Squirrel Cage Induction Motor Fundamentals

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Ac squirrel cage induction motor: here you know complete fundamental related to squirrel cage induction motor. Here you will know the Squirrel cage induction motor working principle, construction, application and advantages of the squirrel cage induction motor.

Introduction

There are two main types of induction motors exists. They are,

  1. Squirrel cage induction motor
  2. slip ring induction motor

In previous, we saw complete details related to slip ring induction motor. Here we know the fundamental details related to squirrel cage motor.

What is squirrel cage induction motor?

In simple words, the induction motor which uses a squirrel cage rotor is called squirrel cage induction motor. The reason behind the name “squirrel cage” is because of the type of rotor used in these motors. In these type of motors, the rotor is simplest and most rugged in construction.

These motors have much higher efficiency than slip ring induction motors. Most of the industries prefer these type of motor due to less maintenance cost, higher efficiency, and their lightweight construction. Let see the construction of squirrel cage induction motor.

Squirrel cage induction motor construction

Any Induction Motor has main two parts: a Stator and a Rotor. The construction of Stator of the induction motor is almost the same as other motors. But the rotor construction differs with respect to the type of motor. Slip ring induction motor is made up from wound rotor and squirrel cage induction motor is made up from the squirrel cage rotor.

Stator

The stator is the outer component of the motor which can be seen. A stator is in all the motor only winding on the stator is vary with types of motor.

Squirrel cage induction motor stator

source

In squirrel cage induction motor, there is a 3 phase winding on the stator slots. Windings are such placed that they are electrically and mechanically 120o apart from in space. These windings are either star connected or in delta connected. The winding on a stator is mounted in such a way that it provides low reluctance path for generated flux by A.C current. The insulation between the windings is generally varnish or oxide coated.

Now let’s move toward the construction of the squirrel cage rotor.

Squirrel cage Rotor

Almost 90 % of induction motors are provided with a squirrel cage rotor because of its very simple, robust and almost instructible construction.

squirrel cage induction motor rotor

source

In this type of motor, the rotor is cylindrical core which is laminated to avoid power losses. Squirrel cage rotor is made up of aluminum or copper bars which are placed parallel to each other and all the bars ( conductors ) are short-circuited with end rings. The rotor conductors and end rings form a complete closed circuit. Here rotor core is laminated to avoid power losses from eddy current and hysteresis.

In motor rating up to 100 kW, the squirrel cage rotor is made up from the aluminum cast. In this type of rotor, conductor bars and end rings are permanently short-circuited so we can’t connect any external resistance in rotor circuit for starting purpose. In previous, we saw we can add external resistance in the rotor in slip ring induction motor.

Other parts of motor: A fan is attached to the back side of the rotor to provide heat exchange, and hence it maintains the temperature of the motor under a limit. Bearings are provided as the base for rotor motion, and the bearings keep the smooth rotation of the motor.

now let’s see how squirrel cage induction motor works?

Squirrel cage induction motor working principle

In D.C motors we need to give supply to stator and rotor both for excitation. But here in the induction motor, we only need to give supply to the stator winding for operation. Actually what happens let’s see.

Actually, when we give supply to stator winding then the current starts to flow in the coil will produce a magnetic flux in the coil. Now, here the rotor windings are short-circuited. Induced flux from the stator winding will cut the coils in the rotor, and As the Faraday’s law of electromagnetic induction will cause the current to flow in the coil of the rotor due to the short circuit of the rotor coil.

When current flows through the rotor circuit it generates rotor flux. So here there is two fluxes, one is the stator flux and the other is the rotor flux, and the rotor flux will lag behind the stator flux. Thus, the rotor will feel the torque that rotates the rotor in the direction of the rotating magnetic flux.

In squirrel cage induction motor starting torque is very low. The speed of the rotor will depend on the AC power, and the speed can be controlled by changing the input supply power.

Some features

here we know some key features related speed, starting current, the direction of rotation, slip and power factor of squirrel cage induction motor.

Speed: Squirrel cage induction motor is generally operating at a constant speed. Or we can say in synchronous speed.

Starting current: These motors require high starting current and provide low starting torque.

The direction of rotation: The direction of rotation of these motors can be reversed if you interchange two power lines out of three.

Slip: Slip is a measure of the difference between the speed of the rotating magnetic field and rotor speed. The frequency of the rotor current = slip × supply frequency

Power factor: Power factor is the ratio of actual power to apparent power. It is expressed in percentage. The power factor is low when the motor is running at no load and it is high when the motor is operating at full load.

