Generation of a magnetic field by an electric current

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This article is about electromagnetism. For chemistry and atomic physics, see electron excitation

In electromagnetism, excitation is the process of generating a magnetic field by means of an electric current.

An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate (excite) the field, otherwise no power is transferred to or from the rotor. Field coils yield the most flexible form of magnetic flux regulation and de-regulation, but at the expense of a flow of electric current. Hybrid topologies exist, which incorporate both permanent magnets and field coils in the same configuration. The flexible excitation of a rotating electrical machine is employed by either brushless excitation techniques or by the injection of current by carbon brushes (static excitation).

A 100 kVA direct-driven power station AC alternator with a separate belt-driven exciter generator, date c.

Excitation in generators

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A self-excited shunt-wound DC generator is shown on the left, and a magneto DC generator with permanent field magnets is shown on the right. The shunt-wound generator output varies with the current draw, while the magneto output is steady regardless of load variations. A separately-excited DC generator with bipolar field magnets. Separately-excited generators like this are commonly used for large-scale power transmission plants. The smaller generator can be either a magneto with permanent field magnets or another self-excited generator. A field coil may be connected in shunt, in series, or in compound with the armature of a DC machine (motor or generator).

For a machine using field coils, as is the case in most large generators, the field must be established by a current in order for the generator to produce electricity. Although some of the generator's own output can be used to maintain the field once it starts up, an external source of current is needed for starting the generator. In any case, it is important to be able to control the field since this will maintain the system voltage.

Amplifier principle

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Except for permanent magnet generators, a generator produces output voltage proportional to the magnetic flux, which is the sum of flux from the magnetization of the structure and the flux proportional to the field produced by the excitation current. If there is no excitation current the flux is tiny and the armature voltage is almost nil.

The field current controls the generated voltage allowing a power system&#;s voltage to be regulated to remove the effect of increasing armature current causing increased voltage drop in the armature winding conductors. In a system with multiple generators and a constant system voltage the current and power delivered by an individual generator is regulated by the field current. A generator is a current to voltage, or transimpedance amplifier. To avoid damage from progressively larger over-corrections, the field current must be adjusted more slowly than the effect of the adjustment propagates through the power system.

Separate excitation

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Alternator of s diesel generating set, with excitation dynamo above

For large, or older, generators, it is usual for a separate exciter dynamo to be powered in parallel with the main power generator. This is a small permanent-magnet or battery-excited dynamo that produces the field current for the larger generator.

Self excitation

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Modern generators with field coils are usually self-excited; i.e., some of the power output from the rotor is used to power the field coils. The rotor iron retains a degree of residual magnetism when the generator is turned off. The generator is started with no load connected; the initial weak field induces a weak current in the rotor coils, which in turn creates an initial field current, increasing the field strength, thus increasing the induced current in the rotor, and so on in a feedback process until the machine "builds up" to full voltage.

Starting

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Self-excited generators must be started without any external load attached. External load will sink the electrical power from the generator before the capacity to generate electrical power can increase.

Variants

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Multiple versions of self-exitation exist:

• a shunt, the simplest design, uses the main winding for the excitation power;
• an excitation boost system (EBS) is a shunt design with a separate small generator added to temporarily provide an energy boost when the main coil voltage drops (for example, due to a fault). The boost generator is not rated for permanent operation;
• an auxiliary winding is not connected to the main one and thus is not subject to voltage changes caused by the change of the load.

Field flashing

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If the machine does not have enough residual magnetism to build up to full voltage, usually a provision is made to inject current into the field coil from another source. This may be a battery, a house unit providing direct current, or rectified current from a source of alternating current power. Since this initial current is required for a very short time, it is called field flashing. Even small portable generator sets may occasionally need field flashing to restart.

The critical field resistance is the maximum field circuit resistance for a given speed with which the shunt generator would excite. The shunt generator will build up voltage only if field circuit resistance is less than critical field resistance. It is a tangent to the open circuit characteristics of the generator at a given speed.

