The advantages of synchronous motor:

• It runs at a constant speed (at synchronous speed)
• It can run over a wide range of power factors both lagging and leading
• Its torque is less sensitive to change in supply frequency
• Synchronous motors can be constructed with wider air gaps than induction motors which makes these motors mechanically more stable
• Electro-magnetic power varies linearly with the voltage
• usually, operate with higher efficiencies especially in low speed and unity power factor applications compared to induction motors

The disadvantages of synchronous motor:

• It requires dc excitation which must be supplied from an external source
• It is inherently not self-starting and needs some arrangement for its starting and synchronizing
• These motors cannot be used for variable speed applications
• It cannot be started on load; Its starting torque is zero
• When loading on the synchronous motor increases beyond its capability, the synchronism between rotor and stator rotating magnetic field is lost and motor comes to halt
• Collector rings and brushes are required resulting in an increase in maintenance
• Synchronous motors cannot be useful for applications requiring frequent starting or high starting torques required

Due to the magnetic locking between stator and rotor poles synchronous motor is not self-starting and there is no starting device to accelerate the rotor to near synchronous speed.

By using the following methods we can start a synchronous motor.

Using Pony Motors: The pony motor drives the main motor up to near synchronous speed. The motor then needs to be synchronised to the supply. The DC excitation is then switched on, the rotor field and stator fields than "lock-in" and the pony motor is disconnected. The main motor then runs at synchronous speed.

Using Damper Winding: When the 3 phase supply is switched on, the motor runs up as a normal squirrel cage induction motor.

Using Small D.C machine coupled to it: A small DC machine is coupled with the synchronous motor. The DC machine is made to act as a DC motor so that the synchronous motor is started.

• Torque angle δ is the angle between Rotor flux and Stator fluxes, both are rotating at synchronous speed.
• When the synchronous machine is connected to an infinite bus, its speed and terminal voltage are fixed and unalterable.
• The control variables are only field current and the mechanical torque or load on the shaft.
• There is a limit to the mechanical load that can be applied to asynchronous motor.
• As the load is increased, the torque angle δ also increases until the condition arises when the rotor is pulled out of synchronism and the motor is stopped.
• The figure below represents the variation of power P with Torque angle δ.
• This power versus load angle curve has a sinusoidal shape and is usually called the power-angle characteristic curve of the synchronous machine.
• The power P, for the generator is taken as positive and therefore, for the motor as negative.

Concept:

The synchronous speed of a synchronous machine is given by

$N_s=\frac{120f}{P}$

Where f is frequency and P is the number of poles.

A synchronous machine always rotates with a synchronous speed.

Calculation:

Alternator:

Number of poles (P_G) = P

Frequency (f_G) = 50 Hz

Synchronous speed, $N_{sG}=\frac{120\times 50}{P}=\frac{6000}{P}$

Synchronous motor:

Let the number of poles is P_m

Frequency (f_G) = 60 Hz

Synchronous speed, $N_{sm}=\frac{120\times 60}{P_m}=\frac{7200}{P_m}$

As the alternator is driven by the synchronous motor, both the machines operate at the same speed.

$\Rightarrow \frac{6000}{P}=\frac{7200}{P_m}$

The damper winding in synchronous motor performs two functions:

• Prevents hunting (damped out oscillations)
• Provides starting torque (made self-starting)

Under normal running conditions, damper winding does not carry any current.

Hunting is the phenomenon that occurs in synchronous motors due to varying load or supply frequency.

Causes of hunting

• Periodic variation of load
• Sudden changes in load
• Faults occurring in the system when supplied by the generator
• Sudden change in the field current
• Cyclic variations of the load torque

Effects of hunting

• It can lead to loss of synchronism
• It can cause variations of the supply voltage producing undesirable lamp flicker
• The possibility of a resonance condition increases. If the frequency of the torque component becomes equal to that of the transient oscillations of the synchronous machine, resonance may take place
• Large mechanical stresses may develop in the rotor shaft
• The machine losses increases and the temperature of the machine rises

• Excitation of synchronous motor refers to the DC supply given to rotor which is used to produce the required magnetic flux
• To get a constant magnetic field we have to apply a DC voltage (at 100-250 V) to a coil
• If AC supply is provided to the rotor instead of DC supply, the rotating magnetic field produced by the rotor will alternate
• 3 phase stator winding will receive alternating magnetic flux and the induced alternating current would be unpredictable and undesired.

