Synchronous Machines in PE Power

Synchronous machines are electric machines that operate at a constant speed, synchronized with the power system’s frequency. Unlike induction machines, synchronous machines maintain a fixed relationship between the rotor speed and the frequency of the AC power supply. Synchronous machines are commonly used as generators in power plants and sometimes as motors in industrial applications.

For this reason, synchronous machines in PE Power are the critical exam topic per the NCEES® exam guidelines. This detailed guide on synchronous machines in PE Power will help you cover all the details of synchronous motors and generators. Let’s study this in detail.

Fundamentals of Synchronous Machines

Let’s start with discovering how synchronous machines are made, which will further help in understanding the workings of synchronous motors in the following guide.

Construction of Synchronous Machines

Stator

Like induction machines, synchronous machines have a stator with a three-phase winding. This stator winding is connected to the power supply.

Rotor

The rotor of a synchronous machine has either a cylindrical shape or a salient pole structure. It contains a winding connected to a DC power source, creating a magnetic field.

Working of a Synchronous Motor

Synchronous machines require a separate DC power source for rotor excitation. The rotor winding’s DC (direct current) creates a constant magnetic field.

Synchronization

The rotor of a synchronous machine rotates at a speed precisely equal to the synchronous speed determined by the power system frequency and the number of poles.

Locked to Power System Frequency

Synchronous machines are designed to operate at a fixed speed, synchronized with the power system frequency. They maintain a constant relationship between the rotor speed and the frequency of the AC power supply.

Power Factor Control

Synchronous generators allow control over the power factor, making them helpful in improving the overall power factor of a system.

Synchronous Machines vs. Induction Machines

synchronous machines vs induction machines

1. Speed

Synchronous Machines – They operate at a constant speed (synchronous speed).

Induction Machines – They operate slower than the synchronous speed (dependent on the load and slip).

2. Excitation

Synchronous Machines – They require a separate DC power source for rotor excitation.

Induction Machines – They do not require a separate excitation source.

3. Starting

Synchronous Machines – They are usually not self-starting and may require external means (e.g., a starting motor).

Induction Machines – They are Self-starting due to the rotating magnetic field induced by the stator.

4. Power Factor

Synchronous Machines – THeir power factor can be controlled by adjusting the excitation, making them useful for power factor correction.

Induction Machines – Their power factor is generally lower compared to synchronous machines.

5. Applications

Synchronous Machines – They are commonly used as generators in power plants, especially in large-scale power generation (suitable for applications requiring precise speed control and power factor correction).

Induction Machines – They are widely used in various industrial applications, such as pumps, fans, and compressors (commonly used as motors when precise speed control is not critical compared to industry-grade applications).

6. Efficiency:

Synchronous Machines – They generally have higher efficiency compared to induction machines.

Induction Machines – They are known for their efficiency and reliability but may have slightly lower efficiency than synchronous machines.

Synchronous Generators and Their Types

Synchronous generators, also known as alternators, convert mechanical energy into electrical energy through electromagnetic induction. They are a crucial component in power generation systems, commonly used in power plants and other applications requiring large-scale electrical power.

types of synchronous generators

There are two types of synchronous generators.

  1. Salient Pole Synchronous Generator
  2. Non-Salient Pole Synchronous Generator

Salient Pole Synchronous Generator

A salient pole synchronous generator is characterized by a rotor design where the poles extend outward, resembling salient features. These poles are physically separated and exhibit a distinctive appearance, creating a pronounced protrusion on the rotor. The rotor structure’s protruding poles contribute to the generator’s unique design.

Operation and Working
Excitation

The rotor of the salient pole synchronous generator is subjected to DC excitation, establishing a magnetic field within the rotor winding. This excitation process is critical in preparing the generator for power generation.

Magnetic Field Concentration

The salient pole structure concentrates the magnetic field in specific rotor regions. This concentration enhances the generator’s ability to produce high reactive power levels, making it well-suited for applications where reactive power support is crucial.

