Synchronous motor is very important part of electrical engineering and these are called constant speed machines. Synchronous machines includes alternators and motors which run at a constant speed in synchronism with the alternating current supply to which they are connected.
An alternator is a machine which has a stationary conductor system called stator and a rotating field system called rotor. The arrangement is very helpful to collect heavy currents at high voltages from stationary terminals. Field is light and so can run with high speed.
Basics of Synchronous Machines
Technically a synchronous machine consists of two major parts viz Armature and Field System. The arrangement of fundamental parts of a synchronous machine is such that armature is placed inside the field winding or frame. This construction is similar to the one used for d. c. Machines wherein the armature winding is placed on the rotor and the field system is housed in the stationary stator. This type of construction is used only for low power synchronous machines and is unsuitable for medium and high power synchronous machines.
In large synchronous machines second type of construction is used. This type of construction uses a stationary armature and a revolving field structure. This method is more economical and convenient construction method.
The revolving field system is almost universal because it has the following advantages :
- Stationary Armature :
This kind of construction permits the use of stationary armature on which the windings can be easily braced and insulated for high voltage.
- Slip Ring Operation :
The operation of slip rings on account of their sliding contracts is unreliable with large currents at high potential differences. The use of slip rings carrying large current at high voltages is, therefore, avoided in the stationary armature construction.
Classification of Synchronous Machines
The synchronous machines are classified into two types based on type of rotor used in construction. These type of synchronous machine are :
- Salient Pole Machine
- Cylindrical Rotor Machine
Salient Pole Machine
A salient pole synchronous machine has salient or projecting poles with concentrated field winding. The salient pole construction is used for generators driven by hydraulic turbines since these turbines operate at relatively low speed and a relatively large number of poles are required to produce the desired frequency ; the salient pole construction is better adopted mechanically to this situation.
Cylindrical Rotor Machine
A cylindrical rotor synchronous machine have their field winding distributed in the slots. The distributed field winding produces a sinusoidal flux distribution in the air gap. Cylindrical Rotor Construction is used for turbo-alternator which are driven by high speed steam or gas turbines.