Motor Starting in PE Power

Motor starting is a critical area of expertise you must master in Power Engineering. Motor Staring in PE Power influences the efficiency and reliability of electrical systems.

This process ensures that motors begin operating with the appropriate energy. It is mandatory to maintain system stability and prevent damage. Understanding motor starting in PE Power is a crucial exam topic per the NCEES® and creates your basis for designing and managing effective power systems.

Effective and smooth motor starting is vital because it influences the performance and longevity of motors, which are central to many power and electrical systems. Improper and turbulent starting can cause high current and voltage draw, potentially harming the motor or the entire power system.

This blog will explore the types and operations of motors starting in PE Power, highlighting their impact on system efficiency and operational effectiveness. Let’s have a look at this in detail.

Types of Motors

There are different types of motors, each with different purposes, characteristics, and elements. The common types of motors that you must know to understand motor starting in PE Power include:

  1. Induction Motor
  2. Synchronous Motor
  3. DC Motor

1. Induction Motors

induction motors

Induction motors, also known as asynchronous motors, operate on the principle of electromagnetic induction, where the rotating magnetic field in the stator induces current in the rotor.

Working Principle

Induction motors, or asynchronous motors, operate on a fundamental principle derived from Faraday’s law of electromagnetic induction. When alternating current (AC) flows through the stator winding, it generates a rotating magnetic field. This rotating field is the heart of the induction motor’s operation.

The key here is that the rotor is not supplied with external electricity. Instead, as the magnetic field rotates, it cuts across the conductive bars of the rotor (typically a squirrel cage rotor in most common induction motors). According to Faraday’s law, this changing magnetic field induces a current in the rotor bars.

These induced currents in the rotor will create their magnetic field, which will interact with the original magnetic field from the stator. Lenz’s Law comes into play here, driving the rotor to move in a direction that opposes the cause of its rotation, which in this case is the rotating magnetic field of the stator.

Thus, the rotor starts to rotate. The fascinating aspect of an induction motor is that the rotor never reaches the magnetic field’s speed, known as synchronous speed; there’s always a slight lag, referred to as ‘slip.’

The motor’s torque results from the interaction between these magnetic fields and the slip. As the load varies, so does the slip, offering a natural form of speed regulation. Induction motors are widely appreciated for their ruggedness and simplicity, as they don’t have brushes or commutators, leading to less maintenance and a longer life.

2. Synchronous Motors

synchronous motors

Synchronous motors operate synchronously with the line frequency, with the rotor speed matching the stator’s rotating magnetic field speed.

Working Principle

Synchronous motors operate such that the rotor rotates exactly at the same speed as the stator’s magnetic field – hence the name ‘synchronous.’ The operation begins similarly with an AC power supply to the stator winding, creating a rotating magnetic field. However, a synchronous motor’s rotor is different; it’s either a permanent magnet or electromagnet (excited by direct current).

For the motor to start, initially, it doesn’t immediately rotate at synchronous speed. It needs assistance to get up to speed, often using an auxiliary induction motor mechanism or other starting methods.

Once the rotor reaches near-synchronous speed, an interesting phenomenon occurs. The magnetic field of the rotor locks in phase with the rotating magnetic field of the stator – a phenomenon known as ‘pull-in.’

Post this synchronization, the motor maintains a constant speed regardless of load variations, dictated solely by the supply frequency and the number of poles in the motor. This characteristic of synchronous motors makes them ideal for applications where a constant speed is necessary.

Moreover, they can operate under various power factors – lagging, leading, or unity – which can be advantageous in power systems for power factor correction purposes.

induction vs synchronous motor

3. DC Motors

dc motors

DC motors convert direct current electrical energy into mechanical energy using magnetic fields generated by the armature and field coils.

Working Principle

DC motors transform direct current electrical power into mechanical power through a different mechanism. The core components are the field coils creating a magnetic field, the armature (rotor), and the commutator with brushes.

When DC power is supplied to the field coils, a magnetic field is established in the stator. Simultaneously, the commutator and brushes supply DC power to the armature coils.

