Contactor, Relays and Overloads Electricity for Refrigeration, Heating and Air Conditioning 7th Edition Chapter 11 Contactors, Relays and Overloads

Contactor, Relays and Overloads Upon completion of this chapter the student will be able to: Explain the parts and operation of contactors and relays. Explain the application of contactors and relays in control systems Correctly install a contactor or relay in a control system Draw a simple schematic wiring diagram using contactors and/or relays to control load in a control system. Understand the types and application of overloads Troubleshoot contactor and relays. Identify the common types of overload used to protect loads. Explain the operation of the common overloads. Determine the best type of overload for a specific application. Draw schematic wiring diagrams using the proper overload to protect loads.

Electrical Safety Upon completion of this chapter the student will be able to: Troubleshoot common types of overloads. Explain the operation of a magnetic starter. Size the overload devices to be used in a magnetic starter for motor protection. Wire a magnetic starter using switches, thermostats, and push-button stations. Troubleshoot magnetic starter and push-button stations.

Key Terms Coil Contactor Contacts Current Overload Fuse Inductive Load Internal Compressor Overload Line Break Overload Magnetic Overload Magnetic Starter Mechanical Linkage Overload Pilot Duty Overload Push-Button Station Relay Resistance Load

Contactors A contactor is used to control an electric load in a control systems. Contactors make or break a set of contacts that control the voltage applied to some load in cooling systems.

Contactors

Coils Coil Characteristics depends on the type of wire and the manner in which it is wound. Coils are typically designed to operate on 24 volts, 120 volts, 208/240 volts and occasionally 480 volts. The coil is identified by the voltage marked on it.

Contacts The contacts of a contractor make a complete circuit when the contactor is energized, allowing voltage to flow to the controlled load. Contractors are rated by the ampere draw they can carry. There are two types of loads that a contractor can control: an inductive load, and a resistive load. Contacts are made of silver and cadmium which resists sticking. The chemical composition of contacts is such that they operate at cool temperatures of up to 125% of their current-carrying capacity.

Contacts The contactor contacts must be in good condition to ensure that proper voltage reaches the load.

Relays Relays are used to open and close a circuit to allow the automatic control of a device or circuit.

Relay Applications Relays can be used to control indoor fan motors, condenser fan motors, damper motors, starting capacitors, and control lockouts.

Overloads An overload is an electrical device that protects a load from a high ampere draw by breaking a set of contacts. The simplest form of overload protection is the fuse. Fuses can be used to protect wires and non-inductive loads, but they provide inadequate protection for inductive loads. A load that is purely resistive in nature with no coils to cause induction is called resistive or non-inductive load.

Over Load Protection

Fuses Fuses consists of two ends or conductors with a piece of wire that will melt and break the circuit if the current passing through it exceeds the amperage rating of the fuse.

Line Break and Pilot Duty Overloads Overloads can be divided into two basic groups: Line break and Pilot duty. The line break overload breaks the power to a motor. A pilot duty overload breaks an auxiliary set of contacts connected in the control circuit.

Line Break Overload One of the most common types of line voltage overloads is the metal disc mounted between two contacts. This is called a bimetal line break overload.

Internal Compressor Overload The most popular line break overload for use in small central residential system is an internal compressor overload. The internal compressor overload is a small device inserted into the motor windings. This overload can sense the current draw of the motor, as well as the winding temperature, more effectively than external overloads.

Internal Compressor Overload

Pilot Duty Overload The pilot duty overload breaks the control circuit when an overload occurs, which would cause a contractor to be de-energized. This type of overload is common on larger systems and still exists on smaller systems currently in the field. Two basic pilot duty overloads are being used in the industry today: The current overload Magnetic overload

Current Overload Works similarly to the line break overload excepts that a pilot duty set of contacts is opened rather than the line.

Magnetic Overload Consists of a movable metal core in a tube filled with silicone or oil. Surrounding the metal tube is a coil of wire. When the current increases, so does the magnetic field of the coil. The overload operates by the magnetic field created by the coil. The device is designed to create a magnetic field that is strong enough to pull the coil up, opening the pilot contacts on overload.

Magnetic Overload