Thermal Overload Relays According to the Motor Power

YouTube：https://youtube.com/shorts/aXp2T5jNRjA

How to Select Circuit Breakers, Contactors and Thermal Overload Relays According to the Motor Power.
1. **Calculating the Motor Current According to the Motor Power**
- **Three-phase AC Motors (Taking the Common Three-phase 380V as an Example)**
- The formula is \(I = \frac{P}{\sqrt{3}U\cos\varphi\eta}\), where \(P\) is the motor power (in Watts, W), \(U\) is the rated voltage of the motor (in Volts, V, here it is 380V), \(\cos\varphi\) is the power factor of the motor (generally between 0.75 - 0.9), and \(\eta\) is the motor efficiency (generally between 0.7 - 0.95).
- For example, for a three-phase motor with a power \(P = 11kW\), assuming the power factor \(\cos\varphi = 0.85\) and the efficiency \(\eta = 0.88\), then its rated current \(I=\frac{11000}{\sqrt{3}\times380\times0.85\times0.88}\approx22.8A\).
- **Single-phase AC Motors (Taking the Common Single-phase 220V as an Example)**
- The formula is \(I=\frac{P}{U\cos\varphi\eta}\). For example, for a single-phase motor with a power \(P = 2.2kW\), assuming the power factor \(\cos\varphi = 0.8\) and the efficiency \(\eta = 0.85\), then its rated current \(I=\frac{2200}{220\times0.8\times0.85}\approx14.7A\).
2. **Selection of Circuit Breakers**
- **Principle**: Circuit breakers are mainly used for short-circuit protection and long-time overload protection.
- **Selection Method**
- For motors with direct starting, considering the motor starting current (generally 4 - 7 times the rated current), the rated current \(I_{n}\) of the circuit breaker should be greater than the rated current \(I\) of the motor. Usually, \(I_{n}\geqslant1.5 - 2.5I\). For example, for the above 11kW three-phase motor with a rated current of approximately 22.8A, a circuit breaker with a rated current of 40A or 50A can be selected (because \(22.8A\times2 = 45.6A\) approximately).
- For motors that start and brake frequently, considering the starting current and the operation frequency, generally \(I_{n}\geqslant2 - 3I\).
3. **Selection of Contactors**
- **Principle**: Contactors are mainly used to control the starting, stopping, forward and reverse rotation of motors, etc. Factors such as their rated current, rated voltage and load type need to be considered.
- **Selection Method**
- In terms of rated current, the rated current \(I_{km}\) of the contactor should be greater than or equal to the rated current \(I\) of the motor. Generally, \(I_{km}\geqslant1.3 - 2I\). For the above 11kW three-phase motor, a contactor with a rated current of 32A or 40A can be selected.
- In terms of rated voltage, the rated voltage of the contactor should be selected according to the rated voltage of the motor to ensure that they match. If the motor is 380V, select a contactor with a rated voltage of 380V or higher (such as 400V, 415V, etc., for voltage fluctuation situations).
- For motors that start, brake and reverse frequently, a contactor with one level higher rated current should be selected, and the service life and operation frequency of the contactor should also be considered.
4. **Selection of Thermal Overload Relays**
- **Principle**: Thermal overload relays are used for overload protection, and their operating characteristics are inverse-time, that is, the greater the overload current, the shorter the operating time.
- **Selection Method**
- The rated current \(I_{th}\) of the thermal overload relay should match the rated current \(I\) of the motor. Generally, \(I_{th}\approx0.95 - 1.05I\). For an 11kW three-phase motor, a thermal overload relay with a rated current between 20 - 32A can be selected.
- The operating current of the thermal overload relay needs to be adjusted appropriately according to the actual operation of the motor. During the normal starting and operation of the motor, there should be no misoperation; but when the motor is overloaded, it should be able to cut off the circuit in a timely and reliable manner to protect the motor. At the same time, there are two reset methods for thermal overload relays, automatic reset and manual reset, which should be selected according to the specific application scenarios. Automatic reset is convenient, but in some cases, it may cause the motor to start frequently. Manual reset requires manual intervention, but it is safer.