A synchronous motor is a type of AC motor that operates at a constant speed, synchronised with the frequency of the current supply. Unlike asynchronous (induction) motors, synchronous motors do not rely on slip to induce current in the rotor. Instead, synchronous motors are designed in such a way that the rotation of the rotor is directly related to the rotating field produced by the stator.
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The construction of a synchronous motor includes two main components: stator and rotor. The stator is the stationary part of the motor which consists of the stator winding and is connected to the AC supply. When an AC current is supplied, the stator windings produce a magnetic field.
The rotor is the rotating part of the motor, it can be either a silent pole (for low speed applications) or a non-silent pole(for high speed applications).
The working principle of the synchronous motor is based on magnetism between the rotating field of the stator and the magnetic field of the rotor. When AC supply is given to the stator winding, it generates a rotating magnetic field. The speed of this rotating magnetic field is called synchronous speed and is determined by frequency supply and number of poles of the stator. The rotor is energised by the DC supply and produces a constant magnetic field.
Synchronous motors are not self-starting motors. To start these motors, an external means is required to bring the rotor close to synchronous speed. Once the rotor reaches its synchronous speed, it gets synchronised with the stator’s magnetic field to ensure the motor operates at synchronous speed irrespective of load.
Synchronous motors find their applications across various industries due to their constant speed and high efficiency. In industrial applications, they are used where constant speed and high torque are required such as compressors, blowers, and conveyors.
In power generation, synchronous motors are used as synchronous condensers to improve power factor and stabilize the power system. Synchronous motors are also commonly used in HVAC systems, pumps, and fans where maintaining a constant speed is required for consistent airflow and temperature control.
What is a Synchronous Motor| Construction, Working Principle and Applications
The synchronous motor can be broadly classified by its rotor’s construction, excitation methods, and applications.
The main types of synchronous motors are:
Wound rotor synchronous motors are the most traditional type of synchronous motor, widely used in various types of industrial applications.
The defining feature of this synchronous motor is its rotor construction. Unlike other types of synchronous motors, its rotor contains wound field wire coils made up of copper wire. These coils are wound around the rotor core. When these coils are energised, it creates an electromagnet. The rotor also includes slip rings, which are connected to these windings and allow for DC supply to the field coils.
The working of the wound rotor synchronous motor starts with the excitation of the rotor. DC current is supplied to the rotor windings, through slip rings and brushes and creates a strong magnetic field within the rotor.
Simultaneously, the stator, which is connected to the three-phase AC power supply, produces a rotating magnetic field. As this field rotates, it interacts with the magnetic field produced by the rotor and causes the rotor to rotate at the same speed.
Wound rotor synchronous motors offer several advantages that make them suitable for various applications. They provide precise speed control, which is crucial in many industrial applications. These motors also boast efficiency when operating at rated load, contributing to energy saving in large applications. It also has the ability to operate as a leading power factor which makes it useful for power factor correction for various applications.
Permanent magnet synchronous motor represents a significant advancement in electric motor technology. These motors have gained popularity across various industries due to their unique design and operational benefits such as high efficiency with excellent performance.
Unlike other synchronous motors that use electromagnets, permanent magnet synchronous motors use permanent magnets on the rotor. These magnets are made up of high-strength rare earth materials such as neodymium or samarium-cobalt. These magnets create a strong magnetic field without the need for external excitation.
The stator of the permanent magnet synchronous motor is similar to other AC motors. It contains winding that produces a rotating magnetic field. The interaction between the rotating magnetic field of the stator and the permanent magnetic field of the rotor results in smooth rotation of the rotor at synchronous speed.
One of the primary advantages of permanent magnet synchronous motors is their high efficiency. By eliminating the need for rotor winding and associated copper losses, these motors achieve high efficiency.
These motors are also known for their excellent performance. They offer rapid acceleration and acceleration which makes them ideal for servo applications that require precise speed control. Another significant benefit is their low maintenance requirement. The absence of slip rings and brushes which are common wear-and-tear items, reduces the need for regular maintenance.
Permanent magnet synchronous motors found their way into many household appliances where energy efficiency is necessary such as air conditioners, refrigerators, and washing machines. They are a key component in many electric and hybrid vehicles, providing the high efficiency and power density needed for automotive applications.
Reluctance synchronous motor is a unique and important type of synchronous motor. This motor operates on a principle distinct from other types of synchronous motors, which depend on magnetic reluctance to generate torque and movement.
The construction of a reluctance synchronous motor is characterised by its rotor design. Unlike other synchronous motors that use wound coils or permanent magnets in the rotor, the rotor of reluctance synchronous motor is made up of magnetically soft materials such as iron. It features a silent pole structure, with alternating regions of high and low reluctance.
The stator of the reluctance synchronous motor is similar to other AC synchronous motors. It contains windings that are energised with alternating current to produce a rotating magnetic field.
