Synchronous machine – APSEEE

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Parallel Operation of Alternators

Parallel Operation of Alternators

When the number of smaller units is connected in parallel instead of installing a large unit is called parallel operation of alternators. There are numbers of reasons, connecting alternator in parallel such as cost become less, maintenance and repair, efficiency and reliability of the power system. All the alternator of the system, work in parallel form a large capacity alternator. In this article, we will discuss about need for parallel operation of alternators

Need for parallel operation

In modern power systems alternators are operated in parallel to supply a common total load. Due to following reasons the alternators are connected in parallel.

  1. The demand of electrical power is huge, and it cannot be met by a single unit and it is difficult to build a large alternator, therefore to meet the demand of electrical power several alternators are connected in parallel.
  2. The parallel operation increases the reliability of the electric supply. If we use single large alternator in the event of fault on alternator or turbine whole the system is paralyzed. But with several alternators work in parallel maintain the continuity of supply rather than breakdown of one unit.
  3. Maintenance and repair of the alternator is more convenient if more number of small capacity alternators are installed at the power station. For repairing of one alternator there is no need to shut down the whole power plant.
  4. With increase the demand of electrical energy, we can install a alternator with existing plant.
  5. Transportation problems are faced with single large unit. But this problem can be eliminated by using small units.
  6. The load on power plant varies, usually having its peak value during the day and its minimum value during the night time.

Thus the number of units operating at a particular time can be varied depending upon the load at that time. If the alternators are connected in parallel the less efficient alternator can be shut down when the load requirement is less.

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Applications of Synchronous Motor

Applications of Synchronous Motor

Synchronous motor is constant speed motor. This motor is not used for drive heavy load. In this topic we will discuss about various applications of synchronous motor.

The Synchronous motors are used for various purposes which are given below:-

  1. The synchronous motors are used in power houses and electrical substations in order to improve power factor. For this purpose the synchronous motors are operated on no load and overexcited.
  2. In large industries the power is lagging since there are large number of induction motors are installed. These motors are run at lagging power factor. To improve this lagging power factor there is over excited synchronous motors are used in the industries.
  3. because of high efficiency and high speed of the synchronous motors, these motors are used for those applications where constant speed is required. Speed above the 500rpm, synchronous motors are used for drives, such as centrifugal pumps, compressors, rubber and paper mills, line shafts, blowers etc. Speed below the 500rpm synchronous motors are used for drives such as ball and tube mills, electroplating, generators, vacuum pumps, metal rolling mills, centrifugal and screw-type pumps etc.
  4. Synchronous motors are used to regulate voltage at the end of the transmission line. The voltage at the end of the long transmission lines changes appreciably due to presence of the large inductive load. When this load is disconnected from the lines suddenly, voltage tends to rise because of capacitance of line is increased. To keep this voltage at normal level synchronous motors are installed. This voltage can be controlled by vary the excitation of the synchronous motors. When load decreased due to inductive load, the motor excitation is increased. If the line voltage is increased due to capacitive effect, then motor excitation is decreased to maintain the line voltage within normal level.

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Construction of Synchronous Machines

Construction of Synchronous Machines

Construction of synchronous motor and synchronous generator is same. In this article, we will discuss about construction of Synchronous machines.

Synchronous machines have following important parts

  1. Stator
  2. Rotor


Stator is outer part of the machine It is the stationary part of a synchronous machine. The stator contains following parts.

construction of synchronous machines

  • Stator Frame

It is a outer body of a machine. It is made up of cast iron. It protects the inner parts of the machine. The stator core is placed in between the frame. Cast iron is used for the construction of stator frame because it has high mechanical strength.

  • Stator Core

The material of a stator core is laminated silicon steel. It is made from number of stamping which are insulated from each other. Laminations are used to reduce eddy current losses and silicon steel is used to reduce hysteresis losses. The function of stator core is to provide an easy path for magnetic flux. The slots are cut on its inner periphery to accommodate the winding.

  • Stator Winding

Stator winding is placed in the slots. Enameled copper is used as winding material.


The rotating part of the machine is called Rotor. From construction point of view there are two types of rotors named as

Salient pole type rotor

Non-salient pole type rotor

  • Salient Pole Type Rotor

Salient pole type construction is suited for medium and low speeds synchronous generators. In this case, projected poles are provided on the rotor. These rotors are usually employed at hydroelectric plants. The speed of these machine is quite low. These rotors are designed with larger diameter and small axial length. The exciting current is supplied by an exciter in fixed on each alternator shaft.

  • Non-Salient Pole Type Rotor

Non-salient pole type construction is suited for the high speeds. In this case rotor is made of silicon steel laminations. These rotors are used for high speed alternators. These rotors have smaller diameter and larger axial length. These types of rotor give noiseless operation and better in dynamic balancing. About 2/3 of the rotor pitch is slotted, leaving 1/3 for the pole centre. The speed may be as high as 3000 r.p.m at 50Hz.


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