What Is The Difference Between Salient Pole and Cylindrical Pole Alternator? . A synchronous generator, also called alternator, is a most widely used electric generator that produces alternating current supply. It is the main component of every power generating station.
Based on the rotor construction, the synchronous generators are of two types namely −
- Cylindrical Rotor Synchronous Generator
- Salient Pole Rotor Synchronous Generator
However, both types of generators are similar in stator construction. The stator of both types of generators consist of a three-phase winding distributed over its inner periphery.
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What is Alternator?
An alternator (or synchronous generator) is an electrical machine that is used to convert mechanical energy into electrical energy in the form of alternating current or AC. In other words, it is an AC generator or also known as a Synchronous generator.
A synchronous generator output frequency depends only on the speed of the prime mover i.e. its output frequency is synchronized with the RPM of the prime mover. Therefore it is used for generating specific frequency voltage.
An alternator or synchronous generator has mainly two parts: a stator (stationary part), and a rotor (rotating part). The stator of an alternator has armature winding where the current is induced and the output is supplied. Whereas the rotor contains the field winding that is used for generating the necessary magnetic field. The field windings are energized by a separate DC source.
The alternators are classified based on the design of this rotor which also affects the performance of the alternator as well i.e. salient pole and cylindrical (non-salient) Pole rotor alternator.
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What is a Salient Pole Rotor Synchronous Generator?
When the synchronous generator uses a projected pole type rotor, it is known as salient pole rotor synchronous generator or salient pole alternator.
In a salient pole alternator, the rotor poles are made of steel laminations and are fixed to the rotor hub. This type of rotor has rotor poles that are physically separated. Each pole carries a concentrated excitation winding. The salient pole rotor is usually used in alternators having 4 poles or more.
The salient pole alternators are mainly used in the applications where the speed of the prime mover is less because at high speeds the centrifugal forces will be large and may damage the poles of the rotor.
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Features of the Salient Pole Alternator
- The poles are protruding and extended outward.
- It has a large number of poles about 4 to 60.
- It has a large diameter
- It has a small axial length
- It has lower mechanical strength. Therefore it is used for only low and medium-speed applications in the range of 100 to 1500 RPM.
- Its design flaw is that it has comparatively high windage losses.
- It cannot handle high speed due to high windage loss.
Applications of Salient Pole Alternators
- Hydropower Plants: Ideal for slow-speed turbines.
- Diesel Generators: Provide reliable energy in off-grid areas.
- Standby Power Systems: Efficient for low-speed backup generators.
Advantages of Salient Pole Alternators
- Cost-effective for low-speed operations.
- Simplified construction and maintenance.
- Superior cooling due to exposed rotor design.
- Operates efficiently in variable-speed environments.
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Limitations of Salient Pole Alternators
- Bulky and heavy construction.
- Limited to low and medium-speed applications.
- Not suitable for high-speed turbines.
What is a Cylindrical Rotor Synchronous Generator?
When the rotor of a synchronous generator has non-salient pole or smooth cylindrical construction, the generator is called cylindrical rotor synchronous generator or cylindrical rotor alternator.
In the cylindrical rotor alternator, the rotor has a smooth cylinder made of solid forged steel and has a number of slots on its outer periphery. The rotor winding is placed in these slots. The cylindrical rotor has smooth surface because there is no poles projected on it. Hence, it maintains a uniform air-gap between the stator and rotor in the alternator.
The cylindrical rotor synchronous generators are usually driven by steam turbines and mostly used in very high speed applications such as turbo alternators.
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Features of cylindrical pole alternator
- It has non-salient poles or its poles are not protruding.
- It has a few number of poles i.e. 2 or 4
- It has a small diameter
- It has a longer axial length.
- Its design allows high mechanical strength making it more durable and have very low windage losses
- It enables it to have the high-speed operation up to 3000 RPM.
- It generates no noise in the system.
Applications of Cylindrical Pole Alternators
- Thermal Power Plants: High-speed requirements match steam turbines.
- Nuclear Power Stations: Consistent performance under constant speed.
- Large Industrial Units: Power supply for high-load equipment.
Advantages of Cylindrical Pole Alternators
- Compact design suitable for high-speed turbines.
- Enhanced mechanical stability and durability.
- Higher efficiency at constant speeds.
- Reduced noise and vibrations.
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Limitations of Cylindrical Pole Alternators
- Requires advanced cooling systems.
- Higher initial cost due to precision engineering.
- Inflexible in variable-speed scenarios.
