors.. thanks for this article sir :)

<p> Electrical Power Distribution - The function of a ship&#39;s electrical distribution system is to safely convey electrical power to every item of equipment connected to it. The most obvious element in the system is the main switchboard. The main board supplies bulk power to motor starter groups (often part of the main board), section boards and distribution boards. Transformers interconnect the HV and LV distribution sections of the system. Circuit breakers and fuses strategically placed troughout the system automatically disconnects a faulty circuit within the network. The main switchboard is placed in the engine controlroom and from there engineroom staf monitor and control the generation and distribution of electrical power. It is very important that every engineer has a profound knowledge of the electrical distribution of the ship&#39;s power. The only way to aquire this knowledge is to study the ship&#39;s power diagrams.</p> <p> Almost all oceangoing ships have an A.C. distribution system in preference to a direct current D.C. system. Usally a ship&#39;s electrical distribution scheme follows shore pratice. This allows normal industrial equipment to be used after being adapted and certi ed where and if necessary, so it can withstand the conditions on board of a ship (e.g. vibration, freezing and tropical temperatures, humidity, the salty atmosphere, etc. encountered in various parts of the ship).</p> <p> Most ships have a 3-phase A.C., 3-wire, 440V insulated-neutral system. This means that the neutral point of star connected-generators is not earthed to the ship&#39;s hull. Ship&#39;s with very large electrical loads have generators operating at high voltages (HV) of 3.3KV, 6.6KV, and even 11KV.</p> <p> The most common power frequency adopted for use on board ships is 60Hz. This higher frequency means tha t generators and motors run at higher speeds with a consequent reduction in size for a given power rating. Lighting and low power single-phase supplies usually operate at 220 V. This voltage is derived from a step down transformer connected to the 440 V system.</p> <p> Grounding systems in shipboard electrical networks.</p> <p> In electrical engineeering, the ground means reference in electrical circuits from which other voltages are measured. The earth point means a solid connection to the earth, which due to its massive section and mass has almost no resistance for electrical current. If the reference for your voltage measurements is the earth the earth becomes your ground. By absense of the earth on board of a ship, the ship&#39;s hull can be used as a substitute for the earth. Depending on the construction of the electrical networks they may or may not be connected to earth potential. In general we can have solidly grounded, reactance grounded, resistance grounded and isolated networks. In isolated networks there is the challenge to detect earth faults. Ships distribution systems are typically isolated in low voltage systems (1000V AC below) and high resistance grounded in high voltage systems (1000V above). High resistance grounding ensures the trip action in case of an earth fault and prevents short circuit faults in the network. High resistance grounding can therefore not guarantee continuity of service.</p> <p> &nbsp;</p> <p> Electrical faults</p> <p> Earth fault - An earth fault is caused by loss of insulation allowing the current to ow to earth potential. Causes of earth faults are typically breakdown or wear of insulation. The majority of earth faults occur within electrical equipment due to an insulation failure or a loose wire, wich allows a live conductor to come into contact with its earthed metal enclosure.</p> <p> To protect against the dangers of electric shock and fire that may result from earth faults, the metal enclosures and other non-current carrying metal parts of electrical equipment must be earthed. The earthing connector connects the metal enclosure to earth (the schip&#39;s hull) to prevent it from attaining a dangerous voltage with respect to earth. Such earth bonding of equipment ensures that its voltage in reference to earth always remains at zero.</p> <p> &nbsp;</p> <p> Open circuit fault - An open circuit fault occurs when a phase conductor is completely or even partialy interupted. Causes of open circuit faults are bad connections or a break in the wire. Open circuit faults when intermittent can cause ashes. Open circuit faults when not completely open (bad connection) can cause a lot of heat and are a re hazzard. Open circuits in three phase circuits can cause motors to run on only two phases and create a motor overload.</p> <p> &nbsp;</p> <p> Short circuit fault - Short circuit faults occurs where two di erent phase conductors are connected together. This can be caused by double break loss of insulation, human error or another abnormal situation. A large amount of current is released in a short circuit, often accompanied by an explosion.