- Types and classes of contactors
- Basic installation concepts
- Connecting the switched load
- Control circuits
- Additional devices
- Basic connection diagrams
Magnetic starters, relays and contactors belong to one of the most extensive groups of panel equipment. For the correct operation of these devices, a number of wiring rules must be followed, knowledge of the basics of relay technology, as well as a competent approach to organizing power supply circuits for electrical appliances.
Types and classes of contactors
Contactors are designed for remote or automatic switching of power lines for high-power electrical appliances. These electrical products include panel mounting devices with virtually unlimited power, as well as modular DIN rail mounting devices. In the latter case, the permissible current is usually no more than 63 amperes. Small-sized (non-modular) contactors for DIN-rail mounting are designed for currents up to 100 A and in fact are panel-mounted products for a rather simple reason: their dimensions do not allow correct installation of the front panel of the panel.
Left: modular contactor for 63 A DIN rail. Right: panel mounted contactor
The generally accepted classification of magnetic contactors implies their division into values corresponding to the size and permissible current load. So, modular devices are limited to the 4th value, there are 7 in total, with the maximum dimensions, the contact group is designed for a current strength of up to 250 A. Outside of the general classification, there are contactors capable of switching circuits with a current of 1000 A and above, but such devices have a narrow industry application and we will not consider them.
Individual models of contactors may differ in the class of electrical insulation and permissible switching voltage. There is also a difference in the operating voltage for which the solenoid coil is designed. Additional differences are:
- the number of switched poles of the power group of contacts (from 1 to 4);
- response time (from 0.01 to 1 s);
- the type and efficiency of arcing devices for different degrees of load inductance;
- permissible number of switching cycles per hour;
- the level of noise and vibration;
- the presence and number of additional low-current contacts.
The device is a three-pole contactor with normally open contacts: 1 – coil; 2 – stationary magnetic circuit (core); 3 – movable core; 4 – fixed contacts; 5 – dielectric holder of movable contacts; 6 – moving contacts
The terms contactor and starter have different meanings. So, the name contactor means a monoblock device only with the set of functions that are provided for by the design. A starter is a set of devices combined within one control assembly. It can include several contactors, as well as additional attachments, protective devices, control elements and a housing with a certain degree of dust and moisture protection. Starters are usually designed to control the operation of asynchronous electric motors.
Combined motor starter
Basic installation concepts
A contactor or starter is almost never the only element in a control circuit. A prerequisite is the presence of a circuit breaker in the circuit, the rating of which is calculated based on the limiting current of the contactor. It is also important to correctly select the current-time characteristic of the protective shutdown, it must correspond to the resistance class of the contactor to inductive loads.
Magnetic contactors are designed for natural air cooling, and therefore the place of their installation must have sufficient internal volume or have ventilation openings. Also, a prerequisite is the absence of vibrations of the base to which the contactor is fixed, otherwise an unintentional throwback of the pull-in rod with subsequent opening of the circuit is possible. Finally, the operating conditions of the contactor must correspond to its protection class against external influences, because the internal mechanism is extremely sensitive to moisture and dust, especially abrasive and conductive.
Connecting the switched load
The connection of the power circuits of the contactor is carried out, as a rule, with screw terminals with a clamping plate or a saddle. When assembling the power circuit, it is recommended to take maximum measures to ensure the maximum contact area of the cable cores with the contact pad. So, it is better to roll single-wire conductors into a half-ring, multi-wire ones – to squeeze with a flat pin tip.
The group of power contacts at each pole is represented by two fixed and two movable, connected by a conductive plate. Thus, the contacts of each phase are located in parallel, their clamping screws are located on the front of the case and are marked with the letter L with the corresponding digital index. The tip of the core is inserted under the clamping plate or into the saddle until it stops, and then clamped with a screw. For rated currents over 63 A, it is recommended to use a torque tool. Power contacts need to be tightened after 48 hours to compensate for permanent metal deformations.
As you can see, the connection diagram of the power section is extremely simple: the contactor switches the phase lines, the operating zero is collected on the common bus or cross-module. The only difference is when assembling circuits with an isolated neutral, in such cases the working neutral conductor is switched by the fourth pole of the contactor.
Electromagnetic contactors are not mechanically latching in the on position. To ensure the retention of the rod during operation, a self-picking scheme is used. This is a fairly convenient technique that allows switching the coil power supply circuit with various protection and automation devices of the electric drive. Exceptions are assemblies controlled by PLC or relay automation..
