What are the two most common types of thermal overload relays? Overload metal pots with bimetal stripping and soldering. These are simple devices that use the bending action of a bimetal strip to open a circuit when it gets hot enough. They're not particularly accurate, but they work well enough for most applications.
Heater-operated switches. These are more sophisticated devices that use the heat from an electrical heater to melt a wax or plastic component inside the switch. The molten material seeps into a channel and opens the circuit. Heater-operated switches are more accurate than metal pot overloads, but they can be expensive. They also require maintenance during welding operations or when replacing the wax or plastic components.
Flashover plugs. These are bulky devices that use the resistance between their two electrodes to melt through a flashcube when it gets too hot. They're not as sensitive as thermal switches, but they do have an advantage over metal pot overloads in that they don't need to be recalibrated each time you weld different materials.
Heat sinks. These are devices that disperse the heat from an electric heater into a large surface area filled with water or other heat-conducting liquid.
Thermal overload relays are low-cost electromechanical main-circuit protection devices. They provide dependable motor protection in the case of an overload or a phase failure. Together with contactors, the thermal overload relay can provide a compact beginning solution. The relay detects overheating of the motor windings and immediately opens both sets of motor contacts to remove power from the overloaded section of the circuit.
Overload protection prevents electrical components within the motor from being damaged by excessive current loads. Electrical motors use electrical energy to turn parts such as shafts, gears, or brushes. If there is any obstruction that prevents the rotor from turning, the motor will try to continue running but will eventually fail due to overloading of its components. This type of failure could happen if someone were to connect a wire to a metal part inside the motor casing. In this case, the wire would become hot because of the current flowing through it to the casing. When the thermal overload relay senses that the temperature of this wire has risen too high, it will open both sets of motor contacts, removing power from the overloaded section of the circuit.
Electromagnetic contactors are controlled electrical switches used to make and break connections between circuits. They consist of a fixed contact attached to one side of a housing and a movable contact attached to an armature that moves within the housing when activated by an electromagnetic force.
Thermal overload relays are safeguards. They are intended to disconnect power if the motor uses an excessive amount of current over a lengthy period of time. Thermal overload relays include a normally closed (NC) relay to do this. If the motor gets too hot, the grease inside the bearings melts, causing them to fail.
The typical thermal overload relay has an adjustable timer that can range from several minutes to many hours. If the motor stays connected even after the timer expires, then power will be disconnected from the motor.
These devices were originally used on motors in air conditioners and heat pumps. An overloaded motor will cause it to break down due to overheating. But an underloaded motor will not perform as expected by its controller, which could lead to poor performance or damage to the motor windings.
Modern thermal overload relays use semiconductor components instead of electromechanical parts. These units are usually more reliable because they cannot burn out like their electromechanical counterparts did. However, they are also more expensive to purchase.
Electromechanical thermal overload relays work by comparing the temperature of the motor's bearing housing to a preset value. If the motor's temperature rises above this threshold, then the relay opens the circuit that powers it, shutting off the motor.
The thermal overload relay, seen in ill. 34, is a basic device that consists of a thermal element and a switch that opens when the temperature rises. The sign for a magnetic overload is the same as the symbol for a relay with a single normally closed contact. The current flow is sent to the overload coil through a relay. When the current reaches a certain level, the coil causes the switch to open and remove the load from the line.
The term "magnetic" means that a magnet is used instead of a spring to provide force against which the contact closes. This type of overload protection is necessary on motors, heaters, and other equipment that can be damaged by excessive currents. Magnetic overload relays are available in industrial, commercial, and home applications.
The term "thermal" means that the overload is caused by overheating. Thermal overloads occur when you pass too much current through an electrical component like a light bulb or heater. The overload protector senses the high temperature and shuts off the power before something else happens to the component.
An electric motor will run fast and hard until it is overloaded. If there is no way to shut off the power, the motor could damage itself or someone could be injured by the spinning parts.
Thermal overload protectors should always be used with electrical components that are subject to heating like lamps, heaters, and electric blankets. These devices use thermistors or bimetallic strips as their sensing elements.
Overload relays of the magnetic type work by monitoring the intensity of the magnetic field created by the current flowing to the motor. The primary distinction between magnetic overload relays and thermal overload relays is that magnetic kinds are not temperature sensitive. If a motor is overloaded and fails, it will continue to draw too much current until either the circuit breaker opens or the overload relay shuts off the power to the motor.
Magnetic overload relays were initially used for industrial applications where the use of heat as an indicator of an electrical fault was undesirable. Now they are also found in residential settings such as home automation systems and custom built computer peripherals.
Thermal overload relays rely on the heat generated by continuous high load currents to activate their internal mechanisms. So they need to be installed close to the source of the heat (motor or fan), which limits their application.
Residential thermal overload relays are usually mounted near electric heaters or air conditioners and actuate when their sensors reach 120 degrees Fahrenheit or below. They can also be located near radiators or other sources of heat that could indicate an electrical problem.
Commercial thermal overload relays are usually placed in vaulted ceilings or inside walls and actuate when their sensors reach 140 degrees Fahrenheit or above. They can also be located near hot water tanks or steam pipes that could indicate an emergency situation requiring immediate repair.