Table of contents | |
Construction of Synchronous Machines | |
Classification of Synchronous Machines according to the form of Excitation | |
Advantages of Stationary Armature | |
Frequently Asked Questions (FAQs) |
A synchronous machine is an ac rotating machine whose speed under steady state condition is proportional to the frequency of the current in its armature.
Diagram of Synchronous Machine
Synchronous machines are commonly used as generators especially for large power systems, such as turbine generators and hydroelectric generators in the grid power supply.
Because the rotor speed is equal to the synchronous speed of stator magnetic field, synchronous motors can be used in situations where constant speed drive is required.
Since the reactive power generated by a synchronous machine can be adjusted by controlling the magnitude of the rotor field current, unloaded synchronous machines are also often installed in power systems for power factor correction or for control of reactive kVA flow. Such machines, known as synchronous condensers, and may be more economical in the large sizes than static capacitors.
The bulk of electric power for everyday use is produced by polyphase synchronous generators (alternators), which are the largest single-unit electric machines in production.
For instance, synchronous generators with power ratings of several hundred megavolt-amperes (MVA) are fairly common, and it is expected that machines of several thousand megavolt- amperes will be in use in the near future.
Like most rotating machines, synchronous machines are capable of operating both as a motor and as a generator. They are used as motors in constant-speed drives, and where a variable-speed drive is required, a synchronous motor is used with an appropriate frequency changer such as an inverter.
As generators, several synchronous machines often operate in parallel, as in a power station. While operating in parallel, the generators share the load with each other; at a given time one of the generators may not carry any load. In such a case, instead of shutting down the generator, it is allowed to "float" on the line as a synchronous motor on no-load.
The synchronous machine has 3 phase winding on the stator and a d.c. field winding on the rotor.
The rotor carries a field winding which is supplied with direct current through two slip rings by a separate d.c. source.
Rotor construction is of two types:
(i) Salient (or projecting) pole type
(ii) Non-salient (or cylindrical) pole type
In this type, salient or projecting poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator as shown in Figure below:
Salient Pole Rotor
Type rotors due to the following reasons:
(a) The salient field poles would cause an excessive windage loss if driven at high speed and would tend to produce noise.
(b) Salient-pole construction cannot be made strong enough to withstand the mechanical stresses to which they may be subjected at higher speeds.
- Since a frequency of 50 Hz is required, we must use a large number of poles on the rotor of slow-speed alternators.
- Low-speed rotors always possess a large diameter to provide the necessary space for the poles.
- Consequently, salient-pole type rotors have large diameters and short axial lengths.
High-speed alternators (1500 or 3000 r.p.m.) are driven by steam turbines and use non-salient type rotors due to the following reasons:
(a) This type of construction has mechanical robustness and gives noiseless operation at high speeds.
(b) The flux distribution around the periphery is nearly a sine wave and hence a better e.m.f. waveform is obtained than in the case of salient-pole type.
- Since steam turbines run at high speed and a frequency of 50 Hz is required, we need a small number of poles on the rotor of high-speed alternators (also called turboalternators).
- We can use not less than 2 poles and this fixes the highest possible speed. For a frequency of 50 Hz, it is 3000 r.p.m.
- The next lower speed is 1500 r.p.m. for a 4-pole machine.
- Consequently, turboalternators possess 2 or 4 poles and have small diameters and very long axial lengths.
Brushes excitation system for a Synchronous Machine
Brushless excitation system for a Synchronous Machine
This arrangement has the following advantages:
(i) It is easier to insulate stationary winding for high voltages , because they are not subjected to centrifugal forces .
(ii) The stationary 3-phase armature can be directly connected to load without going through large, unreliable slip rings and brushes.
(iii) Only two slip rings are required for d.c. supply to the field winding on the rotor. Since the exciting current is small, the slip rings and brush gear required are of light construction.
(iv) Due to simple and robust construction of the rotor, higher speed of rotating d.c. field is possible. This increases the output obtainable from a machine of given dimensions.
Salient Pole Rotor showing the field winding and damper bars
Q.1. What is synchronous condenser?
An over-excited synchronous motor under no load ,used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.
Q.2. State the characteristic features of synchronous motor.
- The motor is not inherently self starting.
- The speed of operation is always in synchronous with the supply frequency irrespective of load conditions.
- The motor is capable of operating at any power factor.
Q.3. In what way synchronous motor is different from other motors?
- All dc and ac motors work on the same principle.
- Synchronous motor operates due to magnetic locking taking place between stator and rotor magnetic fields.
Q.4. Why a synchronous motor is a constant speed motor?
Synchronous motor work on the principle of force developed due to the magnetic attraction established between the rotating magnetic field and the main pole feed. Since the speed of rotating magnetic field is directly proportional to frequency the motor operates at constant speed.
Q.5. How the synchronous motor can be used as s synchronous condenser?
Synchronous motor is operated on over excitation so as to draw leading reactive current and power from the supply lines. This compensates the lagging current and power requirement of the load making the system power factor to be come unity. The motor does the job of capacitors and hence called as synchronous condenser.
23 videos|89 docs|42 tests
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1. What is the construction of synchronous machines? |
2. How are synchronous machines classified according to the form of excitation? |
3. What are the advantages of a stationary armature in synchronous machines? |
4. How does the excitation of synchronous machines affect their operation? |
5. What are the main differences between separately excited and self-excited synchronous machines? |
23 videos|89 docs|42 tests
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