How to calculate wires and switches based on the motor capacity?
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How to calculate the wires and switches based on the motor’s capacity? The methods for selecting circuit breakers, contactors, thermal elements, and wires, depending on the capacity of a three-phase squirrel-cage asynchronous motor, are as follows: 1. If the motor’s capacity is N KW, then its rated current is 2N A; in most cases, this value is very close to the rated current indicated on the motor’s nameplate! If you don’t believe it, you can check the motor manual; this formula is extremely accurate! For example: if the motor power is 7.5 KW, then the rated current is 15 A ； With a motor power of 55 KW, the rated current is 110A ； The rated current is the primary factor in selecting circuit breakers, contactors, and wires! 2. Select the circuit breaker as follows: If the capacity of the motor is N KW, then its rated current is 2N A. Generally, the capacity of the circuit breaker chosen is around 4N A ； For example: if the motor power is 7.5 KW, then the rated current is 15 A ； The rated capacity should be 32 A ； With a motor power of 55 KW, the rated current is 110A ； The rated capacity should be 250 A ； Note: The circuit breakers and contactors for fans and pumps should be selected with a higher capacity, as they require a longer starting time and generate greater starting torque ； 3. The contactor selection is the same as above, namely (3.5—4) N A ； 4. Wire selection: It should be chosen based on the motor’s rated current, typically at 1.5 times the rated current. However, factors such as the installation environment and method must also be taken into account, with an appropriate coefficient applied. How to calculate the wires and switches based on the motor’s capacity?
Resistance: In circuits, resistance is denoted by “R” followed by a number; for example, R15 refers to the resistor with the number 15. The main functions of resistance in circuits include current sharing, current limiting, voltage division, biasing, filtering (when used in combination with capacitors), and impedance matching. Parameter identification: The unit of resistance is ohm (Ω), while the units for multiples include kiloohm (KΩ), megaohm (MΩ), etc. The conversion method is: 1 megohm = 1000 kiloohms = 1,000,000 ohms. There are 3 ways to indicate the resistance value: direct marking, color coding, and numerical marking. The numerical notation is mainly used for small-sized circuits such as surface-mounted components; for example, 472 denotes 47×102Ω (that is, 4.7K) ； 104 indicates that the 100K color-code system is the most commonly used. Examples are as follows: four-color ring resistors, five-color ring resistors (precision resistors). The positions of the color codes on the resistor and their corresponding multiplier values are shown in the table below: Color, Significant digits, Multiplier, Allowable tolerance (%): Silver / 10-2 ±10, Gold / 10-1 ±5, Black 0, 100; Brown 1, 101 ±1, Red 2, 102 ±2, Orange 3, 103; Yellow 4, 104; Green 5, 105 ±0.5, Blue 6, 106 ±0.2, Purple 7, 107 ±0.1, Gray 8, 108; White 9, 109 +5 to -20, Colorless / / ±20