Brushless excitation

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Brushless excitation creates the magnetic flux on the rotor of electrical machines without the need of carbon brushes. It is typically used for reducing the regular maintenance costs and to reduce the risk of brush-fire. It was developed in the s, as a result of the advances in high-power semiconductor devices.[2] The concept was using a rotating diode rectifier on the shaft of the synchronous machine to harvest induced alternating voltages and rectify them to feed the generator field winding.[3][4][5]

Brushless excitation has been historically lacking the fast flux de-regulation, which has been a major drawback. However, new solutions have emerged.[6] Modern rotating circuitry incorporates active de-excitation components on the shaft, extending the passive diode bridge.[7][8][9] Moreover, their recent developments in high-performance wireless communication[10][11] have realized fully controlled topologies on the shaft, such as the thyristor rectifiers and chopper interfaces.[12][13][14][15][16][17][18]

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Advantages and Disadvantages of Synchronous Motors

Synchronous motors maintain constant speed under varying loads through rotor/stator synchronization. They offer high efficiency but require separate excitation power source and have limited speed control capability.

What is a Synchronous Motor

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A synchronous motor is an AC motor that operates at a constant speed. Unlike other motors, synchronous motors do not depend on the principle of induction for their work. It uses a magnetic field to rotate the rotor at a synchronized speed with the frequency of the AC supply. It means its speed is directly proportional to the power supply. 

Therefore a synchronous motor can maintain a synchronous speed under various load conditions. This makes these motors ideal for applications that require constant speed such as industrial machinery and power plants. 

Synchronous motors have unique construction consisting of a stator and a rotor. The rotor consists of a cylindrical core with evenly placed slots. The field winding of the rotor is placed in the slots. On the other hand,  stator has three-phase winding. When three-phase power is supplied to the stator winding, a rotating magnetic field is generated. Then the magnetic field interacts with the rotor and causes the rotor to rotate at a synchronous speed. 

Synchronous motors are commonly used in various commercial and industrial applications such as pumps, compressors, fans, generators, and conveyor belts. It is also used in various power plants to generate energy at synchronous speed. 

Mechtex 1.4 W synchronous motor ensures reliable, efficient performance and is ideal for pumps, compressors, fans, generators, and conveyor belts. 

Also Read

Introduction of Synchronous Motor

Advantages of Synchronous Motors

Synchronous motors offer various advantages such as precise speed control, high efficiency, power factor correction, and stable operation that make them ideal solutions in various industrial, commercial, and residential applications. And allow synchronous motors to continue to drive innovation and efficiency across diverse sectors. 

Let's take a look at some important advantages of synchronous motors:

• Ability to maintain constant speed

One of the major advantages of the synchronous motor is its ability to maintain constant speed under various load conditions. It means these motors can maintain a constant speed even when the amount of load changes making it ideal for various industrial and commercial applications. 

The synchronous motor maintains a constant speed through the synchronization of the rotor and stator. When electrical current flows through the stator, it generates a magnetic field that interacts with the magnetic field produced by the rotor and causes the rotor to rotate at the same speed as of the magnetic field. This ensures a relationship between the stator and rotor, resulting in smooth and steady operation. 

This unique advantage makes them ideal for applications such as pumps, compressors, and fans where precise and constant speed is required. 

• High Efficiency

Synchronous motors maintain a constant speed regardless of load. Unlike other motors, which may experience slip and loss of efficiency at partial load, synchronous motors operate at high efficiency across the entire operating range due to their ability to operate at synchronized speed. 

By adjusting the current supplied to the rotor, synchronous motors actively control their power factor. This allows them to correct any leading and lagging power factor resulting in optimizing efficiency and reducing energy losses.

It makes synchronous motors a cost-effective solution for various applications such as pumps, fans, conveyor belts, and HVAC systems where energy consumption is required. 

Mechtex 12V Small Synchronous Motor is a prime example of a highly efficient and power factor-optimized solution, making it well-suited for various industrial applications such as pumps, fans, conveyor belts, and HVAC systems where energy conservation and consistent performance are crucial.

• High Starting Torque

Synchronous motors generate a significant amount of torque, allowing them to quickly accelerate heavy loads, without the need of any external starting source such as starters or frequency drives. 

The high starting torque ensures smooth and steady acceleration of the motor and minimizes the risk of wear and tear. It results in a longer life span and reduces maintenance costs. 

Synchronous motors with high starting torque are ideal for applications that require high inertia loads such as large fans, pumps, or conveyor belts. They can overcome the load easily and bring them to operate quickly and efficiently. 

• Long LifeSpan

The most significant advantage of synchronous motors is they have a long lifespan. Unlike other motors, synchronous motors have robust and durable construction, which allows them to operate in heavy loads and harsh environments conditions. 

Additionally, synchronous motors have a simple design with fewer moving parts, which reduces the chances of wear and tear and increases lifespan. Furthermore, synchronous motors have a high load-carrying capacity means they can handle heavy loads without affecting the performance and efficiency of the motor. 

