Knowing the motor power allows us to know the motor current, which in turn provides a basis for selecting circuit breakers, contactors, and wires.
We generally call miniature molded case circuit breakers "air switches," which come in two series: type D for power protection and type C for lighting protection. When selecting air switches, the following points should be noted: the selection factor for type C is 1.5-2 times, and for type D it is 1.25-1.5 times. For general-purpose molded case circuit breakers, a selection factor of 1.15 times is sufficient.
Circuit breaker selection: Multiply the rated current of the motor by 2.5, and the set current should be 1.5 times that of the motor. This ensures frequent starting and sensitive short-circuit operation. Contactor: Select an AC contactor with a current of 2.5 times that of the motor. Wire selection: Select according to the current magnitude.
For example, a three-phase asynchronous motor, 7.5KW, 4 poles, has a rated circuit current of approximately 15A.
1. Selection of circuit breaker: Generally, select one with a rated current of 1.5-2.5 times, multiply the rated current of the motor by 2.5 times, and set the current by 1.5 times that of the motor. This ensures frequent starting and sensitive short-circuit operation.
2. Contactor selection: Choose an appropriate size based on the motor power, 1.5-2.5 times the motor current. Also, pay attention to the matching of auxiliary contacts to avoid insufficient auxiliary contacts after purchase. For AC contactors, choose one with a current rating 2.5 times that of the motor. This ensures smooth operation during long-term, frequent operation.
3. Selection of wires: Based on the rated current of 15A of the motor, select wires with appropriate current carrying capacity. If the motor starts frequently, choose a relatively thicker wire, and vice versa. There are relevant calculation rules for current carrying capacity. Here we choose 4 square millimeters.
When selecting a contactor, it's important to distinguish between direct starting and reduced-voltage starting, as well as heavy-load and light-load starting. For direct starting or heavy-load starting, considering the inrush current during startup and the reliability of the product, a selection of 2-2.5 times the motor's rated current is generally sufficient. For reduced-voltage starting, such as star-delta starters, the main contactor can be selected based on the motor's rated current. For star-connected contactors, a smaller configuration is generally acceptable.
We can determine the wire's safe current carrying capacity by referring to tables or by estimating based on experience. We also need to consider the minimum installation diameter and mechanical strength of the copper wire. Generally, we calculate the wire size based on the motor's rated current, using one square millimeter wire per kilowatt (kW). This is for aluminum core wire; for copper core wire, we would need to select a lower grade.
Wire safety current carrying capacity meter
For example, consider the selection of a 15kW motor. The estimated current for a 15kW motor is approximately 15*2=30A.
For circuit breaker selection: for type D circuit breakers, the current rating is 30*1.25-1.5, so a current rating of 50A is recommended; for type C circuit breakers, a current rating of 63A is recommended.
For direct start contactor, select 65A. For star-delta reduced voltage start contactor, select 32A for the main contactor and 25A for the star point contactor.
For direct starting, a 10 square millimeter wire is sufficient; for star-delta reduced voltage starting, a 4 square millimeter wire is sufficient. Both wires should be copper core wires.
Three-phase asynchronous motors are a common type of motor. We know that the power calculation formula for a three-phase asynchronous motor is P=√3*U*I*cosj. This formula is a basic formula but has certain limitations. This article provides a detailed summary of the basic knowledge of this three-phase asynchronous motor power calculation formula.
I. Explanation of the power calculation formula for a three-phase asynchronous motor
Three-phase asynchronous motor power formula: P=√3*U*I*cosj
This formula applies to circuits with three-phase symmetrical loads. This formula is only valid when a three-phase asynchronous motor is used in a three-phase symmetrical load.
P—is the active power input to the motor.
U—is the line voltage of the motor power supply input.
I—is the line current input to the motor power supply.
cosj is the power factor of the motor.
j—is the phase difference angle between the phase voltage and the phase current.
II. Derivation of the Power Calculation Formula for Three-Phase Asynchronous Motors
The total power of a three-phase symmetrical load circuit is equal to 3 multiplied by the power of each phase, i.e., P = 3 * u * i.
in:
u— is the average phase voltage
i — is the average phase current
For Y-type connections: U = √3u, I = i
P=3*1/√3*U*I*cosj=√3UIcosj
In a delta connection: U=u, I=√3i
P=3*U*1/√3*I*cosj=√3UIcosj
III. Basic Concepts of Three-Phase Circuits
A star connection, also known as a Y-connection, is a connection where the ends X, Y, and Z of the three windings are connected together, and the starting ends A, B, and C are brought out. The point where X, Y, and Z are connected together is called the neutral point of a Y-connected symmetrical three-phase power supply, denoted by N.
Star connection (Y connection)
The connection method in which the beginning and end of the three windings are connected in sequence is called a delta connection.
Triangle connection
Terminal wire (live wire): Leads from the starting terminals A, B, and C.
Centerline: The line leading out from the neutral point N; connection method without a centerline.
Line voltage: the voltage between terminals.
Phase voltage: The voltage of each phase of the power supply.
Line current: The current flowing through the terminal line
Phase current: The current flowing through each phase
This formula for calculating the power of a three-phase asynchronous motor is also applicable to other three-phase symmetrical circuits, provided the load is symmetrical: P = √3 * U * I * cosj
