Introduction to Synchronous Motor, Working, Types, Construction, Advantages & Applications

Hello friends, I hope all of you are fine. In today’s tutorial, we are going to have a look at an introduction to synchronous motors. Alternating current motors are such devices in the electrical power system that alter electrical power into mechanical power according to the input supply, whether it is single-phase or 3-phase. The most used alternating motors are induction motors and synchronous motors.

The structure of the synchronous motor is comparable to the synchronous generator; any synchronous machine can work as a synchronous motor or generator. The normal ratings of these motors are between one hundred fifty kilowatts and one megawatt, and their speed of revolution is one hundred fifty to eighteen hundred revolutions per minute (rpm). The main disadvantage of this motor is that it is not self-starting, while the induction motor is self-starting. In today’s post, we will have a look at its working principle, construction, starting method, or other related terms. So let’s get started with an introduction to synchronous motors.

Introduction to Synchronous Motor

• The synchronous motor is such a motor in which the speed of rotation of the rotor is identical to the synchronous speed; synchronous speed is the rotational speed of the magnetic field provided at the stator of the motor.

• The stator of the motor is electromagnetic; it produces a field when current is provided to the stator. The rotating speed of the field is called synchronous speed.

• The rotor of a synchronous motor behaves like a permanent magnet or electromagnet, and its field interacts with the stator field, so it rotates in time with the rotation of the stator field.

• A synchronous motor is also known as the “doubly fed” because its rotor and stator both are connected with separate input supplies.

• Generally used alternating current motors are induction and synchronous motors.

• The main difference between these 2 motors is that a synchronous motor rotates at a constant speed that is similar to the stator’s rotating field speed.

• While in an induction motor, slip is required; that means the speed of the rotation of the rotor is less than the speed of the rotating field at the stator.

• A small rating motor is used in different clocks and tape recorders to provide accurate speed for that device.

• The larger motors that are used in the power system perform two functions: the first is a conversion of electrical power into mechanical; the mechanical second is these motors regulate the power factor of the system to unity or almost one.

Working Principle of Synchronous Motor

• For understanding the working principle of the synchronous motor, let’s take a 2-pole motor shown in a given figure.

• As we know, in the case of a synchronous machine, either it is a motor or a generator; its field windings are at the rotor, and armature windings are at the stator, but in induction machines, it is different.

• When the field current is provided to the rotor, then a field at the rotor is produced.

• Then the 3-phase supply provided at the stator also produces the 3-phase current at the stator.

• Due to 3-phase current at the stator, it generates the phase-rotating field at the stator BS.

• So now there are 2 fields that exist in this synchronous motor, and then the field of the rotor BR will try to follow the field of the stator, similar to how 2 magnets that are close to one another will try to line up.

• As the field of the stator BS rotates, the field of the rotor tries to continuously follow the field of the stator.

• If there is a large angle between these 2 fields, then the force applied on the rotor will be higher, so the rotor moves as its field is trying to catch the field of the rotor.

• The main working principle of synchronous motors is that their rotors continually try to follow the rotation speed of the stator field; in this way, the motor runs.

Equivalent Circuit of a Synchronous Motor

• Almost all aspects of synchronous motors, like construction and working, are similar to synchronous generators, but the difference is that a generator converts mechanical power into electrical, and a motor converts electrical into mechanical.

• Due to this, the power flow direction will be against the generator’s power flow, and the current will also reverse.

• So the equivalent circuitry of a synchronous motor will be similar to a synchronous generator’s equivalent circuit, but the current IA flow direction will be opposite to the generator’s.

• The resultant circuit of a synchronous motor is shown in a given figure.

• The per-phase equivalent circuitry of motor is drawn in this figure. 

• If we apply KVL (Kirchhoff’s voltage law) on the equivalent circuit of a synchronous motor then we have these equations.

Vø= E_A + jX_SI_A + R_AI_A

E_A = Vø – jX_S I_A – R_AI_A

• These equations are similar to those of a synchronous generator but the difference is that the sign is different.

Synchronous Motor from a Magnetic Field Perspective

• For further understanding of synchronous motor working, let’s we assume that the synchronous generator is linked with infinity bus bar (this bus has a constant value of frequency and voltage at different values of load connected with it).
• The prime mover of the generator is providing torque to the generator in the direction of rotation of generator.
• The synchronous phasors diagram connected with large power system (infinity bus bar) is shown in the given figure.

• The resultant field figure is shown here.

• The rotor field B_R generates internal generated voltage (E_A), net field (B_net) generates phase voltage (V_ø) and the stator field (E_S) produces (E_stat = -jXsI_A)
• We can see from field figure and phasor diagram of the generator that their rotation direction is anticlockwise.

