How to calculate the size of the distribution transformer based on the total motor power

The starting current of the motor, Iq, is approximately 2 to 7 times the rated current. According to GB12325-90, the allowable deviation for three-phase supply voltage of 10KV and below is ±7% of the rated voltage. GB-T-3811-2008 Crane Design Specification, clause 7.2.1.2, stipulates that voltage fluctuations must not exceed ±10% of the rated value.

2. Selection of tap changer capacity for direct-starting of single electric motors:

Four-pole or two-pole squirrel-cage motors typically start directly, with a starting current six times the rated current. When the grid voltage drops by 15% — note that this exceeds the ±10% standard — then the transformer secondary capacity = √3 * 380 * (1 - 15%) * Iq / 1000 * cosφ = 1.732 * 323 Iq / 850 = 0.66Iq. The transformer's average power consumption is 7.5%, so the transformer capacity = (1 + 0.075) * transformer secondary capacity = 1.075 * 0.66Iq = 1.075 * 0.66 * 5~7 motor rated current. When taking approximate values, the transformer capacity ≈ 8.5 motor rated power.

3. Selection of tap-changing transformer capacity for simultaneous direct start of N identical electric motors:

Transformer capacity = 1.075 * N * Transformer secondary capacity per motor = N * (1.075 * 0.66) * Iq = 0.71 * N * Iq. Typically, Iq can be taken as 12 times the motor rated power (kW). Therefore, the transformer capacity ≈ 8.5 * N * Motor rated power. According to this principle, when the power of multiple motors is different, they can be calculated separately and then summed up. When they are not started simultaneously, calculate based on simultaneous start-up of motors, and add the motors running normally to determine the transformer secondary capacity required for both types of motors, and then calculate the transformer capacity.

How to calculate the size of the distribution transformer based on the total power of the motor

4. Selection of the voltage reducer transformer capacity for the variable frequency drive startup of a single motor:

Compared to the two previous models, the starting current is controlled at twice the rated current, so the transformer's capacity = (1 + 0.075) * transformer secondary capacity = 1.075 * 0.66Iq = 2.838 motor rated power, meaning the transformer's capacity ≈ 2.8 motor rated power.

5. Similarly, when multiple (N) identical motors are started simultaneously using a variable frequency drive, the selection of the voltage reducer transformer capacity:

The transformer capacity ≈ 2.8 times the rated power of a single motor. Based on this principle, when the power of multiple motors is different, they can be calculated separately and then summed up.

How to calculate the size of the distribution transformer based on the total power of the motor