A synchronous motor is an AC motor that runs at a constant speed synchronised with the frequency of the AC supply. Unlike Induction motors, which experience a slip during the operation. The synchronous operate at a synchronous speed regardless of variation in load. This key feature makes synchronous motors ideal for various industrial applications.
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The construction of a synchronous motor consists of two main components: stator and rotor. The stator is the stationary part of the motor and generates a rotating magnetic field. The rotor is the rotating part of the motor which interacts with the stator’s rotating magnetic field and creates movement.
The working principle of a synchronous motor is based on the interaction between the magnetic fields of the stator and rotor. When AC power is supplied to the stator, it creates a rotating magnetic field. At the same time, the rotor is excited with DC power using external sources and creates a magnetic field. Then the rotor interacts and locks itself with the stator’s magnetic field to ensure synchronous speed movement.
The speed of the synchronous motor is determined by the frequency of the AC supply and the number of poles in the motor using the following formula:
Ns = 120xƒ/P
where,
ƒ is the frequency of supply and P is the number of poles of the motor.
Synchronous motors are widely used in applications where constant speed is essential, such as industrial machines, robotics, conveyors, and precision automation systems. Their ability to maintain a steady speed under varying loads makes them ideal for such environments. Additionally, they are used in power factor correction applications to improve the efficiency of power systems.
A synchronous motor consists of the following components. Read ahead to find out how a synchronous motor functions. Check out the video below by "learnchannel" for a better understanding of how synchronous motors work.
Stator
The stator is the stationary part of the motor and consists of a set of electromagnets, typically evenly spaced around the inner circumference. These electromagnets are energized with AC voltage, creating a rotating magnetic field.
The rotor is the rotating part of the motor and contains a set of permanent magnets or electromagnets. In the case of permanent magnets, they establish a fixed magnetic field. For electromagnets, the rotor field can be excited using direct current (DC) to create a magnetic field.
The synchronous motor rotor must be designed to rotate at the same speed as the rotating magnetic field of the stator. The speed of the rotating magnetic field (synchronous speed) is determined by the frequency of the AC supply and the number of pole pairs on the stator.
When the stator's rotating magnetic field interacts with the fixed magnetic field of the rotor, they try to align with each other due to the magnetic attraction and repulsion forces. This alignment exerts torque on the rotor, causing it to rotate.
As long as the synchronous motor is connected to a stable AC power supply and the load on the motor is within its capabilities, the rotor will maintain synchronisation with the rotating magnetic field of the stator. Thus, the motor will operate at a constant speed, the synchronous speed, regardless of the load.
Synchronous motors need some method to bring them up to synchronous speed when they are initially turned on since they cannot start on their own. This can be done using auxiliary devices like a small induction motor (synchronous motor starting as an induction motor) or external means like DC excitation to establish the rotor's magnetic field before synchronization.
Working Principle of Synchronous Motor
Unlike induction motors, which are self-starting due to the rotating magnetic field produced by the stator, synchronous motors require external means to start. AC synchronous motors consist of a stator with three-phase windings and a rotor that is excited by a DC supply. The three-phase windings in the stator create a rotating magnetic field, while the DC supply provides a constant flux.
However, the torque produced on the rotor of a synchronous motor is pulsating and not uni-directional. This pulsating torque is a result of the varying polarity of the rotor and stator poles. At any given instant, the rotor and stator poles can have the same polarity (N-N or S-S), causing a repulsive force on the rotor.
Where,
Nr= North Rotor Pole
Sr= South Rotor Pole
Ns= North Stator Pole
Ss= South Stator Pole
In the next instant, the polarity can be N-S, resulting in an attractive force. However, due to the inertia of the rotor, it remains in a standstill condition and is unable to rotate in any direction. The direction of the instantaneous torque on the rotor reverses after a half cycle, further preventing the motor from starting on its own.
One key limitation of synchronous motors is that they are not self-starting. Unlike induction motors, synchronous motors require external means to bring the rotor to near synchronous speed before they can be synchronised to the power supply. Let's explore the reasons behind this limitation and examine the different methods used to start synchronous motors.
