Typical circuits of relays and starters in instrumentation and automation are presented and discussed here.
Circuits are quite trivial and widely distributed, but nevertheless they can be of interest for beginning specialists of instrumentation and automation.
A simple control circuit (on / off) a three-phase motor shown in Figure 1.
Figure 1. Simple relay / starter circuit
K1 – relay / starter ~ 220 V AC with 4 normally open contacts.
SB1 – "Start" button with 1 normally open contact
SB2 – "Stop" button with 1 normally closed contact
K1.1 – normally open contact of relay K1
K1.2...K1.4 – contacts of relay K1 for switching power circuits
When the "Start" button (SB1) is pressed, the voltage ~ 220 V AC between the phase and the neutral wire is fed through the normally closed contact SB2 of the "Stop" button to the relay / starter coil K1.
The relay trips and closes three power contacts, connecting the motor to the three-phase circuit, and the locking contact K1.1, which keeps the relay in the ON state.
When the "Stop" button (SB2) is pressed, the supply of the relay coil K1 stops and it returns to its original state by breaking both the power circuit contacts and the blocking contact K1.1.
Although the diagram shows the process of switching on a three-phase electric motor, this circuit is classical and suitable for various purposes, where "Start" and "Stop" buttons are used, with appropriate changes in the power part of the circuit.
Another widely used scheme for switching the relay / starters to control the reversing motor is shown in Figure 2.
Figure 2. Reverse motor control scheme
K1, K2 – relay / starter ~ 220 VAC with 4 normally open contacts and one normally closed.
SB1, SB2 – "Forward", "Reverse" buttons with one normally open contact.
SB3 – "Stop" button with 1 normally closed contact.
When SB1 ("Forward") button is pressed, the voltage ~ 220 Volts is fed through the normally closed contact SB3 of the "Stop" button and the normally closed contact K2.2 of relay K2 to the relay coil K1. It trips and closes its blocking contact K1.1. Relay K1 is in ON state.
In addition, it opens the normally closed contact K1.2 in the circuit of the SB2 ("Reverse") button, thus preventing the K2 relay from operating when the "Reverse" button is pressed. Otherwise, there would be a short circuit between phases "B" and "C".
When the SB3 button is pressed ("Stop"), the supply circuit of the relay coil K1 is broken, it goes to the initial state, disconnecting the power circuits of the electric motor.
When the SB2 button is pressed, the voltage ~ 220 Volts is fed through the normally closed contact SB3 of the "Stop" button and the normally closed contact K1.2 of the relay K1 to the relay coil K2. It closes its K2.1 blocking contact, keeping itself in the ON state.
In addition, it opens the normally closed contact K2.2 in the SB2 ("Forward") button circuit, thereby preventing the K1 relay from tripping when the "Forward" button is pressed.
The power supply circuits of the motor are assembled so that when the relay K2 is activated, the phases "B" and "C" are interchanged and the motor rotates in the reverse direction.
When the SB3 ("Stop") button is pressed, the power supply circuit of the K2 relay coil is broken, it goes to the initial state, disconnecting the power circuits of the electric motor.
To increase the reliability of the circuit, there are industrial control units for the reversible motor, in which, in addition to the electrical blocking of the inclusion of opposite relays / starters, mechanical levers for blocking the simultaneous operation of two relays K1 and K2 are also used. In rare cases, this can happen when the power contacts of one of the relays are burned (stuck).