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A.C. Motors for Ships Machinery- Arrangement and Distribution

Supplying alternating current to a coil: which is free to rotate in a magnetic field will not produce a motor effect since the current is constantly changing direction. Use is therefore made in an induction or squirrel cage motor of a rotating magnetic field produced by three separately phased windings in the stator.

The rotor has a series of copper conductors along its axis which are joined by rings at the ends to form a cage. When the motor is started the rotating magnetic field induces an e.m.f. in the cage and thus a current flow. The current-carrying conductor in a magnetic field produces the motor effect which turns the rotor. The motor speed builds up to a value just less than the speed of rotation of the magnetic field.

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The motor speed depends upon the e.m.f. induced in the rotor and this depends upon the difference in speed between the conductors and the magnetic field. If the load is increased the rotor slows down slightly, causing an increase in induced e.m.f. and thus a greater torque to deal with the increased load.

Squirrel cage induction motor starting
Squirrel cage induction motor starting

The motor is almost constant speed over all values of load. It will start against about two times full load torque but draws a starting current of about six times the normal full load current. The starting current can be reduced by having a double cage arrangement on the rotor. Two separated cages are provided, one below the other in the rotor. When starting, the outer high-resistance cage carries almost all the rotor current. As the motor accelerates the low-resistance inner winding takes more and more of the current until it carries the majority.

A number of different fixed speeds are possible by pole changing. The speed of an induction motor is proportional to frequency divided by the numbers of pairs of poles. If therefore a switch is provided which can alter the numbers of pairs of poles, then various fixed speeds are possible. The number of poles affects the starting characteristics such that the more poles the less the starting torque to full load torque ratio. Only the induction type of a.c. motor has been described, since it is almost exclusively used in maritime work. Synchronous motors are another type which have been used for electrical propulsion systems but not auxiliary drives.

A number of different arrangements can be used for starting an induction motor. These include direct on-line, star delta, auto transformer and stator resistance. Direct on-line starting is usual where the distribution system can accept the starting current. Where a slow moving high inertia load is involved the starting time must be considered because of the heating effect of the starting current. The star delta starter connects the stator windings first in star and when running changes over to delta.

The star connection results in about half of the line voltage being applied to each phase with therefore a reduction in starting current. The starting torque is also reduced to about one-third of its direct on line value. A rapid change-over to delta is required at about 75% of full load speed when the motor will draw about three-and-a-half times its full load current. The auto transformer starter is used only for large motors. It uses tappings from a transformer to provide, for example, 40%, 60% and 75% of normal voltage (Figure below).

The motor is started on one of the tappings and then quickly switched to full voltage at about 75% full speed. The tapping chosen will depend upon the starting torque required with a 60% tapping giving about 70% of full load torque. A smaller percentage tapping will give a smaller starting torque and vice-versa. The stator resistance starter has a resistance in the stator circuit when the motor is started. An adjustable timing device operates to short circuit this resistance when the motor has reached a particular speed.

Modern electronic techniques enable a.c. induction motors to be used in speed-control systems. The ship's supply, which may not be as stable in voltage or frequency as that ashore, is first rectified to provide a d.c, supply. This is then used as the power supply of an oscillator using high-power electronic devices. These may be thyristors (for powers up to 1.5 M W or more) or transistors (for powers up to a few tens of kilowatts). The high-power oscillator output is controlled in frequency and voltage by a feedback system. The motor speed is varied by changing the oscillator output frequency. The motor current necessary to obtain the desired torque (at small angles of slip) is normally obtained by maintaining the voltage almost proportional to frequency.

Certain protective devices are fitted in the motor circuit to protect against faults such as single phasing, overload or undervoltage. Single phasing occurs when one phase in a three-phase circuit becomes open circuited. The result is excessive currents in ail the windings with, in the case of a delta connected stator running at full load, one winding taking three times its normal load current.

A machine which is running when single phasing occurs will continue to run but with an unbalanced distribution of current. An overload protection device may not trip if the motor is running at less than full load. One method of single phasing protection utilises a temperature-sensitive device which isolates the machine from the supply at some particular winding temperature.

Overload protection devices are also fitted and may be separate or combined with the single phase protection device. They must have a time delay fitted so that operation does not occur during the high starting current period. An undervoltage or 'no volts' protective device ensures that the motor is properly started after a supply failure.



Related Info:

  1. A.C. motors for ships machinery
    2. Supplying alternating current to a coil which is free to rotate in a magnetic field will not produce a motor effect since the current is constantly changing direction. Use is therefore made in an induction or squirrel cage motor of a rotating magnetic field produced by three separately phased windings in the stator. ...

  2. Use of A.C. generators
    3. A coil of wire rotating in a magnetic field produces a current. The current can be brought out to two slip rings which are insulated from the shaft. Carbon bushes rest on these rings as they rotate and collect the current for use in an external circuit. Current collected in this way will be alternating, that is, changing in direction and rising and falling in value. To increase the current produced, additional sets of poles may be introduced....

  3. D.C. motors for ships machinery
    4. When a current is supplied to a single coil of wire in a magnetic field a force is created which rotates the coil. This is a similar situation to the generation of current by a coil moving in a magnetic field. In fact generators and motors are almost interchangeable, depending upon which two of magnetic field, current and motion are provided.....

  4. Use of D.C. generators
    5. A current is produced when a single coil of wire is rotated in a magnetic field. When the current is collected using a ring which is split into two halves (a commutator), a direct or single direction current is produced. The current produced may be increased by the use of many turns of wire and additional magnetic fields....

  5. Emergency power supply for ships machinery operation
    6. In the event of a main generating system failure an emergency supply of electricity is required for essential services. This can be supplied by batteries, but most merchant ships have an emergency generator. The unit is diesel driven and located outside of the machinery space .

  6. Maintenance requirement for ships electrical equipment
    7. With all types of electrical equipment cleanliness is essential for good operation. Electrical connections must be sound and any signs of sparking should be investigated. Parts subject to wear must be examined and replaced when necessary. ...

  7. Choice of batteries for ships machinery spaces - Lead acid and alkaline batteries
    8. The battery is a convenient means of storing electricity. It is used on many ships as an instantly available emergency supply. It may also be used on a regular basis to provide a low-voltage d.c. supply to certain equipment.....

  8. Ships battery maintenance guidance
    9. The electrolyte level should be maintained just above the top of the plates. Any liquid loss due to evaporation or chemical action should be replaced with distilled water. Only in an emergency should other water be used. It is not usual to add electrolyte to batteries.....

  9. Operating characteristics of battery for ships machinery spaces
    10. Having been 'discharged' by delivering electrical power a battery must then be 'charged' by receiving electrical power. To charge the battery an amount of electrical power must be provided in the order of the capacity.....

  10. Insulation resistance measurement
    11. Good insulation resistance is essential to the correct operation of electrical equipment. A means must be available therefore to measure insulation resistance. Readings taken regularly will give an indication as to when and where corrective action, maintenance, servicing, etc., is required....

  11. Use of navigational light circuits
    12. The supply to the navigation lights circuit must be maintained under all circumstances and special provisions are therefore made. To avoid any possibility of accidental open circuits the distribution board for the navigation lights supplies no other circuit.....

  12. Ward—Leonard speed control system
    13. As a very flexible, reliable means of motor speed control the Ward-Leonard system is unmatched.The system is made up of a driving motor which runs at almost constant speed and powers a d.c. generator .....

  13. Danger of electric shock to human body
    14. The resistance of the human body is quite high only when the skin is dry. The danger of electric shock is therefore much greater for persons working in a hot, humid atmosphere since this leads to wetness from body perspiration.....





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