The majority of new buildings and all commercial vessels have an AC power generation plant with AC distribution. The generators are synchronous machines, with a magnetizing winding on the rotor carrying a DC current, and a three-phase stator winding where the magnetic field from the rotor current induces a three-phase sinusoidal voltage when the rotor is rotated by the prime mover.
The frequency f [Hz] of the induced voltages is proportional to the rotational speed n [RPM] and the pole number p in the synchronous machine:
f= p/2 * n/60
A two-pole generator will give 60Hz at 3600RPM, a four-pole at 1800RPM, and a six-pole at 1200RPM, etc.
50Hz is obtained at 3000RPM, 1500RPM, and 1000RPM for two-, four-, and six-pole machines. A large medium speed engine will normally work at 720RPM for 60Hz network (10 pole generator) or 750RPM for 50Hz networks (8 pole generator).
The DC current was earlier transferred to the magnetizing windings on the rotor by brushes and slip rings. Modern generators are equipped with brushless excitation for reduced maintenance and downtime, brush-less excitation machine is an inverse synchronous machine with DC magnetization of the stator and rotating three-phase windings and a rotating diode rectifier. The rectified current is then feeding the magnetization windings.
The excitation is controlled by an automatic voltage regulator (AVR), which senses the terminal voltage of the generator and compares it with a reference value. Simplified, the controller has PID characteristics, with stationary limited integration effect that gives a voltage drop depending on the load of the generator.
The voltage drop ensures equal distribution of reactive power in parallel-connected generators.
According to most applicable regulations, the stationary voltage variation on the generator terminals shall not exceed ±2.5% of nominal voltage.
Also, the largest transient load variation shall not give voltage variation exceeding -15% or +20% of the nominal voltage unless other has been specified and accounted for in the overall system design. In order to obtain this transient requirement, the AVR is normally also equipped with a feed-forward control function based on measuring the stator current.
In addition to the magnetizing winding, the rotor is also equipped with a damper winding which consists of axial copper bars threaded through the outer periphery of the rotor poles, and short circuited by a copper ring in both ends. The main purpose of this winding is to introduce an electromagnetic damping to the stator and rotor
dynamics.
A synchronous machine without damper winding is inherently without damping and would give large oscillations in frequency and load sharing for any variation in the load.
The stationary, transient and sub-transient models are known from the theory of synchronous machines.
Simplified one could say that the flux linkages in the damper winding, which are “trapped” and resist changes due to being short-circuited, characterize the sub-transient interval.
This is observed as an apparent lower inductance in the generator, which gives a stiffer electric performance during quick load variations, and helps to reduce
transient voltage variations and the voltage variations due to harmonic distortion in load currents.
This effect is only contributing for dynamic variations faster than characterized by the sub-transient time constant such as the first period of motor start transients and transformer inrush, and for harmonic distorted load currents.
Often, the generators are connected to a propulsion engine’s shaft, i.e. a shaft generator.
The shaft generators are in some applications made for two-directional power flow, which means that it can be run as motor.
This principle may be called a PTI-PTO concept (Power take-in – Power take out). Shaft generators have the disadvantage of forcing the main propeller to work at fixed speed if the generator output shall have constant frequency. This will reduce the efficiency of the propeller in low load applications.
Static converters may be installed to keep fixed frequency for variable speed.