The simplest self-locking circuit includes one additional normally open blocking contact. The coil supply circuit is connected through the normally open contact of the start button. The second circuit is connected in parallel, it consists of a series-connected blocking contact and a normally closed contact of the Stop button. Thus, when the contactor is turned on, a blocking contact is closed, which is held during the entire operation and supplies power to the coil. If it is necessary to stop, the coil power circuit is opened by the “Stop” button.
Contactor self-catching circuit: L1, L2, L3 – three-phase power supply phases; N – neutral; KM – magnetic starter coil; NO13-NO14 – additional normally open contact; M – asynchronous motor
There are also more complex control schemes. So, the use of a normally closed contact of the start button of one contactor can be used to exclude the simultaneous operation of two starters, which, in particular, can be important in the construction of reverse connection circuits or be due to another technological necessity. The same principle can work when using a normally closed blocking contact of one contactor, which is connected in series with the contact of the start button of another..
Reverse engine start-up diagram: KM1, KM2 – coils of magnetic starters; NO KM1, NO KM2 – normally open contacts of starters; NC KM1, NC KM2 – normally closed contacts of starters; KK – thermal relay
Limit switches, dry contact sensors and all kinds of protective devices can also be included in the self-locking circuit. Automatic switching on of the contactor is also possible; for these purposes, the button is replaced or duplicated by parallel switching on of limit switches or sensors. Thus, the complexity and control schemes of an automated electric drive are practically unlimited..
As already mentioned, the contactors themselves are extremely simple in design and can only consist of an electromagnetic retractor and one or more pairs of power contacts. At the same time, there is an impressive number of additional modules that can expand the original functionality far beyond the usual switching..
The most common attachments are with additional blocking contacts. If the contactor does not have such initially, this type of equipment is the only way to implement a self-catching scheme. Also, additional block contacts can be used to implement more complex control, indication and automation schemes.
Thermal releases are another popular type of accessories. Their task is to monitor the load flowing in the circuit and turn off the coil power when the permissible current values are exceeded for a long time. Like thermal trip units of circuit breakers, attachments for contactors have different current-time tripping characteristics for different types of induction motors. Electromagnetic releases are not used as additional attachments because the contactors are not designed for switching short-circuit currents.
Contactor auxiliary devices: 1 – thermal overload relay; 2 – contactors; 3 – time delay prefix; 4 – auxiliary contacts
Time delay prefixes make it possible to implement schemes of slow start and stop of the electric drive. Time relays have the ability to manually set in a certain range, which allows you to fine-tune the compensation of the inertial run of the electric motor before reversing.
Among the additional devices, we should also mention the attachments for mechanical interlocking of the opposite connection, using which you can assemble a reversing starter from two conventional three-pole contactors. If control is carried out directly from a cabinet or panel, you can use starter attachments in which a group of connections for self-picking has already been made and the “Start” and “Stop” buttons are installed. If the contactor coil does not correspond to the actual voltage of the control circuit, it can be easily replaced with another one with suitable parameters. Additional motor protection is provided by monitoring and phase sequence relays as well as surge suppressors.
Basic connection diagrams
In total, there are three power switching circuits, according to which the contactors are connected. The first and simplest is direct phase switching, which is suitable both for one-sided starting of the drive and for controlling an active load. There is nothing remarkable in the circuit, the contactor simply acts as a remote switch.
An example of using contactors in the generator autostart circuit: 1 – input automatic; 2 – counter; 3 – RCD of the main network; 4 – main input contactor; 5 – block for automatic generator start; 6 – gas generator; 7 – RCD of the reserve network; 8 – time relay; 9 – contactor of reserve input
A slightly more complex circuit is used to control the forward and reverse rotation of three-phase asynchronous machines. Two contactors are installed in pairs, the outgoing phase conductors are connected in parallel. In this case, the connection from the supply side is performed with a crossover that changes the sequence of any two phases out of three. When assembling a reversing circuit, it is extremely important to provide two-way protection against reverse connection: both with the help of mechanical interlocking and using interlocking contacts..
The third type of circuit is starting, it is used when controlling high-power induction motors. The general assembly contains two contactors for each direction of rotation of the drive. In each pair, one contactor is a starting one, through which the motor is connected according to the winding connection scheme in a “star”, due to which the starting currents are significantly reduced. After some time required for reaching the rated speed, the second contactor is switched on, through which the connection of the windings in a “delta” is realized. To implement such a connection scheme, it is required to lay six power cores and one working neutral conductor to the motor, as well as install a turn-on delay relay on the main contactors.