what is a motor starter, types of starter and motor starting methods?

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What is a Motor starter

A motor starter is an electrical device used to safely start or stop an electrical motor. This device can be a combination of contactors, relays, capacitors, and timers with other electrical components. A motor starter is like a guard to the electrical motor to protect the motor from damage. The main functions of motor starters are as follows;

  • To safely start and stop a motor when the need arises.
  • To protect the motor from Low voltage and over current.
  • Reversing the directions of a motor.

The are two major components of a motor starter are; the Contactor and the overload protection circuit.

  • The Contractor: This acts are a switch to the electrical motor starter. It makes and break the supply voltage to the motor by turning ON and OFF supply voltage of the motor. By so doing making the motor to start or stop.
  • Overload protection circuit: This section of the motor starter protects the motor from overloads which can lead to breaking down the motor. When a motor is overloaded it drives in a huge amount of current that can damage the winding. So when the overload circuit senses a high current it breaks the power supply.

Why do we need a starter to run an electrical Motor?

An electrical motor starter is very important when running an induction motor. To avoid burning the winding of the electrical motor, a starter is a must-use especially when running a big motor.

The rotor of the electrical motor has a low impedance. The rotor impedance depends on the slip of the induction motor which is the relative speed between the rotor and stator.

The impedance varies inversely with the slip of the motor and it is at its maximum when the motor is in the rest position. i.e. when the motor is not running. At this point, the impedance is at a minimum and it draws a high current called inrush current. This inrush current is what magnetizes the air gap between the rotor and stator and then causes EMF to be induced on the rotor winding. The EMF produces an electrical current on the rotor winding to create a magnetic field before a tongue is generated in the rotor. As the speed of the rotor increases the impedance at the slip of the motor decreases and the current drawn by the motor is reduced.

The inrush current which is same thing as the starting current is about 4 – 10 times the rated current of the motor. The starting current is very high and will cause damage to the electrical motor winding by burning the coils.

An electrical motor starter is used to prevent the inrush/starting current from damaging the motor. So the starter limits the initial starting current for some duration during startup and when the motor attains a certain speed, the normal power supply is then resumed. The starter also offers protection for the motor from low voltage and over-current during motor operations.

Small electrical motors that are below 1 horsepower might not need a motor starter before they can start. They have a high impedance which can withstand the starting current. But they still need high low voltage and high current protections. Because of that, these types of motors use Direct On-Line starter(DOL).

How a motor starter works.

A starter is a motor control that allows us to safely turn OFF or ON our electrical motor. It can be done manually or automatically.

A manual starter is a motor starter that involves turning OFF/ON a motor manually. This is done by pushing the switch ON button to energize the motor or pushing the OFF button to break the supply contact of the motor. This way of starting a motor is not really safe for the motor as it can allow inflow of high current that might damage the motor coil sometimes.

An automatic motor starter is a starter that consists of a contactor, and electromechanical relays that switch ON/OFF of a motor automatically. When a current passes through the contactor coils it energizies the coil and generates an electromagnetic field. This electromagnetic field is what pushes ON or OFF of the supply contact to the motor winding.

A motor starter is designed to have two different switch buttons, one is to start the motor and the second is to stop the motor. When the start button is pushed, the contactor is energized, and current flows through to make the motor start running. As the motor is running the stop button can be pushed to de-energize the contactor and make the motor stop. For the electrical motor to start running, we must push the start button and for us to stop the motor, we push the stop button. It doesn’t start running automatically without pushing the ON or OFF button.

Different Types of Electrical Motor Starting Techniques.

In the industries, many starting techniques are employed in starting an electrical motor. These are different from the types of motor starters. Before discussing the type of motor starter let’s look at the starting technique of an electrical motor.

  • Full Voltage Or Across The Line Voltage: This Type of starter connects the full voltage from the supply to the motor. Motors that use this type of starter have low power ratings and run only in one direction.
  • Full Voltage Reversing Starter: This type of starter is used in 3-phase motors, where running a motor in the forward and revising directions are needed. To achieve this type of starter, contactors are used to interchange the connection of the motor to make it change directions.
  • Multispeed Starter: In a situation where an electrical motor needs to change from one speed to another, this starter is introduced. When varying the AC supply frequency of a motor the speed changes. This type of starter is employed to vary the speed of the electrical motor. Multi-Speed starters run the motor on a pre-select speed to meet its application.
  • Reduced Voltage Starter: This type of starter is widely used and its working technique is to limit the starting current to prevent it from damaging the motor winding

Types of motor starters.

