The induction motor has been a reliable workhorse of the industry ever since it was invented by Nikola Tesla in 1888. It is the most widely used motor because of its simple, brushless, low-cost, and rugged construction. Most induction motors in use are 3-phase in large ratings or 1-phase in small ratings.
The 3-phase induction motor has three stator coils wound with wires, and the rotor in a squirrel-cage configuration. The cage rotor is generally made of cast aluminum bars running along the machine length and two end rings shorting all the bars. The rotor cage in a high-efficiency motor is often made of copper, which has much better conductivity than aluminum.
There is no electrical connection between the stator and the rotor.
The 3-phase current in the three stator coils wound in P-poles configuration
and powered at frequency f creates a magnetic flux that rotates at constant speed ns (called the synchronous speed), which is given by:
The number of magnetic poles created by the stator winding depends on the coil span.
Injecting the rotor current from an outside source, as in the synchronous or dc motor, is not required in the induction motor. The sweeping (cutting) flux of the stator induces current in the rotor bars, which in turn produces the mechanical torque, and hence the name induction motor.
Transfer power from stator to rotor on electric motor
This simplifies the construction to a great deal, as no brushes or slip rings are required. The power transfer from the stator to the rotor is brushless; it is done by magnetic flux as in the transformer.
The cage rotor conductors with end rings constitute numerous shorted coils for the induced currents to circulate. From an analytical view point, the induction motor is essentially a transformer with a shorted secondary coil that can rotate.
Shaft design on ship induction motors
Most induction motors have a horizontal shaft, but vertical-shaft induction motors are also available for places where the footprint space is at a premium, such as on ships.
Special 3-phase motors on ship
Most induction motors have a squirrel-cage rotor, but some special 3-phase induction motors have a rotor wound with wires just like the stator with all three phase terminals brought out through slip rings.
The rotor coil terminals are shorted outside the slip rings in normal operation, or with an external resistance to increase the starting torque or for speed control.
Speed and rotor slip on ship Electrical motor
For 50 and 60 Hz motors, the synchronous speed ns of the stator flux is:
The rotor speed nr is always less than ns, that is, the induction motor
always runs at subsynchronous speed (nr < ns). The motor performance primarily depends on the rotor slip, defined as
Under normal running operation, the slip is typically a small percentage of ns.
If all the load were removed, the motor speed will rise approximately by the slip percentage, although the exact speed regulation is determined by Equation above which applies to any source of mechanical power. Therefore, in the first approximation,
motor speed regulation = slip at rated load
Furthermore, since the rotor sees the stator flux rotating at slip speed, the rotor current frequency is
fr = s × f
where f = frequency of the stator supply (main lines)
Synchronous and supersynchronous ship generator speed
The motor with load on the shaft, no matter how small, cannot run at the synchronous speed. At synchronous speed, s = 0, and there would be no rotor current induced and no torque produced for the motor to run.
At a supersynchronous speed (nr > ns), the rotor sees the flux sweeping in the other direction, reversing the current direction, making the machine work as a generator, converting the shaft mechanical power into electrical power delivered out of the stator terminals to the power lines.
Most wind power installations use induction machine as the generator driven at supersynchronous speed by the low-speed wind turbine with
high gear ratio.