Rotating Machines and Electric Power Devices in PE Power

Rotating machines are fundamental components in electrical engineering, comprising generators and motors, each with distinct functions. This is also the crucial exam topic concerning PE Power per the NCEES ® exam guidelines.

In this blog, we will discuss rotating machines and electric power devices in the PE power exam, with their fundamental concepts and all the technical details. Let’s discuss this in detail.

Rotating Machines

Rotating machines convert mechanical energy into electrical energy or vice versa by employing the principle of electromagnetic induction. These machines play a crucial role in generating and consuming electricity in power systems. Generators and motors are the two primary types of rotating machines in power systems.

Let’s start by discussing these types to understand rotating machines for the Power P.E. Exam.

Types of Rotating Machines

types of rotating machines


Generators are pivotal in converting mechanical energy into electrical energy, governed by Faraday’s Law of electromagnetic induction.

  • Synchronous Generators: These generators produce electricity with a constant frequency synchronized with the grid.
  • Asynchronous Generators (Induction Generators): Commonly used in wind and small hydroelectric power plants, they don’t need synchronization with the grid.
Operating Principle

Faraday’s Law states that a changing magnetic field induces an electromotive force (EMF) in a conductor. In generators, the rotating magnetic field component of the motor within a magnetic field leads to variations in magnetic flux, inducing an EMF in the stator windings. This induced EMF represents electrical power, facilitating the conversion of mechanical energy into usable electrical form.


Motors, conversely, transform electrical energy into mechanical energy, guided by the Lorentz force law.


  • Induction Motors: The most common type, where the rotor is not directly connected to the electrical supply but rotates due to induction from the rotating magnetic field created by the stator.
  • Synchronous Motors: These possess the constant speed of a rotating machine synchronized with the supply frequency.
  • DC Motors: Direct current motors, where the rotor rotates due to the interaction of current and magnetic fields.
Operating Principle

The Lorentz force law asserts that a current-carrying conductor in a magnetic field experiences a force. In motors, the rotor’s current-carrying conductors interact with the magnetic field, inducing a mechanical force that results in the desired rotational motion.

Operating Principles of Generators and Motors

Understanding the foundational principles governing generators and motors is essential for profoundly comprehending their complex frameworks. Let’s uncover the critical fundamental aspects and principles in detail.

1. Faraday’s Law

Faraday’s Law underscores that a changing magnetic field induces an EMF in a conductor.


This principle serves as the basis for generating electrical power in generators. The dynamic interplay of the rotating rotor and the magnetic field induces a varying magnetic flux, leading to EMF induction within the stator windings.

2. Lorentz Force Law

The Lorentz force law is fundamental to converting electrical energy into mechanical energy in motors.


Motors leverage the Lorentz force law, where the interaction between current-carrying conductors in the rotor and the magnetic field induces a mechanical force. This force translates electrical energy into proper rotational motion, defining the motor’s operational dynamics.

Construction and Components of Rotating Machines

The complex and profoundly interconnected design of rotating machines involves harmonizing electric, mechanical, and magnetic components.

Electric Components

The electric components are the stationary windings (stator) and moving windings (rotor).

Windings – These conductive coils play a crucial role in facilitating the flow of electric current, a fundamental aspect of the energy conversion process.

Mechanical Components

The mechanical components, including shafts, bearings, and frames, contribute to the structural integrity and smooth operation of rotating machines.

Shafts – Shafts are responsible for transmitting mechanical power and are integral to the efficient functioning of the machine.

Bearings – Facilitating smooth rotation, bearings reduce friction and wear, ensuring longevity and reliability.

Frames – Providing structural support, frames contribute to the overall stability and durability of the rotating machine.

Magnetic Components

The interaction between the stator and rotor fields is central to creating the rotating magnetic field controlled by field windings.

Rotating Magnetic Field – The rotating magnetic field component generates an electromagnetic field under the torque in a moving coil through the dynamic interaction of the stator and rotor magnetic fields, fostering the essential conditions for energy conversion.

Field Windings – Controlling the intensity of the magnetic field, field windings are instrumental in regulating the overall performance of the rotating machine.

Electric Power Devices

Rotating Machines and Electric Power Devices in PE Power go in parallel. Electric power devices are the primary fueling components that keep the motors and generators running in power systems. Let’s discuss crucial electric power devices like diodes and transformers in detail.


Transformers are essential devices in electrical systems that facilitate the efficient transmission and distribution of electrical energy. They operate based on electromagnetic induction, allowing for voltage transformation between different levels.

Types of Transformers

Step-Up Transformers

The Step-up transformers increase the voltage from the primary to the secondary winding.

Working Principle

When AC voltage is applied to the primary winding, it creates a magnetic field. With more turns than the primary, the secondary winding experiences a proportionally higher induced voltage. This increase in voltage is crucial for long-distance power transmission, reducing energy losses.

Step-Down Transformers

The Step-down transformers decrease the voltage from the primary to the secondary winding.

Working Principle

Like step-up transformers, AC voltage applied to the primary winding generates a magnetic field. With fewer turns, the secondary winding experiences a proportionally lower induced voltage. Step-down transformers are employed near end-users to bring the voltage to a safe and usable level.