This makes synchronous motors reliable and long-lasting solutions for various industrial and commercial applications. 

• Smooth and quiet Operation

Synchronous motors have a unique advantage over the other motors in that they have a smooth and quiet operation. Synchronous motor stator and rotor create a constant magnetic field which allows them to rotate at a constant speed without any load. This results in less vibrations and quiet operation. 

Additionally, synchronous motors have a high power factor means they require less energy during operation which contributes to smooth and quiet operation. 

Furthermore, synchronous motors lack mechanical commutators which eliminate the sparking and friction and cause noise in motors. 

This makes them ideal for use in applications such as medical equipment, audiovisual systems, and residential or office buildings where noise levels must be minimal. 

Disadvantages of Synchronous Motors

Synchronous motors have a number of disadvantages such as high cost, the need for a separate power source for excitation, limited speed control, and difficulty in starting which makes them less ideal for various industrial and commercial applications. 

Let's take a look at some drawbacks of synchronous motors:

• High Cost

One of the most significant disadvantages of synchronous motors is the high cost. These motors involve complex construction and precision engineering making them more expensive as compared to other types of motors. The stator and rotor must be precisely designed and aligned to ensure the synchronous motor runs smoothly at a constant speed.

Additionally, synchronous motors are made up of high-quality materials such as copper and rare earth magnets which makes them expensive in manufacturing as compared to other types of motors. 

This high cost makes the synchronous motors less accessible for small businesses and individuals with minimal budgets. 

• Need a Separate power source for excitation

Another major disadvantage of synchronous motors is they need a separate power source for excitation. It means in addition to the main power source, synchronous motors require a separate power source is required to create a magnetic field that allows motors to operate at constant speed. 

This is the significant drawback of synchronous motors as it not only adds complexity to their construction but also increases the cost of the motor. Furthermore, if excitation fails, the synchronous motor will not function properly resulting in high downtime and potential losses.

Additionally, the use of separate power sources makes synchronous motors less adaptable to various industrial and commercial applications. 

• Limited Speed Control

Another significant disadvantage of synchronous motors is they have limited speed control. Unlike other types of motors, synchronous motors require a constant current supply at a specific frequency to maintain their synchronous speed. It means their speed cannot be varied easily. It makes them less suitable for applications that require precise speed control.

The speed of synchronous motors can be varied within the range of 5% to 10% leading to additional equipment and increased cost to maintain precise speed control. This inability to vary the speed results in reduced efficiency and increased wear and tear on the synchronous motor. 

• Not Self-Starting

One of the major disadvantages of synchronous motors is they are not self-starting. It means they require an external source of power to start and rotate. It can be a significant drawback as it makes synchronous motor structures more complex and adds extra cost to the motor system. 

Furthermore, a synchronous motor requires speed control to maintain its synchronized speed, adding an external power source leads to inability and potential damage to synchronous motors if variations in power supply are not properly managed for synchronized speed. 

Therefore the lack of self-starting capability of synchronous motors limits their usability and adds challenges in operations. 

Also Read 

Why Synchronous motor is not Self Starting

• Sensitivity to changes in load

Another Disadvantage of synchronous motors is they are sensitive when the load is changed. It means any sudden increase or decrease in load on the motor greatly affects the performance and efficiency of the motor. It is due to synchronous motors fixed speed. As synchronous motor operates at a synchronous speed determined by the frequency of the power supply. 

Any change in load may affect its synchronous speed, resulting in a decrease in torque and an increase in power consumption leading to overheating and causing failures in many cases. 

Additionally, it is difficult to control the speed of the motor when the load is changed which creates a problem for applications that require precise speed control.  

Conclusion

Synchronous motors offer various advantages such as precise speed control, high efficiency, stable operation, and long lifespan. This characteristic makes synchronous motors suitable for various industrial and commercial applications where constant speed is required.

However, the synchronous motor also comes with some drawbacks such as high cost, the need for a separate power source for excitation, limited speed control, and difficulty in starting. These drawbacks add challenges to operations and limit the usability of synchronous motors for various applications. 

Despite these limitations, synchronous motors are irreplaceable solutions in various sectors especially where constant speed and efficiency are required. Advanced technology has addressed some of the disadvantages of synchronous motors and made them adaptable to a wide range of applications. Ultimately, the choice of motor type depends upon the need of the application which determines the most suitable solution. 

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