Problems with direct motor starting
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Can a 15KW three-phase asynchronous motor be started directly under a 250KVA transformer? What about 200KVA?
The limit can be set at 30% of the transformer capacity. In other words, other factors are not taken into account. Theoretically, a 50KVA transformer can be started directly. It is presumably derived from the requirement that the minimum voltage should not be lower than 85% of the rated voltage. I have previously made a 1250 transformer. Equipped with 3 low-voltage motors of 280KW each. Practice has proven it. It’s no problem to start the first one directly, but it’s a bit difficult with the second one. The third one immediately brought down the first two :( Fortunately, the process requirements specify that only 2 units can be operated at a time. It is a low-pressure circulating water pump
Hehe, motors over 160kW should use 6kV; the transformers would be overloaded if it were higher. . Hehe
The size of the motor capacity that a transformer can directly start is determined by its short-circuit capacity and the motor’s starting current. For applications over 75KW, soft starters are the most economical choice.
It is not recommended to start it directly; there is an empirical formula: I (full voltage) / I (rated)
A 15KW motor can be started casually, even with your eyes closed.
You say you have some memory of this formula. But this depends on the occasion. The 30% of the transformer’s rated capacity that I mentioned is the limit value (of course, a safety factor is still applied). What you’re referring to is probably the formula for an allowable voltage drop of 5% or 7%. Which motors should be started specifically. It also depends on the actual usage conditions, or in other words, the operating mode and the starting and stopping processes. For example, the 280KW motor I mentioned. It is used for the low-pressure circulating water system. It runs almost continuously. To protect the equipment, on-site operations generally involve reversing the pump direction once a month; equipment of this kind can then be started under the most severe conditions, especially if there are many important or sensitive devices in the vicinity. Such as electronic devices, etc. Of course, when the motor starts frequently, it cannot be started directly at 30% of its capacity.