Let's get to know the contactor more closely, which is one of the indispensable equipment of motor protection and switching circuits. In this article "What is a contactor?" We will answer the question. We will try to give information about contactor types, operating principle and selection.
What is a contactor?
The contactor is similar to the relay in terms of operation; However, it is an electrically controlled switching device that can be controlled remotely, suitable for use at higher currents.
What does a contactor do?
The duty of the contactor is to transmit the energy from one point to another by switching, ie opening and closing.
The difference of the contactor from the circuit breaker and fuse, which are other switching elements, is that it does not protect in case of overcurrent. They only transmit energy from one point to another. Therefore, in electrical installations, it should be used together with circuit elements that provide thermal magnetic protection such as thermal relay, motor protection switch, automatic fuse, fuse load breaker.
Contactor working principle
The control circuit consists of electromagnetism working with a spring system. Magnetism is activated by the current flowing through the coil and the two magnetisms come closer to each other. This movement turns off the two magnetisms. Therefore, the contacts also close and the springs in the system provide contact power. When the control current is cut, the contacts open and the main current is cut. In summary; We can say that it works on the principle of the main contacts closing as a result of the energization of the coil and the opening of the main contacts as a result of the de-energization of the contactor coil. This basic principle has been used for over 100 years and no new alternative has yet been developed.
Where are contactors used?
Generally, contactors are used in motor starting applications to start and stop the motor. The most commonly used type is the 3 pole contactor suitable for use in 3 phase systems. In addition to motor applications, there are also areas of use such as heater, lighting, DC switching. Compensation contactors specially produced for these applications are used in compensation applications.
When you browse the manufacturers' catalogs, it is possible to see many different types. The main varieties are:
Power contactors: It is the most widely available type in the market. They are produced with 3 poles and 4 poles up to 750kW. They are used in all kinds of applications.
Mini contactors: They are smaller in size compared to power contactors. They gain space. They are generally produced up to 5.5 kW.
Silent contactors: They do not create a mechanical noise from the contacts when they are opened and closed. They are used in applications where noise is not desired in indoor installations.
Compensation contactors: These are products specially produced for compensation applications where capacitors are activated.
Bus-type contactors: Products that switch high-power DC and AC loads. They have a completely open structure. They are preferred in heavy industrial environments and special applications such as railways.
Contactor internal structure
Suppose we broke the casing of a contactor. When we look at the contactor parts, we do not encounter a very complex structure. 1) Main contacts 2) Moving contacts 3) Fixed contacts 4) Coil 5) Outer casing 6) core 7) coil terminals 8) We see arc discharge channels.
When choosing a product, it should be selected according to the load we will connect. If the load is a motor with inrush current, ie inrush current, the contactor is selected according to the AC-3 current written in the manufacturers' catalogs. If a resistive heater is desired to be switched, it is selected according to AC-1 current. Parameters such as how many poles of the load, power value, nominal operating voltage, coil voltage, dimensions, number of internal auxiliary contacts are also factors that affect the selection of the product. When choosing the product, special attention should be paid to the coil voltage. Because the voltage supplied by the main contacts and the voltage supplied by the coil may differ from each other. According to the needs of the project, the coil can be fed with different voltages such as 24VDC, 48VDC, 230VAC, 380 VDC, etc.
Contactor usage categories
The contactor should be used in the field according to the type of load to be connected. In other words, their current carrying capacity may vary depending on the type of load they switch. These values must be checked from the manufacturer's catalogs and selected according to the list below for different applications in AC and DC. The usage categories according to the IEC60947-4-1 standard are:
AC-1: They are less inductive or non-inductive loads. Heating furnaces
AC-2: Starting and stopping ring motors
AC-3: Starting and stopping squirrel cage induction motors
AC-4: Pulsed starting of squirrel cage induction motors
AC-5a: Switching discharge lamps
AC-5b: Switching incandescent lamps
AC-6a: Switching transformers
AC-6b: Capacitor switching
AC-8a: In motor control of hermetic refrigeration compressors (with manual reset of thermal relays)
AC-8b: In motor control of hermetic refrigeration compressors (automata of thermal relayswith tick reset)
DC-1: They are less inductive or non-inductive loads. Heating furnaces
DC-3: Starting, pulsing, dynamic braking of shunt motors
DC-5: Starting, pulse starting, dynamic braking of series motors
DC-6: In switching of incandescent lamps