Squirrel cage induction motor advantages

Squirrel cage I.M has some of the main advantages that why Almost 90 % of induction motors are provided with squirrel cage rotor. So let’s see the advantages :

Squirrel cage I.M are

  • Cheap
  • Robust
  • Require less maintenance

In this motor, the rotor is made up from aluminum bars or copper bars, So less material required for rotor construction And this will also reduce copper losses.

  • High efficiency
  • Small and lightweight
  • Due to an absence of brushes, the chances of spark are reduced.
  • These motors are provided with fans, so less heat is generated.
  • Squirrel cage motors operate at nearly constant speed, high overload capacity, and operates at better power factor.

Disadvantages of squirrel cage induction motor

The main disadvantage of squirrel cage induction motor is poor speed control and low starting torque. However, with the variable frequency drives, this disadvantages can be overcome.

 Squirrel cage induction motors have poor starting torque and high starting currents. Starting torque will be in the order of 1.5 to 2 times the full load torque and starting current is as high as 5 to 9 times the full load current. In slip ring induction motors, higher starting torque can be attained by providing an external resistance in the rotor circuits during the starting period of the slip-ring induction motor. This arrangement in slip-ring induction motors also reduces the high inrush currents during starting of an induction motor.

 Squirrel cage I.M.s are more sensitive to the supply voltage fluctuations. When the supply voltage is reduced, an induction motor draws more current. During voltage surges, increase in voltage saturates the magnetic components of the squirrel cage induction motor.

 Speed control is not easy in these motors. We can’t connect any external resistance in rotor circuit because rotor conductors are permanently short-circuited by end rings.

Squirrel cage induction motor Applications

Squirrel cage I.Ms are widely used than slip ring I.M in industries and in the home due to maintenance cost and higher efficiency. Squirrel cage Induction motors are quite common in the industry. You find them almost in every machine, conveyor everywhere nearly.

Squirrel cage I.M is generally used where we require constant speed and no speed control drives.

These motors are used in

  • Fans and blowers
  • Industrial Drives
  • Lathes and turning equipment
  • Pumps

Difference between a squirrel cage I.M and synchronous motor

There are some differences between Squirrel cage and synchronous motor.

The speed of operation :

For a given frequency, induction motor always run at a speed less than synchronous speed ( Ns ).

For a given frequency, synchronous motor always runs at a constant speed called as synchronous speed whatever the amount of load it may be on it.

Effect of load on the speed of the rotor

The speed of the induction motor is always dependent upon the load, the speed is going to decrease with the increase in load.

The speed of the synchronous motor is not dependent on the load thus it remains constant at any load.

Construction of rotor

In squirrel cage I.M cage, Squirrel cage rotor is used.

In synchronous motor salient, non-salient or permanent magnet is used as a rotor.

External supply

Synchronous motors require an additional DC power source for energizing rotor winding. Induction motors do not require any additional power source.

Cost

Induction motors are cheaper than synchronous motors. Synchronous motors are too much expensive to produce than similar rating induction motor.

Starting of motor

All induction motors are self-starting motors. We only need to give supply to the stator winding.

In the synchronous motor, we need to give external supply to rotate rotor near to the synchronous speed.

Starting torque

Induction motors have its own starting torque. The synchronous motor does not have any starting torque we need provide some auxiliary mean to generate require starting torque.

Speed control

Speed control is possible in case of the induction motor.

We can’t control the speed of the synchronous motor.

Power factor

Induction motors always operate with lagging power factor. Changing in power factor is not possible.

By the D.C excitation power factor can be changed from lagging to leading in case of the synchronous motor.

Conclusion

Hope you know everything fundamental related squirrel cage induction motor.

We saw the construction of the Squirrel cage induction motor. The main difference between slip ring and squirrel cage induction motor is the rotor construction. Motors have main two parts in construction: stator and rotor. In these motor, the rotor is squirrel cage type. This rotor is made up from parallel aluminum or copper bars. Rotor conductor bars are short-circuited with two end rings.

We saw the working principle of squirrel cage induction motor which is the same in all induction motor. When we apply A.C supply to stator winding flux generated in the stator winding. Due to this flux rotor current induced and rotor flux also generated. Rotor flux lag behind the stator flux so that torque generates and rotor feels a force in moving direction.

We saw some of the advantages and disadvantages of Squirrel induction motor. These motors have simple and robust construction. These motors have higher efficiency and low cost because of that in 90% application of induction motor this type of motor is used.

We saw Squirrel cage induction motor Applications. These motors are used where we require constant speed and not require speed control.


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