As current flows through the armature coil within this magnetic field, a force (Lorentz force) acts on the coil, creating torque, which causes the rotor to turn. The role of the commutator, in conjunction with the brushes, is critical here.

It reverses the current direction in the armature coils as the rotor turns, ensuring that the torque acts in a constant direction, maintaining continuous rotation.

The speed of a DC motor is primarily controlled by manipulating the voltage applied to the armature or by adjusting the field current. This flexibility in speed control is one of the significant advantages of DC motors.

They can provide high starting torque, which is beneficial in applications like electric vehicles or cranes. However, the presence of brushes and commutators means more maintenance is required compared to AC motors, as these components are subject to wear and need regular replacement.

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Motor Starting Methods

Initiating a motor starting involves a series of sequential steps that lead to a safe and effective motor start and getting maximum & consistent output with various components and stages involved.

Different starting methods address specific challenges associated with motor starting, offering solutions that range from simple to more sophisticated. It all depends on the application requirements and the operational constraints. Let’s look at different motor methods starting in PE Power.

Direct-On-Line (DOL) Starting

Direct-on-line (DOL) starting is the simplest form of motor starting method. It involves applying the full line voltage directly to the motor terminals.

Motor Starting Sequence

Application of Full Voltage to the Motor

This is typically achieved through a DOL starter consisting of a contactor and an overload relay for protection.

Initial High Current Draw

When the total voltage is applied, the motor draws a large inrush current, typically 6-8 times its full load current. This high current is due to the motor’s low initial impedance when at a standstill.

Development of High Torque

Applying full line voltage generates a strong magnetic field in the stator. This, in turn, induces a high current in the rotor (since it’s still at a standstill or low speed), resulting in a high starting torque.

Acceleration to Full Speed

The motor then accelerates towards its full speed. The torque decreases as the motor picks up speed due to increased back electromotive force (EMF).

Steady State Operation

Once the motor reaches nearly its full speed, the current draw drops to the standard full-load current. The motor then continues to operate under its load conditions.

Star-Delta Starting

As the name suggests, Star-Delta starting involves initially connecting the motor windings in a star (Y) configuration. Let’s have a look at how it triggers.

Motor Starting Sequence

Initial Connection in Star Configuration

This reduces the voltage across each winding to 1/√3 (about 58%) of the line voltage, thereby proportionally reducing the starting current and starting torque.

Starting in Star Mode

Upon starting, the motor begins to accelerate. The reduced voltage in the star configuration limits the inrush current and starting torque, easing the mechanical and electrical stress on the motor.

Transition Period

After the motor reaches a certain speed (typically around 70-80% of its rated speed), the winding configuration switches from star to delta. This transition is often achieved through a timer or a speed-sensing mechanism in the starter.

Reconnection in Delta Configuration

Once the transition is initiated, the motor windings are reconfigured into a delta (Δ) arrangement. This reconnection allows the full line voltage to be applied across each winding.

Acceleration to Full Speed

After the reconnection, the motor accelerates to its full speed, with the inrush current being significantly less than it would have been under a DOL start.

Autotransformer Starting

In autotransformer starting, an autotransformer is used to apply a reduced voltage to the motor initially.

Motor Starting Sequence

Initial Reduced Voltage Application

The autotransformer has taps at different voltages, typically providing 50%, 65%, and 80% of the full line voltage.

Starting at Reduced Voltage

The motor starts at one of these reduced voltages, significantly lowering the starting current and torque. This is particularly useful for reducing mechanical stresses and limiting the impact on the electrical distribution network.

Gradual Voltage Increase

As the motor accelerates, the voltage applied to the motor is gradually increased. This can be done in steps (changing taps on the autotransformer) or continuously until it reaches the full line voltage.

Full Voltage Application

Once the motor reaches a substantial speed, the starter switches the motor directly across the line, applying full voltage to the motor terminals.

Completion of Start Sequence

The motor now runs at the full line voltage and accelerates to its rated speed, with the autotransformer being bypassed or removed from the circuit.

Soft Starting

Soft starters use solid-state devices to increase the voltage supplied to the motor gradually.

