Motor Starter - Types of Starter & Advantages and Disadvantages

A motor starter is an electrical device or a combination of devices used to start and stop an electric motor safely and efficiently. It provides the necessary control and protection for motors in various industrial and commercial applications. Motor starters are essential components in electrical systems because they prevent motors from drawing excessive current during startup, protect them from overloads, and enable operators to control their operation.

There are several types of motor starters, each designed for specific applications and motor sizes. Here are some common types of motor starters:

Direct-On-Line (DOL) Starter: This is the simplest type of motor starter. It connects the motor directly to the power supply at full voltage when starting. DOL starters are suitable for smaller motors with low inrush currents and no special startup requirements. However, they can cause a sudden surge of current during motor startup, which may lead to voltage sags and disturbances in the electrical system.
Star-Delta Starter: Star-delta starters are used for three-phase induction motors. They initially connect the motor windings in a star configuration (low voltage, high current) for startup to reduce inrush current. After a predetermined time or when the motor reaches a certain speed, the starter switches to a delta configuration (full voltage) for normal operation.
Autotransformer Starter: Autotransformer starters use an autotransformer to reduce the voltage applied to the motor during startup, thus limiting inrush current. Once the motor reaches a certain speed, the autotransformer taps are switched to provide full voltage for running.
Soft Starter: Soft starters are electronic devices that provide a gradual voltage ramp-up to the motor during startup. This gradual acceleration reduces mechanical and electrical stress on the motor, as well as the demand for current from the power supply. Soft starters are ideal for applications where smooth startup and reduced wear and tear are critical.
Variable Frequency Drive (VFD): While VFDs are primarily used for controlling motor speed, they also offer motor starting capabilities. VFDs can provide soft start and stop functions, precise control over acceleration and deceleration, and protection features. They are particularly useful for applications requiring variable speed control and energy efficiency.

Motor starters typically include protective features like overload protection (to prevent the motor from overheating due to prolonged overcurrent), short-circuit protection (to safeguard against short circuits), and phase-failure protection (to prevent damage caused by phase imbalances). Some motor starters also have control elements like start/stop buttons, indicator lights, and control relays for user-friendly operation.

In summary, a motor starter is an electrical device used to safely start and stop electric motors while providing protection against overloads and other electrical faults. The type of motor starter chosen depends on the specific motor, application requirements, and desired control features.

How to Work Direct-On-Line (DOL) Starter

Table of Contents

A Direct-On-Line (DOL) starter is one of the simplest and most commonly used methods for starting three-phase induction motors. It works by connecting the motor directly to the full voltage of the power supply during startup. Here’s how a DOL starter works:

Components of a DOL Starter:

Contactor: A DOL starter includes a contactor, which is an electromagnetic switch that controls the flow of electrical current to the motor. It typically consists of a coil (for electromagnet activation) and a set of contacts (for switching the motor on and off).
Overload Relay: To protect the motor from overheating due to excessive current draw, a DOL starter often includes an overload relay. The overload relay measures the current flowing to the motor and trips if it exceeds a set threshold, thereby disconnecting the motor from the power supply.

Operation of a DOL Starter:
Here is a step-by-step description of how a DOL starter operates:

Starting the Motor:

When the operator presses the start button, it energizes the coil of the contactor in the DOL starter.
The energized coil generates a magnetic field, causing the contacts of the contactor to close.
With the contacts closed, the full voltage from the power supply is applied directly to the motor terminals.

Motor Startup:

The application of full voltage to the motor causes it to start rotating.
However, during startup, the motor typically experiences a high inrush current, which can be several times its rated current. This inrush current can cause voltage drops in the electrical system and mechanical stress on the motor.

Running State:

Once the motor reaches its full speed, it operates in its normal running state, drawing the rated current from the power supply.

Overload Protection:
The overload relay in the DOL starter plays a crucial role in protecting the motor. If the current drawn by the motor exceeds the preset threshold due to an overload condition, the overload relay will trip. When the overload relay trips, it de-energizes the coil of the contactor, opening the contacts and disconnecting the motor from the power supply. This action prevents the motor from overheating and suffering damage.

