Working Principle of Synchronous Motor

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The working of a synchronous motor is based on the principle of synchronism. Synchronous motors consist of a stationary part called the stator. The stator contains wire coils, supplied with an alternating current(AC) supply, and produces a rotating magnetic field.

"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 working of synchronous motor and appreciate their vital role in powering various applications in the world!

What is a Synchronous Motor?

A Synchronous motor is an electric motor that combines the features of both induction motors and direct current motors. Its name originates from its unique design including elements of both induction and direct current motors to achieve performance and versatility in operation. 

Typically, synchronous motors operate at a constant speed, by efficiently synchronizing with the frequency of the alternating current power supply. This constant speed regulation enables precise control over the rotational speed and makes them ideal for various applications where consistent movements are required. 

Synchronous motors offer high efficiency and power factor correction capabilities. They actively adjust the power factor optimising the energy consumption and reducing power loss. This feature makes synchronous motors ideal for applications where energy utilization is important. Moreover, synchronous motors also offer excellent torque and starting performance. They deliver high torque at low speed to ensure smooth operation across a wide range of applications. 

Common types of synchronous motors include permanent magnet synchronous motors (PMSM), synchronous reluctance motors (SynRM), and 3-phase synchronous motors each offering unique advantages for different applications. These motors come in various sizes and diameters to cater to different torque and power requirements. These motors can be customized to meet specific torque, speed, and size requirements of diverse applications across various industries. 

Synchronous motors are used in a wide range of industrial and commercial applications such as industrial machinery, power generation plants, pumps, textile machines, and renewable energy systems where precise control and energy efficiency are required. 

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Basics of Synchronous Motors

Working of Synchronous Motors

The working of a synchronous motor is based on the principle of synchronism. In this principle, the rotation of the rotor is synchronized with the frequency of the alternating current (AC) supply. Similar to AC motors, synchronous motors also consist of a stationary part called the stator. The stator contains wire coils, supplied with alternating current (AC) supply. When these coils are energised with an AC supply, it produces 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 synchronize the rotation of the rotor with the frequency of AC supply. 

When the rotor rotates with the magnetic field produced by the stator, an electromotive force (EMF) is induced in rotor windings. This EMF creates a magnetic field in the rotor, which aligns itself with the magnetic field of the stator. As a result, the rotor rotates at the same speed as the rotating magnetic field. The speed of synchronous motors is inherently related to the frequency of the AC power supply. The standard frequency of the synchronous motor is 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 live applications.

However, precise speed control is achieved by adjusting the frequency of 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 rotor winding to create the magnetic field produced for synchronization. This process is known as excitation. It helps synchronous motors to maintain their synchronization with the AC power supply. However synchronous motors are not self-starting. Unlike other AC motors, which can start and operate without any external source, synchronous motors require an external source for initial rotation to synchronize with the magnetic field produced by the stator. Once synchronous motors are synchronized, they operate efficiently. 

However synchronous motors are not self-starting. Unlike other AC motors, which can start and operate without any external source, synchronous motors require an external source for initial rotation to synchronize with the magnetic field produced by the stator. Once synchronous motors are synchronized, they operate efficiently.

To know more why synchronous motors are not self-starting. Read our blog https://mechtex.com/blog/why-synchronous-motor-is-not-self-starting

A synchronous motor is one in which the rotor normally rotates at the same speed as the revolving field in the machine. The stator is similar to that of an induction machine consisting of a cylindrical iron frame with windings, usually three-phase, located in slots around the inner periphery. The difference is in the rotor, which normally contains an insulated winding connected through slip rings or other means to a source of direct current (see figure).

The principle of operation of a synchronous motor can be understood by considering the stator windings to be connected to a three-phase alternating-current supply. The effect of the stator current is to establish a magnetic field rotating at 120 f/p revolutions per minute for a frequency of f hertz and for p poles. A direct current in a p-pole field winding on the rotor will also produce a magnetic field rotating at rotor speed. If the rotor speed is made equal to that of the stator field and there is no load torque, these two magnetic fields will tend to align with each other. As mechanical load is applied, the rotor slips back a number of degrees with respect to the rotating field of the stator, developing torque and continuing to be drawn around by this rotating field. The angle between the fields increases as load torque is increased. The maximum available torque is achieved when the angle by which the rotor field lags the stator field is 90°. Application of more load torque will stall the motor.

For more Asynchronous Motor for Mill Machine Manufacturersinformation, please contact us. We will provide professional answers.

One advantage of the synchronous motor is that the magnetic field of the machine can be produced by the direct current in the field winding, so that the stator windings need to provide only a power component of current in phase with the applied stator voltage—i.e., the motor can operate at unity power factor. This condition minimizes the losses and heating in the stator windings.

The power factor of the stator electrical input can be directly controlled by adjustment of the field current. If the field current is increased beyond the value required to provide the magnetic field, the stator current changes to include a component to compensate for this overmagnetization. The result will be a total stator current that leads the stator voltage in phase, thus providing to the power system reactive volt-amperes needed to magnetize other apparatuses connected to the system such as transformers and induction motors. Operation of a large synchronous motor at such a leading power factor may be an effective way of improving the overall power factor of the electrical loads in a manufacturing plant to avoid additional electric supply rates that may otherwise be charged for low power-factor loads.

Three-phase synchronous motors find their major application in industrial situations where there is a large, reasonably steady mechanical load, usually in excess of 300 kilowatts, and where the ability to operate at leading power factor is of value. Below this power level, synchronous machines are generally more expensive than induction machines.

The field current may be supplied from an externally controlled rectifier through slip rings, or, in larger motors, it may be provided by a shaft-mounted rectifier with a rotating transformer or generator.

A synchronous motor with only a field winding carrying a direct current would not be self-starting. At any speed other than synchronous speed, its rotor would experience an oscillating torque of zero average value as the rotating magnetic field repeatedly passes the slower moving rotor. Normally, a short-circuited winding similar to that of an induction machine is added to the rotor to provide starting torque. The motor is started, either with full or reduced stator voltage, and brought up to about 95 percent of synchronous speed, usually with the field winding short-circuited to protect it from excessive induced voltage. The field current is then applied and the rotor pulls into synchronism with the revolving field.

This additional rotor winding is usually referred to as a damper winding because of its additional property of damping out any oscillation that might be caused by sudden changes in the load on the rotor when in synchronism. Adjustment to load changes involves changes in the angle by which the rotor field lags the stator field and thus involves short-term changes in instantaneous speed. These cause currents to be induced in the damper windings, producing a torque that acts to oppose the speed change.

Protection for synchronous motors is similar to that employed with large induction motors. Temperature may be sensed in both the stator and field windings and used to switch off the electric supply. Considerable heating occurs in the rotor-damper winding during starting, and a timer is frequently installed to prevent repeated starts within a limited time interval.

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