</p> <p> &nbsp;</p> <p> Significance of Earth Faults<br /> If a single earth fault occurs on the live line of an earthed distribution system it would be the equivalent to a schort-circuit fault across the generator trough the schip&#39;s hull.</p> <p> The resulting large current would immediatly cause the line protective device (fuse or circuit breaker ) to trip out the faulty circuit. The faulted electric equipment would be immediately isolated from the supply and so rendered safe. However, the loss of power supply, could create a hazardous situation, especialy if the equipment was classed essential e.g. steering gear. The large fault current could also cause arcing damage at the fault location. In contrast a single earth fault occuring on one line of an insulated distribution system will not cause any protective trip to operate and the system would continue to function normally. This is the important point: equipment continues to operate with a single earth fault as it does not provide a closed circuit so no earth fault current will flow. More important is that if a second earth fault occurs on another line of the insulated system, the two faults together would be equivalent to a short- circuit fault (via the ship&#39;s hull) and the resulting large current would operate protection devices and cause disconnection of perhaps essential services creating a risk to the safety of the ship.</p> <p> An insulated distribution system therefore requires two earth faults on two di erent lines to cause an earth fault current to flow.</p> <p> In contrast, an earthed distribution system requires only one earth fault to cause an earth fault current to flow. An insulated system is, therefore more e ffective than an earthed system in maintenance continuity of supply to essential services. Hence its adoption for most marine electrical systems.</p> <p> Note: Double-pole switches with fuses in both lines are necessary in an insulated single-phase circuit.</p> <p> An Electric Power System&#39;s Reliability-<br /> Reliability of an electric system is obtained by sectioning of the distribution system and providing multiple power sources, by providing an emergency power system, subsectioning of the circuits, the choice of the earthing system and the selectivity of the protections.</p> <p> Sectioning of the distribution system and providing multiple power sources. Providing multiple transformators can protect certain users against particular problems. An example can be computer systems wich are sensitive to harmonics.</p> <p> Emergency power systems - Two independant high to low voltage power stations, emergency generators, UPS, independant emergency lighting a.o. have to be placed in well protected area&#39;s enabling them to function in case of an emergency and or accident.</p> <p> <br /> Sectioning of circuits - Essential equipment can take there power from the main or emergency switchboard.&nbsp; This way a fault wich a ects a secondary circuit doesn&#39;t influence a circuit with high priority. Sectioning of circuits is done as demanded by The Rules and the demands of exploitation, providing at least two power sources for all essential equipment.</p> <p> Selectivity - If a fault occurs at any point in an electrical&nbsp; distribution circuit, it is essential that it does not interrupt the supply to essential services. This obvious requirement leads to the necessity of rapidly isolating the defective section without depriving the other users of electrical energy; this is in fact the principle of selective tripping.</p> <p> The protective element (circuit breaker or fuses) which is placed immediately up-stream from the part of the circuit where the fault has occured, and this alone element, must then operate; the other protecting elements must not trip. Conventional selectivity processes (overcurrent and time lag) fulfil l these requirements to a more or less satisfactory degree.</p> <p> Overcurrent Selectivity - This makes use of protective equipment operating instantaneously (rapid circuit breakers or fuses). The selectivity is based on the fact that the shortcircuit current decreases with increasing distance from the power source. It is thus especially for low voltages where the connecting impedances are not negligible.</p> <p> Time lag selectivity - This can make complete selectivity&nbsp; by delaying the tripping of each circuit breaker for durations all the higher as the circuit breaker is nearer the source of energy.</p> <p> &nbsp;</p> <p> <em>Reference:<br /> </em></p> <p> <em>An excerpts from:<br /> </em></p> <p> <em>Antwerp Maritime Academy<br /> Navale Engineering<br /> Marine Electrical Knowledge<br /> </em></p> <p> <em><br /> </em></p>

it is a nice site,, but it can be better when there is a picture in every definition of terms......

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