Types of Induction Motor MCQ Quiz in தமிழ் - Objective Question with Answer for Types of Induction Motor - இலவச PDF ஐப் பதிவிறக்கவும்

AC motors can be divided into two categories: synchronous and asynchronous

• For a synchronous motor, the rotor turns at the same rate as the stator’s rotating magnetic field, which is referred to as the synchronous speed.
• In an asynchronous motor (also referred to as an induction motor), the rotor turns at a speed slower than the stator’s rotating magnetic field.
• Induction motor can be classified as squirrel cage and slip ring induction motor.

Classification of DC machine (generator/motor) is given below:

• Squirrel cage type
• Wound type
   

Squirrel cage rotor:

• It consists of a laminated cylindrical core having parallel slots on its outer periphery.
• One copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings.
• This forms a permanently short-circuited winding which is skewed by some angle.
  
• The rotor is not connected electrically to the supply but has current induced in it by transformer action from the stator.
• Most of the 3-phase induction motors use squirrel cage rotors because of their simple and robust construction enabling them to operate in the most adverse circumstances.
• It suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque. 
   

Slip Ring Rotor or Wound rotor:

• It consists of a laminated cylindrical core and carries a 3-phase star-connected winding.
• The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring.
  
• The three brushes are connected to a 3-phase star-connected resistance by slip ring as shown.
• At starting, the external resistances can be added to the rotor circuit to give a large starting torque.
• These resistances are gradually reduced to zero as the motor runs up to speed.
• The external resistances are used during starting period only.
• When the motor attains normal speed, the three brushes are short-circuited so that the wound rotor runs like a squirrel cage rotor.

Skewing of Squirrel cage Induction motor: 

• Eliminate harmonic
• Noise reduction
• To produce uniform torque
• Reduce the magnetic pulling and eliminate locking tendency of the rotor
• Better voltage form
• Increases resistance of rotor to get high starting torque
• The main purpose of skewing is to reduce the magnetic logging(Cogging) between the starter and the rotor.

Key Points

Rotor skew angle formula:

\( = \frac{{720}}{n}\left( {\frac{p}{2}} \right)\) electrical degree

p = number of pole 

n = order of harmonics

The correct answer is option 3):(To ensure that insertion of external resistance is not possible )

Concept:

• A squirrel-cage rotor consists of thick conducting bars embedded in parallel slots These bars are short-circuited at both ends by means of short-circuiting rings
• Shorting of rotor bars in form of a squirrel cage causes a suitable low-impedance path for rotor-induced currents. The impedance characteristics (R & X) of rotor winding infects the motor performance, starting torque, and total losses consequently motor efficiency.
• The rotor bar of the squirrel cage rotor is permanently short-circuited at the ends to ensure that insertion of external resistance is not possible 
• The rotor slots are usually not quite parallel to the shaft but are purposely given a slight skew It helps to make the motor run quietly by reducing the magnetic hum It helps in reducing the locking tendency of the rotor i.e.The tendency of the rotor teeth to remain under the stator teeth due to direct magnetic attraction between the two

Squirrel-cage Rotor:

• Almost 90 percent of induction motors are the squirrel-cage type because this type of rotor has the simplest and most rugged construction imaginable and is almost indestructible.
• The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor conductors which, it should be noted clearly, are not wires but consist of heavy bars of copper, aluminum, or alloys.
• One bar is placed in each slot, rather the bars are inserted from the end when semi-closed slots are used.
• The rotor bars are brazed or electrically welded or bolted to two heavy and stout short-circuiting end-rings, thus giving us, what is so picturesquely called, a squirrel-case construction.
• It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any external resistance in series with the rotor circuit for starting purposes.
   

Advantages of Squirrel Cage Induction Motor:

• They are low cost.
• Require less maintenance as there are no slip rings or brushes.
• The slot space factor is the ratio of the conductor is per slot and slot area. For Squirrel cage induction motor it has a better slot space factor due to that it has less amount of copper loss as compare to the Wound type rotor.
• Good speed regulation and they are able to maintain a constant speed.
• High efficiency in converting electrical energy to mechanical energy.
• Have better heat regulation.
• Small and lightweight.
• Explosion-proof (as there are no brushes that eliminate the risks of sparking)

The speed of the induction motor can be controlled by any of the following methods:

1.) V/f control or frequency control

• The basic idea of a V/f control is to maintain the stator flux constant. In order to operate the machine under nominal conditions, the stator flux must be nominal.
• This control method is commonly applied to blower fans and centrifugal pumps.

2.) Pole changing method

• Pole Changing Method is one of the main methods of speed control of an induction motor.
• This method of controlling the speed by pole changing is used mainly for the cage motor only because the cage rotor automatically develops a number of poles, which is equal to the poles of the stator winding.
• This method is used in AC locomotives.

3.) Supply voltage method

• The speed control of a three-phase induction motor is obtained by changing the supply voltage until the torque required by the load is developed at the desired speed.
• The torque developed by the induction motor is directly proportional to the square of the supply voltage and the current is proportional to the voltage.
• Therefore, the stator voltage control method is suitable for applications where the load torque decreases with the speed, as in the case of a fan load.