• For the calculation of torque induced in the generator, we can use above given field figure and can write the equation for torque induced as.

t_ind=kB_R x B_net

t_ind=kB_RB_netsinδ

• If we observe field figure than we can note that t_ind direction is clockwise, opposite to the rotation of the generator.
• We can say that this induced torque opposes the torque applied t_app by the prime mover.

Assume that, in its place of rotating the shaft in the direction of motion, the prime mover abruptly drops power and starts to drag on the generator’s shaft. What happens to the generator now?

• Due to this the speed of rotation of the rotor will decrease and lags the field of the generator. It is shown in given figure.

• Due to the slow speed of the rotor field of the rotor (B_R) will also decrease and lags the net field (B_net) due to this, the operation of the generator abruptly varies.

t_ind=kB_R x B_net

• From this equation, we can see that when the rotor field (B_R) lags the net field (B_net) the direction of torque induced also changes and it follows a counter-clockwise path.
• In simple we can say that the torque is now following the rotation path and the generator becomes a motor.
• The increment in the angle of torque (δ) causes to generate higher torque in the direction of motion till finally the induced torque in the motor is similar to the torque of load at its shaft.
• At this point, the generator is working at synchronous speed but now it is behaving like a motor.

Synchronous Motor V Curve

• The V curve of the synchronous motor is a graphical representation of the current flowing through the stator and the field current flowing through the rotor.

• In the given figure, you can see the graph between the Ia armature current and the field current. If its shape is like the alphabetical ‘V’, it’s called a ‘V curve’.

• The value of power factor P.F. can be varied by changing the value of field current. As we discussed, the value of armature current varies with the alteration in field current.

• Let’s suppose that the synchronous motor is operating without load. So if the value of the field current varies or increases, the armature current Ia starts to decrease and get its lowest value.

• At this point, the power fact value of the motor is ‘1’.

Synchronous Motor Construction

• There are 2 main parts of a synchronous motor: the first one is a rotating part called the rotor, and the second is a static part known as the stator.

• The assembly of an induction motor’s stator is similar to a synchronous motor.

• But the difference in rotor construction is that the rotor of a synchronous motor is connected to the external field supply, and an induction motor lacks this supply, and it shows self-starting behavior.

• The stating part of the synchronous motor consists of windings that are placed in different slots at the stator.

• The winding arrangements in this motor are similar to the windings at the stator of a synchronous generator.

Difference between Synchronous and Induction Motor

Synchronous Motor Applications

• These are some applications of synchronous motors that are mentioned here.

• The main purpose of installation in different power systems is to maintain the power factor.

• These motors are also employed in voltage-regulating applications.

• It is used in such systems where less speed and higher loads are working.

• Compressors mostly consist of synchronous motors.

• Cranks, grinders, or places where high power loads are working where these motors are used.

Difference between Synchronous Motor and Asynchronous Motor

• These are some differences between synchronous and asynchronous motors.

Synchronous Motor Ratings

•  As the physical structure of the synchronous motor is similar to synchronous generator so their rating will also similar.
• The main difference is that the internally generated voltage of high value provides leading P.F instead of lagging, so the effect of extreme If denoted as a rating a leading P.F.
• As the output of the motor is mechanical so a rating of motor will be in HP rather than in KWs. In given figure shows the rating of a large motor.

• The important thing is that small-rating synchronous motors also have a service factor mentioned on the nameplate.

• In simple words, a synchronous motor is more favourable for low-speed and high-power-using applications than an induction motor, so these motors are normally used for such applications where high loads are operating.

Synchronous Motor Advantage

• These are some benefits of synchronous motors.

• The main benefit of this motor is it maintains the power factor of the system.

• Its speed always remains constant whether the load is increasing or decreasing; that makes it most effective for industrial use.

• These motors provide more mechanical stability than an induction motor due to a larger air gap.

• Its power shows linear behaviour with the voltage.

• Its efficiency is higher than an induction motor’s, almost ninety per cent.

Synchronous Motor Disadvantages

• With the advantages and benefits, this motor also has some drawbacks that are described here.
• A synchronous motor is not self-excited it needs a special dc source for this process.
• It also not self-start need special arrangements for self-starting.
• Its price of per kilowatt-hour is larger than an induction motor.
• Due to the presence of a slip ring and carbon brushes its price is higher.

That’s all about the synchronous motor almost each and everything related to the synchronous motor is mentioned in this tutorial. If you have any query ask in the comments. see you in the next tutorial Synchronous Motor Torque-Speed Characteristic Curve.

• I also have written some related articles you can also read them are listed here.I
• Introduction to Synchronous Motor
• Synchronous Motor Torque-Speed Characteristic Curve
• Synchronous Motor Starting Method
• What is Synchronous Condenser (Capacitor)
• Synchronous Motor Power Correction. 
• Types of Synchronous Motor
• Effect of Field Current Changes on a Synchronous Motor.