How to choose air switch, contactor, and wire for motor power
Knowing the power of the motor, we also know the current of the motor, which provides the basis for selecting circuit breakers, contactors, and wires.
We generally refer to miniature molded case circuit breakers as circuit breakers, which come in two series: power protection type - D type, and lighting protection type - C; When selecting with a blank switch, please note that the selection coefficient for Type C is 1.5-2 times, and for Type D it is 1.25-1.5 times. The selection factor for the overall molded case circuit breaker should be 1.15 times.
Air switch selection: Multiply the rated current of the motor by 2.5 times, and set the current to 1.5 times that of the motor to ensure frequent starting and sensitive short-circuit action; Contactor: The selection of AC contactor is 2.5 times the motor current; Wire selection: It is based on the magnitude of the current.
For example, a three-phase asynchronous motor, 7.5KW, 4-pole, with a rated circuit of approximately 15A.
1. Selection of air switch: Generally, it is recommended to use 1.5-2.5 times its rated current, multiply the rated current of the motor by 2.5 times, and set the current to 1.5 times that of the motor to ensure frequent starting and sensitive short-circuit action.
2. Selection of contactors: Choose the appropriate size based on the motor power, 1.5-2.5 times, and also pay attention to the matching of auxiliary contacts. Do not buy enough auxiliary contacts when the time comes. The selection of AC contactors is 2.5 times the motor current. This can ensure long-term and frequent work.
3. Selection of wires: Based on the rated current of 15A of the motor, choose wires with appropriate current carrying capacity. If the motor starts frequently, choose wires that are relatively thicker, otherwise they can be relatively thinner. The current carrying capacity is determined by relevant calculations. Here, we choose 4 square meters.
When selecting a contactor, it is necessary to distinguish whether it starts directly or with reduced voltage, and also to consider whether it starts with heavy load or light load; For direct starting or heavy load starting, considering the impact current during the starting process and the reliability of product quality, it is generally recommended to choose 2-2.5 of the rated current of the motor. For step-down starting, such as star delta starter, the main contactor should be selected from the rated current of the motor, and the contactor connected to the star can be configured one level smaller.
We can calculate the safe current carrying capacity of the wire based on a table, or estimate the selection based on experience. At the same time, we also need to consider the minimum installation diameter and mechanical strength of the copper wire. Generally, we calculate the wire based on the rated current of the motor, with one KW per square wire. This wire is an aluminum core wire, which needs to be downgraded by one level when converted to a copper core wire.
Wire safety current carrying capacity meter
For example: Selection of 15KW electric motor. The estimated current for a 15KW electric motor is around 30A, which is 15 * 2.
The selection of air switches is: D-type current is 30 * 1.25-1.5, and it is advisable to choose a current of 50A, while C-type should choose 63A.
Select 65A for direct starting of the contactor, star delta voltage reduction for starting, 32A for the main contactor, and 25A for the star point contactor.
The direct starting wire is 10 square meters, and the star delta voltage reducing starting wire is 4 square meters. The above wires are selected as copper core wires.
Three phase asynchronous motor is a common type of motor, and we know that the power calculation formula for three-phase asynchronous motor is P=√ 3 * U * I * cosj. This formula is a basic formula with certain limitations. This article provides a detailed summary of the basic knowledge of the power calculation formula for this three-phase asynchronous motor.
1、 Explanation of power calculation formula for three-phase asynchronous motor
Power formula for three-phase asynchronous motor: P=√3*U*I*cosj
This formula is applicable to circuits with three-phase symmetrical loads, and it only holds true when three-phase asynchronous motors are used in three-phase symmetrical loads.
P - is the active power input to the motor
U - is the line voltage of the motor power input
I - is the line current of the motor power input
Cosj - is the power factor of the motor
J - is the phase difference angle between phase voltage and phase current
2、 Derivation of power calculation formula for three-phase asynchronous motor
The total power of a three-phase symmetrical load circuit is equal to 3 times the power of each phase, i.e. P=3 * u * i,
Among them:
U - is the average phase voltage
I - is the average phase current
When using Y-shaped connection: U=√3u，I=i
P=3 * 1/√ 3 * U * I * cosj=√ 3UIcosj
When using a delta type connection: U=u，I=√3i
P=3 * U * 1/√ 3 * I * cosj=√ 3UIcosj
3、 Basic concepts of three-phase circuits
Take the end X of the three windings, Y. The method of connecting Z together and leading the starting points A, B, and C out is called star connection, also known as Y connection. X. The point where Y and Z are connected together is called the neutral point of a symmetrical three-phase power supply connected by Y, represented by N.
Star Connection (Y Connection)
The connection method of sequentially connecting the beginning and end of three windings is called triangular connection.
Triangular connection
End wire (live wire): The starting end A, B, and C lead out wires
Neutral line: Neutral point N leads out the line, and there is no neutral line in the connection method
Line voltage: the voltage between terminal lines
Phase voltage: the voltage of each phase of the power supply
Line current: the current flowing through the terminal wire
Phase current: the current flowing through each phase
The power calculation formula for this three-phase asynchronous motor is also applicable to other three-phase symmetrical circuits, as long as the load is symmetrical, P=√3*U*I*cosj
