1. Calculation of motor power and wiring diameter

First, the line current for a 100KW load is calculated.

For a three-phase balanced circuit, the formula for calculating the power of a three-phase circuit is: P=1.732IUcosφ.

From the three-phase circuit power formula can be derived:

Line current formula: I=P/1.732Ucosφ

In the formula: P is the circuit power, U is the line voltage, the three phase is 380V, cosφ is the inductive load power factor, generally take 0.8 of your 100KW load line current:

I = P / 1.732 Ucos phi = 100000/1.732 * 380 * 0.8 = 100000/526.53 = 190 a

The current value is also corrected according to the nature and quantity of the load.

If the load is large and the motor is large, because the starting current of the motor is very large, which is 4 to 7 times the working current, it is also necessary to consider the starting current of the motor, but the starting current time is not very long, generally only considered by the coefficient of 1.3 to 1.7 when selecting the wire.

If we take 1.5, then the current is 285A. If the number of 60KW load is large, we are not used at the same time, you can take the coefficient of use 0.5 to 0.8, here take 0.8, the current is 228A. You can select wire, circuit breaker, contactor, thermal relay and other equipment according to this current. So the step of calculating the current cannot be omitted.

Wire selection:

According to the wire allowable current carrying meter, choose 50 square copper rubber wire, or choose 70 square copper plastic wire.

Transformer selection:

Transformer selection also has many conditions, here is a simple total capacity divided by the power factor and then rounded. S = P/cos phi = 100/0.8 = 125 kva

Choose a transformer larger than 125KVA.

How much current a 50 square copper core cable can withstand also depends on the laying method and ambient temperature, as well as the structural type of the cable and other factors.

50 square 10/35KV XLPE insulated cable long-term allowable carrying capacity Air laying long-term allowable carrying capacity

(10KV three-core cable)231A(35KV single-core cable)260A directly buried long-term allowable current (soil thermal resistance coefficient 100° C.m /W)(10KV three-core cable)217A(35KV single-core cable)213A

Second, according to the power distribution cable simple calculation

The rated power of the known motor is 22KW and the rated voltage is 380V. The transformer is 400 meters away from the well site. What is the cross-sectional area of the cable line?

(Resistivity of copper 0.0175)(a) with rated capacity, calculate the rated current of the motor at rated power

Solution: From P=S×COSφ, S=P/COSφ=22/0.8=27.5KVA, P is the rated power,COSφ is the power factor, 0.8 is taken according to the motor name

With S=I×U, the rated current I=S/U=27500/380=73A at rated power is calculated from the calculation formula

Estimation formula:

Two and a half times nine, minus one straight up.

Thirty-five times three and a half. Pairs minus five.

Conditions change and convert, high temperature 10% discount copper upgrade.

Through the pipe number two, three, four, eighty-six fold full load flow.

Instructions:

(1) The current carrying capacity (safety current) of various insulated wires (rubber and plastic insulated wires) is not directly pointed out, but is expressed by "cross section multiplied by a certain multiple", obtained by mental calculation.

The multiple decreases with the increase of the section.

"Two and a half times down to nine, minus a line up to go" is said to be 2.5mm 'and below a variety of cross-section aluminum core insulated wire, its carrying capacity is about 9 times the number of sections. For example, 2.5mm 'wire, the carrying capacity is 2.5×9=22.5(A). The multiple relationship between the carrying capacity and the number of sections of the wires from 4mm 'and above is arranged upward along the line number, and the multiple is successively reduced by l, that is, 4×8, 6×7, 10×6, 16×5, 25×4.

"Thirty-five times three and A half, two groups minus five points", said that the 35mm" wire carrying current is 3.5 times the number of sections, that is, 35×3.5=122.5(A). From 50mm 'and above the wire, the multiple relationship between the carrying capacity and the number of sections becomes two two line numbers into a group, and the multiple is reduced by 0.5 in turn. 50, 70mm '

The carrying capacity of the conductor is 3 times the number of sections; 95, 120mm" wire carrying capacity is 2.5 times the number of its section, and so on.