Applications

The enhanced capability of salient pole generators to adjust reactive power makes them suitable for systems with fluctuating loads. They can efficiently respond to changes in the load profile and maintain stable voltage levels. For instance, they are commonly deployed in power systems where the power factor is low. Their ability to provide substantial reactive power support aids in improving and stabilizing the power factor.

Non-Salient Pole (Cylindrical Rotor) Synchronous Generator

Unline salient pole generators, a non-salient pole synchronous generator features a cylindrical rotor and lacks the protruding poles characteristic of salient pole generators. The poles in this design are evenly distributed around the rotor, resulting in a more uniform appearance without significant protrusions.

Operation and Working
Excitation

Similar to the salient pole generator, the non-salient pole synchronous generator rotor is subject to DC excitation. This excitation process establishes a magnetic field within the rotor winding, initiating the generator’s readiness for power generation.

Uniform Magnetic Flux Distribution

The absence of protruding poles contributes to a more uniform distribution of the magnetic flux across the rotor. This uniformity is advantageous in specific applications where a consistent magnetic field is sufficient.

Applications

The uniform magnetic flux distribution makes non-salient pole generators suitable for applications with stable and predictable load conditions. They can efficiently generate power under relatively constant load profiles. Therefore, they are typically employed in applications where the emphasis is not primarily on high levels of reactive power support. This makes them a perfect fit for general power generation applications where reactive power demands are moderate.

Salient Pole vs. Non-Salient Pole Generator

Salient Pole Synchronous Generator

Voltage Generation

As the rotor starts to rotate at a synchronous speed, it cuts through the magnetic flux produced by the stator windings. This cutting action induces a voltage in the stator windings, following Faraday’s law of electromagnetic induction. The generated voltage in the stator windings becomes the generator’s electrical output, ready to be utilized or fed into the power grid.

Reactive Power Capability

One distinctive advantage of the salient pole structure is its ability to provide substantial reactive power support to the power system. Concentrating magnetic flux around the protruding poles enhances the generator’s reactive power generation capabilities.

This feature makes salient pole synchronous generators particularly suitable for applications where maintaining or improving the power factor is essential, such as in power systems with low power factors or those experiencing fluctuating loads.

Non-Salient Pole Synchronous Generator

Voltage Generation

As the rotor rotates at the synchronized speed, it cuts through the magnetic flux generated by the stator windings. This cutting action induces a voltage in the stator windings, adhering to the principles of electromagnetic induction.

The induced voltage becomes the generator’s electrical output, ready to be utilized for various applications or supplied to the power grid.

Uniform Magnetic Field

The distinctive feature of the non-salient pole generator is the distributed nature of its rotor poles. The cylindrical rotor lacks protruding features, resulting in a more uniform magnetic flux distribution across the rotor surface.

This uniformity contributes to a consistent and evenly distributed magnetic field, which may be advantageous in applications where a more uniform magnetic field distribution is sufficient.

Remote Sensing and Control in Induction or Synchronous Machines

SCADA systems are critical in monitoring, controlling, and optimizing power generation and distribution systems in synchronous and asynchronous machines. With the combination of sensors and communication protocols, SCADA servers and a user-friendly interface contribute to:

  • Improved efficiency
  • Reduced downtime
  • Informed decision-making

Before moving further, read the first part of this series on Induction Machines in PE Power.

How do SCADA Systems Work?

let’s provide a concise summary of the step-by-step instructions on how SCADA systems work:

Data Acquisition

Sensors and RTUs/PLCs collect data from field devices, including temperature sensors, pressure sensors, and flow meters.

Data Transmission

Communication protocols like Modbus, DNP3, or OPC facilitate secure data transmission from RTUs/PLCs to the central SCADA system using communication networks such as Ethernet or radio frequency.

The RTUs and PLCs will be discussed in detail in the following sub-section.

Data Processing

SCADA servers at the central control center receive and process the data, utilizing specialized software for analysis, storage, and visualization.

Human-machine interface (HMI)

SCADA systems provide a user-friendly interface (HMI) for operators to visualize real-time data, historical trends, and control parameters, including graphical displays, charts, and alarms.