The working of these synchronous motors is based on the tendency of magnetic systems to minimise reluctance. When the stator’s rotating magnetic field is established, the salient pole rotor attempts to align itself with the stator’s magnetic field in a way that creates a path for the least magnetic reluctance. This alignment generates the torque. As the stator field rotates, the rotor continuously seeks the minimum reluctance and results in sustained operation.
One of the key advantages of reluctance synchronous motors is their simple design. The absence of rotor windings or permanent magnets makes it robust and reduces manufacturing costs. These motors also offer high efficiency at high speed. The lack of rotor winding is a significant source of power loss which makes reluctance synchronous motors a popular choice for applications where energy efficiency is necessary.
Reluctance synchronous motors are used in diverse applications. They are traditionally used in textile machines where precise speed is paramount. They are also found in various pumps, and fans where their efficiency at higher speeds can be advantageous.
Hysteresis synchronous motors are a unique and specialised type of three-phase synchronous motor. These motors operate at the principle of magnetic hysteresis, which sets them apart from the other types of synchronous motors.
The construction of a hysteresis synchronous motor is simple. The stator is similar to other types of AC synchronous motors and consists of polyphase windings. However, the rotor is truly different from other synchronous motors. It is made up of hard magnetic materials such as chrome or cobalt steel alloy. This rotor composition is important for its operation and performance.
The working of the hysteresis synchronous motor is based on the magnetic hysteresis of rotor material. When the stator generates a magnetic field it continuously magnetises and demagnetizes the rotor material.
Due to the nature of magnetic hysteresis, there is a lag between the applied magnetic field and the resulting magnetic flux in the rotor. This lag produces a constant torque that causes the rotor to follow the stator field synchronously, giving the motor its defining operational speed.
One of the most notable advantages of hysteresis synchronous motors is their smooth operation. They provide constant torque with minimum vibrations resulting in smooth and quiet rotation. Another notable advantage of these synchronous motors is they are self-starting. They can start and accelerate to synchronous speed while under load, without the need for additional starting mechanisms.
The applications of hysteresis synchronous motors are diverse and found in applications that require constant speed, smooth operation, and low noise.
Hysteresis synchronous motors are used in clock and timing devices where their ability to maintain constant speed is crucial. These synchronous motors are also found in small fans and blowers where quiet operation is essential. They are found in medical equipment where their smooth operation and constant speed capability are valuable attributes.
Each type of synchronous motor has its own set of advantages and limitations, making them suitable for different applications. Factors such as starting requirements, efficiency needs, speed control precision, power factor control, and load characteristics all play a role in selecting the appropriate type of synchronous motor for a given application.
"In synchronous motors, the interaction of magnetic fields ensures rotation stays in perfect rhythm with the alternating current, a testament to precision engineering."
Synchronous motors are a remarkable innovation in electrical engineering, as they combine the elements of both induction motors and direct current motors to deliver exceptional performance and versatility. In this blog, we will uncover the workings principle of synchronous motors, also shedding light on how they synchronize their rotation with the frequency of the alternating current (AC) power supply. Whether you're a beginner or an expert, read this blog to demystify the workings of synchronous motors and appreciate their vital role in powering various applications in the world!
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A synchronous motor is a type of AC motor that operates at a speed directly proportional to the frequency of current supplied. It helps the synchronous motor to maintain synchronous speed regardless of changes in load.
Unlike induction motors, which rely on the principle of electromagnetism and experience a change in speed due to a change in load. However, synchronous motors maintain their synchronous speed with the frequency supply irrespective of changes in load making them ideal for applications that require constant speed control such as clocks, timers, and various industrial processes.
The construction of a synchronous motor includes two main components: stator and rotor. The stator is the stationary part of the motor and consists of three-phase windings similar to the induction motor. While the rotor is the rotating part of the motor, it can be either a salient pole or cylindrical.
Check out a Video by "Electrical Deck" about synchronous motors.
Synchronous motor works on the principle of interaction between the stator and rotor. When a three-phase AC supply is provided to the windings of the stator, they generate a rotating magnetic field (RMF) which interacts with the magnetic field of the rotor, locks the rotor and causes it to rotate at synchronous speed.
However, unlike induction motors, synchronous motors are not self-starting. At idle conditions, the rotor’s magnetic field experiences alternating torque due to the rotating magnetic field (RMF) of the stator, which is insufficient to initiate the rotation of the rotor. To start the rotation, auxiliary methods such as damper windings or an external motor are used to bring the rotor up to synchronous speed before DC excitation is applied.
What is a Synchronous Motor | Construction, Working Principle and Applications
Mechtex MTR5 is a 2.1W synchronous motor with voltage options spanning 12V, 24V, 48V, 110V, and 230V and 1.35 Ncm torque at 50/60Hz rated frequencies, making it well-suited for industries such as Pumps and Actuators, and Industrial Machinery.
