When we choose refrigeration equipment contactor or air conditioner contactor, we need to select the contactor and circuit breaker based on the click power and current relationship. What is the relationship between them? How should we choose the right air compressor contactor and circuit breaker?
Relationship between power and current
Commonly used three-phase asynchronous motors generally have two connection methods, one is star and the other is angular. Except for smaller motors, most are triangular. If it is a 7.5KW three-phase asynchronous motor powered by 380V, its rated working current per phase is about 15A. The actual calculation may be slightly smaller than this, we can choose the cable according to this current. If it is a 380V powered 4KW three-phase asynchronous motor, then its rated working current per phase is about 8A. Perhaps we can see the rule that a power of 1KW needs about 2A, and the rated operating current of a 75KW motor is about 150A. Calculated by the formula method, the results are relatively close.
How to choose cable according to the current
How to choose a cable with a large cross-sectional area according to the current during work? The cable we chose is a copper core cable. For example, if you wire an 18.5KW motor, you can calculate that its rated current is 37A, and a copper wire of 1 square mm can pass a current of 4~6A, and its median value is 5A, then its cross-sectional area cable should be 37 /5=6.4 square millimeters.
Our standard cable is 6 square millimeters for a total of 10 square millimeters. To ensure reliability, we chose a 10 sq. ft. cable. In fact, we may choose 6 square meters in our specific choice. This should be considered holistically. How much power is consumed when the load is working. If only 60% of the rated power is less than 6 square meters, we can choose 6 square meters.
How to choose circuit breakers and thermal relays according to motor power
For example:a three-phase asynchronous motor, 7.5KW, 4 poles (generally 2, 4, 6 levels, the number of different levels, the rated current is also different), rated circuit about 15A.
1. Selection of circuit breaker: Generally, the rated current is 1.5-2.5 times, the rated current of the motor is multiplied by 2.5 times, and the setting current is 1.5 times of the motor. This ensures frequent startup and sensitive short-circuit action.
2. The choice of cable: according to the rated current of the motor 15A, choose the appropriate current-carrying wire. If the motor starts frequently, choose a relatively thicker wire, otherwise it can be relatively thinner. The current-carrying capacity has relevant calculations. Here we choose 4 square.
3. Selection of AC contactor: Choose the appropriate size according to the motor power, 1.5-2.5 times, also pay attention to the matching of auxiliary contacts, do not buy auxiliary contacts when it is not enough. The AC contactor selection is 2.5 times the motor current. This can ensure frequent and long-term work.
4. The scope of the thermal relay: its setting current can be adjusted, generally adjusted to 1-1.2 times the rated current of the motor.
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