Industrial Motor Starters and Starting Methods

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What is a Motor Starter?
A Motor starter is an electro-mechanical device that is used to start and stop an electric motor either manually or automatically. They also provide overload protection to the motor. Therefore, a motor starter has two basic functions: (i) to switch the power automatically or manually to a motor (ii) protect the electric motor against overload.

Why does an electric motor require a motor starter?
When a motor is energized or started, a very large current – five to ten times the full-load current – flows initially. The resulting large initial current transient is known by various names - inrush current, starting current, or locked-rotor current. This surge current drops as the motor accelerates up to its running speed. A starter is therefore needed to limit this initial large starting current to protect the motor and limit secondary effect on other facilities connected to the same power supply system as the motor.

Different starting methods are used to reduce the starting current to comply with local laws and regulations. Naturally, avoiding huge voltage drops on the mains which could have adverse effect on other electronic systems or users is a major objective during electric motor starting.

Types of Electric Motor Starters and Starting Methods
There are various types of methods used in starting industrial motors however, the list below is the most common types usually employed:
(i) Direct-on-line (DOL) starting as applied in DOL Motor starters

(ii) Star-delta starting as applied in Star-Delta Motor Starters

(iii) Auto-transformer starting as applied in Auto-transformers Motor Starters

(iv) Soft starting as applied in Soft Motor Starters

(v) Frequency converter starting as applied in Frequency converter motor starters

Most manual motor starters used for small to medium electric motors (Single phase and 3 phase) are DOL starters. DOL, Star-Delta and Auto-transformer Motor starters are often referred to as Magnetic Motor starters because they employ the principle of electromagnetism in their operations.

Direct-On-Line Starting
Direct-on-line starting means that the motor is started by connecting it directly to the supply at the rated full voltage. Direct-on-line starting, (DOL), is suitable for stable supplies and mechanically stiff and well-dimensioned shaft systems – and pumps qualify as examples of such systems. DOL starting is the simplest, cheapest, and most common starting method for small to medium sized single- and three-phase AC motors

DOL starters are used for motors that start and stop frequently and they often have some kind of control system, which consist of a contactor and overload protection such as a thermal relay. A simple DOL starter configuration is shown below:

Star-Delta Starting
This starting method is commonly used in 3 phase induction motors. The main objective of this starting method is to reduce the starting current. In starting position, current supply to the stator windings is connected in star (Y) for starting. In the running position, current supply is reconnected to the windings in delta (Δ) once the motor has gained speed. 

Normally, low-voltage motors over 3 kW will be configured to run at either 400 V in delta (Δ) connection or at 690 V in star (Y) connection. The flexibility provided by this design is usually used to start the motor with a lower voltage.  Star-delta connections give a low starting current of only about one third of that found with direct-on-line starting. Star-delta starters are particularly suited for high inertias, where the loads are initiated after full load speed.

Below is the typical configuration of a star-delta motor starter:

Auto-Transformer Starting
Auto-transformer starting makes use of an autotransformer coupled in series with the motor during starting. The autotransformer contains transformers, often featuring two voltage reductions, which reduce voltage to provide low-voltage starting by tapping off the secondary voltage of the autotransformer, usually at approximately 50 - 80 per cent of full voltage. Only one tapping is used, depending on the starting torque/current required. Of course, reduced voltage to the motor will result in reduced locked rotor current and torque, but this method gives the highest possible motor torque per line ampere.

At no point in time is the motor not energized, so it will not lose speed as is the case with star-delta starting. The time of the switch between reduced and full voltage can be adjusted to suit specific requirements.

Soft Starting
A soft starter is a device which ensures a soft start of a motor. Soft starters are based on semiconductors via a power circuit and a control circuit, these semiconductors reduce the initial motor voltage. This results in lower motor torque. 

During the starting process, the soft starter gradually increases the motor voltage, thereby allowing the motor to accelerate the load to rated speed without causing high torque or current peaks. Soft starters can also be used to control how processes are stopped.

Frequency Converter Starting
Frequency converters are designed for continuous feeding of motors, but they can also be used for start-up only.

The frequency converter makes it possible to use low starting current because the motor can produce rated torque at rated current from zero to full speed. Frequency converters are becoming cheaper all the time. As a result, they are increasingly being used in applications where soft starters would previously have been used. The implementation of frequency converter motor starter is shown below:

Comparison of the Common Motor Starters 
A comparison of the various motor starters discussed above is shown here. Each motor starting methods have their advantage and disadvantage and this is summarized in the table below:

Starting Method



Direct-On-Line (DOL)    

       ·       Simple and cost-efficient.

       ·       Safe starting.

       ·       Highest possible starting          torque

High locked-rotor current

Star-Delta Starting

Reduction of locked-rotor      current by a factor 3.

      ·       High current pulses                    when switching over                from star to delta.

       ·       Not suitable if the                      load has a low inertia.

       ·       Reduced locked-rotor                torque.


Reduction of locked-rotor         current by V2, where V is the voltage reduction e.g., 60% = 0.60.

       ·       Current pulses when                switching from reduced          to full voltage.

       ·       Reduced locked-                          rotor torque.

Soft Starter

      ·       Provides “Soft” starting             of  motors

      ·       No current pulses.

      ·       Less water hammer                   when  starting a pump.

      ·       Reduction of locked-                 rotor  current as                         required, typically, 2-3             times.

Reduced locked-rotor torque.

Frequency Starter                     


      ·       No current pulses.

      ·       Less water hammer                   when  starting a pump.

      ·       Reduction of locked-                 rotor  current as                         required, typically to                 around   full-load current.

      ·       Can be used for                           continuous feeding of               the  motor.


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