IMPORTANT REQUIREMENTS FOR ELECTRICAL MACHINARY

 

Machinery requirements

It is a standard requirement that all propulsion and auxiliary machinery fitted should be capable of operating when upright and when inclined at an angle of list up to 15o either way under static conditions and 22 ½o under dynamic conditions either way and when simultaneously inclined dynamically 7 1/20 by bow and stern. The emergency generating sets shall be capable of functioning when the ship is inclined 22 1/2o from upright and inclined 100 bow to stern. The two main factors of concern are lubrication and the functioning of contactors, switchgear and relays having unsymmetrical or unbalanced magnetic systems when the magnetic pull required to operate increases with tilt. Apparatus, such as transformers or switches, containing oil could be affected.

Temperature effects

Extremes of temperature will affect the performance and the effective life of the electrical apparatus. Devices which depend on electromagnetic operation by shunt coils will find resistance of the coil increases with temperature so with less current both the ampere turns and the field strength is reduced.

Contactors and relays may fail to operate correctly if overheated.

The total temperature is determined partly by ambient air temperature and partly by heating effect of the current windings.

This heating effect gives a temperature rise and this is always about the same for similar load.

The total temperature, which will affect the life of the insulation and the performance of the equipment, will be maximum at the maximum ambient temperature. For unrestricted service the cooling air temperature is 45^oC . For restricted service and vessels intended for northern and southern waters outside of the tropical belt the temperature is 40^oC.

Adequate ventilation and avoidance of hot pockets where electrical apparatus operates is important.

When considering suitable operating temperature for a device the 'hot spot temperature' is important. In the field coil the hot spot is somewhere in the centre of the winding and there is a temperature gradient form there to the surface. Previous recorded surface temperature values corresponding to specified hot spots temperature are acceptable for recording the machines performance. Another method is to record changes in resistance due to temperature in the winding.

When carrying out temperature tests on machines the maximum surface temperature of the windings is found just after the machine has stopped and it is no longer cooled by windage. The temperature bulb should be covered by a pad of felt to prevent heat loss when the surface winding readings are taken. 

Installation and maintenance

To reduce end play and avoid hammering during rolling machines should be installed with their axis of rotation in the fore and aft direction or vertically. If unavoidable that the machine is placed athwartships suitable thrust bearings should be provided against the hammering effect. Special attention should be paid to the lubrication of ring lubricated sleeve bearings.

The main cause of overheating in electrical joints is loose connections usually due to vibrational problems. All screws and nuts should be locked and periodically checked and tightened if necessary.

Heavy current circuits, control and shunt field circuits should all be checked.

Machine rating

The recognised standard is the Continuous maximum rating (C.M.R.), motors and generators are seldom if ever called upon to operate under sustained overload.

Momentary overloads (15s for test purposes) of 50% in generators is allowed. Motor overload is determined by function and size.

C.M.R. machines will still carry moderate overloads for reasonable duration's. An example of this may be an oil pump on start up may experience high loads as the oil is initially cold. 

Circuit protection

For example;

• Motor drawing 100A on 220v supply 218v measured at motor terminals giving a 2 volt drop across cables
• Cable resistance therefore is 0.02 Ohm's.

If the motor is bypassed the PROSPECTIVE SHORT CIRCUIT current would be 11,000A

The main circuit breaker may be protected by fuses or a circuit breaker having at least the necessary breaking capacity and fast enough operative time. This is 'back up' protection. Generator circuit breakers must not be used for this purpose.

In motor circuits the breaking capacity of motor starters is usually very limited and does not greatly exceed the starting current of the motors, If a fuse is fitted for 'back up' protection of the motor starter it should be able to carry the starter current for the time necessary to start the motor plus a suitable margin. If correctly chosen it will not blow except under maximum mechanical fault or electrical fault or overload conditions. It will still give protection should the fault current exceed what the motor starter can handle.

If A.C. generators and their excitation systems undergo steady short circuit conditions they should be capable of maintaining a current of at least three times its rated value for 2 seconds unless requirements are made for a shorter duration. The safety of the installations must be insured.

Performance

The standard condition for generator performance is based on the starting kVA of the largest motor, or group of motors which can be started simultaneously and this kVA should not exceed 60% of the generator capacity.

Voltage should not fall below 85% or rise above 120% of the rated voltage when such a load have a power factor from zero to 0.4 is thrown on or thrown off the board.

Voltage must be restored to within 3% of the rated voltage within 1.5s. For emergency generators 4% in 5s is allowed. The transient effect when a load is suddenly thrown on is to cause a voltage dip. This dip may be made less if the generator is designed to have a lower reactance during transient conditions. However, too low a reactance with a smaller voltage dip may involve high short circuit currents in excess of capabilities of the available protective devices.

The designer must consider the opposing conditions of low transient voltage dip and low short circuit currents and balance these conditions against possible increase in machine size, weight and cost.

Functional systems generally operate faster than error operation systems. Nevertheless most functional systems use an A.V.R. for trimming purposes because of practical difficulties of maintaining normal voltage within narrow limits. Methods normally supplied will maintain voltages within +/- 2 ½ % with many attaining +/- 1 ½ %