In modern electrical circuits, protective contactors are widely used. They protect engines and others, for example, heating devices from overloads and sudden voltage drops. Contactors are also designed for stationary connection and disconnection of single-phase and three-phase equipment. One of these contactors is the magnetic starter PME-211.
Magnetic starter
Magnetic starter PME-211 consists of a collapsible housing, an electromagnet, contacts, both power and auxiliary (blocking). In the lower part of the carbolite body there is a core with a retracting coil. Power contacts are located on top, which are connected to the movable core and, when the coils are triggered, are retracted, while closing the power contacts. The magnetic starter PME-211 also has auxiliary contacts, or, as they are also called, interlock contacts. When the power is closed, they block the power on the coil of the contactor. This eliminates the need to constantly press the start button.
The magnetic starter on top has a protective cover that covers the power contacts. The core consists of layers of special ferromagnetic steel, interconnected in a single shape.
Production
The production of PME-211 magnetic starters in Russia is almost over. Due to the difficult economic conditions, the production of contactors has almost ceased, however, over the past five years, several competitive plants have been restored and managed to load them at full capacity. This type of starter gradually begins to be replaced by cheaper and easier to maintain contactors of both domestic and foreign production.
The starter case is made of carbolite, this material does not conduct electric current and is the most convenient. The core, as described above, consists of plates of electrical steel, isolated from each other, to avoid the occurrence of eddy currents. The contact and contact surfaces are made smooth to avoid buzzing.
Power and auxiliary contacts are made of brass with silver brazed. Silver helps to avoid an arc, and also has greater resistance to shock during switching on and off.
The PME-211 magnetic starter retractor is made of copper wire. The diameter of the wire and the number of turns varies depending on the rated voltage at which this starter will work. It should be remembered that it is necessary to supply a voltage corresponding to the rated voltage to the coil, otherwise the coil will fail as well as the contactor itself.
220 V starter
The magnetic starter PME-211 for 220 V has a pull-up coil with a nominal supply voltage of 220 V. This means that the control network voltage should also be 220 V. These starters are mainly used to control the connection of low-power motors and heating devices, which require the use of contactor.
380V starter
The magnetic starter PME-211 for 380 V has a pull-up coil designed for a supply voltage of 380 V. Moreover, supplying a lower voltage can damage the starter. Starters with this voltage are used to control equipment that is more powerful than at 220 V.
Connection Features
Connection of the PME-211 magnetic starter is carried out depending on the requirements of the circuit and technical conditions. A single connection is provided provided that there is no need for reverse switching. Typically, a single connection is used to control heating elements.
When reversing the connection, two paired magnetic starters are connected by wires so that when you press the start button, the engine starts to rotate in a certain direction, depending on which starter trips. This connection scheme is called reversible and is widely used in the assembly of control circuits on almost all machine-building equipment. With this scheme, the locking contacts protect against the simultaneous operation of magnetic starters, which can damage the motor.
Often, magnetic starters are paired with a thermal relay protecting the equipment from overloads and the disappearance of one phase. The thermal relay is structurally designed so that mechanical connection with a magnetic starter is possible. The relay has a bimetallic plate, which, with increasing current, heats up and bends to turn off the power to the starter coil, thereby breaking the power circuit.
In control circuits with magnetic starters, fuses and circuit breakers are also used for protection. Thermal relays have a special adjustment of the trip current. Over time, it is necessary to add value, since the equipment is worn out and this adjustment is necessary to avoid constant tripping, but do not forget that with an increase in the tripping current, the motor can be damaged.
Application
In almost any electrical circuit, magnetic starters are used. They are used to control induction motors in almost any equipment, and they are also used to control the connection of powerful heating devices. Magnetic starters are easy to maintain, which makes them common to use, since motors and heating elements are more expensive, they must be protected from failure.