Using Pony Motors:
The pony motor method involves using a separate motor, known as a pony motor, to drive the main synchronous motor up to near synchronous speed. The pony motor is initially used to bring the rotor to a speed close to the synchronous speed. Once the rotor is near synchronous speed, the main motor is synchronised to the power supply.
The DC excitation is then switched on, causing the rotor field and stator fields to "lock in" and establish magnetic locking. At this point, the pony motor can be disconnected, and the main motor continues to run at synchronous speed.
Utilizing Damper Windings:
Another method to start a synchronous motor is by utilising damper windings. When the three-phase supply is switched on, the motor initially runs up as a normal squirrel cage induction motor. The damper windings, also known as amortisseur windings, consist of short conductors embedded in the rotor.
These windings provide starting torque to the motor and help it reach synchronous speed. Once the motor reaches synchronous speed, the DC excitation is applied to the rotor, and the motor operates as a synchronous motor.
Coupling with a Small DC Machine:
In some cases, a small DC machine can be coupled with the synchronous motor to enable starting. The DC machine is initially made to act as a DC motor, driving the synchronous motor. This helps bring the synchronous motor to near synchronous speed.
Once the rotor is close to synchronous speed, the DC excitation is switched on, and the synchronous motor continues to run at synchronous speed.
Using External Means to Rotate the Rotor:
Another approach to starting a synchronous motor is by using external means to rotate the rotor at a speed near or equal to the synchronous speed. This can be achieved by using an external device such as a diesel engine to drive the rotor. Once the rotor is rotating at the desired speed, the DC excitation is applied, and the motor enters synchronous operation.
To start a synchronous motor, a specific procedure needs to be followed.
Here is a general step-by-step guide:
1. Provide a three-phase AC supply to the stator windings of the synchronous motor. This creates a rotating magnetic field rotating at synchronous speed (Ns).
2. Use an external means, such as a diesel engine, to drive the rotor at a speed very close to or equal to the synchronous speed. This ensures that the rotor is rotating in the same direction as the rotating magnetic field.
3. Switch on the DC supply to the rotor, which creates rotor poles. At a particular instant, the stator and rotor, unlike poles, will face each other, and their magnetic axes will be near each other. This results in a force of attraction between the poles, pulling them into magnetic locking conditions.
4. Once magnetic locking is established, the rotor and stator poles continue to occupy the same relative positions. The rotor experiences a uni-directional torque in the direction of the rotating magnetic field, causing it to rotate at synchronous speed.
5. Once synchronism is achieved, the external device used to rotate the rotor can be removed. The rotor will continue to run at the same speed as the rotating magnetic field, thanks to magnetic locking.
It is important to maintain the DC excitation of the rotor from a DC power supply as long as the motor is operating to ensure the magnetic locking is maintained.
Synchronous motors offer several advantages in terms of efficiency and control. However, their self-starting limitation necessitates the use of external means to bring the rotor to near synchronous speed before synchronization with the power supply can occur.
Methods such as using pony motors, damper windings, coupling with a small DC machine, or employing external means to rotate the rotor are employed to start synchronous motors. By following the proper starting procedure, synchronous motors can be successfully brought to synchronous speed and operate efficiently in various industrial applications.
An electrical machine is a general term used for an electromagnetic device that is used for conversion between electrical and mechanical energy. It can be used as a generator to develop electrical energy or as a motor to develop mechanism energy. A synchronous motor is a type of motor that is used in industries for its constant speed.
Electric Motor
An electric motor is a machine that converts electrical energy into mechanical energy. It has a stator (stationary part) and rotor (moving part). When electrical energy is supplied, the rotor rotates generating rotational mechanical energy. It works completely opposite to that of a generator.
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Electrical motors can be classified into AC and DC motors. While the AC motors are further classified into induction motors and synchronous motors.
A synchronous motor is a type of AC motor whose rotor rotates at the same speed as the rotating magnetic field. The stator’s magnetic field revolves at a speed that depends on the supply frequency known as synchronous speed. Hence the name synchronous motor. The rotor of the synchronous motor is synchronized with the frequency of the supplied current.