Motor Starting Sequence

Gradual Voltage Ramp-Up

As a “Slow Start,” the start sequence begins with a very low voltage, significantly reducing the starting current and torque.

Controlled Acceleration

The voltage is then ramped up in a controlled manner. This ramp-up can be adjusted in terms of time and voltage increase profile, allowing precise control over the acceleration of the motor.

Reduction of Mechanical Stress

This method of starting is gentle on mechanical components, reducing wear and tear on gears, shafts, and couplings.

Reaching Full Voltage

The voltage continues to increase until it reaches the full line voltage. The duration of this ramp-up is set based on the load characteristics and the desired acceleration profile.

Transition to Normal Operation

Once full line voltage is reached, the motor operates directly online, but with the added benefit of a much smoother start, reducing the overall electrical and mechanical stress on the system.

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Factors to Consider for Motor Starting

For a smooth and reliable motor starting that ensures equipment safety and durability, it is necessary to keep an eye on some crucial factors. Let’s look at these fronts from the standpoint of the industry’s best practices and SOPs.

Voltage and Current Requirements

Standard Industry Ratings

Motors are rated for specific voltages and currents, crucial to optimal performance. Industry standards specify these ratings, such as those set by the International Electrotechnical Commission (IEC) or the National Electrical Manufacturers Association (NEMA). For instance, standard voltage ratings for industrial motors include 230V, 460V, and 575V in North America and 400V in Europe.

Procedures and Precautions

When starting a motor, it’s crucial to ensure that the supply voltage matches its rated voltage. A voltage that is too low can cause the motor to underperform, while a voltage that is too high can lead to excessive current draw and potential damage.

The inrush current at startup is another critical factor; it can be several times the motor’s total load current, which may cause voltage drops in the supply network, affecting other equipment.

Impact of Voltage and Current

The correct voltage and current are imperative for efficient motor operation and to prevent overheating, which can lead to insulation failure. Maintaining the proper voltage helps achieve the desired torque and speed performance of the motor.

Mechanical Stress

Standard Industry Procedures

The mechanical stress on a motor during startup is significant. Industry practices often involve using soft starters or variable frequency drives (VFDs) to reduce this stress. These devices gradually ramp up the motor to its full operating speed, minimizing the mechanical shock and wear on components like bearings, gears, and couplings.

Precautions and Directions

Manufacturers often provide guidelines on ramp-up times and torque settings to minimize mechanical stress. Following these guidelines can extend the lifespan of the motor and the mechanical systems it drives.

Impact of Mechanical Stress

Sudden starts can cause mechanical shock, leading to premature wear or failure of mechanical components. By controlling the startup process, the mechanical stress is significantly reduced, enhancing the reliability and lifespan of the motor and the driven machinery.

Power Quality

Industry Standards

Power quality refers to maintaining a stable and distortion-free power supply. Standards such as IEEE 519 provide guidelines on harmonics and other power quality issues. Motors and their starting methods can significantly impact power quality, especially in facilities with other sensitive equipment.

Procedures and Precautions

Soft starters or VFDs can help mitigate power quality issues by reducing harmonics and voltage drops. It’s also important to correctly size the power supply and consider the cumulative impact of multiple motors starting simultaneously.

Impact of Power Quality

Poor power quality can lead to malfunctions or damage to sensitive electronic equipment. Harmonics generated by motor starts can cause overheating in other electrical components. Voltage drops during motor startup can lead to flickering lights and affect the performance of other machinery.

Maintaining high power quality is essential for the overall stability and efficiency of the electrical and power systems.

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You clearly understand how different types of motors starting in PE power, and the factors affecting them are crucial for the efficient, safe, and reliable operation of electric motors.

Adherence to industry standards and choosing the best motor starting method based on usage and equipment needs ensures not only the longevity of the motor itself but also the overall health of the electrical and mechanical systems in which they operate.

We recommend you read our separate guide on Codes and Standards in PE Power for a detailed account of safety and precaution SOPs.

For PE Power exam preparation, don’t forget to check out valuable resources and tailored exam preparation courses at Study for FE – your go-to place for all things PE.


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.