What are different types of motor starting methods?

Three-phase induction motors of large power ratings draw high starting currents which can affect the grid voltage and cause a voltage drop to other electrical loads on the same grid.

That’s why we use motor-starting methods. A motor starting method is a method that is used for the safe and smooth starting of an electric motor.

That’s exactly what happened with us in my work, when trying to start a large induction motor, the large voltage drop value caused the motor under-voltage protection to stop starting. We used a Soft Starter to run the motor.

So, the main functions of a motor starter include a safe start and protection from the high starting current impact.

Why is it important to use motor motor-starting method?

Using a motor starting method is important for several reasons, primarily to ensure the efficient and reliable operation of electric motors and to prevent damage to the motor itself and other connected equipment. Here are some key reasons why using a motor-starting method is essential:

  1. Limiting Inrush Current: Starting methods help in limiting the high inrush current that occurs when a motor is started directly across the line. This prevents voltage drops in the power supply system and reduces the risk of electrical disturbances and equipment damage.
  2. Reducing Mechanical Stress: By controlling the torque during startup, starting methods help in reducing the mechanical stress on the motor and connected machinery. This prevents wear and tear, extending the lifespan of the equipment and minimizing the need for frequent maintenance or repairs.
  3. Preventing Voltage Fluctuations: Certain starting methods help in preventing voltage fluctuations and dips in the power supply system, ensuring a stable and reliable power supply for other connected devices and equipment.
  4. Energy Efficiency: Some starting methods, such as soft starters and variable frequency drives (VFDs), help in reducing energy consumption by controlling the speed and torque of the motor more efficiently. This leads to energy savings and improved overall system efficiency.
  5. System Protection: Implementing a suitable starting method protects the motor from electrical faults, overcurrent, and other potential issues that could lead to motor burnout, system downtime, or even safety hazards in the working environment.

By using the appropriate motor starting method, you can ensure a smooth and controlled start, minimize operational risks, improve energy efficiency, and extend the lifespan of the motor and associated equipment, ultimately contributing to a more reliable and efficient industrial or commercial operation.

Motor starting methods

Motor starting methods are techniques used to start electric motors. These methods are employed to control the starting current and torque of the motor, prevent mechanical stress, and minimize voltage dips in the power supply. Some common motor-starting methods include:

  1. Direct On Line (DOL) Starting: The motor is directly connected to the power supply, applying full line voltage to the motor from the beginning. This method is simple and cost-effective but can cause high inrush currents.
  2. Star-Delta Starting: This method is commonly used for large three-phase induction motors. Initially, the motor is started with the windings connected in a star configuration, reducing the starting current. Once the motor reaches a certain speed, the windings are switched to a delta configuration for normal operation.
  3. Auto-Transformer Starting: An auto-transformer is used to reduce the voltage applied to the motor during starting. This method helps reduce the starting current and is suitable for motors that require reduced voltage during start-up.
  4. Soft Starter: A soft starter is an electronic device that limits the starting current and torque by controlling the voltage applied to the motor during the starting process. This method ensures a smooth start and is commonly used for medium-sized motors.
  5. Variable Frequency Drive (VFD) Starting: A VFD is used to control the motor speed by varying the frequency and voltage supplied to the motor. It allows for precise control over the motor’s acceleration and deceleration, as well as speed regulation during normal operation.
  6. Stator Resistance Starter: A Stator Resistance Starter (SRS) is a starting method used for three-phase induction motors. It involves the insertion of an external resistance in the stator circuit during motor starting.

The selection of the appropriate starting method depends on factors such as the motor type, size, application requirements, and the specific constraints of the power system.

Each method has its advantages and disadvantages, so the choice should be based on the specific needs of the application. Let’s discuss each in more detail.

Stator Resistance starter

A Stator Resistance Starter (SRS) is a starting method used for three-phase induction motors. It involves the insertion of an external resistance in the stator circuit during motor starting.

This method is commonly used for controlling the starting current and torque of the motor, particularly in applications where a smooth start and reduced mechanical stress are required.

The Stator Resistance Starter typically consists of a set of resistors that are gradually cut out from the circuit as the motor accelerates to full speed.

By initially inserting resistance in the stator circuit, the starting current is reduced, preventing excessive voltage drops in the power supply system. As the motor gains speed, the resistance is gradually reduced, allowing the motor to operate at its normal speed without the resistance.

While this method helps in achieving a controlled start and reduces mechanical stress, it also leads to energy losses in the form of heat dissipated by the resistors. Therefore, it is important to properly size the resistors and consider the energy losses when implementing a Stator Resistance Starter.

In many applications, modern soft starters or variable frequency drives have largely replaced the use of Stator Resistance Starters due to their more efficient and sophisticated control over the motor’s speed and torque. These advanced methods offer better control and energy efficiency during both starting and normal operations.

Autotransformer Starter

An Autotransformer Starter is a type of reduced-voltage starting method used for starting three-phase induction motors. It involves an autotransformer, which is a single-winding transformer where part of the winding acts as both the primary and secondary winding.

This method is used to reduce the starting current and voltage during motor start-up, minimizing the impact on the power supply system.

