For 380V three-phase squirrel-cage motors, this is an old question that seemingly had answers decades ago, yet it remains almost like a new problem as most contemporary answers fail to withstand the question "why?" or lack practicality in real-world applications.
Motor power greater than 10kW;
Motor power greater than 50kW;
Motor power exceeding 20% of the transformer's capacity;
For motors that frequently start, the voltage drop during starting exceeds 10%;
For motors that start occasionally, the voltage drop during starting exceeds 15%.
For items 1 and 2, it seems there is no solid basis, as in reality, motors with hundreds or even thousands of kilowatts have been directly started at full voltage.
Regarding item 3, is the transformer operating at no-load conditions?
As for items 4 and 5, how are they practically controlled in real-world applications? Is it through actual measurements?
The primary objective of adopting a reduced-voltage start, in fact, is to avoid disrupting the normal operation of other equipment.
If there are no other devices present, only the transformer and the motor, then the motor's power can approach the transformer's capacity. Considering factors such as power factor and efficiency, generally, a motor with a power up to 80% of the transformer's capacity can be directly started, or the transformer and motor can be started simultaneously. In reality, when most large motors start, the transformer will have other loads to varying degrees, and the amount of these other loads is also one of the factors to consider.
Because the starting current of a squirrel-cage motor is 5-7 times its rated current, this high current can cause a significant voltage drop. When the voltage drop reaches a certain threshold, other electrical equipment may stop working or malfunction. Therefore, there are regulations stating that for motors that frequently start, the voltage drop should not exceed 10%, and for those that do not frequently start, the voltage drop should not exceed 15%.
It is because the transformer cannot provide the large current required for the starting of a large motor. If the transformer could supply the large current needed for motor starting, there would be no voltage drop or only a minimal voltage drop. This indicates that the size of the transformer's capacity also determines the magnitude of the voltage drop. Therefore, in practical situations, one cannot discuss the need for reduced-voltage starting of a motor without considering the transformer's capacity, regardless of the motor's size.
The current load on the transformer is also one of the factors that determine the voltage drop.
Imagine starting the same motor when the transformer is at no-load versus full load; the resulting voltage drop would definitely be different. Thus, it is insufficient to determine whether a motor requires reduced-voltage starting solely based on the motor's power as a percentage of the transformer's capacity without considering the transformer's current load.
In conclusion, to determine if a motor requires reduced-voltage starting, at least three factors must be considered: motor power, transformer capacity, and current load, and none can be overlooked. Now, let's examine the various so-called methods of determination to see if they comprehensively consider these three factors.
To simplify, let's use the parameter of current. Essentially, the core of this question is: What level of current will cause a specific voltage drop? In other words, it's a matter of current versus voltage drop.
As we know, the starting current of a squirrel-cage motor is 5-7 times its rated current. For a transformer, what level of load current will result in a voltage drop of 10% or 15%? This requires the use of the impedance voltage parameter. Typically, the impedance voltage of a power transformer is around 5%, meaning that when the transformer's output current is at its rated value, the voltage drop is 5%. This is why a transformer designed to output 380V has an output voltage of 400V, which drops to exactly 380V when fully loaded.
Assuming that the transformer's output voltage drop changes linearly with increasing load current, then when the transformer's output current is twice the rated current, the voltage drop is 5%; at three times the rated current, it's 10%; and at four times, it's 15%. For motors that frequently start, the allowable voltage drop is 10%. Therefore, if the sum of the motor's starting current and the existing load current is less than three times the transformer's rated current, the motor can be directly started. This is the simplest and most practical method for determining how to start an electric motor.。