【20150823】

Total current X voltage X power factor X coefficient times square root of 3? I can’t do this

1. According to the \"Power Engineering Design Manual,\" the capacity of a transformer should be determined based on the calculated load. For a single transformer supplying power to a steady load, the load factor is generally around 85%. That is: β = S/Se, where: S —— calculated load capacity (kVA) ； Se———Transformer capacity (kVA) ； β———Load factor (usually taken as 80%–90%). 2. Calculate the maximum power per phase of the load: Add up the load powers for phases A, B, and C separately; for example, if the total load power for phase A is 10 KW, that for phase B is 9 KW, and that for phase C is 11 KW, then the maximum value, which is 11 KW, is taken as the result. (Note: The power of a single-phase device is calculated based on the maximum value indicated on its nameplate; for three-phase devices, the power is divided by 3 to obtain the power per phase for that device.) )For example: Total power of Phase C load = (300W per computer × 10 units) + (2KW per air conditioner × 4 units) = 11KW. 3. Calculate the total three-phase power: 11KW × 3 phases = 33KW (total three-phase power of the transformer). Divide this value by 0.8 – this is the most important step. Over 90% of the transformers available on the market have a power factor of only 0.8, so it is necessary to divide by this power factor. 33KW / 0.8 = 41.25KW (total power of the transformer); 41.25KW / 0.85 = 48.529KW (power of the transformer that needs to be purchased). Therefore, it is sufficient to choose a 50KVA transformer when making the purchase. 4. Points to note: First, the rated capacity of a transformer refers to the maximum apparent power under specified operating conditions that allows the transformer to function properly. This apparent power is also the output power of the transformer, as well as the maximum apparent power load it can handle. When the transformer is operating at its rated capacity, its output apparent power equals this rated capacity; whereas when it is operating at its rated capacity, its input apparent power is greater than the rated capacity.

You can calculate it using the following method: 1. Determine the total apparent power output of the transformer based on the actual electrical demand. If the secondary side has multiple windings, then the total apparent power output is the sum of the apparent powers output by each winding on the secondary side: PS = U2I2 + U3I3 + … + UnIn, where U2, U3, …, Un are the effective voltages of each winding on the secondary side (in V)； I2 I3……In——RMS voltage of each winding on the secondary side (V) ； 2. For the calculation of apparent power PS1 and input current I1, when the transformer is under load, part of the input power is lost due to heating losses from the winding resistance and core losses. Therefore, the relationship between the transformer’s input power and output power is given by: PS1 = PS/η (W), where η represents the efficiency of the transformer. η is always less than 1; for transformers with a power of less than 1 KW, η = 0.8–0.9. Once the apparent power PS1 at the transformer’s input is known, the input current I1 can be calculated using the formula I1 = PS1 / U1 × (1.1–1.2) (A). In this formula, U1 represents the effective value of the voltage on the primary side (V), which is generally the voltage of the external power supply ； 1.1～1.2——Empirical coefficient considering the magnitude of the transformer’s no-load excitation current. 3. Determine the core cross-sectional area S; the dimensions of the E-type core commonly used in small single-phase transformers are shown in Figure 1. The size of its central column cross-sectional area S is related to the total apparent power output by the transformer, that is: S = K0√PS (cm²), where PS represents the total output power of the transformer (W)

Answer: It can be calculated using the following method: 1. Determine the total apparent power output of the transformer based on the actual electrical demand. If the secondary side has multiple windings, then the total apparent power output is the sum of the apparent powers output by each winding on the secondary side: PS = U2I2 + U3I3 + … + UnIn. Here, U2, U3, …, Un represent the effective voltages of each winding on the secondary side (in volts)； I2 I3……In——RMS voltage of each winding on the secondary side (V) ； 2. For the calculation of the apparent power PS1 and the input current I1, when the transformer is under load, part of the input power is lost due to heating losses from the winding resistance and core losses. Therefore, the relationship between the transformer’s input power and output power is: PS1 = PS/η (W), where η represents the efficiency of the transformer. η is always less than 1; for transformers with a power of less than 1 KW, η = 0.8–0.9. Once the apparent power PS1 at the transformer’s input is known, the input current I1 can be calculated using the formula: I1 = PS1 / U1 × (1.1–1.2) (A). Here, U1 represents the effective value of the voltage on the primary side (V), which is generally the voltage of the external power supply ； 1.1～1.2——Empirical coefficient considering the magnitude of the transformer’s no-load excitation current. 3. Determine the core cross-sectional area S; the dimensions of the E-type core commonly used in small single-phase transformers are shown in Figure 1. The size of its central column cross-sectional area S is related to the total apparent power output by the transformer, that is: S = K0√PS (cm²), where PS represents the total output power of the transformer (W)

1. Conventional method: According to the Power Engineering Design Manual, the capacity of a transformer should be determined based on the calculated load. For a single transformer supplying power to a steady load, the load factor is generally around 85%. That is: β = S/Se, where: S —— calculated load capacity (kVA) ； Se———Transformer capacity (kVA) ； β———Load factor (usually taken as 80%–90%). 2. Calculate the maximum power per phase of the load: Add up the load powers for phases A, B, and C separately; for example, if the total load power for phase A is 10 KW, that for phase B is 9 KW, and that for phase C is 11 KW, then the maximum value, which is 11 KW, is taken as the result. (Note: The power of a single-phase device is calculated based on the maximum value indicated on its nameplate; for three-phase devices, the power is divided by 3 to obtain the power per phase for that device.) )For example: Total power of Phase C load = (300W per computer × 10 units) + (2KW per air conditioner × 4 units) = 11KW. 3. Calculate the total three-phase power: 11KW × 3 phases = 33KW (total three-phase power of the transformer). Divide this value by 0.8 – this is the most important step. Over 90% of the transformers available on the market have a power factor of only 0.8, so it is necessary to divide by this power factor. 33KW / 0.8 = 41.25KW (total power of the transformer); 41.25KW / 0.85 = 48.529KW (power of the transformer that needs to be purchased). Therefore, it is sufficient to choose a 50KVA transformer when making the purchase.