Here’s how an Autotransformer Starter generally works:

  1. The motor is initially connected to the autotransformer, which provides a reduced voltage during the starting period.
  2. The autotransformer helps in reducing the voltage applied to the motor, thus limiting the inrush current and torque during start-up.
  3. Once the motor reaches a certain speed or reaches its full speed, the autotransformer is bypassed, and the motor is then connected directly to the full voltage.

The autotransformer allows for a smooth and controlled start, which helps minimize the mechanical and electrical stresses on the motor and the power system. This method is particularly useful for applications where the starting current must be limited to prevent voltage drops and disturbances in the power supply network.

While Autotransformer Starters are effective in reducing starting current, they do have some disadvantages, including increased cost and size, as well as energy losses in the autotransformer itself.

Therefore, they are typically used for larger motors where reducing the starting current is crucial for the operation of the motor and the stability of the power system.

Motor Soft Starter

motor soft starter
motor soft starter

A motor soft starter is an electronic device that limits the inrush current and torque applied to an electric motor during startup.

It achieves this by controlling the voltage applied to the motor gradually, ensuring a smooth and controlled acceleration. Soft starters are primarily used with AC induction motors to prevent mechanical shock, reduce wear and tear, and minimize disturbances to the power supply system.

Here are some key features and functions of motor soft starters:

  1. Voltage Ramp Control: Soft starters use a voltage ramp to gradually increase the voltage applied to the motor, limiting the inrush current and reducing the mechanical stress on the motor and connected mechanical equipment.
  2. Current Limitation: They help in controlling the starting current, and protecting the motor windings and other electrical components from damage.
  3. Torque Control: Soft starters provide control over the torque applied to the motor, ensuring a smooth and controlled acceleration without sudden jolts.
  4. Built-in Protection: Many soft starters come with built-in protections such as overload protection, under-voltage protection, and overcurrent protection, safeguarding the motor from various electrical faults and abnormalities.
  5. User-Adjustable Parameters: They often allow users to adjust various parameters such as starting voltage, ramp-up time, and current limit to suit the specific requirements of the motor and the application.

Soft starters are widely used in applications where a controlled and gradual start-up is essential, such as conveyor belts, pumps, fans, compressors, and other industrial machinery.

They help extend the lifespan of the motor and the connected equipment while ensuring a stable power supply during the starting process. Additionally, they contribute to energy savings by minimizing the peak current demand during startup.

Motor Star Delta Starter

A motor star delta starter is a type of reduced-voltage starter that is used to minimize the starting current and torque in three-phase induction motors.

This method is commonly employed for applications where a smooth start is necessary, and it is particularly suitable for larger motors. The star delta starter is named for the connection configuration of the motor windings during the starting process.

Here’s how a motor star delta starter typically functions:

  1. Starting in Star Configuration: Initially, the motor windings are connected in a star configuration, which reduces the voltage applied to each winding and consequently reduces the starting current and torque.
  2. Transition to Delta Configuration: After a specific period, once the motor reaches a certain speed or after a predetermined time delay, the windings are switched to a delta configuration for normal operation.

This transition helps in reducing the initial high inrush current, minimizing voltage dips in the power supply, and preventing excessive mechanical stress on the motor and connected equipment during the starting process.

While the star delta starter is effective in reducing starting current and torque, it does require additional switching devices and control circuitry, which increases the complexity and cost of the starter system.

This method is commonly used in applications such as pumps, compressors, and other heavy-duty industrial machinery where a controlled start is necessary to prevent equipment damage and power supply disturbances.

How Does A Star Delta Connection Work?

Star and delta connection
Star and Delta connection

During motor startup, the motor windings are initially connected in a star configuration and then switched to a delta configuration.

This switching process is facilitated by a control and power circuit employing three contactors: one for the delta connection, another for the star connection, and the third for the main contactor.

Further in this article, you will come across the star delta starter control and power diagram.

Now, let’s find out How much star delta connection reduces the starting current.

At the star connection line Current is equal to the phase Current, While Voltage is not the same,

  • IL = Iph
  • Vph = VL/√3

Starting the motor in the Star connection reduces the voltage at starting to be just VL/√3, which is 58% of the motor-rated delta voltage.

How does the line current reduce? During star connection, the current is,  ILY = IphY = VL/(Z√3)

When the motor is switched into delta:

  • VL = Vph
  • IL∆    = √3 Iph =  √3 VL/ Z

Now let’s compare the line current in the star and delta connections of the same motor.

ILY / IL∆ =(VL / √3 Z) / (√3 VL/ Z)

ILY = 1/3 IL∆

This answers the important question: How much star delta connection reduce the starting current?

The answer is, that the star delta starter reduces the motor starting current to a third of the starting current of the delta connection ILY = 1/3 IL∆

N.B: Reducing the voltage also reduces the starting torque by 1/3 of the motor torque, because the torque is proportional to square the voltage.