Advantages and Disadvantages of DOL Starters:
Advantages:

Simplicity: DOL starters are straightforward and easy to install.
Low Cost: They are cost-effective for smaller motors.
High Starting Torque: DOL starters provide high starting torque.

Disadvantages:

High Inrush Current: The high inrush current during startup can cause voltage sags and disturbances in the electrical system.
Mechanical Stress: The abrupt application of full voltage can subject the motor and connected equipment to mechanical stress.
Not Suitable for Large Motors: DOL starters are not suitable for large motors due to the high inrush current.

In summary, a Direct-On-Line (DOL) starter directly connects the motor to the full voltage of the power supply during startup. While it is a simple and cost-effective starting method for smaller motors, it may not be suitable for larger motors where soft starters or other methods are used to reduce inrush current and mechanical stress.

How to Work Star-Delta Starter

A Star-Delta Starter is a common method used to start three-phase induction motors, especially for motors with a higher power rating. This starter reduces the initial high inrush current that occurs during motor startup by initially connecting the motor windings in a star configuration (low voltage, high current) and then switching to a delta configuration (full voltage) for normal operation. Here’s how a Star-Delta Starter works:

Components of a Star-Delta Starter:

Contactor: Like other motor starters, a Star-Delta Starter includes contactors, which are electromagnetic switches used to control the flow of electrical current to the motor.
Timer or Control Logic: A Star-Delta Starter incorporates a timer or control logic that determines the sequence of switching between star and delta configurations.

Operation of a Star-Delta Starter:
Here is a step-by-step description of how a Star-Delta Starter operates:

Star Configuration (Starting State):

When the motor is to be started, the operator initiates the start sequence by pressing the start button.
The control logic or timer in the starter first closes the main contactor’s contacts (the main contactor is connected to the power supply).
Simultaneously, it closes the contacts of the star contactor, which are connected to the motor’s windings in a star (Y) configuration. In this configuration, each motor winding receives a reduced voltage (typically 1/√3 or approximately 58% of the line voltage). As a result, the motor experiences a reduced starting current.
The motor starts rotating in this star configuration with reduced voltage and current.

Transition to Delta Configuration (Running State):

After a preset time or when the motor has reached a certain speed (determined by the control logic or timer), the control system opens the star contactor’s contacts.
It simultaneously closes the contacts of the delta contactor, which are connected to the motor’s windings in a delta (Δ) configuration. In this configuration, each motor winding receives the full line voltage, and the motor continues running at full speed and torque.
The motor is now operating in the delta configuration, drawing full current from the power supply.

Advantages and Disadvantages of Star-Delta Starters:
Advantages:

Reduced Inrush Current: Star-Delta Starters significantly reduce the high inrush current that occurs when starting a motor at full voltage, which helps prevent voltage sags and disturbances in the electrical system.
Mechanical Stress Reduction: The gradual starting process reduces mechanical stress on the motor and connected equipment.

Disadvantages:

Complexity: Star-Delta Starters are more complex than Direct-On-Line (DOL) starters, requiring additional components and control logic.
Cost: They are more expensive due to the need for extra components.
Limited to Certain Motors: Star-Delta starters are typically used for medium to large motors and are less suitable for smaller motors.

In summary, a Star-Delta Starter starts a three-phase induction motor in a star configuration with reduced voltage and current to reduce inrush current and mechanical stress during startup. After a specified time or motor speed is reached, it transitions to the delta configuration for normal operation at full voltage and current.

How to Work Autotransformer Starter

An Autotransformer Starter is a type of motor starter used to reduce the voltage applied to a motor during startup. This reduction in voltage helps limit the inrush current and mechanical stress on the motor during the initial moments of operation. Here’s how an Autotransformer Starter works:

Components of an Autotransformer Starter:

Autotransformer: The primary component of an Autotransformer Starter is an autotransformer. An autotransformer is a single-winding transformer with multiple taps or connections along the winding. It allows for variable voltage output based on the selected tap.
Contactor: Like other motor starters, an Autotransformer Starter includes a contactor, which is an electromagnetic switch used to control the flow of electrical current to the motor.
Timer or Control Logic: Some Autotransformer Starters incorporate a timer or control logic to automate the switching process.

