Softstarters and frequency converters are digital drive control systems that have significantly dominated the market of safety electronics and building automation. Among their many advantages, one can appreciate such as a gentle start-up curve or the ability to manage the engine speed. In order to ensure correct functionality, it should still be borne in mind that they still need components for power distribution in the form of, among others, magnetic switches.
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In addition, contactors are still an important part of modern industrial systems for switching motors – they directly control the drive. This article will broadly discuss the role of contactors and motor switches in effective control and protection of drives. It will also discuss the main issues regarding the selection of protections and their configuration. A thorough familiarization with the principle of operation of devices is crucial for the implementation of reliable and effective drive control systems in wide applications appearing in industry.
How does safety equipment protect against failures?
In the case of electric motors, two types of failures are mainly distinguished: short circuit or overload. They carry great danger for the systems and due to the different characteristics of both phenomena, devices protecting the system must be able to independently detect both cases.
The main cause of short circuits is damage to the insulation of circuits, which can occur when very high currents flow through the installation, which can then result in immediate damage to the equipment connected to the system. The first commonly used means of protection against this phenomenon were fuses, which, when a high current occurred, interrupted the system by burning the insert. Due to the limitations of single use, these solutions have currently been replaced by automatic devices. These include magnetic switches or combined and hybrid devices.
Drive overload is caused by exceeding the technical parameters of the machine manufacturer regarding the working environment or the operating parameters of the engine. The consequence of exceeding these limits may be overload, which may cause a drop in speed or a complete interruption of the machine's operation. In both situations, the current drawn by the motor is higher than the basic value. Due to the high risk of overheating in such conditions, the machine is not able to continue working without exposing itself to permanent damage. Soft start systems and inverters have built-in overload protection. In the case of direct start, the only effective protection is provided by thermal relays or other types of thermal protection in the form of the previously mentioned combined or hybrid systems.
Magnetic switches
Magnetic switches perform the basic function of protecting the motor from damage due to a short circuit. These protections, by their design, enable a quick reaction and interruption of the system when excessive current values occur. Magnetic switches can be extended with additional functions using accessories, which include fault indication modules or remote control. It is important to be particularly aware that in order to obtain full protection in a direct start system, the magnetic switch must be extended with a thermal element!
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Thermal and combined magnetic-thermal protection
Thermal relay is a basic overload protection, which in its basic form is a separate device. In order to provide full short-circuit and overload protection, the thermal relay should be coordinated with the previously described magnetic protection.
The thermal protection module also exists in the form of an integrated protection with a magnetic module, where such a solution is called a magnetic-thermal switch, a combined switch or most commonly: a motor switch. In each of the above configurations, the integrated bimetallic system interrupts the power supply due to overload, in order to limit the possible overheating that may result from it.
Motor switches (combined) can be used with a pushbutton or rotary drive, depending on your preferences. Both variants are available from Schneider Electric Polska
Contactors, or how to easily turn on the engine?
In the next step after voltage protection, it is necessary to provide functions for controlling the operation of the system. To perform this function, the main element is a contactor, which, when voltage is applied to its electromagnetic coil, switches on the main paths of the device, allowing current to flow.
For applications that are simple and undemanding, contactors from the basic product range, such as the Easy series from Schneider Electric, are suitable. It stands out for its excellent price-quality ratio. This series is highly popular with customers thanks to its good price availability combined with a durability of 10 million cycles.
Industrial Contactor Working Environment
For more extensive applications, which translate into a range of electrical and industrial requirements, the devices must comply with strict conditions regarding durability and with the option of expanding the functions through accessories. In such a situation, the solutions from the Schneider Electric TeSys Deca series are ideally suited. This range of contactors stands out with a mechanical life of 30 million cycles at 60 cycles per minute, constituting a long-term, failure-free functional element of the system.
Thanks to the low-current coil, the contactors are ideal for coordination with PLC units without the need to add relays. The standard auxiliary NO + NC contacts and additional contacts in the form of optional modules are in mirror form, which guarantees that the contact signal will always indicate the actual state of the device even in the event of the main paths sticking together. This functionality allows the use of TeSys Deca solutions in safety applications. Thanks to the numerous certificates and declarations of conformity for this family of products, this series guarantees reliability and many years of operation.
Monoblock cameras for more advanced applications
When planning control systems for multiple engines, higher requirements for protection appear. In such circumstances, it is worth considering the use of combined devices that perform the thermal element control functions simultaneously with the short-circuit protection function.
The combined device in the form of a monoblock enables the achievement of the so-called full coordination (no obligation to test and replace the device after a short circuit). Precise operation combined with resistance to ambient temperature ensures the implementation of electronic thermal protection setting in the protection. The monoblock also means maintaining more space and saving work time during installation. Schneider Electric offers a range of TeSys Ultra hybrid systems, the basic function of which as a drive can be extended with a wide range of additional modules for starting bidirectional motors.
IIoT contactors - direct start in the era of Industry 4.0
TeSys™ Island is the latest version of devices enabling digital, intelligent load management, data transmission to achieve higher levels of efficiency of industrial engines and systems while providing easier control. The system is distinguished by high durability and versatility through market-leading contactors - TeSys Deca. The system enables precise recognition and classification of drive failures (with distinction into blockage, overload, dry running, etc.).
The built-in electronic module operates according to digital settings. The contactor island can communicate with the automation system via EthernetIP, ModbusTCP, Profinet or Profibus protocols. This series allows for a reduction in the costs of cabinet assembly and wiring (up to 20%), system component costs (limiting the number of controller I/O modules, relays, energy meters) and maintenance costs thanks to more efficient and reliable diagnostics of the source of failure and system status.
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