Part.1 Given the capacity of the three-phase motor, find its rated current
Formula:
Divide the capacity by the number of kilovolts and multiply the quotient by the coefficient of seventy-six.
Three-phase 220 kW motor , 3.5 amps.
Commonly used 380 motors, one kilowatt and two amps.
Low voltage 660 kW motor, 1.2 amps.
High voltage three thousand volt motor, four kilowatts per amp.
High voltage six thousand volt motor, eight kilowatts per amp.
If the capacity is larger, reduce it a bit; if it is smaller, add a bit more.
Accurately calculate the current I=P/U×√3×cosφ (A)
Supplement: To be precise, it should also be multiplied by the motor efficiency, which is generally 0.9. Our common three-phase motor rated voltage (U) is 380v. The power factor (COSφ) is generally 0.85, which is marked on the motor nameplate.
Specific algorithm for rated current of 10KW three-phase motor:
I=10000÷(380×1.73×0.85×0.9)=19.8A.
Part.2 Measure the secondary side current of the power transformer and calculate its load capacity
Formula:
The secondary voltage of the distribution transformer is known, and the measured current is calculated in kilowatts.
The voltage level is 400 volts, 0.6 kilowatts per amp.
The voltage level is 3,000 volts, 4.5 kilowatts per amp.
The voltage level is six thousand volts, and one ampere is equal to nine thousand watts.
The voltage level is ten thousand volts, one ampere is fifteen thousand watts.
The voltage level is thirty-five thousand, and one amp is fifty-five kilowatts.
Part.3 Measure the current of the incandescent lighting circuit and calculate its load capacity
Formula:
The lighting voltage is two hundred and two hundred and twenty watts per ampere.
Regardless of power supply or distribution lines, just use a clamp ammeter to measure the current value of a phase line, and then multiply it by a factor of 220. The product is the load capacity of the phase line. Measuring the current to find the capacity number can help electricians quickly adjust the unbalanced three-phase load capacity of the lighting main line. It can also help electricians analyze the reasons why the protective melt in the distribution box often fuses, the reasons why the distribution wires heat up, etc.
Part.4 Measure the no-load current of the 380V single-phase welding transformer without nameplate and calculate the rated capacity
Formula:
Three hundred and eighty welding machine capacity, no-load current multiplied by five.
The no-load current of the transformer is generally about 6% to 8% of the rated current (the state stipulates that the no-load current should not be greater than 10% of the rated current). This is the theoretical basis for formulas and formulas.
Part.5 Given the capacity of the 380V three-phase motor, find the rated current and setting current of the overload protection thermal relay element
Formula:
Motor overload protection, thermal relay thermal element;
The current capacity is two and a half times, and the kilowatt setting is twice that.
The setting current of the thermal element is calculated according to "twice the kilowatt number"; the rated current of the thermal element is calculated according to "two and a half times the current capacity"; the model specification of the thermal relay, that is, its rated current value should be greater than or equal to the rated current value of the thermal element.
Part.6 Given the capacity of the 380V three-phase motor, find the rated current level of its remote control AC contactor
Formula:
Remote control motor contactor, double capacity depends on grade;
Step by step, start forward and reverse, and rely on the level to level up.
Part.7 Given the capacity of a small 380V three-phase cage motor, find the minimum capacity of its power supply equipment, load switch , and protection melt current value
Formula:
Direct starting motor, capacity does not exceed 10 kilowatts;
Six times the kilowatt switch is selected, and five times the kilowatt is equipped with the melt.
The power supply equipment needs to be three times larger in kilowatts than kilovolt amperes.
illustrate:
The direct starting motor described in the formula is a small 380V squirrel cage three-phase motor. The starting current of the motor is very large, usually 4 to 7 times the rated current. The maximum capacity of the motor started directly by the load switch should not exceed 10kW, generally below 4.5kW, and the open load switch (plastic cover porcelain bottom isolating switch) is generally used for small capacity motors of 5.5kW and below for infrequent operation. Direct starting; enclosed load switch (iron case switch) is generally used for infrequent direct starting of motors below 10kW.