Control and Monitoring

Operators in the control room use HMI displays to take control actions, such as adjusting setpoints or activating alarms, based on real-time information. Automated control processes respond to predefined logic and algorithms.

Reporting and Analysis

SCADA systems include reporting tools for generating customized reports and visualizations, supporting performance analysis, compliance reporting, and decision-making.

Alerts and Notifications

SCADA systems can send alerts and notifications to operators and maintenance personnel in case of abnormal conditions, equipment failures, or critical events.

Maintenance and Optimization

Remote access capabilities allow engineers and maintenance personnel to diagnose issues, troubleshoot, and implement software updates without physical presence. Continuous optimization is achieved through the analysis of historical data and ongoing performance monitoring.

Let’s see how SCADA systems are applied in both synchronous and asynchronous machines:

Synchronous Machines

1. Data Acquisition

Field Devices: Sensors, including those for voltage, current, and temperature, are installed on synchronous generators to capture operational parameters.

RTUs/PLCs: Remote Terminal Units (RTUs) or Programmable Logic Controllers (PLCs) are deployed at the generator site to acquire data from sensors and control devices.

2. Data Transmission

Communication Protocols: Protocols like IEC 61850, Modbus, or DNP3 facilitate the transmission of real-time data from RTUs/PLCs to the central SCADA system.

Secure Data Transmission: Security measures, such as encryption, ensure the secure transfer of critical data between the synchronous generator and the central SCADA system.

3. Data Processing

SCADA Servers: Data received from synchronous generators is processed by SCADA servers at the central control center. SCADA software is employed to analyze electrical parameters and monitor machine health.

Database Storage: Processed data is stored in databases for historical analysis, trending, and generating performance reports.

4. Human-Machine Interface (HMI)

HMI Displays: SCADA systems offer a user-friendly interface for operators and engineers to visualize real-time data and monitor the status of synchronous generators.

Control Room Displays: Operators in the control room utilize large displays for monitoring synchronous generators viewing alarms, trends, and key performance indicators.

5. Control and Monitoring

Control Actions: Operators adjust setpoints, control the excitation system, and coordinate the generator’s operation with the power grid based on information displayed on the HMI.

Automated Control: SCADA systems can automate control processes for synchronous generators, modifying excitation levels, reactive power output, or synchronization parameters through predefined logic and algorithms.

6. Reporting and Analysis

Reporting Tools: SCADA systems include reporting tools for performance analysis, compliance reporting, and decision-making regarding the operation of synchronous generators.

Alerts and Notifications: Operators receive alerts and notifications in case of abnormal conditions, equipment failures, or other critical events related to synchronous machines.

Asynchronous (Induction) Machines

1. Data Acquisition

Field Devices: Sensors capturing parameters like current, voltage, and temperature are placed on induction motors for data acquisition.

RTUs/PLCs: Similar to synchronous machines, RTUs or PLCs acquire and transmit sensor data to the central SCADA system.

2. Data Transmission

Communication Protocols: Common communication protocols transmit data from RTUs/PLCs to the central SCADA system.

Secure Data Transmission: Security measures ensure the secure transfer of data between induction motors and the SCADA system.

3. Data Processing

SCADA Servers: SCADA servers process data received from induction motors, analyze electrical parameters, and monitor motor performance.

Database Storage: Processed data is stored in databases for historical analysis, trending, and generating performance reports.

4. Human-Machine Interface (HMI)

HMI Displays: SCADA systems provide a user-friendly interface for operators to visualize real-time data and monitor the status of induction motors.

Control Room Displays: Operators use large displays in the control room for monitoring induction motors viewing alarms, trends, and key performance indicators.

5. Control and Monitoring

Control Actions: Operators adjust setpoints and control parameters based on information displayed on the HMI.

Automated Control: SCADA systems can automate control processes for induction motors, adjusting voltage and frequency through Variable Frequency Drives (VFDs) for optimal speed control and energy efficiency.

6. Reporting and Analysis

Reporting Tools: SCADA systems include reporting tools for performance analysis, compliance reporting, and decision-making related to the operation of induction motors.

