Answer MET Question 55

 Question:  Draw and Explain a Variable Frequency Drive used for optimization of energy efficiency of auxiliary machineries on board vessels.

Answer: Variable frequency drive or VFD

VFDs are specific types of variable speed drives, in which speed is varied and controlled by controlling frequency of the input a.c supply to the motor. Ship auxiliaries that can run on reduced output when not really necessary to run at maximum speed can be controlled to run at desired output level thus reducing the power consumption of the equipment which leads to the reduction in fuel consumption. 

Equipment that can be run on reduced load are:

1. Main Engine Auxiliary blowers.

2. Engine room air supply fans.

3. Boiler and incinerator FD fans.
4. Cooling water system pumps for engines. etc.

Since an induction motor rotates near synchronous speed is the most effective and energy-efficient way to change the motor speed is to change the frequency of the applied voltage. VFDs convert the fixed-frequency supply voltage to a continuously variable frequency, thereby allowing adjustable motor speed. The convertion to a new frequency takes place in two stages: the rectifier stage and the inverter stage.

The conversion process incorporates three functions:

1. Rectifier stage A full-wave; solid-state rectifier convert three- phase 50 Hz power from a standard 440 V or higher utility supply to either fixed or adjustable DC voltage. The system may include transformers If higher supply voltages are used.

2. Inverter stage Electronic switches - (power transistors or thyristors) switch the rectified DC on and off, and produce a current or voltage waveform at the desired new frequency.The amount of distortion depends on the design of the inverter and filter.

3. Control system: An electronic circuit receives feedback information from the driven motor and adjusts the output voltage or frequency to the selected values. Usually the output voltage is regulated to produce a constant ratio of voltage to frequency (V/Hz). type