Pithy formula of the most popular calculation form

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Common calculation formula for electricians (pithy formula)

Description: applicable to any voltage level

in daily work, some electricians are only involved in the calculation of transformer rated current of oneortwo voltage levels. Simplifying the above formula, we can deduce the formula for calculating the rated current at the side of each voltage grade:

multiply the capacity coefficient

given the transformer capacity, quickly calculate the current value of its primary and secondary protection fuse links (commonly known as fuses)

formula b:

high voltage fuse link of distribution transformer, capacity and voltage comparison

low voltage fuse link of distribution transformer, capacity multiplied by 9 divided by 5


correct selection of fuse links is of great importance to the safe operation of transformers. When the fuse is only used as the high and low voltage side protection of the transformer, the correct selection of the melt is more important. This is a problem that electricians often encounter and need to solve

if the capacity of three-phase motor is known, calculate its rated current

formula C: divide the capacity by kilovolts, and multiply the factor by point 76


(1) the formula is applicable to the calculation of rated current of three-phase motor at any voltage level. From the formula and formula, it can be explained that the rated current of motors with the same capacity and different voltage levels is different, that is, the voltage kilovolts are different. If the same capacity is removed, the quotient obtained is obviously different. If different quotients are multiplied by the same coefficient of 0.76, the current value obtained is also different. If the above formula is called general formula, a special formula for calculating the rated current of 220, 380, 660, 3.6kV voltage level motors can be derived. When calculating the rated current of a three-phase motor with the special formula, the relationship between the capacity kW and the current ampere is directly multiplied, eliminating the capacity divided by the kilovolt number, and the quotient multiplied by the coefficient of 0.76

three phase two hundred two motors, kW 3.5 amps

commonly used 380 motors, one kW two amps

low voltage 660 motor, kW 1.2 amps

high voltage three thousand volt motor, four kilowatts and one ampere

high voltage 6kV motor, eight kilowatts and one ampere

(2) when pithy formula C is used, the unit of capacity is kW, the unit of voltage is kV, and the unit of current is A. This must be noted

(3) the coefficient 0.76 in formula C is the comprehensive value calculated by considering the motor power factor and efficiency. The power factor is 0.85, and the efficiency is not 0.9. These two values are applicable to motors above tens of kW, but they are larger for commonly used motors below 10kW. Therefore, there is an error between the rated current of the motor calculated by formula C and the value marked on the motor nameplate. This error has little impact on the rated current of the motor below 10kW

(4) use pithy calculation skills. When calculating the rated current of a commonly used 380V motor with a formula, first multiply the capacity (kw) by 0.76 and quotient 2 with the power supply voltage 0.38kv of the motor. In case of 6kV motor with large capacity, the kW of capacity is exactly a multiple of 6kV, then divide the capacity by kV and multiply the quotient by 0.76 coefficient

(5) error. The coefficient 0.76 in formula C is calculated by taking the motor power factor as 0.85 and the efficiency as 0.9, so there is an error in calculating the rated current of motors with different power factors and efficiencies. For the five special formulas derived from formula C, the multiple of capacity (kw) and current (a) is the quotient of the number of voltage levels (kV) minus the coefficient of 0.76. The special pithy formula is simple and easy to calculate, but it should be noted that its error will increase. Generally, if the kilowatt number is large, the calculated current is slightly larger than that on the nameplate; For those with smaller kilowatts, the calculated current is slightly smaller than that on the nameplate. In this regard, when calculating the current, when the current reaches more than ten amperes or tens of amperes, it does not need to be calculated after the decimal point. It can be rounded without rounding, and only take an integer, which is simple and does not affect practicality. For smaller currents, it is only necessary to calculate to one decimal place

* measure the current to calculate the capacity

measure the no-load current of the motor without nameplate, estimate its rated capacity


the capacity of the motor without nameplate, measure the no-load current value,

multiply ten by eight to calculate, close to the kilowatt number of the grade

Description: the pithy formula is for a three-phase asynchronous motor without nameplate. If you don't know its capacity kW, you can estimate the motor capacity kW by measuring the no-load current value of the motor

measure the secondary side current of the power transformer and calculate its load capacity


know the secondary voltage of the distribution transformer, and measure the current to calculate kW

voltage level 400 V, 1 a 0.6 kW

the voltage level is 3000 volts, one ampere, 4.5000 watts

voltage level: 6kV, one ampere integer: 9kw

the voltage level is 10kV, one ampere and fifteen thousand watts

voltage level is 35000, one ampere, fifty-five thousand watts


(1) electricians often encounter superior departments and managers in their daily work to ask about the operation of power transformers and how much load is it? Electricians often need to know the load of the transformer. The load current is easy to know. Directly look at the ammeter set on the power distribution device, or measure it with the corresponding clamp ammeter. The load power can not be seen and measured directly. This needs to be calculated by this pithy formula, otherwise it will be complex and time-consuming to calculate with conventional formula

