1. Understand the nature and installed capacity of the plant, and calculate its total variable capacity.
The factory has a variable kilovolt-ampere (kVA) and an estimated kilowatt-hour (kWh). For metallurgical, textile, and cement plants, the kilowatts are equivalent to kVA. In mechanical manufacturing and processing plants, half of the kilowatts are considered as kilowatts. In light industrial and chemical plants, the power is generally around seven thousand kVA.
2. Know the rated capacity of the shunt capacitor and calculate its capacitance.
The relationship between the compensation capacity and the microfarads: divide the thousand by the square of the voltage, and multiply by three points two times the microfarad method. Commonly used shunt capacitors follow specific microfarad calculation rules. If the rated voltage is 203 volts, then 60 microfarads are needed. At 400 volts, 20 microfarads per 1000 VA. At 6.3 kV, 8 microfarads per 1000 VA. At 10.5 kV, 3 microfarads per 1000 VA.
3. Understand the distribution transformer capacity and calculate the maximum motor capacity that can be directly started.
For a three-phase asynchronous motor, the full-voltage starting power in kW is calculated as: transformer capacity (kVA) multiplied by 5.9.
4. Know the current transformer capacity and calculate its secondary rated impedance.
Common current transformers have a rated load impedance. For a secondary rated current of 1 A, the impedance is in volt-amperes. For a 5 A secondary rating, the capacity is multiplied by 4 and divided by 100.
5. Knowing the 380V single-phase AC welding machine, calculate the transformer capacity, power cable cross-section, fuse size, and load switch capacity.
A 380V welding machine requires a two-core power cable. For 80% aluminum cross-section, the copper section is halved. The fuse should be three times the capacity, and the load switch must match the fuse rating.
6. Know the incandescent bulb capacity and calculate its thermal resistance.
To calculate the thermal resistance of a bulb, divide the wattage by the square of the voltage. For common bulbs, at 220V, the resistance is 48 ohms. At 127V, it's 1600 ohms. At 101V, it's 1200 ohms. At 36V, it's 1300 ohms.
7. Know the capacitor capacity in an AC circuit and calculate its capacitive reactance.
The capacitive reactance for a power frequency capacitor is calculated by dividing 3200 by the microfarad value.
8. Know the load resistance in a power frequency rectifier circuit and calculate the filter capacitor's capacitance.
In a power frequency rectification circuit, the filter capacitor's capacitance is calculated as: 1000 divided by the load resistance (in ohms).
9. Know the motor capacity and number of poles, and calculate the rated torque.
The motor's rated torque is calculated as ten times the number of poles, multiplied by three, and divided by six. The unit is Newton-meters (N·m).
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