Some Important Facts You Need to Know About Motor Starters: Functions, Coordination, and Importance

What is a Motor Starter?

A motor starter is an electrical device that is used to start, stop, and protect electric motors. Motor starters are typically used in industrial and commercial applications where electric motors are used to drive machinery, pumps, fans, and other types of equipment.

Related Article: Electric Motor Control in Industrial Plants

Electric motors are widely used in a variety of industrial, commercial, and residential applications, from powering machinery and equipment to driving pumps, fans, and compressors. However, motors can be vulnerable to a range of electrical faults that can cause damage, reduce efficiency, and compromise safety. Motor starters are devices that are specifically designed to address these issues and ensure that motors operate as intended.

Each type of starter has its advantages and disadvantages, depending on the application requirements. For example, DOL starters are simple and economical, but they can cause voltage dips in the power supply, which can affect other equipment. In contrast, the wye-delta starter and autotransformer starter are more expensive but are more suitable for large motors, reducing the starting current and protecting the motor from damage. Therefore, it is essential to choose the appropriate type of starter for each application to achieve optimal performance and reliability.

What are the Basic Functions of Motor Starters? 

A motor starter has four basic functions:

  • Isolation
  • Short-circuit protection.
  • Overload protection
  • Control 


A motor starter provides a means of isolating the motor from the power supply. This is important for maintenance and repair work, as it allows the motor to be safely disconnected from the power source to prevent accidental start-up while work is being carried out. The isolation function is typically provided by a switch that is part of the motor starter.

This isolation must be designed according to the specifications, which require:

  • simultaneous all-pole breaking 
  • conformity with specified clearances according to the supply voltage
  • interlocking
  • visible, or fully apparent, circuit-breaking

The term “visible breaking” signifies that the opening of the poles can be viewed directly by an operator apparent breaking is identified either by the position of the actuator, or by a position indicator which, according to the standard, must not indicate the “off” position unless the contacts are actually separated by a sufficient distance specified in the standards. Manufacturers offer numerous devices to perform this function. The isolation and short-circuit protection functions are often combined in a single device (e.g. fused isolator). For this purpose, some basic devices have to be supplemented with an additional device, such as a plug-in support. 

Short-circuit protection.

A motor starter also provides protection against short circuits. A short circuit is a fault that occurs when an electrical current flows through an unintended path, such as a fault in the motor winding or a damaged cable. Short circuits can cause high levels of current to flow, which can damage the motor and other components of the electrical system. The short-circuit protection function is typically provided by a fuse or a circuit breaker that is part of the motor starter.

This function requires the detection of overcurrents following short circuits (generally more than 10 times the nominal current) and the opening of the faulty circuit. It is provided by fuses or magnetic circuit-breakers.

Overload protection

A motor starter also provides protection against overloads. An overload occurs when the motor draws more current than it is designed to handle, which can cause the motor to overheat and eventually fail. Overload protection is typically provided by a thermal overload relay or electronic overload protection device that is part of the motor starter. These devices monitor the current flowing to the motor and trip the starter if the current exceeds a certain level for a certain period of time.

This function requires the detection of excess currents following overloads (Ir < Ioverloads < Im) and the opening of the faulty circuit. It is provided by electromechanical or electronic devices (overload relays) combined with a breaking device (circuit-breaker or contactor) or incorporated in electronic starters or variable speed drives. It also protects the motor line against thermal overloads. 

  • Ir: setting current of the overload protection; this must match the nominal current (In) of the motor to be protected. 
  • Im: trip current of the time-delayed magnetic protection. 
  • Iinst: trip current of the instantaneous magnetic protection. This can vary from 3 to 17 times Ir, but is generally close to 10 Ir. 
  • Ics: rated breaking capacity in short circuit Icu: ultimate (maximum) breaking capacity in short circuit.


Finally, a motor starter provides a means of controlling the on and off operation of the motor. The control function is typically provided by a set of contacts that are operated by a control circuit. The control circuit may be operated manually or automatically, depending on the application. In addition to on-off control, some motor starters also provide other types of control, such as reversing, speed control, and soft starting.

In the design of a motor starter, the functions are distributed in different ways as shown in Figure 1 below: 

Figure 1. Various Combinations of Motor Starter

The term “Control” signifies the closing (making) and opening (breaking) of an electrical circuit under load. The control function is provided by switches or possibly by motor circuit-breakers, starters or variable speed drives. However, the contactor is the product most commonly used for this function, since it allows remote control. For motors, this control device must permit a large number of operations (electrical durability) and must conform to IEC 60947-4-1.

