Basically, the rotating part of an electrical machine is the rotor. The rotor is used to house the rotor winding and magnetic poles. The electrical power is supplied to the rotor through slip rings and brushes. Based on the construction the rotor of an electrical machine is classified into two types. They are,
- Salient pole type or projected pole type rotor (low speed, more number of poles, kaplan turbines)
- Non-salient type or smooth cylindrical type rotor (high speed, 2 or 4 poles, pelton wheel)
Salient Pole Type Rotor :
The salient pole type construction of rotor is shown below is used in low and medium speed (125 to 500 rpm) electrical machines. The poles are made of steel laminations rivetted together and are fixed to the rotor by a dovetail joint. In synchronous machines, the damper winding is provided on the slots, provided on the pole faces (also known as squirrel cage winding).
The damper bars are short-circuited at both ends by copper rings. These dampers are used in preventing hunting and to provide starting torque in synchronous motors. The rotor windings are placed on the pole-pieces supported by pole shoes. The ends of the winding are taken out through slip-rings for electrical connection.
The salient pole type structure has the following special features,- They have a large diameter and short axial length.
- The pole shoes cover about ⅔ of the pole pitch.
- Poles are laminated to reduce eddy current losses.
- These are employed with hydraulic turbines or diesel engines.
Non-Salient Pole Type Rotor :
The construction of a non-salient type rotor also called a smooth cylindrical type. It is used in very high-speed applications (like turbo-alternators) usually driven by steam turbines. The rotor consists of a smooth solid forged steel cylindrical having a number of slots on its outer periphery (parallel to the shaft) for accommodating rotor winding.
The surface of the rotor of this type is smooth due to the absence of projection of poles, thus maintaining uniform air-gap between stator and rotor. Such rotors are designed mostly for 2-pole or 4-pole machines. The un-slotted portions from the pole face is shown in the below figure.
The non-salient pole type structure has the following special features,- They are of small diameter and of very long axial length to reduce the peripheral velocity
- This type of construction offers fewer windage losses
- High speeds are obtained i.e., 1500 to 3000 rpm.
- Better in dynamic balancing and quiter in operation.
- This type of construction is mechanically very strong compared to the salient pole type.
Difference Between Salient Pole and Non-salient Pole Type Rotor :
| S.no. | Salient Pole Type Rotor | Non-salient Type Rotor |
|---|---|---|
| 1. | Poles are projected. | Outer surface is smooth (poles are not projected). |
| 2. | More number of poles are present. | Two or four poles are present on the rotor |
| 3. | Large diameter (≈ 10m) and small axial lengths (≈ 3m). | Small diameter (≈ 1m) and large axial lengths (≈ 10m). |
| 4. | Used for low and medium speeds. | Used for high speeds (1500 to 3000 rpm). |
| 5. | More windage losses. | Less windage losses (due to smooth surface). |
| 6. | Water turbines are preferred. | Steam turbines are preferred. |
| 7. | Construction is complicated. | Easy to construct. |
| 8. | Perfectly not balanced. | Perfectly balanced. |
| 9. | Air gap is present in between poles. | No air-gap in between poles. |
| 10. | Flux is not uniform due to interpolar gap. | Flus is uniformly distributed. |
