Electric motors are used in the vast majority for drive mechanisms and independent units. When a change in the direction of rotation of its shaft is required, a reversing starter is used for starting, the connection diagram of which is the subject of study by professionals and ordinary people.

• • Operating principle

How is the starter arranged and why is it needed?

As can be logically determined from the name, this device is designed to start electric motors of various drive mechanisms and equipment. This is a specific equipment that is necessary for switching power purposes with large loads, both on direct and alternating current. The starter has a wider functionality than the basic contactor and, in addition to providing frequent starts and stops, can act as a protective barrier in case of overloads. In addition, reversing and non-reversing starters, for example, the PML series, have found their application in organizing remote control circuits, starting pumping, ventilation, crane units, air conditioners, etc.

Any magnetic starter consists of the following main parts:

• electromagnetic part. It consists of a coil and disconnected magnetic circuits - a fixed core and a movable armature;
• Block of main contacts. They are needed to close / open power powerful loads. Taking into account the parameters of the starter, it can have up to 5 pairs of contacts. One half of them is located on the anchor traverse, and the other half is on the upper part of the hull;
• blocking contacts. They are used when switching the control circuits of the circuit, for example, when turning on / off is done with start buttons. There is a blocking of the main contacts, which means that the need to hold the control button is eliminated;
• return mechanism. In fact, this is just a spring, which, when the contacts open, returns the armature to its original position, providing the necessary clearance between the pairs.

The difference between direct and reversing starters

The main difference between non-reversible and reversible starting devices is the connection diagram. The package is also changing. The direct type contactor is single, while the reversing contactor is block type, consisting of two straight lines combined in one housing. Visual differences between these two relays can be seen in the comparison of PML-1100 (left) and PML-1500 (right):

In this case, one extremely important condition must be observed: the reversible connection of the starters must completely exclude the possibility of their simultaneous operation. This will inevitably lead to a short circuit phenomenon.

The connection diagram of the reversible magnetic starter of electric motors is divided into two main types:

1. Connection to a network with a voltage of 220 V;
2. 380 V contactor start.

Type and operation of the reverse circuit for 220 V

On this wiring diagram, you can see the following main elements (indicated by numbers):

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1. Blocking or block contacts;
2. Coils of magnetic starters, designed for a supply voltage of 220 V;
3. Thermal or current protection contacts (relay elements);
4. Power contacts of starters.

In addition, alphanumeric designations are distinguished:

• MP-1, MP-2 - magnetic starters. Their boundaries in the diagram are marked with dashed lines;
• Stop, Start - controls (the block itself is highlighted with a dashed line). Separately, only the Stop button is highlighted. The starting buttons (forward and reverse) are designated as two pairs of contacts associated with starters MP-1 and MP-2;
• M - electric motor.

Operating principle

As you can see, three different phases from a 380 V network are connected to the power contacts of the starters. There is no designation in the above diagram, but in other cases you can find the symbols A, B, C or L1, L2, L3. A block connection is organized by a direct jumper of the central phases of the relay, as well as diagonal jumpers of the side phases (conditionally 1 phase MP-1 is connected to 3 phase MP-2, etc.).

After that, the wires go to the electric motor M. At this interval, a thermal relay is connected to the circuit break. It monitors two of the three phases to cut off the power to the motor in the event of an overload.

The control unit with start buttons is connected from one of the central phases to the break of the thermal relay, and the neutral wire (ground) from the coils. Protection against simultaneous activation of starters is organized by cross-connecting the contacts of the start / reverse buttons with the blocking contacts of the opposite contactor.

When switched on from the forward stroke control unit, the contacts are closed to the first starter, which starts the engine. At the same time, the contacts of the second starter open, and the proper voltage is not supplied to the coil.

The reverse is turned on after the engine is stopped by the Stop button, followed by pressing the reverse. Thus, we have on the coils the lateral phases changed in some places, which leads to the rotation of the engine in the opposite direction. The blocking of the first starter occurs according to a similar principle.

Type and operation of the reverse circuit for 380 V

Here we have, in fact, all the same elements that are used for PML at 220 V, but the starter coils are designed for a higher voltage (they have more turns). In addition, the difference from the previous scheme is the connection of the control unit not through one, but through two phases, without using a common zero.

Where else are reversing starters used?

The scope of double starting relays is quite wide. It is not limited to electric motors alone. The need to change the direction of rotation or move the drive mechanisms may also arise in other cases.

For example, each person has a water supply and heating system at home, where there is always a place for various shut-off valves. For industrial scale, with high flow rates, pipeline diameters, high accuracy of flow control, ordinary taps are indispensable. It uses electric gate valves, as well as a mechanical control system for the working body. The rotation of the disk or the movement of the valve occurs in different directions, which means that the use of reverse start-up schemes is justified.

Without going far, you can find reversing starters such as PML or others in the lifting system of elevators. The up and down movement occurs due to a change in the direction of rotation of the main drum.

Changing the direction of rotation of the engine, the actuators associated with it, is a rather popular procedure. In this case, power from a three-phase network occurs through an intermediate switching relay - a reversing magnetic starter type PML 1500 or any other.