When two synchronous generators are running in parallel operation and the prime mover of one generator is shut down. The generator will still run while taking power from the generator line. The alternator supplies the obtained power from the line to its losses. When a mechanical load is connected to it, the machine will run at constant speed. When a machine runs and operates the way mentioned above, it is known as a synchronous motor.
Unlike the induction motor, the synchronous motor does not rely on the induced rotor current instead the rotor has either permanent magnets or field windings that are energized using an external source. In an induction motor, the stator windings generate a rotating magnetic field (RMF) that also induces a current in the rotor. In a synchronous motor, the stator only generates RMF. The rotor’s magnetic field magnetically locks with the revolving RMF and rotates at the same speed known as synchronous speed.
Click image to enlarge
Good to Know: A synchronous motor is the same machine as Alternator or synchronous generator. For example, A synchronous motor can be operated as a synchronous generator (alternator) without changing the rating and design. i.e.
The synchronous motor is structurally similar to an alternator. It operates at a constant speed called synchronous speed, NS. It depends on the supply frequency and the number of poles in the rotor. Synchronous speed is given by
NS = 120f ÷ p
Where
The number of poles depends on the design of the motor and cannot be changed during operation. Therefore, the speed of the synchronous motor only varies with the supply frequency.
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The construction of a synchronous motor is similar to an Alternator or Synchronous generator. it differs from an asynchronous or induction motor based on its rotor design.
A Synchronous motor has two main parts
The stator is the stationary part of the motor. Just like an induction motor, the stator core is made of thin laminated sheets of steel or cast iron of good magnetic quality to reduce hysteresis and Eddy current loss. The core has axial slots for holding the three-phase alternating stator field winding called armature winding.
The stator’s armature winding is supplied with 3-phase power through its input terminal. It is responsible for generating the rotating magnetic field (RMF).
The rotor is the rotating part of a synchronous motor. It has a cylindrical shape and holds the field winding, which is responsible for generating the magnetic field or poles. The rotor is energized using slip rings and a brush assembly, typically with a DC source. Usually, a small DC generator connected to its shaft is used for excitation.
The rotor of a synchronous motor can be designed in one of the following ways.
The term ‘salient’ means ‘pointing outward’. The salient Pole rotor has protruding or projecting poles toward the armature winding. The rotor core is made of a laminated steel sheet for reduction in hysteresis and Eddy current. The field windings are wounded around each pole.
The salient Pole rotor has a large number of poles. It is not suitable for high-speed operation due to its high windage losses (at high speed). It is used in low and medium-speed synchronous motors. Physically, it has a large diameter and small axial length.
Slip rings and brush assembly is used to provide electrical connection between the stationary circuit and rotating part of a machine. It is used to energize the field winding using a DC source.
Such type of rotor has a cylindrical shape rotor made of laminated steel. The core has slots for field windings that are secured with the help of wedges to prevent by pulled out. While the unslotted portion of the core becomes magnetic poles.
It has less number of poles, having smaller diameter and longer axial length. It is costlier than the salient pole rotor. However, the rotor design helps in uniform distribution of flux, mechanical strength, robustness, etc. Therefore such synchronous motors are used for high speed.
Modern synchronous motors use a permanent magnet rotor where the rotor has permanent magnets mounted on its surface. There are no field windings. These magnets generate the necessary field without the requirement of an excitation source. The permanent magnet is made of neodymium-boron-iron since they are easily available and cost-effective. Such rotor does not have a slip ring or brush assembly.
The disadvantage of a permanent magnet rotor is that the motor is not self-starting due to the rotor’s inertia, it can’t follow the fast revolving RMF right at the startup. Therefore VFD (variable frequency drive) is necessary for its operation.
Synchronous motor works on the principle of magnetic locking between the stator RMF (rotating magnetic field) and the rotor magnetic field. As we know, opposite poles attract each other, therefore the RMF poles attract the opposite rotor poles generating a rotating motion.