Below are the different types of motor starters that are used in starting our motor. Here we will explain and talk more about their advantage and disadvantages. The different types of electrical motor starters include;

  • Direct- Online Starter (DOL)
  • Startor Resistance Starter
  • Rotor Resistance/ Slip Ring Motor Starter
  • Autotransformer Starter
  • Star Delta Starter
  • Soft Starter
  • Variable Frequency Drive (VFD)

The electrical motor starter has many types and all types can be classified into two categories, manual or Magnetic starter.

manual starter.

The manual starter is the type of starter that involves manually switching of motor by pushing the switching buttons to turn the motor ON or OFF. The mechanism for this type of starter is that when the OFF button is pushed, it cuts off the supply voltage to the motor causing the motor to stop. When the ON button is pushed, it connects the supply voltage to the motor to start it.

This type of starter applies to small motors which don’t need much power to start up. The starter provides the motor with overload protection but doesn’t protect it against low voltage protections. During power failure the starter doesn’t break up the circuit instead the circuit will be ON and restart immediately when power is restored. An example of this type of starter is a direct online starter(DOL).

Magnetic Starter.

This type of starter is employed in all high-power AC motors. It operates electromagnetically just like a relay that breaks and makes it’s contacts using magnetism.

starter that uses magnetic starter provides protections such as low are over-current protection. Magnetic starters limit the starting current during motor starting. When there are power failures the magnetic starter breaker the circuit and needs to start properly when the power is restored unlike manual starters.

The magnetic starter consists of two circuits which are;

  • The Power Circuit: This circuit consists of an electrical contact that turns ON/OFF the power supply from the supply line of the motor through the overload relay. The work of the power circuit is to deliver power to the motor.
  • The Control Circuit: This controls the power circuit by making it break or make up the circuit. With the help of a magnetic coil, it energizes or de-energizes to pull or push the electrical contact using magnetic control.

Direct Online (DOL) Starter

DOL Direct Online motor starter is the simplest form of starter which connects a motor directly to the power supply. It consists of a contactor and overload relay that connects the contact of the motor to the supply and protects the motor against high currents. DOL does not reduce the starting current of a motor, it starts with full voltage which is why it applies to only a small motor below 5hp ratings. It only has an overload relay which helps to detect and trip OFF supply when there is a high current on the motor.

DOL starter has a simple two-push button for starting and stopping the motor. When the start button is pressed the contactor coil gets energized and pulls the switch to close the circuit for current to flow through the motor. On pressing the stop button the contactor coil is de-energized leaving the contact button open, by so doing stopping the motor operations.

Advantages of DOL Starter

  • It is simple with cost-effective design.
  • It has a very simple design that is very easy to understand and operate.
  • It has a high starting torque because of its high starting current.

Disadvantages Of DOL Motor Starter.

  • It has a high starting current which can cause the in rush current to damage the motor windings.
  • DOL starters are not safe for heavy motors.
  • The high inrush current causes cause voltage dip in the supply line.
  • it not always safe for our electrical motor.

Startor Resistance Starter.

This type of Motor starter uses the RVS – Reduce Voltage Starter technique to start a motor. This is done by by adding an external variable resistance in series with each phase of the 3-phase of the motor. The work of the resistance here is to reduce the voltage applied to the stator and subsequently reduce the initial current to the motor.

When starting the motor, the variable resistor is tuned to the maximum resistance. This is to make the voltage across the motor to be at a minimum as there will be a voltage drop on the connected resistors before going into the motor. The excess of the low voltage to the stator is to limit the starting inrush current that can place the motor windings in danger. As the motor gradually starts running, the resistance is reduced to a minimum and the stator phase directly connects to the supply power line.

A two-times reduction in starting voltage decreases the torque by 4 times because the current is directionally proportional to voltage and torque varies to the square of the current. Starting torque when using a Stator resistance starter is very low and needs to be maintained.

Advantages Of Stator Resistance Starter.

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

Disadvantages of Stator Resistance starter.

  • The resistors dissipate power through heat.
  • Because the starting voltage is reduced, the starting torque is very low.
  • The resistors for stator resistance starter is expensive for large motor.

Rotor Resistance or Slip Ring Motor Starter.

In the case of a rotor resistance starter, it uses a full voltage starting technique. And is only applicable to slip-ring induction motors and that is why It is also referred to as slip-ring motor starters.

In a slip-ring motor starter, an external resistance is connected with the rotor in star form through the slip ring. The work of this resistance is to limit the rotor current and increase the torque. Which will result in reducing the starting inrush current and also improving the motor power factor.

When the motor starts running and attends to a rated speed, the resistors are then removed from the rotor. Is only when starting the motor that they connect the rotor resistor.