Center-Tapped Transformers

The enter-tapped transformers are designed with a center tap on one of the windings, creating two equal halves.

Working Principle

This type of transformer is commonly used in applications requiring a dual-voltage output. When an AC voltage is applied across the primary winding, the center tap is a reference point. The voltage induced in each half of the secondary winding is equal but opposite in phase.

The resulting output consists of two voltages, each half of the total secondary voltage, and can be used individually or combined to provide a center-tapped output.

Semiconductor Diodes

Diodes are semiconductor devices that allow current to flow in one direction only. The most common type is the PN junction diode.

Working Principle

A PN junction is formed by combining a P-type semiconductor (with excess positive charge carriers or “holes”) and an N-type semiconductor (with excess negative charge carriers or electrons).

When a voltage is applied across the diode in the forward bias direction (positive to the P-side and negative to the N-side), it allows current to flow. This occurs as the electric field across the PN junction reduces the potential barrier, enabling the flow of majority carriers (holes from the P-side and electrons from the N-side).

In reverse bias, the potential barrier increases, impeding the flow of majority carriers and preventing current.


thyristors in pe power

Thyristors, or silicon-controlled rectifiers (SCRs), are three-terminal semiconductor devices with anode, cathode, and gate terminals.

Working Principle

Thyristors have an additional control terminal (gate) compared to diodes. In their off-state, no current flows between the anode and cathode.

By applying a positive pulse to the gate terminal, the thyristor enters the “on” state. Once turned on, the gate loses control, and the thyristor conducts until the current drops below a threshold or is reversed.

Thyristors are widely used in power control applications such as motor drives and voltage regulators.

Transistors Made with Diodes as NPN or PNP


Transistors are semiconductor devices with three layers of semiconductor material, forming either NPN (Negative-Positive-Negative) or PNP (Positive-Negative-Positive) configurations.

1. NPN Transistor

The NPN transistor consists of a layer of P-type semiconductor (base) sandwiched between two layers of N-type semiconductor (collector and emitter).

Working Principle

In typical operation, the base-emitter junction is forward-biased, allowing current to flow from the emitter to the base. This causes the NPN transistor to conduct, permitting current flow from the collector to the emitter.

The base current controls the current flow between the collector and emitter, making NPN transistors useful for amplification and switching applications.

2. PNP Transistor

The PNP transistor consists of a layer of N-type semiconductor (base) sandwiched between two layers of P-type semiconductor (collector and emitter).

Working Principle

In regular operation, the base-emitter junction is forward-biased, allowing current to flow from the base to the emitter. This causes the PNP transistor to conduct, permitting current flow from the emitter to the collector.

Like the NPN transistor, the base current controls the current flow between the emitter and collector, making PNP transistors suitable for amplification and switching.

Voltage Regulators and Stabilizers

Voltage regulators and stabilizers are critical in ensuring motors and generators’ efficient and stable operation. Their primary functions include maintaining a constant voltage supply, protecting the equipment from voltage fluctuations, and enhancing the overall reliability of electrical systems.

Voltage Regulators in Motors:

Motor voltage regulators are crucial for maintaining a consistent voltage supply to the motor windings. Fluctuations in voltage can adversely affect the motor’s performance, efficiency, and lifespan.

How Voltage Regulators Work in Motors?
  • Sensing Voltage – The voltage regulator continuously monitors the voltage supplied to the motor.
  • Reference Voltage – A reference voltage represents the desired voltage level for optimal motor performance.
  • Error Detection – Any deviation between the sensed and reference voltage is considered an error. The voltage regulator detects this error.
  • Control Element – The voltage regulator utilizes a control element, often a feedback loop or electronic circuit, to adjust the voltage supplied to the motor. This adjustment compensates for the detected error and ensures the motor receives a stable voltage.
  • Feedback Loop – The feedback loop continuously compares the actual voltage at the motor terminals with the reference voltage. The error signal from this comparison is used to adjust the control element, maintaining the voltage within the desired range.
  • Stabilizing Operation – By regulating the voltage supplied to the motor, the voltage regulator ensures that the motor operates under optimal conditions, improving efficiency and reducing wear and tear.

Voltage Regulators in Generators

Voltage regulators in generators are essential for maintaining a steady and controlled output voltage, regardless of changes in the load or speed of the generator.

How do Voltage Regulators work in Generators?

Sensing Output Voltage – The voltage regulator continuously monitors the output voltage generated by the generator.

Reference Voltage – A reference voltage is set as the target output voltage. This reference voltage represents the desired level for a stable and reliable electrical power supply.

Error Detection – Any deviation between the generated and reference voltage is considered an error. The voltage regulator detects this error.

Control Element – Similar to the motor application, the voltage regulator employs a control element, often in a feedback loop or electronic circuit, to adjust the generator’s output voltage. This adjustment compensates for the detected error, maintaining a stable output.

Feedback Loop – The feedback loop continuously compares the output voltage with the reference voltage. The error signal from this comparison is used to adjust the control element, ensuring that the generator output remains within the desired range.