Operation of an Autotransformer Starter:
Here is a step-by-step description of how an Autotransformer Starter operates:

Reduced Voltage Starting State:

When the operator initiates the motor’s startup sequence by pressing the start button, the Autotransformer Starter begins the process in a reduced voltage state.
The main contactor closes, connecting the motor to the autotransformer.
The autotransformer initially applies reduced voltage to the motor’s windings by using one of its taps that is designed for lower voltage output. This tap provides a voltage less than the full supply voltage. As a result, the motor starts with reduced voltage, which reduces the inrush current during startup.

Ramping Up Voltage:

After a predetermined time or when the motor reaches a certain speed, a timer or control logic in the Autotransformer Starter increments the voltage to the motor.
It does this by switching to a tap on the autotransformer that provides a higher voltage output. This gradual increase in voltage is intended to accelerate the motor smoothly and limit the current draw.

Full Voltage State (Running State):

When the motor has reached its full speed or when the control logic determines that it’s time to do so, the Autotransformer Starter switches to a tap that provides the full supply voltage to the motor.
At this point, the motor is running at its normal speed and full torque, drawing the rated current from the power supply.

Advantages and Disadvantages of Autotransformer Starters:
Advantages:

Reduced Inrush Current: Autotransformer Starters significantly reduce the high inrush current that occurs when starting a motor at full voltage, helping prevent voltage sags and disturbances in the electrical system.
Mechanical Stress Reduction: The gradual starting process reduces mechanical stress on the motor and connected equipment.
Suitable for a Range of Motors: Autotransformer starters can be used with a variety of motor sizes and are especially useful for medium-sized motors.

Disadvantages:

Complexity: Autotransformer starters are more complex than Direct-On-Line (DOL) starters, requiring additional components and control logic.
Cost: They are more expensive due to the need for an autotransformer and additional control components.
Less Energy Efficient: Autotransformer starters are not as energy-efficient as Variable Frequency Drives (VFDs) for applications that require variable speed control.

In summary, an Autotransformer Starter starts a motor with reduced voltage to limit inrush current and mechanical stress during startup. It gradually ramps up the voltage to the motor until it reaches full voltage and full speed, at which point the motor operates normally.

How to work Soft Starter

A Soft Starter is an electronic device used to control the acceleration and deceleration of electric motors, particularly three-phase induction motors. Its primary function is to reduce the mechanical stress and electrical inrush current during motor startup and stoppages. Soft starters are commonly employed in applications where smooth and controlled motor operation is essential. Here’s how a Soft Starter works:

Components of a Soft Starter:

Solid-State Semiconductor Devices: The core of a Soft Starter consists of solid-state semiconductor devices, such as thyristors or insulated-gate bipolar transistors (IGBTs). These devices are used to control the voltage and current supplied to the motor.
Control Circuitry: Soft starters include control circuitry that manages the operation of the semiconductor devices. This circuitry can be programmed to provide specific acceleration and deceleration profiles.

Operation of a Soft Starter:
Here is a step-by-step description of how a Soft Starter operates:

Motor Startup:

When the operator initiates motor startup, the control circuitry in the Soft Starter gradually increases the voltage supplied to the motor over a predetermined time period.
Initially, the semiconductor devices are triggered to allow only a fraction of the full voltage to be applied to the motor. This reduces the initial inrush current, which is significantly lower than what would occur in a Direct-On-Line (DOL) starter.
Over time, the voltage supplied to the motor is incrementally increased, allowing the motor to accelerate smoothly and gradually reach its full speed. The control circuitry ensures that the motor follows a predefined acceleration profile.

Motor Operation (Running State):

Once the motor reaches its full speed, the Soft Starter supplies the motor with the full rated voltage, enabling it to operate at its intended speed and torque.

Motor Deceleration (Shutdown):

When the operator initiates motor shutdown or stoppage, the Soft Starter reverses the process. It gradually reduces the voltage supplied to the motor, allowing it to decelerate smoothly and avoid abrupt stops.
This controlled deceleration reduces mechanical stress on the motor and connected equipment.

