What is Motor Starter?

What is Motor Starter? . A motor starter is an essential device used to safely start, stop, and control electric motors. Acting as an intermediary between the power supply and the motor, it safeguards motors from overcurrent damage, ensures efficient power usage, and facilitates smooth operation. Motor starters play a critical role in industrial machinery, HVAC systems, and household applications.

Why Are Motor Starters Important?

Motor starters are vital for controlling and protecting motors during operation. They prevent sudden voltage spikes and reduce mechanical stress, ensuring the motor’s longevity. Let’s explore their significance in detail:

Protecting Motors from Electrical Damage

Electrical surges or overcurrent can severely damage motors. Motor starters limit the inrush current during startup, protecting the motor windings from overheating and failure.

Facilitating Motor Control

Motor starters enable precise control, allowing users to start, stop, or reverse motor operation with ease. This is particularly crucial in automated and industrial settings.

Types of Motor Starters Based on Starting Methods & Techniques

In industries, various starting techniques are used to start an induction motor. Before discussing the types of motors, here are some of the techniques used in motor starters.

Full Voltage or Across The Line Starter

Such starters directly connect the motor with the power line providing the full voltage. The motors connected through such starters have low power ratings so that they do not create a huge voltage drop in the power line. They are used in an application where motors have low ratings & need to run in one direction.

Full Voltage Reversing Starter

3 phase induction motor’s direction can be reversed by swapping any two phases. Such a starter incorporates two mechanically interlocked magnetic contactors with swapped phases for forward & reverse direction. It is used in an application where the motor needs to run in both directions & the contactors are used to control it.

Multispeed Starter

In order to vary the speed of an AC motor, you need to vary the AC supply frequency or vary the number of poles (by reconnecting the windings in some) of the motor. Such types of starter run the motor in a few pre-selected speeds to meet its applications.

Reduced Voltage Starter

The most common type of starting technique is to reduce the voltage at the starting of the motor to reduce the inrush current that could damage the windings of the motor & also cause a huge dip in voltage. These starters are used for high rated motors.

Types of Motor Starters

Direct Online (DOL) Starter

DOL aka Direct Online Starter is the simplest form of motor starter that connects the motor directly to the power supply. It consists of a magnetic contactor that connects the motor with a supply line & an overload relay for protection against overcurrent. There is no voltage reduction for safe starting a motor. Therefore the motor used with such starters has below 5 hp rating. It has two simple push buttons that start & stop the motor.

Pressing the start button energizes the coil that pulls the contactors together to close the circuit. And pressing the stop button de-energizes the contactor’s coil & pushes its contacts apart thus breaking the circuit. The switch used for turning ON/OFF the power supply can be of any type such as rotary, level, float, etc.

Although, this starter does not provide safe starting voltage the overload relay provides protection against overheating & overcurrent. The overload relay has normally closed contacts that energize the contactor’s coil. When the relay trips, the contactor’s coil de-energize and break the circuit.

Read More : Transformer vs. Induction Motor

Advantages of DOL Motor Starter

it has a very simple & cost-effective design.
It is very easy to understand & operate.
it provides high starting torque due to the high starting current.

Disadvantages of DOL Motor Starter

The high inrush current can damage the windings
The high inrush current causes voltage dip in the power line.
It is not suitable for heavy motors
It can decrease the lifespan of a motor

Stator Resistance starter

Stator resistance starter uses the RVS (reduced voltage starter) technique to start a motor. External resistance is added in series with each phase of a 3 phase induction motor’s stator. The resistor’s job is to reduce the line voltage (subsequently reducing the initial current) applied to the stator.

Initially, the variable resistor is kept at maximum position offering maximum resistance. Therefore the voltage across the motor is minimum (in safe level) due to the voltage drop across the resistor. The low stator voltage limits the starting inrush current that can damage the motor windings. As the motor picks up the speed, the resistance is reduced & the stator phase is directly connected to the power lines.

As the current is directly proportional to the voltage & torque varies to the square of the current, a 2 times decrease in the voltage decreases the torque by 4 times. Thus the starting torque using such a starter is very low & needs to be maintained.

Advantages of stator resistance motor starter

It provides flexibility in starting characteristics.
The variable voltage supply allows smooth acceleration
It can be connected to both star or delta connected motor.

Disadvantages of stator resistance motor starter

The resistors dissipate the power
The starting torque is very low due to voltage reduction
The resistors are quite expensive for large motors.

Rotor Resistance or Slip Ring Motor Starter

This type of motor starter works on a full voltage motor starting technique. It works only on a slip ring induction motor that is why it is also known as a slip ring motor starter.

External resistances are connected with the rotor in star combination through the slip ring. These resistors limit the rotor current & increase the torque. This, in turn, reduces the starting stator current. It also helps in improving the power factor

The resistors are only used during the starting of the motor & it is removed once the motor picks up its rated speed.

Rotor-Resistance-or-Slip-Ring-Motor-Starter

Advantages of Rotor Resistance Motor Starter

It provides a low starting current using full voltage.
Due to high starting torque, the motor can be started under load
This method improves the power factor.
It provides a wide range of speed control

Disadvantages of Rotor Resistance Motor Starter

It only works on slip ring induction motor
The rotor is expensive & heavier.

Autotransformer Starter

Such type of motor starters uses an autotransformer as a step-down transformer to reduce the voltage applied to the stator during the starting stage. It can be connected to both star & delta connected motors.

The autotransformer’s secondary is connected with each phase of the motor. The multiple tapings of autotransformer provide a fraction of the rated voltage. During starting, the relay is at the start position i.e. the tap point providing a reduced voltage for the startup. The relay switches between the tap points to increase the voltage with the speed of the motor. At last, it connects it with the full rated voltage.

