Marine AC Generators

AC Generator:

Main generator power rating range from 100KW to 2MW at 440v 60Hz AC driven by diesel,steam/gas turbine or propulsion shaft driven.An emergency generator is usually 20KW to 200KW at 440V.Basic principle of an AC generator is simple.
Pairs of electromagnetic poles are driven past fixed coils of wire on stator by a primemover.An alternating emf which has a sinusoidal wave form is induced into each stator phase winding.Size of emf generated depends on strength of magnetic flux and rate at which flux cut the coil. E=n.∅ n is rotational speed of poles, ∅ is magnetic flux.
Frequency f of emf is the no.of waveform cycles per second.This depends on no.of poles and rotational speed. f=n.p

In practice speed is maintained constant by generator's primemover which fixes the output frequency.Constant speed then allows size of generated emf to be controlled by size of pole flux.Emf generated in each phase is 120° out of phase with each other.One end of the each 3 phase windings are connected each other to form neutral or star point.Other end of the windings are connected to outgoing conductors called lines.

 Two main parts of generator are stator and rotor.

Fabricated steel stator frames support stator core and its windings.It is assembled from laminated steel with windings housed in slots around inner periphery of cylindrical core.
Rotor are of 2 types- salient pole type[<1800rpm] and cylindrical type[1800-3600rpm].Salient pole type has projecting poles bolted to shaft hub.Field excitation windings are fitted around each poles.Cylindrical rotor have excitation windings wedged into axial slots around steel rotor.

Inorder to prevent bearing damage from circulating shaft current all non driving end bearings are insulated from earth by a thin layer of insulating material [PTFE-polytetraflouroethylene or teflon] beneath bearing pedestal.Pedestal holding down bolts also must be insulated.
Pedestal bearing is lubricated by an oil ring.



Generator cooling are of two types - 1.air to water cooling [HV system] 2.air to air cooling
Cooling air is forced through ventilation ducts in stator core, between rotor poles and through air gap between stator and rotor.
Temperature detectors are used to monitor the temperature of stator windings ,bearings and the cooling air/water.
While generator is stopped space heaters are provided to avoid internal condensation forming on winding insulation.

Shaft Generator:

Shaft driven generators are driven by main engine to supply power to the mains.The mains must be supplied by constant voltage and frequency by shaft generator even at changing speed of main engine.
Advantages :

• reduced emission
• lower fuel consumption
• reduced noise levels
• reduction in maintenance and lub oil cost

Apparent disadvantage of shaft generator is that it does not have direct frequency control and is determined by ME speed.Frequency must be separately regulated.

• 3∅ rectifier converts AC into DC
• 3∅ controlled inverter converts DC back to fixed output frequency AC.[achieved by sequenced thyristor switching]
• A DC link inductor coil smoothens the current flow[reduce ripples] and act as a current limiter in the event of short circuit fault
• Inverter thyristor is turned ON by +ve current pulse to its gate.Thyristor is turned OFF when it's current is reduced to zero.This is a problem for inverter thyristors when driving into ship's inductive load[about 0.8 pf lagging].Thyristor can switch off only when p.f is unity ie, voltage and current pass through '0' point at the same time[in phase].To over come this leading p.f is injected using synchronous compensator.Addition of synchronous compensator solves problem as sytem works in unity.Hence shaft generator must only supply active power KW.At every instant leading KVAr +Q must match with lagging KVAr -Q of ship's load.Compensation must be automatically controlled and it is done by including a synchronous motor as synchronous compensator whose regulating pf is controlled by regulating it's DC field current.Synchronous compensator also dampens harmonics.Synchronous motor running at no load act as variable capacitor when it's excitation is changed hence called synchronous condensers.

Harmonics:
Harmonic components are periodic wave having a frequency that is integral multiple of fundamental frequency.They are voltages and currents at integral multiple of fundamental frequency 50Hz. ie, 100Hz-150Hz-200Hz etc.
Harmonics are designated by harmonic number.Harmonic with a frequency of 150Hz is 3rd harmonics.They super impose on fundamental frequency distorting it and changing it's frequency.
Harmonics causes overheating of transformers,cables,motors,generators,capacitors etc.Electronic displays and lighting will flicker,breakers can trip,computers may fail etc.
Harmonic filters avoid/bypasses harmonics generated.