Difference between Cylindrical Rotor and Salient-Pole Rotor Synchronous Generator
The following table highlights all the significant differences between a cylindrical rotor alternator and a salient-pole alternator
| Basis of Difference | Cylindrical Rotor Synchronous Generator | Salient-Pole Synchronous Generator |
|---|---|---|
| Description | A synchronous generator that has its rotor which is cylindrical in shape is known as cylindrical rotor synchronous generator. | A synchronous generator whose rotor has poles projected on the rotor surface is called salient pole rotor synchronous generator. |
| Pole | In a cylindrical rotor alternator, the portion without the slots of the cylindrical rotor acts as poles. Thus, the poles are non-projecting type. | In salient pole alternator, the rotor poles are projecting out from the surface of the rotor. |
| Rotor diameter | The diameter of the cylindrical rotor alternator is relatively smaller. | The diameter of the salient-pole rotor alternator is larger than that of cylindrical rotor alternator. |
| Axial length | Cylindrical rotor alternator has large axial length. | The axial length of the salient-pole rotor alternator is small. |
| Air gap | In cylindrical rotor alternator, the air-gap between stator and rotor is uniform because it has smooth cylindrical periphery. | Salient pole rotor alternator has non-uniform air gap because the poles are projecting out from the surface. |
| Also called | Cylindrical rotor alternator is also called “non-salient rotor alternator” or “non-projected pole type alternator”. | Salient pole alternator is also known as “projected pole type rotor alternator”. |
| Rating (For the same size) | The rating of the cylindrical rotor alternator is higher than salient pole alternator. | Salient pole alternator has smaller rating than the cylindrical type alternator. |
| Mechanical strength | The mechanical strength of the cylindrical rotor alternator is high. | Salient pole alternator has relatively less mechanical strength. |
| Damper winding | Cylindrical rotor alternator does not have separate damper winding. | Salient pole alternator have separate damper winding. |
| Suitability | Cylindrical rotor alternator is suitable for high speed applications. Where, the speed ranges from 1500 to 3000 RPM. | Salient pole rotor alternator is most suitable for low speed applications, where the rotor speed ranges from 125 to 500 RPM. |
| Prime mover | The prime movers used for running the cylindrical rotor alternator are steam turbines used in thermal, gas and nuclear power plants. | The prime movers used for driving the salient pole alternators are hydro turbines and IC engines. |
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Main Differences Between Salient Pole and Cylindrical Pole Alternator
Definition
- The salient pole alternator is a type of alternator whose rotor has salient or projecting poles.
- The non-salient or cylindrical pole alternator is a type of alternator whose rotor is cylindrical having non-protruding poles.
Rotor
- The salient rotor has protruding or projecting poles on the surface of the laminated steel core. Excitation windings are wound around the polearm
- The cylindrical pole rotor has a solid steel core with slots for excitation windings. Whereas the unslotted portion forms the poles.
Poles
- The salient pole rotor has a larger number of poles. Usually, the number of poles reaches up to 64.
- The non-salient pole rotor has fewer number of poles between 2 and 4.
Size
- The salient pole alternator has a large diameter with a small axial length.
- The non-salient pole alternator has a smaller diameter with a long axial length.
Damper windings
- The salient pole alternator requires separate damper winding to prevent haunting.
- The cylindrical pole alternator does not require damper winding.
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Haunting is a phenomenon that occurs due to variations in load. When the load is changed, the rotor rotates back a few degrees to find another synchronous point causing oscillation.
Windage loss
- The salient pole rotor has high windage loss due to the large air gap in it. The loss increase with speed. Therefore, salient pole alternators are not suitable for high speed.
- The cylindrical pole rotor has a small air gap and has a cylindrical periphery. Therefore it has low windage loss.
Speed
- The salient pole alternator is used for low to medium-speed applications where the prime mover has 100 RPM to 1500 RPM speed.
- The cylindrical pole alternator allows it to have high speed where the prime mover can attain up to 3000 RPM.
FAQs
Q1: Which alternator is better for hydropower plants?
Salient pole alternators are better suited for hydropower plants due to their ability to operate efficiently at low speeds.
Q2: Why are cylindrical pole alternators preferred for high-speed applications?
Cylindrical pole alternators have a streamlined design that ensures stability and efficiency at high rotational speeds.
Q3: Can salient pole alternators handle high-speed operations?
No, their bulky construction and mechanical limitations make them unsuitable for high-speed operations.
Q4: What are the cooling requirements for cylindrical pole alternators?
They require advanced cooling systems due to reduced surface exposure.
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