(2) voltage level 400 V, 0.6 kW per generator. When the load current at the secondary side of the power transformer (voltage class 400V) is measured, the load power kW is obtained by multiplying the ampere value by the coefficient of 0.6

measure the current of incandescent lamp lighting line, and calculate its load capacity

lighting voltage is 220, one ampere, 220 watts

note: 220V incandescent lamps are mostly used for lighting in industrial and mining enterprises. The lighting power supply line refers to the line from the distribution board to each lighting distribution box. The lighting power supply main line is generally three-phase four wire, and single-phase can be used when the load is below 4kw. Lighting distribution line refers to the line from lighting distribution box to lighting facilities such as illuminators or sockets. No matter the power supply or distribution line, as long as the current value of a phase line is measured with a clamp ammeter, and then multiplied by the 220 coefficient, the product is the load capacity of the phase line. Measuring the current and calculating the capacity can help the electrician quickly adjust the imbalance of three-phase load capacity of the lighting trunk line, and help the electrician analyze the reasons for the frequent fusing of the protective melt in the distribution box and the heating of the distribution wire, etc

measure the no-load current of 380V single-phase welding transformer without nameplate, calculate the base rated capacity


three hundred and eight welding machine capacity, and multiply the no-load current by five

single phase AC welding transformer is actually a step-down transformer for special purposes, and its basic working principle is roughly the same as that of ordinary transformer. In order to meet the requirements of the welding process, the welding transformer works in the short-circuit state, and it is required to have a certain arc striking voltage during welding. When the welding current increases, the output voltage drops sharply. When the voltage drops to zero (that is, the secondary side is short circuited), the secondary side current will not be too large, etc., that is, the welding transformer has the external characteristics of steep drop, which is obtained by the voltage drop generated by the reactance coil. When no-load, because no welding current passes through, the reactance coil does not produce voltage drop. At this time, the no-load voltage is equal to the secondary voltage, that is, when the welding transformer is no-load, it is the same as when the ordinary transformer is no-load. The no-load current of the transformer is generally about 6% to 8% of the rated current (the no-load current specified by the State shall not be greater than 10% of the rated current). This is the theoretical basis of formulas and formulas


given the capacity of 380V three-phase motor, calculate the rated current and setting current of its overload protection thermal relay element

pithy formula:

motor overload protection, thermal relay thermal element

two and a half times of the current capacity and twice the kilowatt number setting


(1) for motors that are prone to overload, TBEA will make full use of this advantage. If the starting or self starting conditions are serious and the starting may fail, or the starting time needs to be limited, overload protection should be installed. Overload protection should also be installed for motors that operate unsupervised for a long time or motors of 3KW and above. The overload protection device generally adopts the time-delay overcurrent release of thermal relay or circuit breaker. At present, the thermal relay produced in China is suitable for motor overload protection of light load starting, long-term operation or intermittent long-term operation. [first electrician]

(2) the structure and principle of thermal relay overload protection device are very simple, but the optional heat regulating elements are very subtle. If the grade is selected to be large, it must be adjusted to the low limit, which often causes the motor to stop secretly, affects production, and increases maintenance work. If the grade is selected to be small, it can only be adjusted to the high limit, and the motor often does not act when overloaded, or even burns the motor. (3) To correctly calculate and select the overload protection thermal relay of 380V three-phase motor, it is still necessary to understand that the thermal relay of the same series of models can be equipped with thermal elements with different rated currents. The setting current of the thermal element is set by twice the kW; The rated current of the thermal element is calculated and selected according to two and a half times of the signal current capacity; The model and specification of thermal relay, that is, its rated current value should be greater than or equal to the rated current value of thermal element

given the capacity of 380V three-phase motor, calculate the rated current level of its remote control AC contactor

pithy formula:

remote control motor contactor, twice the capacity depends on the level

start the forward and reverse rotation in steps, and upgrade one level based on the level


(1) at present, the commonly used AC contactors include CJ10, cj12, cj20 and other series, which are more suitable for the starting control of general three-phase motors

given the capacity of small 380V three-phase cage motor, calculate the minimum capacity of its power supply equipment, load switch, protective melt current value

pithy formula:

directly start the motor, and the capacity does not exceed 10 kW

six kW selector switch, five kW with melt

power supply equipment KVA, which requires three times the number of kilowatts


(1) the direct starting motor described in the pithy formula is a small 380V squirrel cage three-phase motor, and the starting current of the motor is large, which is generally 4~7 times the rated current. The maximum capacity of the motor directly started by the load switch should not exceed 10kW, which is generally below 4.5kw, and the open load switch (rubber covered porcelain bottom disconnector) is generally used for infrequent direct starting of small capacity motors of 5.5kW and below; Closed load switch (iron shell switch) is generally used for infrequent direct starting of motors below 10kW. Both of them need melt for short-circuit protection, and the motor power is not greater than 30% of the capacity of the power supply transformer. In short, remember that it is conditional for the motor to start directly with the load switch

(2) the load switch is composed of simple disconnector knife and fuse or melt. In order to avoid the large current when the motor starts, the capacity of the load switch, that is, the rated current (a); The rated current (a) of the melt for short-circuit protection is selected according to six times of kW and five times of kW with fused parts. Since the iron shell switch and the rubber covered porcelain bottom disconnector are all manufactured according to certain specifications, the current value calculated by the formula needs to be close to the switch specification. Similarly, the melt should be selected according to the product specification

given the capacity of cage motor, calculate the action time and thermal element setting current of star delta starter (QX3, qx4 Series)


motor starts star delta, and the starting time is easy to set

capacity square multiplication

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