Why relay is used in motor control

When the contact changes its state it interrupts the flow of current by opening its contacts. With respect to magnetic contactors, the armature or plunger is the movable part of the magnetic circuit. Used when additional auxiliary contacts are needed in a control circuit, a control relay is a magnetic contactor which is not designed for the energization of motors, and does not have built in overload protection.

Loads connected in parallel will experience the same potential difference voltage , but may draw different values of current depending upon their individual resistance. The difference in electric potential between two points, which is defined as the work needed per unit of charge to move a test charge between the two points.

It is measured in volts V. A diagram that shows how a circuit works logically and electrically. It uses symbols to identify components and interconnecting lines to display the electrical continuity of a circuit. It is often used for troubleshooting purposes. Also known as a ladder diagram. Circuits with low-voltage protection will not automatically turn back on when voltage is restored following a power outage.

Examples include the microwave or power tools. A moderate and gradual rise in the value of current over a relatively long period of time that is caused by excessive amounts of current drawn by a motor due to too much load being put on the motor. In contrast to the Power Circuit, the Control Circuit consists of inputs, in the form of switches, pushbuttons or pilot devices, which when activated, can either directly, or through a magnetic motor starter, energize a load.

The Control Circuit often operates at a lower voltage than the Power Circuit for safety and ease of installation. Skip to content Circuits. The initial high value of current produced when an inductive load is first energized.

In electrical terms, refers to a connection where current has only one path to flow. A device for making or breaking the connection in an electric circuit. In electrical terms, refers to a connection where current has more than one path to flow. Previous: Pressure Circuit.

Share This Book Share on Twitter. An eight-pin plug-in-style ice cube relay is shown in Ill. This relay contains two separate single-pole double-throw contacts. Because the relay plugs into a socket, the wiring is connected to the socket, not the relay. The numbering on the socket base designates a terminal with the corresponding pin position.

Care must be taken not to confuse the base numbers with the wire reference numbers used to label control wires. Relay options that aid in troubleshooting are also available. A manual override button, mechanically connected to the contact assembly, may be used to move the contacts into their energized position for testing purposes. Use caution when exercising this feature, as the circuit controlling the coil is bypassed and loads may be energized or deenergized without warning. Like contactors, relay coils and contacts have separate ratings.

Relay coils are usually rated for type of operating current DC or AC , normal operating voltage or current, permissible coil voltage variation pickup and dropout , resistance, and power. Sensitive relay coils that require as little as 4 mA at 5 V DC are used in relay circuits operated by transistor or integrated circuit chips. Relays are available in a wide range of switching configurations.

Like switch contacts, relay contacts are classified by their number of poles, throws, and breaks. The single-pole contact can conduct current through only one circuit at a time while a double pole contact can conduct current through two circuits simultaneously. The single-throw con tact can control current in only one circuit while the double-throw contact can control two circuits.

All relay contacts are constructed as single break or double break. Single-break contacts have lower current ratings because they break the current at only one point. In general, relay contact ratings are rated in terms of the maximum amount of current the contacts are capable of handling at a specified voltage level and type AC or DC. Current ratings specified may include:. The load-carrying capacity of contacts is normally given as a current value for a resistive load.

Lamp filaments are resistive, but change in value by a large factor from their cold state to their operating state resistance. This effect is so great that the inrush current can be expected to be 10 to 15 times greater than the steady-state value.

Normal practice is to de-rate contacts to 20 percent of their resistive load capabilities for a lamp load. Inductive loads, such as transformers, act as energy storage devices and can cause excessive contact arcing when the relay breaks the circuit. For inductive type loads contacts are normally derated to 50 percent of their resistive load capacity. Relay contacts often have two ratings: AC and DC.

These ratings indicate how much power can be switched through the contacts. One way to determine the maximum power capacity of relay contacts is to multiply the rated volts times the rated amperes.

This will give you the total watts a relay can switch. For instance, a 5-A relay rated at V AC can also switch 2. Similarly, a 5-A relay rated at 24 V DC can switch 2. A relay involves two circuits.

Name the two circuits and explain how they interact with each other. Outline three basic ways in which control relays are put to use in electric and electronic circuits. A Control Relay is also known as a Relay, is a switch, an electromagnetic switch. A control relay allows electrical current to flow through a conducting coil that opens or closes a switch. It also protects the circuit current.

With a control relay, users do not need to manually turn the switch to isolate or change the state of an electric circuit. They are electronic parts which power electronic parts such as motor, power plants, power supply system, transistor and many more. There are different types of Control Relays depending on the operating principle and structural features.

Solid State Relays — It uses solid-state components to perform the switching operations without moving any parts. Contactor — A large relay used to switch a large amount of electrical power through its contacts. Electromagnetic Relays — Constructed with electrical, mechanical and magnetic components and have operating coils and mechanical contacts. Hence, when the coil is activated by a supply system, the mechanical contact is either open or close. The supply system has 2 types of AC and DC.

Thermal Overload Protection Relay — works on the principle of the thermal effect of electrical energy. When excessive current flows through the circuit, the circuit opens due to the bimetallic strip experiencing an increase in temperature. Control Relay circuit diagram. Control Relays permit a low current circuit to control a high current circuit.

Using the above diagram, when an electrical current goes through the coil, it generates an electromagnetic field which will attract the switch downwards.