This is an old question for 380V, 3-phase cage motors, it seems that the answer was given decades ago, but it is almost a new question, and most of the answers now do not stand up to asking why? Or there is no operability in practice.
Let's summarize the existing answers first: (Please add if you haven't said everything)
1. The motor power is greater than 10kW;
2. The motor power is greater than 50kW;
3. The power of the motor is greater than 20% of the transformer capacity;
4. The voltage drop caused by the motor that starts frequently is greater than 10%;
5. For the motor that is occasionally started, the voltage drop caused by the start is greater than 15%.
There seems to be an empirical formula, and it is determined whether to start the pressure reduction after substituting the formula into the calculation, and I hope that friends who know the formula will add.
There seems to be no basis for 1 and 2, and in reality, hundreds or hundreds of kilowatts of motors have full voltage direct start.
For 3 items, is the transformer no-loaded?
For items 4 and 5, how to control them in practice? Is it actually a measurement?
In fact, the main purpose of using step-down start is to avoid affecting the normal operation of other equipment. If there is no other equipment, only transformer and motor, then the power of the motor can be close to the capacity of the transformer, considering the power factor, efficiency and other factors, the general motor power is 80% of the transformer capacity, can be started directly, or the transformer and the motor start at the same time.
In reality, when the vast majority of large motors are started, the transformer has more or less other loads, so the amount of other loads is also one of the factors to be considered.
Why does the motor affect the operation of other electrical equipment when it is started?
Because the starting current of the cage motor is 5-7 times of its rated current, the large current will cause a large voltage drop, and when the voltage drop reaches a certain value, other electrical equipment may stop working or cause failure. Therefore, it is stipulated that the voltage drop caused by the motor with frequent start should not exceed 10%, and the voltage drop caused by infrequent start should not exceed 15%.
Why is there a voltage drop?
This is because the transformer cannot provide the large current required for the start of the large motor, and if the transformer can provide the large current required for the start of the large motor, there will be no voltage drop or the voltage drop generated will be very small. This means that the size of the transformer capacity also determines the size of the voltage drop. In practice, we must not talk about the capacity of the transformer, but only talk about how big the motor should be started with a step-down.
The size of the existing load of the transformer is also one of the conditions that determine the size of the voltage drop. Imagine that when the transformer is no-load and fully loaded, the same motor is started separately, and the voltage drop caused by it must be different. Then it is not feasible to only talk about the power of the motor is a few percent of the capacity of the transformer, and not mention the existing load of the transformer, to judge whether the motor wants to step down and start.
At this point, in order to judge whether the motor is started with step-down, at least three factors should be considered: motor power, transformer capacity, and existing load, all of which are indispensable! So look at the various so-called judgment methods now, do you take these three into account?
How do you consider these three factors? In order to facilitate the use of this parameter, the current is used. In fact, the essence of the question is: how much current? How much voltage drop will it cause? That is, the problem of current and voltage drop.
As we all know, the starting current of the cage motor is 5-7 times the rated current.
For transformers, how much load current will cause the voltage drop to reach 10% or 15%? The general impedance voltage of the power transformer is about 5%, that is, when the output current of the transformer is the rated value, the voltage drop is 5%, which is the output of 380V transformer, the design output voltage is 400V, and it is just 380V when fully loaded.
Assuming that the voltage drop of the transformer's output voltage changes linearly as the load current increases, the voltage drop is 5% when the output current of the transformer is 2 times the rated current, 10% when the current is 3 times, and 15% when the current is 4 times. Then the allowable voltage drop of the frequently started motor is 10%, that is, the starting current of the motor plus the existing load current, as long as it is less than 3 times the rated current of the transformer, it can be started directly.
This is the simplest and closest to the actual way to determine which method to start the motor.