What Is Synchronous Motor and Its Advantages and Disadvantages
What is synchronous motor
Synchronous motor is an AC motor that generates a torque by a DC-powered excitation magnetic field that interacts with a rotating magnetic field of the armature to rotate at a synchronous speed.
The structure of the synchronous motor is basically the same as that of the synchronous generator, and the rotor is also divided into a salient pole and a hidden pole. But most synchronous motors are salient. The installation form is also divided into horizontal and vertical.
Characteristics
Since the synchronous motor can operate under the advanced power factor by adjusting the excitation current, it is beneficial to improve the power factor of the power grid. Therefore, large-scale equipment, such as large blowers, water pumps, ball mills, compressors, rolling mills, etc., are commonly driven by synchronous motors. This is especially true when low-speed, large-scale equipment uses synchronous motors.
The running stability of the synchronous motor is also relatively high. Synchronous motors are typically operated in an overexcited state with greater overload capability than the corresponding asynchronous motor.
Advantages of Synchronous Motor
It can be operated under a wide range of power factors both lagging and leading. Hence, it can be used for power factor correction, in addition to supplying torque to drive loads.
Most synchronous motors are rated between 150 kW and 15 kW and run at speeds from 150 to 1800 rpm.
It is less costly in certain kW and speed ranges i.e. for 35 to 350 kW rating at speeds less than 500 rpm.
It usually runs at higher efficiencies, especially in low-speed unity power factor
It can be constructed with wider air gaps than induction motors, which makes it batter mechanically.
Disadvantages of Synchronous Motor
It is not self-starting. Special methods are adopted to make it self-starting.
It needs frequent maintenance.
External DC source is necessary for providing excitation.
Additional damper winding is necessary.
Hunting takes place if the load is changed suddenly.
Application of Synchronous Motor
Synchronous motors are mainly used in constant speed applications, some of the applications are:
An overexcited synchronous motor operates at leading power factor and takes a leading current from the bus bars, so it can be used to raise overall the power factor of the installation. When a synchronous motor is run without load with over-excitation for improving the power factor of an installation it is called as the synchronous capacitor or synchronous condenser.
Advantages of Rotating Field System in Synchronous Machines
Only in small synchronous machines (i.e. synchronous motors and synchronous generators) the field system is placed on stator and armature winding on rotor but in higher rating machines the field winding is placed on the rotor and armature winding is placed on the stator. Following are the important advantages of rotating field system over stationary field system:
The armature winding is more complex than the field winding. Therefore, it is easy to place armature winding on a stationary structure.
The size of the armature conductors is much more to carry heavy currents, therefore, high centrifugal stresses are developed. Thus it is preferred to place them on a stationary structure.
In the modern alternators (synchronous generators), a high voltage is generated, therefore, heavy insulation is provided and it is easy to insulate the high voltage winding when it is placed on the stationary structure.
The size of slip-rings depend upon the magnitude of flow of current, therefore, it is easy to deliver small current for excitation through slip rings of a smaller size when the rotating field system is used.
It is easier to build a properly balanced high speed rotor when they carry the field system.
The weight of the rotor is small when the field system is provided on the rotor and as such friction losses are reduced.
Batter cooling system can be provided when the armature is kept stationary.