Starting of Synchronous motor:

• A synchronous motor is called a doubly excited machine because both its rotor and stator are excited.
• To achieve magnetic locking between stator and rotor, double excitation needed.
• The stator of 3 phase synchronous motor is excited with a 3-phase supply while the rotor is with DC Supply.
• That's why 3 phase synchronous motor needs 3 phase AC supply as well as a DC supply.

Synchronous motor:

V curves for synchronous motor gives the relation between armature current and DC field current. The curves are shown below.

A synchronous motor is capable of operating at all types of power factor i.e. either UPF, leading, or lagging power factor.

• Lagging power factor: If field excitation is such that Eb < V the motor is said to be under excited and it has a lagging power factor.
• Leading power factor: If field excitation is such that Eb > V the motor is said to be over-excited and it draws leading current. So that the power factor improves.
• Unity power factor: If field excitation is such that Eb = V the motor is said to be normally excited.

Synchronous generator:

A synchronous generator or alternator is capable of operating at all types of power factor i.e. either UPF, leading or lagging power factor.

• Leading power factor: If field excitation is such that Eb < V the alternator is said to be under excited and it has a leading power factor.
• Lagging power factor: If field excitation is such that Eb > V the alternator is said to be over-excited and it draws lagging current. 
• Unity power factor: If field excitation is such that Eb = V the alternator is said to be normally excited.
• V -curve for synchronous generator or alternator is shown below

Synchronous motors:

• Synchronous motors are constant speed motors. They run at the synchronous speed of the supply.
• They are generally used for constant speed operation under no-load conditions such as to improve the power factor.
• Synchronous motors have fewer losses than induction motors at a given rating.

Speed control in synchronous motors:

The speed of a synchronous motor is given by

N_s = 120 f  / P

Where,

N_s is the synchronous speed

f = supply frequency

p = number of poles.

• The synchronous speed depends on the frequency of the supply and the number of poles of the rotor.
• Changing the number of poles is not easy, so this method is not used.
• However, with the invention of solid-state devices, the frequency of the current fed to the synchronous motor can be varied.
• The speed of the synchronous motor can be controlled by changing the frequency of the supply to the motor.
• V/f control is one of the methods of speed control in the synchronous motor.

There are two types of V/f control:

 (1) V/f constant:

• Torque will be constant in this region and the power of the motor is variable.
• Hence, it is called a constant torque variable power drive.
• This control is used for frequency under rated frequency.
• Therefore, speed will be less than synchronous speed
• Both voltage and frequency are varied but maintained constant.

(2) V/f is not constant:

• Power will be constant in this region and the torque of the motor is variable.
• Hence, it is called a constant power variable torque drive.
• This speed control is used for frequency greater than rated frequency
• Therefore, the speed will be more than synchronous speed.
• Voltage is kept constant as it can't be varied greater than the rated voltage, only frequency is varied.
  ​

Torque speed Characteristics of V/f control is as follows:

• Synchronous motor runs at a constant speed at a given frequency irrespective of the load.
• But synchronous motors can be run at ultra-low speeds by using high power electronic converters which generate very low frequencies.
• They can be used to drive crushers, rotary kilns, and variable speed ball mills.

A synchronous motor is not a self-starting machine. Therefore the various methods to start the synchronous motor are:

• Using a small D.C machine coupled to it
• Using pony motors
• Using damper winding
• As a slip ring induction motor

Damper winding:

The damper winding in synchronous motor performs two functions:

• Prevents hunting (damped out oscillations)
• Provides starting torque (made self-starting)

Under normal running conditions, damper winding does not carry any current

The starter is used for any high rated machines to limit the starting current. So, here to limit starting current start-delta starter is preferred.

Therefore starting the synchronous motor is not so easy. 

Hence, we use these motors at specific applications where the load is to be driven at a constant speed and in the applications of infrequent starting.

Applications of synchronous motor:

• Power factor correction device
• Conveyor belts, paper-making machines, ball mills
• Phase modifier, to bring down the terminal voltage