As compared to other voltage reduction techniques, it offers high voltage for a specific starting current. It helps in providing a better starting torque.

Advantages of Autotransformer Starter

It provides a better starting torque.
It is used for starting large motors with a significant load.
It also offers manual speed control.
It also offers flexibility in starting characteristics.

Disadvantages of Autotransformer Starter

Due to large size of the autotransformer, such a starter takes too much space.
The circuit is complex & relatively expensive than other starters.

Star Delta Starter

This is another common starting method used in industries for large motors. The windings of 3 phase induction motor are switched between star and delta connection to start the motor.

To start the induction motor, it is connected in star using a triple pole double throw relay. The phase voltage in star connection is reduced by the factor 1/√3 & it reduces the starting current as well as the starting torque by 1/3 of the normal rated value.

When the motor accelerates, a timer relay switches the star connection of the stator windings into the delta connection, allowing the full voltage across each winding. The motor runs at rated speed.

Read More : Servo Motor vs. Stepper Motor

Advantages of Star Delta Starter

Its design is simple & cheap
It does not require maintenance
Provide a low surge current.
It is used for starting large induction motors.
It is best for long acceleration time.

Disadvantages of Star Delta Starter

It works on delta connected motor
There are more wire connections.
It offers low starting torque which cannot be maintained.
There is very limited flexibility is starting characteristics.
There is a mechanical jerk while switching from star to delta.

Soft Starter

The soft starter also uses the voltage reduction technique. It uses the semiconductor switches like TRIAC to control the voltage as well as the starting current supplied to the induction motor.

A phase-controlled TRIAC is used to provide variable voltage. The voltage is varied by varying the conduction angle or firing angle of the TRIAC. The conduction angle is kept at minimum to provide reduced voltage. The voltage is increased gradually by increasing the conduction angle. At maximum conduction angle, the full line voltage is applied to the induction motor & it runs at rated speed.

It provides a gradual & smooth increase in the starting voltage, current as well as the torque. Thus there is no mechanical jerk & provide a smooth operation that increases the life span of the machine.

Advantages of Soft Starter

It provides better control over starting current & voltage
It offers smooth acceleration, thus no jerks.
It reduces the power surges in the system.
Extends the life span of the system
Provide better efficiency & lack the need for maintenance
Its size is small

Disadvantages of Soft Starter

It is relatively expensive
There is energy dissipation in the form of heat

Variable Frequency Drive (VFD)

Just like the soft starter, a Variable frequency drive (VFD) can vary the voltage as well as the frequency of the supplying current. It is mainly used for controlling the speed of the induction motor as it depends on the supply frequency.

The AC from the supply line is converted into DC using rectifiers. The pure DC is converted into AC with adjustable frequency & voltage using pulse width modulation technique through power transistor like IGBTs.

It provides full control over the motor speed from 0 to rated speed. The speed adjust option with the variable voltage provides a better starting current & acceleration.

Variable-frequency-drive-VFD-based-motor-starter

Advantages of Variable Frequency Drive

It provides a better and smooth acceleration for large motor
It offers full speed control with smooth acceleration & deceleration.
It increases the life span due to the absence of electrical & mechanical stress
It offers forward & reverse operation of a motor

Disadvantages of Variable Frequency Drive

It is relatively expensive unless speed control is necessary
There is heat dissipation
VFDs create harmonics in the electric lines which can affect electronic equipment & power factor.

How Motor Starters Work

The manual starter is used for smaller motors where the hand operated lever is manually operated (move the contacts position) to the ON or OFF position. The disadvantage of these kinds of starters is that they need to switch ON after power frailer. In other words, they need manual control for each (ON or OFF) operation. Sometimes, this operation may leads to flow high currents in the motor winding which may burn the motor. This is why it is not recommended in most cases where other alternative motor starters with protection are used such as automatic starters.

Applications of Motor Starters

Motor starters are used across various industries and applications, including:

Manufacturing Plants: To control heavy machinery.
HVAC Systems: For starting and stopping air conditioning compressors.
Water Pumps: To regulate irrigation and water supply systems.
Elevators and Escalators: For smooth and safe operation.
Home Appliances: In devices like washing machines and mixers.

Advantages of Using Motor Starters

Enhanced Motor Protection: Prevents electrical and mechanical damage.
Energy Efficiency: Reduces power wastage during startup.
Longevity: Extends the life of the motor by minimizing wear and tear.
Ease of Use: Simplifies motor control with intuitive interfaces.

Challenges and Limitations

While motor starters are indispensable, they come with some challenges:

Initial Cost: Advanced starters like soft starters can be expensive.
Complex Installation: Some types require professional setup.
Maintenance Needs: Regular upkeep is necessary to ensure optimal performance.

Maintenance Tips for Motor Starters

Regular Inspection: Check components like contactors and relays for wear.
Clean Connections: Ensure terminals are free from dust and corrosion.
Test Overload Relays: Verify their response to overcurrent situations.
Lubricate Moving Parts: Prevent mechanical wear in starters with moving components.

FAQs About Motor Starters

What are the main functions of a motor starter?

Motor starters protect motors from electrical damage, facilitate smooth startups, and allow for easy control of motor operation.

How does a DOL starter differ from a soft starter?

DOL starters connect the motor directly to the power supply, while soft starters gradually increase voltage, minimizing mechanical stress.

What industries commonly use motor starters?

Industries such as manufacturing, HVAC, agriculture, and transportation rely heavily on motor starters.