The direct on line ship motor will rotate with a speed directly determined by the ship network frequency.
For propulsion, thrusters, pumps, winches, etc., there might be significant savings in ship power or fuel consumption of ship generators by reducing the no-load dependent losses in operations. Also, controllability of the driven load will be greatly enhanced by controlling the speed of the ship motor. The penalty is primarily economically, by introducing additional investment costs, and also components that require maintenance. Reduced operating costs or increased earnings should pay back the additional investment if the investment shall be justified. With energy cost (including fuel, maintenance, taxes, etc.) of 1NOK per kWh for generated power onboard a vessel, one will save 8.76 million NOK (approx 1.1 million USD) annually for a 1MW average power reduction.
The most commonly used ship motor drives are:
– Voltage source inverter (VSI) type converters for AC motors, i.e. asynchronous, synchronous and permanent magnet synchronous motors.
– Current source inverter type (CSI) converters for AC motors, normally synchronous motors
– Cycloconverters (Cyclo) for AC motors, normally for synchronous motors
– DC converters, or SCR (Silicon Controlled Rectifier) for DC motors
In ships, the most used variable speed drives uses AC motors. Most ship drives, except the cycloconverter, will consist of a rectifier, which rectifies the line voltage, and an inverter, which generates the variable voltage source for the ship motor.
A ship motor controller contains the speed control, and the control of motor currents by controlling the switching elements of the rectifier and/or inverter. An interface to an overriding ship control system, vessel management system, ship maneuvering control, or ship dynamic position control is normally required.
The vessel motor controller acquires measurement signals and feedback signals from sensors in the drive, and motor. Typically ship motor currents, motor speed, and in some cases temperatures and voltages are measured.
Schematics of a variable speed drive, showing a frequency converter with DC Link, typically for VSI and CSI type converters.
The ship semiconductor components of the ship power electronic circuits are either uncontrollable (diodes) or controllable (thyristors, IGBTs, IGCTs). Low voltage IGBT module, containing all switching elements for a 690V inverter module, and a discrete medium voltage IGCT used in 3300V inverters.
A motor can, if designed for it, run in both directions, with either a driving (motoring) or braking shaft torque. In order to categorize what conditions the ship motor drive is designed for, the quadrant terms are often applied. The quadrants refer the four quadrants of a speed-torque diagram.
The quadrants refer to the torque-speed diagram ship motors
The ship motor is motoring, i.e. running the load with power input to the load shaft in quadrants I and III. Oppositely, the ship motor is braking, i.e. mechanical power is transferred from the load to the drive, when operating in quadrants II and IV.
The ship motor drive normally comprises a speed control function, and the output from this control function can be interpreted as a torque command or reference, which is the input to the ship motor control algorithms. These algorithms use a more or less advanced ship motor model to control the ship motor currents and voltages by turning on or off the switching elements of the rectifier (if controllable) and the inverter.
In principle, the controllers will normally have a control block diagram. Torque control of ship motor is achieved by removing the speed control loop, and give torque reference as a direct input to the ship motor drive as shown with dashed lines. Ship motor speed is normally measured, but new ship motor controllers are equipped with ship motor speed estimator, which eliminate the need for a dedicated speed sensor for most ship applications.
Generic and typical control block diagram for a ship motor drive controller
For most practical reasons, the speed control loop of a ship motor drive can be regarded as a PI (or PID) controlled closed loop with an inner closed torque control loop, which for control purposes be regarded as a first order time lag.
For simulations and synthesis of overriding control loops, the simplified block diagram should be applicable.
Block diagram for a ship motor drive controller
Simplified block diagram for simulations and synthesis of overriding control loops