Advantages of Rotor Resistance Motor Starter.

  • It provides a motor with a low starting current using full voltage.
  • The use of a rotor resistance starter improves the motor power factors.
  • It can start a motor under load because of its high starting torque.
  • It provides speed control of the slip ring induction motor.

Disadvantage of Rotor Resistance Starter.

  • It is only applicable to slip-ring induction motors.
  • The rotor is very expensive and heavier.

Autotransformer Starter.

In this type of starter, an autotransformer is used to step down the starting voltage applied to the stator of the motor. The autotransformer starter can be used to start both delta and star-connected motors.

The secondary side of the autotransformer is connected to each phase of the motor. The transformer is constructed to have multiple taping, which produces different reduced voltages for the starting. A relay switches from one taping to another and by so doing increases the voltage and the speed of the motor. When the motor finally starts running at a certain speed, the relay switches to the full rated voltage of the motor.

Compared to other starters using voltage-reduced technique, autotransformer offers high voltage for a specific current which helps to provide a good starting torque.

Advantages of Autotransformer Starter.

  • It provides the motor with good starting torque.
  • The speed of a motor can be manually controlled when using this starter.
  • It is used in starting a large motor with a significant load.
  • It offers flexibility in starting characteristics.

Disadvantages of Autotransformer Starter.

  • It has a complex circuit and is expensive when compared to other starters
  • The starter occupies a large space. So it needs a good space when using this type of starter.

Star Delta Starter.

Star delta starter is a motor starter that is popularly used in industries to start up a large motor. The motor windings of the 3-phase motor change from star to delta connection to start the motor.

A triple pole double throw relay is used to switch the motor connections from star to delta connection when starting a motor. The motor is first allowed to start in star connections and as time goes on, the relay automatically switches the motor connection from star to delta connections. And then, the full voltage is supplied to all the windings befor the motor runs at its rated speed.

In star connections, the phase voltage is reduced to 1/√3 and the starting current with the starting torque reduced by 1/3 of the rated voltage of the motor. As the motor starts, the timer relay switches the star connection of the stator winding into a delta connection, and full voltage is then supplied to the motor.

Advantages of Star Delta Starter.

  • It is best for starting a large induction motors, especially in the industries.
  • It is easy to design and set up.
  • It provides a low surge current.
  • It doesn’t always require maintenance compared to other starters.
  • It is Good for long acceleration time.

Disadvantages of Star Delta Starter.

  • It is only applicable on motors are connected in delta.
  • It has a low starting torque that can not be maintained.
  • The wiring circuit is more complex compared to other starters.

Soft Starter.

This type of starter uses voltage reduction techniques when starting a motor. With the help of semiconductor switches like TRIAC, to control voltage and current supplied to the induction motor.

With the help of phase-controlled TRIAC, the voltage supply can be varied. When the conduction angle or firing angle of the TRIAC is varied, the voltage is equally varied. The conduction angle of the TRIACs is kept at a minimum to provide a reduced voltage. When starting the motor, the conduction angle of the TRIAC is gradually increases then, causing the voltage to equally increase from the minimum. By so doing the motor will start to run from low speed and gradually increase its speed as the conduction angle keeps increasing to a maximum. Until the Conduction angle reaches the maximum before the full voltage is supplied to the induction motor.

soft starter provides a very smooth and gradual increase of starting voltage, current, and torque. In this type of starter, there is no mechanical jerk and it provides a smooth operation of the motor.

Advantages Of Soft Starter

  • It provides the motor with smooth acceleration with no mechanical jerk.
  • Provides a good starting current and and voltage.
  • it increase the life span of the system.
  • It reduces power surges in the system.
  • It small and portable size.
  • It has good efficiency and does not always need maintenance.

Disadvantages Of Soft Starter

  • Energy dissipation in form of heat.
  • relatively expensive.

Variable Frequency Drive (VFD).

VFD varies the frequency of the supplying current to control motor speed. Variables Frequency drive mainly depends on varying the frequency of supply current to the motor to vary the motor speed.

VFD provides full control of motor speed from 0 to the rated speed. The speed adjustment options and variable voltage provide a good starting current and acceleration.

Advantages of Variable Frequency Drive (VFD).

  • It can be used to run a motor forward or revise direction.
  • There is no electrical or mechanical stress when using this type of starter.
  • It provide full control of the motor speed with smooth acceleration and declaration.
  • It provide smooth acceleration for big motors

Disadvantages of Variable Frequency Drive (VFD).

  • it is very expensive.
  • Heat dissipation
  • It creates harmonic on the line which can affect the power factor and other electronic components.

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