"Conditions change plus conversion, high temperature 10% discount copper upgrade".

The above formula is an aluminum-core insulated wire, and it is arranged at an ambient temperature of 25 ° C.

If the aluminum core insulated wire is placed in the area where the ambient temperature is higher than 25 ° C for a long time, the conductor carrying capacity can be calculated according to the above formula calculation method, and then discounted by 10%;

When the use is not aluminum wire but copper core insulated wire, its carrying capacity is slightly larger than the same specification aluminum wire, can be calculated according to the above formula method to increase the carrying capacity of a line number than aluminum wire.

Such as the carrying capacity of 16mm 'copper wire, you can calculate 16×5=80 according to 25mm2 aluminum wire

The minimum allowable working voltage of the motor is 360V, the sub-side voltage of the transformer is 380V, the maximum allowable voltage drop under rated power is △U is 20V, and the allowable resistance under rated power is

R line =△U/I=20/73=0.27Ω

The cross-sectional area of the wire is calculated from R = Rho L/S: S= Rho L/R

=0.0175×400/0.27Ω=24.62mm squared

Conclusion: 25 square copper cable should be selected

Third, how to choose circuit breakers, thermal relays

How to choose how much cross-sectional area of the cable according to the current, the cable we choose is a copper cable.

For example, we want to give an 18.5KW motor wiring, we can calculate its rated current of 37A, but also according to the experience of 1 square millimeter copper wire can pass 4~6A current, we take the middle value 5A, then the cross-sectional area of the cable line should be 37/5=6.4 square millimeters. Our standard cables are 6 square mm and a total of 10 square mm, in order to ensure reliability, we choose 10 square cable. In fact, in the specific choice, we may also choose 6 square, which should be considered comprehensively, how much power is consumed when the load is working, if only less than 60% of the rated power, you can choose this, if you basically want to work near the rated power, it can only choose 10 square cable.

How to consider the rated voltage and current wiring of the motor according to the power of the motor, select circuit breakers and thermal relays

Three-phase 220 motor, KW 3.5 amps.

Commonly used 380 motor, a kilowatt two amps.

Low voltage 660 motor, kilowatt 1.2 amps.

High voltage 3,000 volt motor, four kilowatts, one amp.

High voltage six thousand volt motor, eight kilowatts, one amp.

A three-phase motor, in addition to knowing its rated voltage, but also must know its rated power and rated current, such as: a three-phase asynchronous motor, 7.5KW, 4 poles (commonly used 2, 4, 6 levels, the series is different, its rated current is also different), its rated circuit is about 15A.

1, circuit breaker:

Generally choose its rated current 1.5-2.5 times, commonly used DZ47-6032A

2, wire:

According to the rated current of the motor 15A, choose the appropriate flow load wire, if the motor starts frequently, choose a relatively thick line, on the contrary can be relatively thin, the flow load has a relevant calculation, here we choose 4 square.

3. Ac contactor:

According to the motor power to choose the appropriate size on the line, 1.5-2.5 times, the general selection manual has the model, here we choose Zhengtai CJX2- 2510, but also pay attention to the matching of auxiliary contacts, do not buy back auxiliary contacts when the time is not enough.

4. Thermal relay

Its setting current can be adjusted, generally adjusted to 1-1.2 times the rated current of the motor.

(1) Multiple motors with wires: Add the total power of the motor by 2 is their total current.

(2) Within 50 meters of the line, the cross section of the wire is: total current divided by 4.(and put a little margin appropriately)

(3) The line length over 50 meters of wire cross-section: the total current in addition to 3.(and appropriate to put a little way)

(4) The current density of large cables above 120 square meters is lower

Iv. Summary

Circuit breaker selection:

The rated current of the motor is multiplied by 2.5 times, and the setting current is 1.5 times that of the motor, so as to ensure frequent start-up and sensitive short-circuit action.

The setting value of the thermal relay is 1.1 times the rated current of the motor.

Ac contactor:

Ac contactor selection is 2.5 times the motor flow.

This ensures frequent work for a long time

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