Stable Power Supply – By regulating the generator output voltage, the voltage regulator contributes to a stable and controlled power supply, critical for various applications, including industrial processes, electrical grids, and emergency power systems.

Uninterruptible Power Supplies (UPS) and their Functionality

An Uninterruptible Power Supply (UPS) is critical in ensuring a continuous and reliable power supply to electronic devices, protecting them from power disturbances and outages. UPS systems safeguard sensitive equipment such as computers, servers, and other critical infrastructure.

Critical Components of a UPS

Battery – The battery is a fundamental component of a UPS. It stores electrical energy and is a backup power source during mains power interruptions.

  • Inverter – The inverter converts DC power from the battery into AC power, providing a seamless transition to battery power when needed.
  • Rectifier/Charger – The rectifier/charger converts AC power from the primary source to DC power to charge the battery. It ensures that the battery is ready to provide backup power when necessary.
  • Static Bypass Switch – The static bypass switch allows the UPS to transfer the load directly to the main power in case of a malfunction or overload. This ensures uninterrupted power flow even during UPS maintenance or failure.
  • Automatic Voltage Regulator (AVR) – The AVR stabilizes the output voltage, compensating for fluctuations or sags in the incoming power. It ensures a constant and reliable voltage supply to connected devices.
  • Monitoring and Control System – The monitoring and control system oversees the UPS’s operation, continuously monitoring input power, battery status, and output power. It provides alerts, alarms, and status information to users.

How Does UPS Work?

Regular Operation (Mains Power Available)

When the main power is available during regular operation, the UPS utilizes the rectifier to convert AC power to DC power to charge the battery. Simultaneously, the inverter converts DC to stable AC power to supply the connected devices.

Power Interruption (Mains Power Failure)

When the UPS detects a mains power failure, the inverter draws power from the battery. This transition is almost instantaneous, ensuring an uninterrupted power supply to connected devices.

Battery Backup

The battery provides backup power during outages, allowing critical systems to continue operating until the mains power is restored or a controlled shutdown is initiated.

Automatic Voltage Regulation (AVR)

The AVR component monitors the quality of incoming power. If there are voltage fluctuations or sags, the UPS adjusts the voltage to maintain a stable output, protecting connected devices from potential damage.

Graceful Shutdown

UPS systems often include software that communicates with connected devices. In the event of an extended power outage, the software initiates a graceful shutdown of connected systems to prevent data loss or damage.

Static Bypass

In case of UPS failure or maintenance, the static bypass switch allows the connected load to be directly powered by the mains, ensuring continuous operation even when the UPS is temporarily out of service.

Concepts to Master Rotating Machines and Electric Power Devices in PE Power

Rotating Machines and Electric Power Devices in PE Power are obeyed by some critical laws and principles. Understanding these principles is crucial for designing, operating, and optimizing the performance of rotating machines. Let’s dive deep into the details.

Electrical Aspects or Parameters

Voltage (V)

Principle: According to Faraday’s Law of electromagnetic induction, the voltage generated in a coil is proportional to the rate of change of magnetic flux through the coil.

Further Study: Faraday’s Law, Ohm’s Law (V = I * R)

Current (I)

Current flow results from the voltage applied and the impedance of the circuit.

Further Study: Ohm’s Law (I = V / R)

Power Factor

The power factor is the ratio of real power to apparent power in an AC circuit. It reflects the efficiency of power utilization.

Further Study: Power factor correction principles involving capacitors to compensate for inductive loads.


Principle: Efficiency is the ratio of output power to input power, indicating how effectively a machine converts input energy to helpful output energy.

Further Study: Energy conservation, efficiency calculations, electrical and mechanical components losses.

Mechanical Aspects

Torque (τ)

Principle: Torque is the turning force applied to a rotating machine, causing it to rotate. In electrical machines, torque is produced by the interaction between the magnetic field and current-carrying conductors.

Further Study: Torque equation in motors and generators.

Speed (N)

Principle: the speed of a rotating machine is its rotational velocity. It is influenced by the applied voltage, load torque, and the machine’s design.

Further Study: Relationship between speed and frequency in AC machines.

Inertia (J)

Principle: Inertia is the resistance of a rotating body to changes in its state of motion. In electrical machines, the rotor’s inertia affects the machine’s response to changes in load or speed.

Further Study: Newton’s laws of motion, the relationship between inertia and acceleration.

Electromagnetic Aspects

Electromagnetic Induction

Changing magnetic fields induces electromotive force (EMF) in conductors, a fundamental principle governing the operation of generators.

Magnetic Fields and Flux

The interaction of magnetic fields in the stator and rotor produces a rotating magnetic field essential for the operation of induction motors and synchronous machines.


Examining rotating machines and power devices in the PE Power exam revealed crucial insights into their control, protection, and fundamental electrical-mechanical aspects.

Integrated into principles like electromagnetic induction, Ohm’s Law, and energy conservation, these vital elements and devices play crucial roles in converting, safeguarding, and supplying power for diverse applications in electrical engineering.

If you are gearing up for PE Power exam preparation, look no further than Study for FE. We have all the resources and tailored PE preparation courses per the NCEES® exam course guidelines. Stay connected with Study for FE to ace your PE exam.


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.