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. An 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 an auto-transformer to improve the torque and current characteristics.
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What is an Induction Motor?
An induction motor or asynchronous motor is an AC motor in which the electric current in the rotor needed to produce torque is obtained by electromagnetic induction from the magnetic field of the stator winding.
There are basically two types of an induction motor:
1. Single Phase Induction Motor:
Single phase induction motor: As the name suggests, this type of motor is supplied with single phase power supply. AC current is carried in the main winding of the motor. The type of single phase induction motor used depends on the starting arrangement they use as an auxiliary as they are not self-starting.
Single phase induction motors are mostly used in low power applications, some of those are mentioned below:
- Small pumps
- Small portable compressors
- Small fans
- Drilling machines
2. 3 Phase Induction Motor:
These 3 phase motor is supplied with 3 three-phase AC supply and is widely used in ships for heavier loads. 3 phase induction motors are of two types, squirrel cage and slip ring motors. Squirrel cage motors are widely used on ships due to their rugged construction and simple design, few e.g. of their applications are:
- Large capacity exhaust fans
- Engine Auxiliary pumps
- Engine blower fan motor
- Engine room heavy load pumps –Ballast, Fire, Freshwater, Sea Water etc.
- Winch motor
- Windlass motor
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Construction of 3 Phase Induction Motor
The main body of the Induction Motor comprises of two major parts:
The stator is made up of a 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.
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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
The rotor consists of a cylindrical laminated core with parallel slots that carry conductor bars.
Conductors are heavy copper or aluminium bars which fit in each slot. 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 reasons:
- They reduce magnetic hum or noise
- They avoid stalling of the 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 the 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.
Advantages of Induction Motor
The motor construction and the way electric power is supplied give the induction motor several benefits such as:
– They are robust and simple in construction with very few moving parts
– They can efficiently operate in a rugged and harsh environment such as in seagoing vessels
– The maintenance cost of 3 phase induction motor is less and unlike that of DC or synchro motor, they do not have parts like brushes, commuters or slip rings etc.
– An induction motor can operate in an intrinsic environment as they do not have brushes which can cause the spark and can be dangerous to such atmosphere
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– 3 Phase induction motor does not need any additional starting mechanism or arrangement as they can generate self-starting torque when three-phase AC supply is provided to them, unlike synchronous motors. However, single-phase induction motor needs some auxiliary arrangement for the starting torque
– The final output of a three-phase motor is nearly 1.5 times the rating (output) of a single phase motor of the same size.
Disadvantages of 3 phase Induction motor:
– During starting, it draws high initial starting current when attached to a heavy load. This causes a dip in voltage during the starting period of the machine. Soft starting methods are connected to the 3 phase electric motor to avoid this problem.
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– Induction motor operates at lagging power factor which results in increased I2R losses and efficiency reduction, especially at low load. To correct and improve the power factor, static capacitor banks can be used with this type of AC motor.
– Speed control of 3 phase induction motor is difficult as compared to DC motors. A variable frequency drive can be integrated with the induction motor for speed control.
Problems in 3 phase Induction motor:
Like any other machinery, a 3 phase induction motor can face various types of challenge which can be broadly classified as:
A) Environmental related faults: The harsh environment of the sea may take a toll on ship’s machinery at an early stage if they are not properly maintained. The ambient temperature and the moisture content in the air at sea will affect the operational performance of the induction motor.
The motors are installed on other big machinery (main engine) having their own vibrating frequency, which affects the motor parts.
A wrong installation or loose foundation of the motor or the load it is connected to may also lead to the reduction in the motor efficiency and on a longer run- a failure of the motor.
B) Electrical-related faults: Problem occurs in the motor due to electrical supply faults such as the unbalanced supply of current or the line voltage, earth fault in the system, the problem of single phasing, short circuit etc. Different types of electrical faults are:
Winding Fault: The winding provided in the stator may fail due to the problem in the insulation caused leading to short circuit.
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Single Phasing Fault: When any one or more than one phase of the 3 phase supply is lost, a running 3 phase motor will continue to run but at elevated parameters of temperature and loss. This condition is known as single phasing.
Crawling: This is a mix of electrical and mechanical fault where the induction motor runs at a lower speed (nearly 1/7th of its synchronous speed) even at full load application. It is the result of abnormal magnetomotive force or high harmonic content in the power supply to the motor.
C) Mechanical-related faults: The motor consists of several mechanical parts, and their alignment with each other and with the load plays an essential role in motor efficiency. Some of the prominent mechanical related motor faults are:
- Imbalance Rotor: The rotor is the only moving part in a 3 phase induction motor. If there is an imbalance in between the shaft rotational axis and weight distribution axis of the rotor, it will create vibration, additional heat and efficiency loss in the system.
The unbalance can be due to defect in the rotor, internal misalignment, bending of the shaft, uneven loading and problem in the motor and load coupling.
- Fatigue Failure: If the maintenance schedule is not proper or the parts used in the motor are of poor quality, weakening of material may lead to fatigue failure which is usually caused by repeatedly applied loads.
- Bearing failure: The motor is fitted with two bearings at rotor’s each ends for supporting and freely rotating the shaft. The bearing can fail if timely maintenance is not done or due to overloading, wrong installation, contaminated lube oil and operating at an excessive temperature.
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- Corrosion: The surrounding of the motor installed in a ship is highly corrosive. As the motor consists of several mechanical parts like the rotor, bearing etc. the moisture present in the atmosphere or the water present in the lubrication (grease) will corrode the bearings, motor shaft and rotors. The insulation can also be affected by the corrosion, and lead to a short circuit between the windings
- Lubrication problem: Lack of lubrication or contamination of the lubricant can lead to increase friction between parts and bearings may worn out quickly.
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Protections for 3 phase induction motor
Single phasing protection: To tackle this problem, protection devices are used for 3 phase induction motor. All motors above 500 KW are to be provided with protection devices or equipment to prevent any damage due to single phasing. The details of these devices can be found here.
Over-temperature: The winding of the motor can be heated due to problems like overloading or single phasing. Fuses, relays etc. are used to protect the motor from overheating
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Soft Starting: As described above one of the disadvantages of 3 phase induction motor is the high current it draws during the starting period. To protect it from this problem, different starting method are used by integrating the motor with soft starter, DOL, Star delta starter, auto transformer etc.
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By using a soft starter for an induction motor, the mechanical and electrical stresses are reduced, protecting the motor during the time of starting.
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