Advantages and Disadvantages of Soft Starters:
Advantages:

Reduced Inrush Current: Soft Starters significantly reduce the high inrush current that occurs when starting a motor at full voltage, helping prevent voltage sags and disturbances in the electrical system.
Mechanical Stress Reduction: The controlled acceleration and deceleration reduce mechanical stress on the motor, gears, belts, and other connected equipment.
Extended Motor and Equipment Lifespan: By minimizing mechanical and electrical stress, Soft Starters can extend the lifespan of motors and associated components.

Disadvantages:

Cost: Soft Starters are more expensive than DOL starters.
Complexity: They require additional control circuitry and semiconductor devices.
Limited to Fixed Speed: Soft Starters do not provide variable speed control; they only control the acceleration and deceleration of the motor to its full speed.

In summary, a Soft Starter gradually controls the voltage supplied to a motor during startup and stoppage, enabling smooth acceleration and deceleration while reducing inrush current and mechanical stress. This makes them suitable for applications where controlled motor operation is critical, such as conveyor systems, pumps, and compressors.

How to Work Variable Frequency Drive

A Variable Frequency Drive (VFD), also known as an adjustable frequency drive or inverter drive, is an electronic device used to control the speed, torque, and direction of an electric motor. VFDs are commonly used in industrial and commercial applications where precise control of motor speed is required. Here’s how a VFD works:

Components of a Variable Frequency Drive:

Rectifier: VFDs typically have a rectifier circuit at the input that converts the incoming AC power (usually three-phase AC) into DC voltage. This rectified DC voltage is then used to generate variable-frequency AC.
DC Bus: The rectified DC voltage is smoothed and stored in a DC bus, often with a capacitor bank. The DC bus serves as an intermediate energy storage component.
Inverter: The inverter is the key component of the VFD. It takes the DC voltage from the DC bus and converts it back into AC voltage, but with an adjustable frequency and voltage level. The inverter uses semiconductor devices like insulated-gate bipolar transistors (IGBTs) to rapidly switch the voltage output.
Control Logic and Microcontroller: VFDs include control logic and microcontrollers to monitor and adjust various parameters, such as motor speed, voltage, and current. The control logic uses feedback from sensors to maintain the desired motor performance.

Operation of a Variable Frequency Drive:
Here is a step-by-step description of how a VFD operates:

Motor Speed Control:

When the motor is started, the VFD initially applies a low-frequency, low-voltage output to the motor. This ensures a smooth and gradual startup, reducing inrush current and mechanical stress on the motor.
As the motor accelerates, the VFD gradually increases the frequency and voltage supplied to the motor. The control logic adjusts the output to maintain the desired motor speed. The VFD can control the motor’s speed across a wide range, from very slow to full speed.
The VFD continuously monitors the motor’s performance using feedback from sensors, such as speed sensors or current sensors. It adjusts the output frequency and voltage as needed to maintain the desired motor speed and torque, even when the load conditions change.

Direction Control:

VFDs can also change the direction of motor rotation by reversing the phase sequence of the output voltage.

Energy Efficiency:

VFDs are known for their energy-saving capabilities. By adjusting the motor speed to match the load requirements, they reduce energy consumption compared to running the motor at a constant speed.

Soft Start and Stop:

VFDs provide soft start and stop functions, gradually ramping up the motor’s speed during startup and slowing it down during shutdown. This reduces mechanical stress and wear on the motor and connected equipment.

Protection and Diagnostics:

VFDs often include built-in protection features. They can monitor motor conditions and respond to faults or abnormal situations, such as overcurrent, overheating, or phase imbalances, by shutting down the motor or generating alarms.

Harmonic Mitigation:

VFDs can help mitigate harmonic distortion in the electrical system. When motors start and stop abruptly, they can introduce harmonics into the power supply. VFDs can smooth out these voltage fluctuations, improving the overall quality of the electrical power in the system.

In summary, a Variable Frequency Drive (VFD) controls the speed, torque, and direction of an electric motor by adjusting the frequency and voltage of the electrical power supplied to the motor. VFDs offer precise motor control, energy efficiency, soft start and stop capabilities, protection features, and harmonic mitigation, making them valuable in various industrial and commercial applications.