3.1. The Stator
- Regarding a stator frame, it has a substantial as well as a symmetrical casting with an integral free cast. The stator core has an electrical sheet steel stamping whose aspect is dependent on 2 factors.
- Low loss
- High grade
- These stampings are congregated in this stator frame, that too under hydraulic pressure. In case of small induction motors, stator laminations are clipped in a single piece. These laminations or stampings are usually of a certain thickness, i.e., between 0.35 mm and 0.6 mm.in case of huge sized induction machines, the static core is put together by taking them from different segmental laminations.
- In order to accommodate former woundcoils,the slots are designed with parallel walls which can also be termed as open type slots. Although, this is envisioned as theideal slot type, but as per usual practice, an effective length reduction across theair gap is possible when the slots are enclosed.
- In order to make the stator winding electrically and mechanically sound, it is essential to take its utmost care. A proper care of these winding ensures higher efficiency and a long life.To retain these qualities, few steps are thoroughly maintained.
- A stator winding is placed in its appropriate position.
- After itspositioning and still remaining in its heated condition, that winding is thoroughly dried.
- After the completion of these 2 processes, this dried winding is fully immersed in a special varnish comprising of high grade synthetic resin.
- Finally, it is passed through various procedures to make it alkali, oil, moisture, and acid proof.
- For the machines which are medium in size, the windings are usually double layer lap. The shape of the used coils is mostly in adiamond
- In case of small motors, mush windings are utilized which work at single layer and at ordinary voltages.
- If the winding selection is for larger motors, the common ones used at high voltages are mainly single layer concentric windings.
- Stator cores are mainly accommodated frames of electrical machinery. Induction motors which have frames in medium and small size have their forms in hollow and cylindrical structure. This is in comparison to larger motors whose shape is similar to a circular box.
- A frame diameter can be near to 150 cm when the size consideration is about small induction motor. A beneficial factor about these frames is its endurance to the end shields.
- Induction motors having large dimensions should have rigid and strong frames. This is considered as an essential aspect regarding these types of frames. The importance of this factor is the lead cause resulting in smaller length in air gap (extremely small). In case this frame is not found to be firm, its result will be an unbalanced magnetic pull due to theirregular air gap. This gap around the equipment leads to the stated outcome.
- Rolled steel plates are generally the frames component for large and medium sized machinery.
- For small machinery, their frames are usually cast, and its production is based on a single unit.
3.2. The rotor
There are mainly 2 types of rotors.
- Wound rotor
- Squirrel cage
In these types of rotors, the stamping or lamination is slotted, and windings are priorly wound. In case of induction motors where its requirement is mainly focused on extremely high starting torque value as well as control in speed. Similar to the insulation of stator windings, wound rotors too have full insulation in the form of copper windings. The connection of these windings can be either with delta or with star and at 3 slip rings; all the 3 ends can be seen when brought out. At those 3 slip rings, a collection of current is made with carbon ends. The current is then assembled with the help of carbon brushes. Later it is used for resistance purpose and then for starting the rotor.
In certain scenarios like the constant running of motor, only with the help of a collar, the slip rings can be short circuited. This collar joins all the slip rings and pushes it along its shaft in the innermost part of the motor. The provision of these carbon brushes is with a certain device which enables the brushes to be lifted up from slip rings. This usually happens after the motor gets started, thereby resulting in frictional losses and damages due to wear.
If comparison is made between stator and rotor, the number of slots in both the cases should not be equal in any term. In case this factor does not come to application, see can see reluctance in variation in the magnetic path, that too from maximum to minimum. This variation will mainly be in rotor teeth in opposite slots. Its outcome will be a flux pulsation that will have a higher frequency. This is because the interval period in the periodic time regarding the tooth occupancy will be same to its position in the opposite direction of t successive teeth.
This positioning tends the rotor to be locked with stat or as well as be the reason for excessive iron loss. This happens at that time when the opposite teeth are with the starting teeth. It can be one of the best ideas that can make the number if rotor teeth and stat or teeth prime to one another.
Made up of laminations or stampings, a squirrel cage rotor is a product which is keyed straight to its shaft. Its windings comprise of copper bars which are embedded along with short circuited rings that are made up of brass. Squirrel cages have partially closed slots. Being a highly robust rotor, this equipment can be stated to indestructible (nearly).
If we consider the production of induction motors in the present scenario, most of the manufacturing companies produce motors having squirrel cage rotors. Being a common production practice, its windings are mainly made up of cast aluminum. Few steps are employed in the case of its construction.
- Initially, the rotor laminations or stampings are assembled in one place.
- He terms given to that place is mould where the molten aluminum is forced intoside that place to form cooling fans, end rings, and bars. This formation can also end rings extension. Aluminum’s forceful insertion takes place when high pressure is applied. This procedure and result are called die cast rotor. It is widely accepted and manufactured. It is because this rotor type does not have any joints. And where there is absence of joint, such rotors does not offer the possibility of producing high contact resistance.
- If it is carefully noticed, then squirrel rotors do not have slots that are parallel to each other. Instead of having parallel slots in the shafts these rotors are skewed. The reason for such structure is due to 2 factors.
- To minimize the rotor’s locking tendency
- To enable the motor for running quietly, that can be achieved by decreasing its magnetic hum.
Comparisons that highlights the beneficial aspects of squirrel cage motor, which is better than a phase wound induction motor
If the beneficial factors are compared, then the following things come across in this case.
- Rugged and cheaper in construction
- Squirrel cage motor has ample space in rotor slots. It also ensures less copper loss and even a lesser overhanging factor.
- It has a higher efficiency. But in comparison to the latter one, the change is slightly more.
- The cooling condition of Squirrel cage motor is better than the other motor. This is due to its large fan space and its bare end rings.
- For a better power factor, squirrel cage motor has a minuscule rotor overhand leakage. This aspect also sees through overload capacity and higher pull out
- A squirrel cage motor does not have the following components to start a rheostat. They are:
- Brush gear
- Rotor terminals
- Slip rings
- Short-circuiting devices
Disadvantages of Squirrel cage motor
- The rotor of a Squirrel cage motor has a large starting current.
- It also has a small starting torque.
- It is nearly impossible in rotor circuit to insert the resistance for sake of increasing its starting torque.
In case of wound rotor machines, its slip rings are created with metal which is either,
- Phosphor bronze
With the help of moulded silica insulation, these rings are shrunk on a specific place known as cast iron sleeve. After all these things are assembled together, they are passed on a rotor shaft. Rotation of these slip rings usually takes place in the following areas,
- Between bearing and core, or
- Between shaft extension and core
In the second case, the structure of shaft is designed in such a way that its interior is hollow. This structure from rotor to those slip rings allows 3 connections which pass via bearings.
3.4. Shaft and bearings
- If we see the structure of an induction motor, the space for anair gap is made in the smallest size possible. This is the reason why the shaft size is also small.
- In case the rotor may suffer from an insignificant deflection, it is made sure that the creation of shaft fulfills 2 criteria – stiff in structure and short in size.
- This is kept in primary consideration because even a minute deflection has the tendency to create major irregularities in air This condition results in an adverse outcome in the form of unbalanced magnetic pull.
- There is also a different and distinct possibility between stator and rotor which can foul one another.
- The general utilization of roller bearing and ball bearing is mostly one important reason. Both of them use accurate centering over journal bearing. This is because former is a much simpler to use than latter one.
- This previous point is also responsible for overall reduction in machine’s length.
- For heavy rotors which are larger in size, the commonly used bearings are journal.
- In case a motor is smaller in size, a ball bearing can be used in the non-driving end and for its driving end, using a roller bearing is the generalized condition.