This section provides overview, applications, and principles of thermal relays. Also, please take a look at the list of 3 thermal relay manufacturers and their company rankings.
A thermal relay is a component that outputs a contact when a current exceeding a set value flows into an electrical circuit. They are mainly used to prevent overloading of motors and wiring. By incorporating thermal relay into circuits, problems such as circuit burnout can be prevented.
Thermal relays are mostly used for motor protection. When a motor is subjected to a torque in excess of its rating, a current in excess of the rating will flow. This phenomenon is called overcurrent.
When a motor is in an overcurrent state for an extended period of time, the internal windings heat up. This causes the internal varnish to melt or the windings to burn out. This is called motor burnout. When a current exceeding the rated value flows, the thermal relays shuts off the power supply by means of a contact output to protect the motor.
In most cases, bimetals are used as circuit conductors inside thermal relays. Bimetal is a material that combines two types of metals with different coefficients of thermal expansion. When heat is generated by an electric current, it is biased and deformed due to the difference in thermal expansion coefficients.
The bimetal inside the thermal relays uses this bias to drive the contact point and output the contact. Thermal relays are available with different current settings depending on the thickness of the bimetal and other factors.
When a motor starts, the current is more than twice the rated value. Since the bimetal heats up and deforms, there is a delay time until it warms up. Due to this delay time, they do not react to the high starting current of the motor.
The output contact of a thermal relays are fixed by a spring, and once activated, in most cases the output continues to be output until the reset button is pressed. This prevents overloaded equipment from restarting.
Thermal relays are selected according to the current rating of the motor to be protected. There are two types of motor protection elements: instantaneous and time-limited.
The momentary element is a protective element that shuts off the motor power supply instantaneously to protect the upper circuit in the event of a motor malfunction. Power fuses and motor relays are used for high-voltage motors, while shock relays and breakers are mainly used for low-voltage motors.
A time-limiting element is a protective element that detects motor overload and shuts off the power supply to protect the motor. Overcurrent relays are used for high-voltage motors, and thermal relays are used for low-voltage motors.
For low-voltage motors, a breaker, drive unit, and thermal relays should be selected in conjunction with the above. The breaker should be at least twice the motor's rated current to avoid tripping due to starting current. The drive and thermal relays should be selected according to the motor's rated current, referring to the manufacturer's catalog.
Thermal relays are classified into two types: bimetal type and electronic type.
The thermal relays has two types of contacts: a normally open contact (a-contact) for monitoring and a normally closed contact (b-contact) for breaking the circuit.
When an overload condition persists and the thermal relays operates, the output contact of the thermal relays must be deactivated after the cause is removed. This procedure is called "recovery" or "resetting".
There are two types of reset procedures: manual reset and automatic reset. The manual return type is simple, press the reset button after removing the cause of the overload. By pressing the reset button, the output contact is released.
The automatic reset type does not require pressing the reset button, but the thermal relays itself automatically resets itself. It is used in special applications where it is difficult for people to approach the thermal relays
Although it varies from manufacturer to manufacturer, the recommended replacement period for thermal relays is approximately 10 years. However, since periodic replacement of thermal relays are costly, in most cases only periodic replacement of thermal relays for critical loads is required.
Since thermal relays are composed of bimetals and resins, which do not deteriorate easily, they rarely deteriorate naturally. The following four factors are known to cause forced deterioration.
Thermal relays are often used as part of electromagnetic switches. In such cases, the primary side of the main circuit is usually connected with a copper bar for power supply in commercial products. The secondary side wiring should be designed to have an allowable current greater than the rated current of the thermal relays.
The allowable current for wiring varies depending on the type of wiring, but each type is defined by the internal wiring regulations. Wiring sold in Japan complies with the extension regulations.
*Including some distributors, etc.
Sort by Features
Sort by Area
This is the version of our website addressed to speakers of English in the United States. If you are a resident of another country, please select the appropriate version of Metoree for your country in the drop-down menu.