Both need to have melt for short circuit protection, and the motor power must not be greater than 30% of the capacity of the power supply transformer. In short, remember that there are conditions for direct starting of the motor with a load switch!
Part.8 Motor starting star-delta , the starting time is easy to adjust
Formula:
Multiply the square root of the capacity by two and add four units of seconds to the product.
Motor starting star-delta, overload protection thermal element;
To set the current phase current, multiply the capacity by eight divided by seven.
When adjusting the time relay, do not connect the motor for operation temporarily, and test whether the action time of the time relay can be consistent with the starting time of the controlled motor. If it is inconsistent, the action time of the time relay should be fine-tuned and then tested. However, the interval between two tests must be at least 90s to ensure that the bimetal time relay automatically resets.
Part.9 Knowing the capacity of the cage motor, calculate the setting current of the circuit breaker release that controls it
Formula:
The tripper of the circuit breaker has times of the set current capacity;
Instantaneous is generally twenty, smaller motors are twenty-four;
The delay trip is three and a half times, and the thermal tripper is twice as long.
The calculation of the set current value of the circuit breaker tripper is a problem often encountered by electricians. The formula gives the multiple relationship between the set current value and the kilowatt capacity of the cage motor being controlled.
"Three and a half times the delay tripping, and twice the thermal tripping" refers to the automatic circuit breaker as an overload protection. The current setting value of the delay tripping device can be selected according to 1.7 times the rated current of the controlled motor. That is 3.5 times the kilowatt number selection. The current setting value of the thermal tripper should be equal to or slightly greater than the rated current of the motor, that is, selected based on twice the kilowatt capacity of the motor.
Part.10 Given the capacity of the asynchronous motor, calculate its no-load current
Formula:
Find the no-load current of the motor, and the capacity is about 20% off;
The new big pole is 40% off, and the old small pole has more kilowatts.
Generally, the no-load current of small motors is about 30% to 70% of the rated current, and the no-load current of large and medium-sized motors is about 20% to 40% of the rated current. The specific no-load current of a certain motor is generally not marked on the motor's nameplate or product manual. But electricians often need to know what this value is, and use this value to judge the quality of the motor repair and whether it can be used. It complies with "the no-load current of a motor is generally 1/3 of its rated current". At the same time, it is in line with practical experience: "The no-load current of the motor can be used if it does not exceed the capacity of kilowatts" (referring to old, small-capacity motors after overhaul). The formula "ask for about 20% off the capacity" means that the no-load current value of a general motor is about 0.8 times the rated capacity of the motor in kilowatts. The no-load current of medium-sized, 4- or 6-pole motors is 0.8 times the kilowatt capacity of the motor; for new series, large-capacity, small-pole 2-level motors, the no-load current calculation is based on the "New large pole number is 40% less" "; For old, old-fashioned series, smaller capacity, more than 8-pole motors with a large number of poles, the no-load current is calculated according to "small pole multi-kilowatts", that is, the no-load current value is approximately equal to the capacity kilowatts, But it is generally less than kilowatts.
Part.11 Given the capacity of the power transformer, calculate the setting current value of the instantaneous release of the automatic circuit breaker on its secondary side (0.4kV).

Formula:
For power supply on the secondary side of the distribution transformer, it is best to use a circuit breaker;
Instantaneous trip setting value, triple capacity kVA.
Part.12 Judgment of in-phase and out-of-phase
Formula :
To determine whether two lines are the same or different, hold a pen in each hand.
The two legs are insulated from the ground, and the two pens each touch a main line.
Look at a pen with your eyes. If it does not light up, it will be different.
During this test, remember that both feet must be insulated from the ground. Because most of the power supply in our country is 380/220V, and the neutral point of the transformer is generally directly grounded, so when doing the test, the human body must be insulated from the earth to avoid forming a loop and misjudgment; when testing, two pens are on and off . The lights show the same, so just look at one.
Part.13 Determine the positive and negative poles of DC power
Formula:
Use an electric pen to determine the positive and negative poles, and be careful when observing the atmosphere tube.
