There are many types of relays, including time relays, safety relays, solid-state relays, and organic relays. To enhance your understanding of relays, this article will introduce the correct usage of relays and the current setting of thermal relays. If you are interested in relays, please continue reading.
I. What are the different types of relays?
Relays can be categorized into time relays, voltage relays, intermediate relays, and thermal relays, among others. A time relay is a circuit relay that controls the opening or closing of a circuit when its output is delayed for a certain period of time after an input signal is applied or removed.
Intermediate relays are designed to amplify signals and power by increasing the number of control circuits or the contact breaking capacity. Current relays have their coils connected in series with the load, while thermal relays are protective electrical devices that activate when current passes through a heating element, causing it to bend and trigger an actuation mechanism.
II. How to use relays correctly
When using relays, it is essential to follow the instruction manual correctly and ensure that each parameter is within its specified range to guarantee safety. Furthermore, the operating time and load capacity of a relay cannot be guaranteed; they will vary depending on the surrounding environment and the specific load conditions.
When using relays, a rectangular wave should be used if the relay is used indoors. If an AC relay is used indoors, a sine wave should be used. To ensure the relay functions properly, it must not be damaged or subjected to impacts, otherwise its internal components will be damaged.
When using a relay, it's crucial to ensure the ambient temperature is neither too high nor too low, and the humidity is moderate. Furthermore, the air must be free of harmful substances, as these can easily affect the relay's performance. If it's a polarized relay, the voltage characteristics of its coil must be determined to ensure its proper operation.
III. Thermal relay current setting
(a) What does the current setting value of a thermal relay refer to?
The setting current of a thermal relay refers to its operating current, which is adjusted between 0.95 and 1.05 times the rated current of the protected motor. If the motor is overloaded and exceeds its setting current, the thermal relay will trip in about 3 to 5 seconds to protect the motor.
The working principle of a thermal relay is that the current flowing into the thermal element generates heat, causing the bimetallic strip with different coefficients of expansion to deform. When the deformation reaches a certain distance, it pushes the linkage to act, disconnecting the control circuit, thereby de-energizing the contactor, disconnecting the main circuit, and realizing overload protection for the motor.
(II) Adjustment of the setting current of the thermal relay
1. Adjust the thermal relay setting current to 1-1.15 times the motor's rated current. The thermal element of the thermal relay has thermal inertia; under overload current, the thermal relay contacts require a certain amount of time to operate. This is the protective characteristic of the thermal relay. Therefore, reasonable overload is permissible during motor operation.
2. Based on protection characteristics, the setting current of the thermal element is usually set to be equal to the rated current of the motor. However, if the motor drives an impact load, or the starting time is long, or the driven equipment cannot be stopped, it can be adjusted to 1.1-1.15 times the rated current of the motor.
3. The working principle of a thermal relay is to first connect the thermal relay in series in the main circuit, and connect the normally closed contact in series in the control circuit. When the motor is overloaded, the current flowing through the resistance wire exceeds the setting current of the thermal relay, the resistance wire heats up, and the bimetallic strip expands due to heat.
4. Due to the difference in thermal coefficients, the bimetallic strip bends, causing the transmission mechanism to operate. This opens the normally closed contact, de-energizes the control circuit, and de-energizes the contactor coil, thus cutting off the power supply to the motor. This achieves protection. With no power in the main circuit, the resistance wire stops heating, the bimetallic strip cools and resets, the normally closed contact closes, and the motor can start again.