Alerts and Notifications: Operators receive alerts and notifications in case of abnormal conditions, equipment failures, or other critical events related to asynchronous machines.

RTUs and PLCs – Everything You Need to Know

Remote Terminal Units (RTUs) and Programmable Logic Controllers (PLCs) are crucial components in SCADA systems, serving as interfaces between field devices and the central SCADA control center. Here’s a detailed explanation of how RTUs and PLCs work in the context of SCADA systems:

RTUs (Remote Terminal Units)

RTUs are deployed in the field near the sensors and other remote devices. Their primary function is to acquire data from various field instruments, such as sensors and meters. RTUs collect analog and digital data, converting these signals into a format that can be transmitted to the central SCADA system.

Let’s have a look at their key characteristics.

Signal Conditioning

In many cases, the raw signals from sensors need conditioning to ensure accuracy and reliability. RTUs may include signal conditioning capabilities, such as amplification or filtering, to prepare the data for transmission.

Processing and Scaling

RTUs process the acquired data locally, scaling analog signals to engineering units and applying any necessary calculations. This preprocessing minimizes the data transmitted over the communication network, optimizing bandwidth usage.

Communication Protocols

RTUs use communication protocols, such as Modbus, DNP3, or IEC 60870, to transmit the processed data to the central SCADA system. These protocols ensure standardized communication and compatibility with SCADA software.

Security Features

RTUs implement security features to protect against unauthorized access and cyber threats. Encryption, authentication, and secure communication channels are often employed to safeguard the integrity and confidentiality of data.

Remote Monitoring and Control

RTUs provide the capability for remote monitoring and control of field devices. Operators in the central control center can send commands to the RTUs to initiate control actions or retrieve additional data from the field.

Distributed Control

In distributed SCADA architectures, multiple RTUs work collaboratively to monitor and control different segments of the overall system. This distributed control approach enhances scalability and redundancy.

PLCs (Programmable Logic Controllers)

PLCs are designed for real-time control of industrial processes. They execute logic control algorithms written in ladder logic or other programming languages. These algorithms define the behavior of the control system based on input conditions.

Let’s see why PLCs are integral in SCADA systems.

Input/Output (I/O) Modules

PLCs are equipped with input and output modules that interface with field devices. Input modules receive signals from sensors, while output modules send control signals to actuators, valves, and other devices.

Scanning Process

PLCs operate in a continuous scanning process. During each scan, the PLC reads input signals from the field, executes the control logic, and updates output signals based on the defined control algorithms.

Programming

PLCs are programmed using specialized software that allows engineers to define the control logic. The programming environment typically includes a graphical interface where users can create and edit control programs.

Communication Interfaces

PLCs feature communication interfaces to exchange data with other devices, including HMI systems, other PLCs, and SCADA systems. Communication protocols, such as OPC, facilitate seamless integration with SCADA software.

Redundancy and Reliability

In critical applications, redundant PLC configurations are often employed to enhance system reliability. Redundant PLCs operate in parallel, with one serving as a backup in case of a failure in the primary unit.

Diagnostic and Fault Detection

PLCs include diagnostic features to detect faults, errors, and abnormalities in the control system. Alarms and error messages generated by the PLC provide valuable information for troubleshooting and maintenance.

Integration with SCADA

PLCs are integral components of SCADA systems. They provide the local control and data acquisition capabilities necessary for monitoring and managing industrial processes. The SCADA system communicates with PLCs to collect data, send control commands, and visualize real-time information.

Conclusion

Now, you have a rich understanding of synchronous machines in PE Power. At the end of this study guide, you will be familiar with the workings of synchronous motors and why they are crucial.

For more reading and a detailed comparison of synchronous and asynchronous (Induction) machines, read our detailed study guide of this series on induction machines in PE Power.

If you are gearing up for PE Power exam preparation – look no further than Study for FE. It is recommended to check all the exam preparation resources and tailored courses offered by Study for FE – Your go-to platform for all things PE.

wasim-smal

Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.