HESB Synchronous-machines-TEST-3

Phrases, linking verbs, etc.

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Theory

Synchronous machine

• Definition: Its motor rotates the same (synchronous) speed as the rotating magnetic field of the stator does.

• Used for generation of electric energy

  • as electric power sources in: cars, aircrafts, military engineering, on-site power generators

• Less widespread

• 3-phase + 1-phase synchronous machines

• Synchronous generator = Alternator

Main features

1. Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronized.

2. The speed of operation of is in synchronism with the supply frequency and hence for constant supply frequency they behave as constant speed motor irrespective of load condition

3. This motor has the unique characteristics of operating under any electrical power factor. This makes it being used in electrical power factor improvement.

More: https://www.youtube.com/watch?v=80DKO7m4d7Y

Advantages and disadvantages

Pros

1. These motors can be made to operate at leading power factor and thereby improve the pf of an industrial plant from one that is normally lagging to one that is close to unity.

2. This motor operates at a constant speed, irrespective of load, from no-load to full load.

3. Electromagnetic power varies linearly with the applied voltage.

4. These motors can be constructed with wider air gapes than induction motors making them mechanically better.

5. Efficiency of operation is usually high, especially in the low speed and unity power factor ranges.

Cons

1. These motors cannot be used for variable speed jobs as there is no possibility of speed adjustment

2. It requires external source for supplying dc excitation

3. It cannot be started under load, the starting torque being zero

4. It has a tendency to hunt.

5. It may fall out of synchronism and stop when over-loaded.

6. Collector rings and brushes are required.

7. For some applications these motors are not desirable as for driving shafts in small work-shops having no power available for starting and in cases where frequent starting or strong starting torque is required.

Design

• Machine consist of:

  • Stator (three-phase winding)

  • Rotor (excitation winding)

• 2 types:

  • Machines with salient poles

    • driven by slow-speed water turbines

    • large diameter + small axial length

  • Machines with a cylindrical rotor

    • driven by fast-speed steam or gas turbines

    • smaller diameter + larger axial length

Principle of operation

• Due to the interaction of the magnetic fields of the stator + the rotor.

• Synchronous motor is a doubly excited machine i.e two electrical inputs are provided to it.

• It’s stator winding which consists of a 3 phase winding is provided with 3 phase supply and rotor is provided with DC supply.

• The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux (-> rotating magnetic field). The rotor carrying DC supply also produces a constant flux.

• At a particular instant rotor and stator poles might be of same polarity (N-N or S-S).

• The rotor locks in with the rotating magnetic field and rotates along with it -> the rotor is synchronized.

• But due to inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. IT IS NOT SELF-STARTING!!

• To overcome this inertia, rotor is initially fed some mechanical input which rotates it in same direction as magnetic field to a speed very close to synchronous speed.