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 is, 1 kilowatt power requires about 2A current, and the rated working current of a 75KW motor is about 150A. Calculated by the formula method, the results are relatively close.
How to choose cable according to 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: To wire a 18.5KW motor, it can be calculated that its rated current is 37A, 1 square millimeter copper wire can pass the current of 4~6A, and its median value is 5A, then the cross-sectional area of the cable should be 37 /5=6.4 square millimeters.
Our standard cables are 6 square millimeters and a total of 10 square millimeters. To ensure reliability, we choose a 10 square cable. In fact, we may choose 6 square meters in the specific choice. This should be considered comprehensively. 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 breaker and thermal relay according to the power of the motor
For example: a three-phase asynchronous motor, 7.5KW, 4 poles (usually there are 2, 4, 6 levels, the number of levels is different, the rated current is also different), the rated circuit is 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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