The thermal relay is a critical component used for overload protection in electric motors. It consists of several key parts: a heating element, a bimetallic strip, contacts, and a mechanism for transmission and adjustment. The heating element, typically a low-resistance wire, is connected in series within the motor's main circuit. When current flows through it, it generates heat, which is then transferred to the bimetallic strip.
The bimetallic strip is made by combining two metal layers with different coefficients of thermal expansion. In most designs, the lower layer has a higher coefficient, meaning it expands more when heated. As the temperature rises due to an overload condition, the bimetal bends upward, disengaging from the gusset and opening the normally closed contact. This action interrupts the control circuit of the motor, causing the contactor coil to de-energize. As a result, the main contact of the contactor opens, cutting off power to the motor and providing overload protection.
Once triggered, the thermal relay must be manually reset after the bimetal has cooled down. This ensures that the system does not automatically restart until the issue is resolved.
Thermal relays are primarily designed for overload protection, not for short-circuit protection. This is because the bimetal takes time to heat up and bend, making the relay unsuitable for fast-acting faults like short circuits. However, they are effective for protecting motors against prolonged overloads.
When selecting a thermal relay, several factors should be considered:
1. **Motor Characteristics**: The relay should match the motor’s rated current, starting conditions, load type, and allowable overload capacity.
2. **Insulation Class and Motor Design**: Motors with higher insulation classes can withstand greater temperatures, so their thermal relays may need to be set differently. Closed motors, for example, tend to retain more heat than open ones, requiring lower setting currents.
3. **Starting Current and Time**: The starting current of a motor can be 5–7 times its rated current. If the start-up time is short (under 6 seconds), the relay can be selected based on the motor’s rated current.
4. **Phase Loss Protection**: For Y-connected motors, standard thermal relays can provide phase loss protection if properly adjusted. However, for Δ-connected motors, a specialized relay with differential phase-break protection is required.
5. **Environmental Conditions**: If the motor cannot stop immediately due to operational constraints, the relay’s setting current should be slightly higher than the motor’s rated current.
Installation of the thermal relay also plays a vital role in its performance. The direction of installation matters, as improper placement can affect the speed at which the bimetal heats up. The relay should be installed away from other heat sources and in line with the manufacturer’s specifications. The ambient temperature around the relay should be similar to that of the motor to ensure accurate operation.
The connecting wires also influence the relay’s behavior. If the wires are too thin, they may conduct excess heat to the bimetal, causing premature tripping. Conversely, thick wires can slow down the response. Therefore, it is important to use wires with the correct cross-sectional area as specified by the manufacturer.
Before commissioning, the setting current of the thermal relay must be adjusted to match the motor’s rated current. For example, a 10kW, 380V motor with a rated current of 19.9A might use a JR20-25 relay with settings of 17–21–25A. Initial settings can be fine-tuned based on actual motor performance.
Finally, wiring diagrams are essential for proper installation. These diagrams illustrate how the thermal relay connects to the motor and control circuit, ensuring safe and effective operation.
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