10-04-2017, 08:01 PM
POWER ELECTRONICS IN SHIP PROPULSION ELECTRIC MOTORS
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INTRODUCTION :
The predominant type of prime movers for DP propulsion plant is Electric Drive .Practically every DP propulsion device installed in newly constructed vessels as well as in most of the conversions is driven by an electric motor. In recent years, variable speed AC Drives has become available and has been used in some applications for DP propulsion. The most commonly used motor drives are:
DC converters, or SCR for DC motors.
Cyclo converters for AC motors ,normally for Synchronous motors .
Current Source Inverters (CSI) for Ac motors (Synchronous Motors )
Voltage Source Inverters ,(VSI)for AC motors ,i.e. asynchronous ,synchronous and permanent magnet synchronous motors.
SCR CONTROLLED DC DRIVE AND FIXED PITCH PROPELLER
Ac controlled produced by Diesel generators at constant voltage (600 VDC max) and frequency.
A fixed pitch propeller is driven by the DC propulsion motors .Varying the propeller RPM and reversing the rotation of the propeller shaft allow the thrust to be controlled in magnitude and direction.
DISADVANTAGES:
The limitation of voltage (max voltages are 600V AC/750V DC) =>leads to heavy, expensive motors , cables runs .
Commutator wear =>higher maintenance requirements of the DC motors.
Practical limit for DC motor drives is 2-3 MW.
The power factor will vary from 0-0.92( =15 degrees) correspondent to 0-100% propeller rpm.
APPLICATIONS:
Introduction of the high current SCR in the late 1960s, the AC/DC electric propulsion drive has become quite popular.
Vessels with this type of propulsion include fishing trawlers and factory vessels, research vessels, ice breakers, off shore supply vessels, conventionally moored and dynamically positioned drill vessels, and semisubmersibles .