Ship electric motor can be directly connected to the network, and such direct-on-line (DOL) motors are normally three-phase asynchronous, or induction motors. The ship asynchronous motor has a rugged and simple design, where the three-phase stator windings are similar to a generator stator winding. The rotor is cylindrical, with a laminated iron core and a short-circuited winding similar to the damper winding in a synchronous machine.
At no-load, the voltages imposed to the stator winding will set up a magnetic field in the ship electric motor, which crosses the air gap and rotates with a speed given by the frequency of the imposed voltages, called synchronous frequency, fs. The synchronous speed ns is hence:
ns = fs*60 / p/2 [RPM]
As the shaft gets loaded, the rotor speed will decrease, and there will be induced currents in the rotor winding since they are rotating relatively to the synchronous rotating magnetic field from the stator windings. One defines the slip, s, as the relative lag of motor speed to the synchronous speed ns:
s= ns-n / ns
Hence the slip varies from 0 (no-load) to 1 (blocked rotor)
The slip at rated load is normally below 0.05 (5%) for most motor designs, and even lower (2-3%) for large motors.
Stator currents and shaft torque for an asynchronous motor on ship power system, which is connected to a fixed frequency and stiff ship network, as function of rotor speed or slip. It also indicates the load curve for a typical CPP ship thruster application at zero pitch and full pitch. Start-up of a ship thruster motor should always be done with zero pitch in order to make sure that sufficient torque margin is available to secure start-up, and to minimize the starting time.
Under stationary conditions, the motor speed is close to synchronous speed, and the induced rotor currents are nearly proportional to the slip, and also the shaft torque.
When the slip approaches the peak torque and higher, the assumptions are not valid any longer, since the effects from neglecting leakage inductances becomes considerable, and the load stator current will typically follow a characteristics as shown, typically with a quite flat current amplitude of approximately five times nominal current (locked rotor current).
Connecting Modes (Soft Starters) of Ship Electric Motors
Due to the high starting current of ship asynchronous machines, it will often be necessary to install devices for soft starting. Soft starters typically reduce the locked rotor currents from 5 times to 2-3 times nominal current, and there by also reduce the voltage drop on ship network. Soft starters on ship electrical system must always be adapted to the load characteristics, as their principle is based on reducing the ship motor voltage at start-up, and hence reducing the torque capability of the motor.
The most commonly used are:
Direct On-Line (DOL) starting Ship Electromotor:
– Start and running: A switch closed
Used for motor ratings relatively low compared to running ship generator capacity, where electrical start-up transients are within acceptable limits and the load permits the starting torque transients.
Star-Delta (Y-D) or Wye-Delta coupling start Ship Electromotor:
– Start: A switch and C switch closed, B switch open
– Running: A switch and B switch closed, C switch open
Reduces the starting current to about 1/3 of direct on line starting, but also reduces the starting torque similarly. The method can be used to reduce electrical start-up transients to
The starting torque of the load must be low in order to ensure acceptable acceleration.
Autotransformer start Ship Electromotor:
– Start: B switch and C switch closed, A switch open
– Running: A switch closed, B switch and C switch open
Reduces the starting current, dependent on the ratio of the ship autotransformer. The current on ship network is smaller than the motor current, given by the ratio of the ship transformer.
The starting torque is being reduced and must be checked to ensure acceptable acceleration of the load.
Semiconductor (thyristor) soft-starters of Ship Electromotor:
– Start: B switch closed, soft-starter used to control motor voltage.
– Running: Normally a by-pass switch A switch to reduce losses.
Used to reduce the electrical starting transients in the ship electrical network.
The controls of the soft-starter can be utilized to program various acceleration ramps and adapt to possible constraints of the load. Since the starting torque is being reduced by the square
of the motor voltage, the start characteristics must be carefully tuned in order to ensure acceptable acceleration.
During start-up, the soft starter generates 6-pulse harmonic distortion of the ship supply voltage.
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