The working principle of a synchronous motor is based on the concept of synchronism. In this principle, the rotation of the rotor is synchronised with the frequency of the alternating current (AC) supply. Similar to other AC motors, synchronous motors also consist of a stationary part called the stator.
The stator contains wire coils, supplied with an alternating current (AC) supply. When these coils are energised with an AC supply, they produce a rotating magnetic field. This rotating magnetic field helps the rotor to rotate.
The rotor is the rotating part of the motor. It contains either electromagnets or permanent magnets arranged in specific patterns. These magnets are then arranged with a rotating magnetic field produced by the stator. As a result of this interaction, the rotor starts to rotate.
The most important feature of synchronous motors is their ability to maintain constant speed. They have the ability to synchronise the rotation of the rotor with the frequency of the AC supply.
When a three-phase AC supply is provided to the stator, it gets energised and creates a rotating magnetic field (RMF). Then, the DC excited rotor interacts with the rotating magnetic field (RMF) and then locks itself, which causes the rotor to rotate at constant speed. This constant speed is known as synchronous speed.
The Formula for Synchronous Speed - Ns = 120 x f / P,
where
This formula shows that the speed of a synchronous motor is inherently related to the frequency of the AC power supply, which is typically 50Hz or 60Hz.
Mechtex has a 110V Synchronous motor and a 230V Synchronous Motor that operates at 50Hz or 60Hz frequency. These synchronous motors are used in various industrial machinery, pumps, compressors, wind turbines, and daily life applications.
However, precise speed control is achieved by adjusting the frequency of the AC supply. By increasing or decreasing the frequency of supply, the motor speed can be adjusted to meet the requirements of specific applications.
In some synchronous motors, an additional DC supply is required for the rotor winding to create the magnetic field produced for synchronisation. This process is known as excitation. It helps synchronous motors to maintain their synchronisation with the AC power supply. However, synchronous motors are not self-starting.
A synchronous motor is not self-starting as its rotor requires an external source to rotate at synchronous speed. At a standstill condition, the stator of a synchronous motor generates a rotating magnetic field, which results in the creation of alternating torque for the rotor.
Hence, the rotor experiences no motion. Therefore, in order to bring the rotor into motion, various starting methods are used to start a synchronous motor. Some of the key methods are:
In this method, copper bars are embedded in the rotor poles, forming a squirrel cage-like structure called damper winding. When the motor starts, it operates as an induction motor using this winding to generate torque. As the rotor reaches near synchronous speed, DC excitation is applied to the field winding and allows it to lock with synchronous speed.
In this method, a small external motor is coupled with the shaft of the synchronous motor. This external motor is called a pony motor or auxiliary motor. These motors are used to bring the synchronous motor near to the synchronous speed. Once the synchronous motor reaches the synchronous speed, the pony motor is disconnected from the synchronous motor. The DC excitation is applied to the rotor winding of the synchronous motor to maintain its synchronous speed.
A variable frequency drive (VFD) is used to start a synchronous motor by gradually increasing the supply frequency from zero to its rated value. This controlled acceleration of frequency allows the rotor to speed up and reach its synchronous speed. Once the rotor reaches its synchronous speed, a DC excitation is applied to allow the synchronous motor to run efficiently.
Why Synchronous Motor is not Self-Starting
Synchronous motors are classified based on rotor construction and excitation method. These types cater to various industrial applications where constant speed and high efficiency are essential. Types of Synchronous Motors are:
This synchronous motor uses a rotor with variable reluctance. The rotor aligns with the rotating magnetic field to minimise the reluctance, which makes it suitable for low-power applications used for precision control tasks.
This synchronous motor has permanent magnets embedded in the rotor to create a rotating magnetic field. This eliminates the need for external excitation and produces a constant magnetic field. This motor offers high efficiency, compact design and excellent performance. PMSM motors are widely used in applications that require precise control, such as robotics, electric vehicles, CNC machines, and HVAC systems.
This synchronous motor uses a rotor made up of hard magnetic materials that retain magnetism. In this motor, the torque is generated through the hysteresis effect as the rotor lags behind the stator field. It offers smooth, silent and vibration-free operation with any DC excitation, which makes them ideal for applications like electric clocks, timing devices, audio equipment, and precision instruments.
Types of Synchronous Motor
A synchronous motor is an electric motor that operates at a constant speed with the frequency of AC supply irrespective of change in load, which makes it ideal for various applications.
Here are the most common applications of synchronous motor
Synchronous motor plays an important role in driving industrial machinery such as conveyor systems, mixers, extruders, machine tools, printing presses, and packaging machinery, where constant speed operation is essential for efficiency.
Synchronous motors are used in power plants to drive large generators or pumps and improve the power factor correction to enhance the efficiency of the power generation plants.
Synchronous motors are used in the pump system to drive the rotor of the pump and maintain constant fluid flow.
Synchronous motors are used in clocks and timers to ensure accurate time-keeping over extended periods without any fluctuations.
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