The rotor was invented in the early 1800s and it was discovered by the naval officers R. P. C. Spengler and Theo A. van Hengel. Earlier it was used in Rotor dynamics for the encryption of files especially used in cryptography. The invention of this type is also supported by Gustaf de Laval who is working for the development of turbine machinery. This type was supposed to be called Laval type as per his name Gustaf de Laval. In this article, we shall discuss what is a Rotor, types of work, how it works and working principle behind it, functions, and applications.
What is a Rotor?
It is the rotating part of the machine. It is attached to a shaft that is connected with the key at the end as a lock. This rotating part is considered as the rotor that rotates inside a magnetic field when torque is developed.
The squirrel cage and slip ring IM differ only with respect to their rotating construction whereas the stator construction for both the machines is the same. A machine is made of laminations and these laminations made of the stator are of thinner and are made of thicker laminations. The conductors are made of copper and these are short-circuited on either side with end rings so that it forms a closed path. These bars are skewed to reduce the eddy current losses. These copper bars are also forged to increase the mechanical strength such that more current can pass through it. The copper bars are wound according to the requirement on the armature core and are considered as the winding which is shown in the figure below.
The reason behind the usage of only copper is we get less ohmic losses and the efficiency can also be increased. But if we use aluminum conductors it has high resistance, then ohmic losses will be more and the efficiency will be less. The commutator, armature, laminated soft iron magnetic core, shaft, winding, and ball bearing all collectively considered as a rotating part. Because all these internal parts collectively rotate inside the magnetic field. The core of this type is shown in the figure below.
Usually, in a generator, it rotates when a conductor is rotated inside the magnetic flux it induces an emf that further produces a torque. This torque, in turn, rotates the rotating part.
There are different types of rotating parts. They are squirrel cage and slip ring type. The squirrel cage type and stator are considered as the squirrel cage induction motor whereas the slip ring type and stator together are considered as the slip ring induction motor. The slip ring motor is shown in the figure below.
There is no specific number of poles for the construction it depends on the number of stator poles. If the stator has 4 poles, then the rotating part should also have 4 poles. This type of rotating part response will change automatically with the change made in the stator. The number of rotating phases is equal to the number of copper bars per pole. For example, if the three-phase motor IM is of 4 poles and 32 stator slots and 28 rotating slots, then the number of copper bars per pole = 28/4 = 7. The operation of this type of motor is not possible if the number of stators and rotating slots is not equal. But the operation is possible even if the stator and rotating part phases are not equal.
Advantages of Squirrel-Cage
- Due to the increase in the length of the rotating bars, the starting characteristics of the IM is improved.
- Harmonics are eliminated and air gap flux distribution becomes uniform.
- Crawling and cogging is eliminated.
- Low starting torque as resistance is less.
- Starting current is more since the resistance is less.
These drawbacks can be overcome by making the changes in the cross-sectional area of the rotating conductor bars. This achieved by skewing the rotating bars of the conductors.
To improve the starting torque, we connect external resistance in series with the machine that acts as an additional resistance helpful in increasing the starting torque. Usually, in a slip ring, the winding is wound in star in order to increase the starting torque. Since star connection employment has more starting torque than delta connection. In delta connection, the effective resistance will be low that results in the decrease of starting torque. The rotating core of a slip ring is shown in the figure below.
Difference between a Stator and a Rotor
|It is the stationary part||It is the rotating part|
|Yoke, core, and winding collectively considered as the stator||Armature, Commutator, rotor core collectively considered as the Rotor|
|The winding arrangement is complex||The winding arrangement is not complex|
|Supply given to stator is three-phase||It is self-excited by the induction principle|
|Frictional losses are more. It requires more insulation because of carrying the heavy current||Frictional losses are less. It requires less insulation.|
We had studied what is the rotor and its other parts in this discussion. It is described as the rotating part of the machine that is used in any rotating machine for the function of either generating or delivering power. In addition to this, we had also discussed the different types of rotating parts available and its working, advantages, disadvantages, difference between a stator and rotor, and applications. Here is a question for the readers what is the function of the Air type?