4.) Cascade control method

• ​In this method of speed control, two motors are used. Both are mounted on the same shaft so that both run at the same speed.
• One motor is fed from a 3-phase supply and the other motor is fed from the induced emf in the first motor via slip-rings.

Slip Ring Induction Motor:

• The slip ring induction motor is one of the types of 3-phase induction motor and is a wound rotor motor type.
• Because of various advantages like low initial current, high starting torque, and improved power factor, it is used in applications that require high torque, cranes, and elevators.
   

Pipe ventilated machine: A machine in which there is a continuous supply of fresh ventilating air, the frame being so arranged that the ventilation air may be conveyed from the machine through pipes or ducts attached to the enclosing case

DC Compound Motor:

• The compound motor has high starting torque and nearly constant speed.
• Because of that Compound motors are used where we require high starting torque and constant speed.
• A compound motor is used in Presses, Shears, Conveyors, Elevators, Rolling Mills, Heavy Planners, etc.
   

Application of Squirrel Cage Induction Motor:

• Squirrel cage induction motors are commonly used in many industrial applications.
• They are particularly suited for applications where the motor must maintain a constant speed, be self-starting, or there is a desire for low maintenance.
• These motors are commonly used in:
  • Centrifugal pumps
  • Industrial drives (e.g. to run conveyor belts)
  • Large blowers and fans
  • Machine tools
  • Lathes and other turning equipment

Additional Information

Advantages of Squirrel Cage Induction Motor:

• They are low cost
• Require less maintenance (as there are no slip rings or brushes)
• Good speed regulation (they are able to maintain a constant speed)
• High efficiency in converting electrical energy to mechanical energy (while running, not during startup)
• Have better heat regulation (i.e. don’t get as hot)
• Small and lightweight
• Explosion-proof (as there are no brushes that eliminate the risks of sparking)

• Squirrel cage type
• Wound type
   

Squirrel cage rotor:

• It consists of a laminated cylindrical core having parallel slots on its outer periphery.
• One copper or aluminum bar is placed in each slot. All these bars are joined at each end by metal rings called end rings.
• This forms a permanently short-circuited winding which is skewed by some angle.
  
• The rotor is not connected electrically to the supply but has current induced in it by transformer action from the stator.
• Most of the 3-phase induction motors use squirrel cage rotors because of their simple and robust construction enabling them to operate in the most adverse circumstances.
• It suffers from the disadvantage of a low starting torque. It is because the rotor bars are permanently short-circuited and it is not possible to add any external resistance to the rotor circuit to have a large starting torque.
   

Slip Ring Rotor or Wound rotor:

• It consists of a laminated cylindrical core and carries a 3-phase star-connected winding.
• The open ends of the rotor winding are brought out and joined to three insulated slip rings mounted on the rotor shaft with one brush resting on each slip ring.
  
• The three brushes are connected to a 3-phase star-connected resistance by slip ring as shown.
• At starting, the external resistances can be added to the rotor circuit to give a large starting torque.
• These resistances are gradually reduced to zero as the motor runs up to speed.
• The external resistances are used during starting period only.
• When the motor attains normal speed, the three brushes are short-circuited so that the wound rotor runs like a squirrel cage rotor.

• The frame of an induction motor is made up of closed grained cast iron.
•  The shaft of the Induction motor is made up of mild steel/carbon steel.

Various parts of the  induction motor are

• Stator
• Rotor
• Fan
• Bearing

Stator:

• It consists of a 3 phase winding with a core and metal housing.
• Windings are such placed that they are electrically and mechanically 120° apart from in space.
• The winding is mounted on the laminated iron core to provide a low reluctance path for generated flux by AC currents.
• The air gap between the stator and rotor is uniform and symmetrical.

Rotor:

• It is the part of the motor which will be in a rotation to give mechanical output for a given amount of electrical energy.
• The rated output of the motor is mentioned on the nameplate in horsepower.
• It consists of a shaft, short-circuited copper/aluminum bars, and a core.
• The rotor is made of non-insulated copper conductors in rod or bar formats & short-circuited with end rings. So it is also called a short-circuited rotor.
• Therefore in the case of the squirrel cage induction motor, it is not possible to add any external resistance in series with the rotor circuit for starting purposes.
• The rotor core is laminated to avoid power loss from eddy currents and hysteresis.
• It does not require a slip ring and brush assembly. 
• Conductors are skewed to prevent cogging during starting operation and give a better transformation ratio between stator and rotor.

Fan: The fan is attached to the backside of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit.

Bearing and shaft assembly:

• The bearings support the rotor.
• It maintains the air gap and transfers the shaft load to the motor casing.
• The rotor core-winding assembly is mounted over the shaft.
• The shaft extends outside the casing and transfers the mechanical energy from the rotor to the load.
• In case of maintenance of motor, Bearings require maximum attention as they are more exposed to wear & tear.

• Bearings acts as a medium between stator body and rotor due to which rotor is able to rotate