A synchronous motor is a doubly excited machine i.e. it requires AC and DC supply for both parts stator as well as rotor to achieve synchronism. A three-phase AC is supplied to the stator’s windings to generate RMF. The stator is designed to have the same number of poles as the rotor. These poles rotate at the speed that is in sync with the input frequency f is called synchronous speed. It is given by
NS = 120f / p
A DC supply is provided to the rotor’s windings to generate a fixed magnetic field. As the DC source supplies constant current, the rotor’s magnetic field does not vary. Magnetic poles are generated at the opposite ends of the rotor. The rotor’s poles interact with the RMF of the stator and rotate at the same speed as it attains the synchronous speed.
If the rotor rotates at the same speed as the stator RMF, there is no load torque. The rotor and stator poles align with each other. If a mechanical load is applied, the rotor starts oscillating about its new equilibrium position, this phenomenon is known as ‘hunting‘. The rotor lags a few degrees behind the stator RMF and starts developing torque. As the load is increased the angle between them is increased until the rotor field lags by 90° behind RMF. At this point, the motor provides the maximum available torque called breakdown torque. If the load exceeds this limit, the motor stalls.
A synchronous motor isn’t inherently Self-starting due to the rotor’s inertia. When power is applied, the stator RMF instantly starts revolving with synchronous speed. However, the rotor cannot keep up. To provide the necessary speed for the rotor to synchronize, the following methods are used.
A damper winding is used in the salient pole rotor. It is a short-circuited winding just like in an induction motor. The RMF induces a current in this winding and generates its own magnetic field that interacts with the RMF and produces the required starting torque. When the rotor reaches near synchronous speed, the DC excitation is applied to the rotor field winding and the motor locks into synchronism.
In this method, the motor initially starts as an induction motor using the damper winding. This winding also helps in damping the oscillations due to sudden changes in load.
A pony motor is a small induction motor or DC shunt motor that is connected with the shaft of a synchronous motor. It helps in providing the necessary starting torque. The DC excitation is not applied until the rotor attains speed near synchronous speed. The rotor magnetically locks with RMF and the power supply to the pony motor is cut off.
VFD or variable frequency drive is a device that provides power with adjustable frequency. As we know, synchronous speed depends on the supply frequency. Initially, the frequency is set to a minimum to reduce the synchronous speed. The speed is gradually increased to the desired value or normal speed.
The synchronous motor is mainly classified into two categories based on rotor magnetization.
In such synchronous motor, a DC source is used to excite its rotor through a slip ring. The rotor includes field winding that is magnetized to generate a constant magnetic field that interacts with the stator RMF.
Non Excited Motor
The rotor of such a synchronous motor does not require external excitation to generate the magnetic field. Instead, it is made of material that generates its own field such as in a permanent magnet or with the help of the stator field. Cobalt steel is usually used due to its high retentivity (material that stores magnetic properties).
Non Excited Motor can be further classified into the types
As the name suggests, the rotor is made of a permanent magnet that generates a constant magnetic field. There are no windings, slip rings and brushes. The rotor field locks with the stator RMF and rotates at synchronous speed. Since they are not self-starting and there are no windings in the rotor, it requires VFD to provide gradual increases in starting speed.
The rotor of such synchronous motor is made of a material having high hysteresis loss such as chrome, and cobalt steel. It is a self-starting single-phase motor that runs at synchronous speed. It has two stator windings i.e. ‘main winding’ and ‘auxiliary winding’ to generate the stator RMF. The cylindrical rotor starts rotation due to induced Eddy current, thus it starts like an induction motor. Once it achieves near synchronous speed, the stator RMF locks the rotor into synchronism.
Such synchronous motor works on the principle of reluctance. Under the influence of a magnetic field, a ferromagnetic material will move to complete the magnetic circuit where there is minimum reluctance. Magnetic field lines follow a low reluctance path just like current follows a low resistance path.
Therefore, a squirrel cage type rotor is used with some teeth removed to form a salient pole as well as less reluctance path. The stator is similar to the hysteresis motor having main and auxiliary windings to generate RMF. At startup, the rotor tries to align with RMF and starts revolving in its direction. But due to rotor inertia, RMF passes the alignment position and tries again during the next revolution. Thus the speed gradually increases and eventually reaches the synchronous speed and magnetically locks with the RMF.
Advantages
Here are some advantages of synchronous motor
Disadvantages
Here are some disadvantages of synchronous motor
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Here are some common applications of synchronous motors.
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