Figure 2. The different operating categories of contactors accordig to IEC 60947-1 | Source: Schneider Electric

Motor Starter Coordination

Since motor starters are composed of different components, it is important to consider the coordination of these elements. The coordination of different motor starter components is important for ensuring that the motor and the electrical system it is connected to operate safely and reliably. Motor starters typically consist of several different components, including contactors, overloads, fuses, and circuit breakers, each of which has a specific function in protecting the motor and the electrical system from faults.

Effective coordination of motor starter components helps to prevent nuisance tripping, which occurs when a protective device such as a fuse or circuit breaker trips unnecessarily, causing the motor to shut down unexpectedly. Nuisance tripping can result in unnecessary downtime and maintenance costs, and can also lead to reduced productivity and safety hazards.

In addition to preventing nuisance tripping, proper coordination of motor starter components can also help to reduce the risk of electrical faults such as overloads and short circuits. These faults can cause damage to the motor and other components of the electrical system, and can also create safety hazards such as fire and electrical shock.

As explained above, the main functions required from a motor starter (isolation, control and protection against short circuits and overloads) can be provided by different devices. 

There are three possible combinations of devices (see Figure 3) which enable a motor starter to provide all these functions correctly, but they require compatibility between the characteristics of each of the combined devices. 

 Figure 3. The three possible combinations of devices for a motor starter | Source: Schneider Electric

  1. The “all in one” solution - The three functions are combined in a single device; its overall performance is guaranteed by the manufacturer. For the user, this is the simplest solution, from the design phase through to installation: it is easy to install (less wiring) and can be chosen immediately (no special research needed). 
  2. The “two device” solution - Thermal-magnetic circuit-breaker + contactor Compatibility between the characteristics of the two devices must be checked by the user. 
  3. The “three device” solution - Magnetic circuit-breaker + contactor + overload relay This can cover a wide power range. This combination requires a compatibility study for the selection of the devices and an installation study for their mounting on a chassis or in an enclosure. 

This nature of this operation (compatibility, choice and installation) is not always obvious to users, since it is necessary to combine the characteristics of the different devices and have the skill to compare them. For this reason, manufacturers investigate the combinations of devices and then offer them in their catalogues. As part of this operation, they attempt to find optimal combinations of protection devices: this is the “coordination” concept. This coordination is the optimal combination of different protection devices (for short circuits and overloads) and the control device (contactor) which make up a motor starter. 

Figure 4. Principle of Coordination | Source: Scheider Electric

Figure 4 simply demonstrates that for a motor starter to work properly, all of its components must work together correctly. There are three main requirements for this coordination:

  • the overload relay must protect the magnetic circuit-breaker in the overload zone : its curve “1” must pass below that of the thermal withstand of the circuit-breaker; 
  • and, conversely, in the short-circuit zones, in order to protect the thermal relay, the short-circuit trip curve must pass below that of the thermal withstand of the relay;
  • finally, in order to protect the contactor, its thermal withstand limit must be above the curves of the two trip releases (thermal, “1”, and magnetic, “3”) (or the fuse, “2”). 

Note that the standard specifies the limit test currents:
  • up to 0.75 Ic only the thermal protection must operate;
  • above 1.25 Ic only the short-circuit protection must operate.

In any installation including electric motors, different kinds of faults can occur. But, whether they originate in the motors (short circuits between phases, etc.) or are related to the operation of the motor (rotor locking, prolonged starting, etc.) or to the power supply (overvoltage,unbalance, etc.), their effects on the motors can include short circuits and/or overheating of the windings, which may destroy them.

Accordingly, in order to avoid these mishaps or limit their effects, every motor should be protected from:

  • short circuits: by fuses, magnetic circuitbreakers, etc.
  • and overloads: by thermal or electronic overload relays, multi-function relays, etc.

In a motor starter, these protection elements are combined with an isolation device and a control device. To ensure that they perform their functions correctly, their coordination must be ensured. This is an operation which is often difficult for the designer of the installation or machine, since he has to take into account not only the type of motor, but also its mode of operation and the characteristics of the installation. 

View or Download PDF

  • Title: Protection of LV Motors | Motor Starters (pp. 20-27)
  • Source: Schneider Electric

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