The bright front end is the negative pole, and the bright rear end is the positive pole.
Part.14 Determine alternating current and direct current
Formula:
The electric pen determines AC and DC, AC is bright and DC is dark,
The AC neon tube is bright all over the body, and the DC neon tube is bright at one end.
Part.15 Determine whether the DC power supply is grounded or not, and the difference between positive and negative grounding
Formula:
The DC coefficient of the substation does not light up when touched by an electric pen;
If the light is close to the tip of the pen, there is a ground fault in the positive terminal;
If the light is close to the finger tip, the ground fault is in the negative pole.
Part.16 Calculation of safe carrying capacity of copper core cable conductors
Formula:
10 is five, 100 is two, 16 and 25 are four, 35 and 50 are three, 70 and 95 are two and a half times.
For pipes, the temperature is 80% or 90% off, and for bare wires, add half. Copper wire upgrade counts.
The relationship between the Arabic numerals and multiples in the formula is as follows:
For wires of 1.5, 2.5, 4, 6, and 10mm2, multiply the cross-sectional area by 5 times.
For 16 and 25 mm2 wires, multiply the cross-sectional area by 4 times.
For wires of 35 and 50 mm2, the cross-sectional area can be multiplied by three times.
For wires of 70 and 95mm2, the cross-sectional area can be multiplied by 2.5 times.
For wires of 120, 150, and 185 mm2, multiply the cross-sectional area by 2 times.
The copper wire area is calculated by upgrading one level.

Part.17 Selection of circuit breaker
Because the circuit breaker is for circuit break protection, it can be selected to be larger than the rated current of the motor. It is usually 1.2 times the rated current of the motor and is conservatively 1.6 times. The thermal relay usually chooses 0.95~1.05 times the rated current of the motor. Personally, I prefer 1 times.
Part.18 Selection of AC contactor
(1) Continuously operating equipment. The contactor is calculated as 67-75%, that is, a 100A AC contactor can only control equipment with a maximum rated current of 67-75A or less.
(2) Equipment that operates intermittently. The contactor is calculated as 80%, that is, a 100A AC contactor can only control equipment with a maximum rated current of less than 80A.
(3) Equipment that works repeatedly for short periods of time. The contactor is calculated as 116-120%, that is, a 100A AC contactor can only control equipment with a maximum rated current of 116-120A or less.
Also consider the working environment and the structure of the contactor.
Part.19 Electric welding machine
1) 380V electric welding machine I=1000S/U=1000S/380=2.63S
220V electric welding machine I=1000S/U=1000S/220=4.55S can be summarized as:
380 welding machines, 2.6 times kVA;
A two hundred and two welding machine, four and a half times kVA.
2) Electric welding machines are usually divided into two categories: arc welding and resistance welding . Resistance welding (butt welding, spot welding, seam welding, etc.) takes shorter time. As mentioned above, their wiring can be smaller. The specific method is:
First, change (lower) the capacity according to the formula of "20% off for solitary soldering, and half for soldering mask". That is, the capacity of arc welding machines will be discounted by 20%, and the capacity of resistance welding machines will be discounted by half (multiplied by 0.5), and then power distribution will be carried out according to the changed capacity.
(1) For a 30 kVA AC arc welding machine, according to "20% off for solitary welding", then 30×0.8=24, that is, the capacity can be changed to 24 kVA during power distribution. When connected to 380 volt single phase, the power distribution can be 24×2.5=60A.
(2) For a 30 kVA spot welding machine, press "solder mask half", then 30×0.5=15, and you can distribute power at 15 kVA.
When it is 380 volt single phase, the power distribution is 15×2.5=37.5A.
Part.20 Motor contactor selection for star-delta start
1) The rated current marked on the motor nameplate refers to the line current.
2) The rated power on the motor nameplate refers to the power under the specified connection method.
3) For motors that use star-delta starting, the two working contactors can only bear 0.58 times the line current after conversion.
4) For motors that are stipulated to use delta connection, the motor power will drop significantly when changed to star connection. At this time, the line current also dropped to 0.58 times its original value.