Relays and Contactors

Relays and Contactors

Relays

A relay is an electromagnet placed close to a movable armature. The relay armature is connected to a set of contacts. When the coil is energized, the relay armature is attracted to the coil and the contacts change state.

Fig. 5 With the relay de-energised, there is a circuit between the Common and Normally Closed

With the switch open there is a complete path from the Common (C) terminal to the Normally Closed (NC) terminal. This path is called a circuit.

When the switch is closed, the electromagnet attracts the armature. There is now a circuit between the Common terminal and the Normally Open (NO) terminal.

Fig. 6 With the relay energised, there is a circuit between the Common and Normally Open contacts.

The standard symbol for a relay is shown below.

Fig. 7 Standard symbol for a relay

A typical relay is shown below.

Fig. 8 Typical relay and symbol

When voltage is applied to the coil (5), an electric current flows through the winding; a magnetic field builds up and pulls the armature (6) against the core (7) of the coil. The armature is mechanically joined to a contact (1) and is pulled against contact (4). This switching position is maintained as long as the voltage is applied. When the voltage is removed the armature is restored to it's original position by a spring (3). In the initial position the contact (2) is active.

In practice, symbols are used to represent relays in circuit drawing. K1 relay in Fig. 8 operates four NO contacts.

Relay configurations

Relay contacts may be of the Normally Open, Normally Closed or Changeover type.

Fig. 9 Contact configurations

A relay may have a combination of these contacts.

Relays can be wired in a configuration known as a latch. A latched relay remains energized until a signal is received to de-energize.

Fig. 10 Latching relay configuration

Industrial relays are typically mounted on a DIN rail. These relays are made in two parts - a base and a body. All wiring is connected to the base. The relay body contains the coil and moving armature, and plugs into the base. Being detachable makes these relays easy to replace in the case of a fault.

The state of the relay (energized or de-energized) can be seen from the position of the moving armature. To assist in troubleshooting, visual indicators such as mechanical flags and LEDs can be incorporated in the relay body.

Typical faults suffered by relays include welded contacts caused by switching arcs, and burnt out coils from prolonged or incorrect use.

Contactors

A contactor is very similar to a relay. The main difference lies in the amount of current each device can carry. The contacts on a contactor are designed to carry much larger currents than those on a relay. This is especially important for electric motors where starting currents are six or seven times that of running current.

The contacts on a contactor may be normally open, normally closed or a combination of both. To protect the device being switched, contactors may be mechanically interlocked. This arrangement makes it physically impossible for contacts to remain closed when they should be open, and vice versa. Without this protection, failure of the contactor to switch properly could damage the motor or its wiring.

Relays and contactors are used in motor control circuits. Contactors are used where large voltages or currents have to be switched.

Fig. 11 Symbol for a contactor

Fig. 12 Relays and contactors can allow a small control signal to switch a much larger power circuit