The popularity of 3 phase induction motors on board ships is because of their simple, robust construction, and high reliability factor in the sea environment. A 3 phase induction motor can be used for different applications with various speed and load requirements.
The ship’s generator 3 phase AC supply can be connected to the AC induction motor via a starter or any other arrangement like auto transformer to improve the torque and current characteristics.
The main body of the Induction Motor comprises of two major parts:
Stator is made up of number of stampings in which different slots are cut to receive 3 phase winding circuit which is connected to 3 phase AC supply.
The three phase windings are arranged in such a manner in the slots that they produce a rotating magnetic field after AC supply is given to them.
Usually, windings are kept at different pitch circle with 30 % overlap to each other.
The windings are wound for a definite number of poles depending upon the speed requirement, as speed is inversely proportional to the number of poles, given by the formula:
Where Ns= synchronous speed
f = Frequency
p = no. of poles
Rotor consists of cylindrical laminated core with parallel slots that carry conductor bars.
Conductors are heavy copper or aluminium bars which fits in each slots. These conductors are brazed to the short circuiting end rings.
The slots are not exactly made parallel to the axis of the shaft but are slotted a little skewed for the following reason:
- They reduces magnetic hum or noise
- They avoid stalling of motor.
Principle and working
When 3 phase supply is given to the motor, the resulting current generates a magnetic flux “Ø”.
Due to the switching sequence of 3 phase current in R, Y and B, the generated flux rotates around the rotor conductor.
According to Faraday’s law which states that –“an emf induced in any closed circuit is due to the rate of change of magnetic flux through the circuit”.
Emf is induced in the Copper bar and due to this, current flows in the rotor. The direction of rotor can be given by Lenz law which states that – “the direction of induced current will be in the opposite of the motion causing it”
Here the relative velocity between the rotating flux and static rotor conductor is the cause of current generation; hence the rotor will rotate in the same direction to reduce the cause i.e. the relative velocity, thus rotating the rotor of the induction motor.
Book on Marine electro-technology by Mc George