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| slip ring induction motor and double cage induction motor [figure 1] |
Induction Motor
Induction motor is very popular in industrial applications and also traction . Popularity of induction motors is reliability. We know 85% of our system works on AC supply . AC is also always available this tis the main advantage of Induction Motor.
3 phase Induction Motor works on the basis of principle electromagnetic induction.we brief some working principle of induction motor. We know induction motors have two parts: stator and rotor. Stator can make rotation magnetic field which can induce rotor coil . Rotor coil generates emf at the resulting flow current through the coil and the coil can fill a magnetomotive force .
That magnetomotive force sometimes attracts each other or repulses each other. It is not over yet because we need to know how a rotating magnetic field is made in a stator . Because we have 3 phase power supply and we know between phases have some Electrical 120° angle Difference.
It means stator when a coil made mmf then 120° apart another coil made mmf . When the first coil attracts the rotor pole the other coil repulsed Rotor pole. This can continue again. Torque is created for this event.
Types of Induction MotorConstructionaly Induction Motor is two types
1) Slip Ring Induction Motor
II) Squirrel Cage Induction Motor
Slip Ring Induction Motor
The Rotor of Slip ring Induction motor Short circuited by Slip ring and brush . With brush a external Resistance is Installed for speed control this called rotor resistance speed control.
The external Resistance helps to make starting torque with low starting current . The Constructionaly length of the slip ring Induction motor is less than the Squirrel cage Induction Motor. Copper loss is high . efficiency is low compared to Squirrel cage Induction Motor.
It uses high torque required applications such as drilling machines , punch machines etc.
Advantages
> High Starting Torque
> low starting current
> Rotor resistance speed control
Disadvantages
> Market price is High
> Required regular maintenance
> Using the slip ring brushes causes High sparking problem
Squirrel cage Induction Motor
The Rotor of Squirrel cage Induction Motor is short circuit by ending ring . This type of Rotor not support any external Resistance it means . We can not control speed by external Resistance. Squirrel cage induction motor speed control only controls voltage , frequency and changing the poles .
But all speed control method most economical and popular speed control is Voltage and frequency control by Variable Frequency Drive (VFD). Variable frequency control provide precious speed control . The squirrel cage Induction Motor is Copper loss is low and High efficiency . And starting current is high due to low starting torque .
This type of motor use exhaust fan low starting torque required application. The Economical value of Squirrel cage Induction Motor is very cheap Compared to the slip ring induction motor.
The Squirrel cage Induction Motor Rotor slot is skewed because of good magnetic interlocking. If Squirrel cage Induction motor use For heavy load . Then the induction motor rotor uses a double Squirrel cage for better magnetic interlocking.
Advantages
> Less maintenance required.
> Market price is very cheap.
Disadvantages
>Low starting torque .
>Rotor resistance speed control not available.
Double Squirrel cage Induction Motor
Double Squirrel cage Induction Motor is also similar Squirrel cage motor . But the Difference is that it is use for high Starting Torque purpose like a slip ring Induction motor. Here have a two Squirrel cage and both cage are short circuited by end ring .
Constructionally two type Squirrel cages are used, the inner Squirrel cage and outer square cage . Outer cage made by brush material or aluminum for made high resistive and inner cage made by copper for made low resistive.
Main aim of increasing starting torque of any type of Rotor can use either it's slip ring type or Squirrel cage or double cage is low current because low current proportional to low speed and high Torque . That will be see Speed torque characteristics of induction motor .
Double cage Induction Motor outer cage is highly resistive when stator flux cuts first outer cage then inner cage and due to low resistance the starting torque of double cage induction motor is increasing.
If we consider outer cage and inner cage a parallel resistance so the equivalent resistance is not too low or not too high . So the induced current is similar to a slip ring.
The basis of this theory is that a double cage induction motor must have a high Starting Torque and low starting current induction motor .
Advantages
> Cheap price compared with slip ring induction motors.
>Less maintenance.
> High Starting Torque with low current.
Disadvantages
> Loss is compared to other types is huge
>Power factor is high.
Can we use a double cage induction motor instead of a slip ring induction motor?
Answer is yes . Sometimes a double cage induction motor is an alternative to the slip ring induction motor . And about speed control , if we change the frequency of double cage Induction Motor Speed can control.
*Note
[Let's discuss the topic of two kinds of motor first, one slip ring type induction motor and another one double cage type induction motor.
Mainly the similarity of both kinds of Motor is torque and speed, basically both motors are constant speed motor and high starting torque.
We know motors are mainly determined by motor torque , speed and speed control. So the first two specifications are the same for both kinds of motor. Now the question is speed control , in general the induction motor is a constant speed motor.
But we can use voltage , frequency and rotor resistance for speed control. In slip ring induction motor speed control by rotor resistance and double cage type induction motor speed control possible by variation of frequency with VFD (variable frequency drive).
Here another contract is Double cage induction motor starting current is 4 to 6 times full load current and slip ring induction motor starting current is twice of load current.
This arrangement must follow the protection system of the Motor due to starting current. This backup arrangement is appropriate from financial point of view and the work situation point of view. ]
Conclusion
This management can be used until a new replacement arrives. This temporary backup system is very much helpful for factory production. From a production point of view the extra money for replacement is negligible. So, this arrangement is full proof economical..
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