Which means,

TY / T = (VL/(√3))2 / (VL)2 = 1/3

Circuit Components of Y/∆ Starter

The star delta starter circuit consists of:

  • Three contractors (Main contactor, Star connection, and Delta connection contractors)
  • One timer
  • Thermal overload for motor protection
  • Start green colored push button.
  • Stop the colored push button.
  • Indicator lamps, Red, Yellow, and green for three-phase power. And one more orange for overload protection.
  • Three-phase voltage volt-meter, and current-meter.

Star Delta Wiring and Circuit Diagram:

The Star Delta starter diagram is divided into power and control wiring. Both are in the following wiring diagram.star delta starter diagram

Contactors Selection

Star Delta Starter Contactor Selection is important because, the currents are not the same as the motor’s full current, and also the wiring diagram has three contactors with three tasks.

Contactors’ sizes are smaller than the Direct Line single contactor, because the fact that they are controlling winding currents only, not the line currents.

I’ve written a detailed article about Star Delta vs DOL Starter, read it now for more information.

Main and Delta Contactors

The currents through the motor winding are (58%, where Iph  = IL∆   /√3) of the line current. So, the main and delta contactors of the star delta starter circuit are, AC3 with 58% of the motor current rating.

Star Contactor:

, In the case of star connection, ILY = 1/3 IL∆,  So, the sizing of the star contactor is 1/3 of the full load current of the motor.

Because the star contactor carries star current only, the current in the star configuration is 1/3 of the full voltage (delta) current of the motor.

Y/∆ starter applications

This starting method is not applicable for loads that require high starting torque because the starting torque of the motor is reduced to a third of its full-rated torque. Centrifugal compressors and pumps are an example of this motor-starting method.

DOL starter for motors

DOL starter or Direct online starter is the simplest method used for starting induction motors.

A DOL (Direct-On-Line) starter is a simple starting method for electric motors, where the motor is directly connected to the power supply. It involves applying full line voltage to the motor terminals during startup. The DOL starter is commonly used for smaller motors, typically in applications where the starting current and torque requirements are not critical.

Here’s how a DOL starter generally works:

  1. Simple Connection: The motor is connected directly to the power supply using a contactor and an overload relay.
  2. Full Voltage Application: Upon starting, the contactor is energized, allowing the full line voltage to be applied to the motor windings.
  3. High Inrush Current: This method results in a high inrush current, which can cause voltage dips in the power supply and mechanical stress on the motor and connected equipment.

While the DOL starter is straightforward and cost-effective, it may not be suitable for larger motors or applications where the high starting current could cause issues in the power system.

In such cases, more sophisticated starting methods such as soft starters or variable frequency drives (VFDs) are used to reduce the starting current and provide a smoother and more controlled start for the motor.

DOL Starter circuit components:

Direct online starter is one of the simplest types of motor starters for the induction motor. The device consists mainly of protective devices such as a circuit breaker, main contactor & an overload relay.

The electromagnetic contactor is operated using a thermal overload relay normally closed contact. The Contactor is coil-operated and is controlled by the start and stop push buttons.

What is the simplest of all starting methods for a motor?

The simplest starting method for a motor is the Direct On Line (DOL) starting method. In this method, the motor is directly connected to the power supply, applying full line voltage to the motor from the beginning. This method is straightforward and easy to implement, requiring no additional control circuitry or devices.

However, it’s important to note that DOL starting can cause high inrush currents, which may lead to voltage dips in the power supply and mechanical stress on the motor. For larger motors, this method might not be suitable as it can cause disturbances in the power system. In such cases, soft starters or variable frequency drives (VFDs) are used to reduce the starting current and provide a smoother start for the motor.

When selecting a starting method for a motor, it’s essential to consider the motor size, load, and the specific requirements of the application.

Which type of motor starting method should I use?

The choice of the motor starting method depends on various factors such as the type and size of the motor, the specific requirements of the application, and the constraints of the power supply system. Here are some guidelines to help you decide which motor-starting method to use:

  1. DOL Starter (Direct-On-Line): Use this method for smaller motors where the starting current and torque requirements are not critical, and where the power supply system can handle the inrush current.
  2. Star-Delta Starter: Employ this method for larger motors, typically in applications where a reduced starting current is necessary to prevent voltage dips and mechanical stress. It is suitable for applications like pumps, compressors, and other heavy-duty industrial machinery.
  3. Soft Starter: Opt for a soft starter when you need a smooth and controlled start to reduce mechanical stress and prevent voltage disturbances. It’s suitable for applications such as conveyor belts, fans, and pumps, where a gradual ramp-up of speed is essential.
  4. Variable Frequency Drive (VFD): Choose a VFD when precise control over the motor speed is required throughout the operation, along with the ability to adjust the torque and acceleration. VFDs are suitable for applications where energy efficiency and speed control are crucial, such as in HVAC systems, pumps, and fans.
  5. Autotransformer Starter: Consider using this method for larger motors that require a reduced voltage during startup to limit the starting current. It is suitable for applications where controlling the inrush current is critical, such as in heavy machinery or industrial equipment.

Evaluate the specific requirements of your motor and the application, along with the constraints of your power system, to determine the most appropriate starting method that balances performance, energy efficiency, and system reliability. If you are unsure, consulting with a qualified electrical engineer or a motor expert would be beneficial in making an informed decision.

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