Whether an electric motor needs to be started with reduced voltage depends on a comprehensive assessment of the following key factors:
Key factors
Motor power
It is generally believed that reduced-voltage starting should be considered when the motor power exceeds 10kW or 50kW, but in reality, there are cases where motors with power exceeding 100kW are started directly.
Transformer capacity and load
It is necessary to consider whether the transformer capacity meets the starting current requirements, as well as the impact of the existing load on voltage drop. For example, starting a motor when the transformer is fully loaded may cause a greater voltage drop.
Start-up frequency and voltage drop
Frequent starts (e.g., multiple times per hour): Voltage drop must be kept below 10%.
Occasional startup (e.g., once a day): Voltage drop not exceeding 15%. 12 Practical Judgment Methods
Measure voltage drop
During startup, measure the bus voltage. If the voltage drop exceeds the aforementioned threshold, a reduced-voltage startup is required.
Empirical Formula Reference
In some scenarios, empirical formulas can be used for estimation, but these need to be verified in conjunction with actual parameters.
System evaluation
If there are other sensitive devices in the system (such as precision instruments, frequency converters, etc.), even if the motor power is not high, reduced voltage starting should be considered to avoid interference.
01 Reduced-voltage starting conditions for electric motors ◆ Overview and background
The issue of reduced-voltage starting for 380V, 3-phase squirrel-cage motors, though seemingly decades old, remains a topic worthy of discussion. Many existing solutions often lack comprehensiveness or practical applicability, failing to meet real-world needs. Below, we will provide a brief overview of existing solutions and encourage contributions and improvements.
◆ Commonly Used Judgment Conditions
When the power of the electric motor exceeds 10kW;
When the motor power is greater than 50kW;
The power of the electric motor accounts for more than 20% of the transformer capacity;
For motors that start frequently, the voltage drop during startup exceeds 10%.
For motors that start occasionally, the voltage drop during startup exceeds 15%.
Furthermore, there appears to be an empirical formula that, by substituting relevant parameters, can further determine whether a reduced-voltage start-up is necessary. I hope someone familiar with this formula can provide more details.
Regarding items 1 and 2, there seems to be a lack of clear evidence, because in reality, there are indeed cases where electric motors of hundreds or even hundreds of kilowatts are directly started at full voltage.
Regarding item 3, whether the transformer is unloaded is also a question worth considering.
As for items 4 and 5, how can they be accurately controlled in practice? Should they rely on actual measurements or other methods?
◆ The impact and control of voltage drop
In practice, the main purpose of using reduced-voltage starting is to avoid affecting the normal operation of other equipment. When only a transformer and motor are present, considering factors such as power factor and efficiency, the motor's power can typically reach about 80% of the transformer's capacity, allowing for direct starting or simultaneous starting with the transformer. However, in practical applications, the transformer often needs to bear other loads, so the amount of these other loads is also an important factor to consider.
When an electric motor starts, it generates a large starting current, approximately 5-7 times its rated current. This large current causes a voltage drop, and if the voltage drop reaches a certain level, it may affect the normal operation of other electrical equipment or even cause malfunctions. Therefore, regulations stipulate that the voltage drop caused by frequently starting motors should not exceed 10%, and the voltage drop caused by infrequently starting motors should not exceed 15%.
So, why does voltage drop occur? This is mainly because the transformer cannot provide the large current required when the motor starts. If the transformer can provide a sufficiently large current, the voltage drop will be very small or almost non-existent. This also illustrates that the size of the transformer has a decisive influence on the generation of voltage drop. Therefore, in practical applications, we must not only consider the motor's power to determine whether reduced-voltage starting is necessary, but also take into account both the transformer's capacity and the existing load conditions.
◆ Comprehensive judgment on the relationship with current
Therefore, we can conclude that determining whether a motor needs reduced-voltage starting requires comprehensive consideration of three key factors: motor power, transformer capacity, and existing load. However, do the various so-called judgment methods currently available adequately consider these three aspects?
For ease of analysis, we can use current as a parameter to explore this problem. In essence, the core of this question is: how much starting current will result in how much voltage drop? In other words, what is the relationship between current and voltage drop?
As is well known, the starting current of a squirrel-cage motor is typically 5-7 times its rated current. For transformers, when the load current reaches a certain level, the voltage drop will correspondingly reach 10% or 15%. Here, we introduce the important parameter of impedance voltage, which represents the voltage drop of a transformer under rated current. Generally, the impedance voltage of a power transformer is about 5%, meaning that when the transformer is fully loaded, its output voltage will exactly reach the design value.
Assuming the transformer's output voltage drop changes linearly with increasing load current, then when the output current is twice the rated value, the voltage drop is 5%; at three times, it's 10%; and at four times, it's 15%. Based on this, we can derive a practical criterion: for frequently starting motors, the permissible voltage drop is 10%. Therefore, as long as the sum of the motor's starting current and the existing load current does not exceed three times the transformer's rated current, the motor can be started directly without voltage reduction.
The Necessity of Reduced-Voltage Starting for Electric Motors: In industrial production, electric motors play a crucial and indispensable role. However, a challenge arises: as electric motor power increases, ensuring a smooth and safe starting process becomes paramount. This article aims to explore when reduced-voltage starting measures are necessary and provides practical methods and techniques to help readers make judgments and calculations based on specific situations.
❒ High-power motors require voltage reduction for starting.
When the power of an electric motor exceeds 10kW, reduced-voltage starting is generally recommended. This is because the current of a high-power motor during startup may exceed the rated load capacity of the transformer, leading to transformer overload or even damage. Reduced-voltage starting effectively controls the starting current, ensuring smooth motor startup while protecting the transformer from damage.
When the power of the motor increases further, for example, exceeding 50kW, reduced-voltage starting becomes particularly necessary. Although these motors can theoretically be started directly at full voltage, in practice, to avoid excessive impact on the power grid and affecting the stable operation of other equipment, we usually recommend using reduced-voltage starting.
❒ Reduced-voltage starting and transformer capacity
In addition to the methods mentioned above, we can also determine whether reduced-voltage starting is necessary based on 20% of the transformer capacity. Specifically, when the motor power exceeds 20% of the transformer capacity, reduced-voltage starting may become necessary.
However, it should be noted that this method is not always reliable. In practical applications, multiple factors such as motor efficiency and power factor must be considered comprehensively. Generally, to ensure that the voltage does not drop excessively and affect the stable operation of other equipment, the motor power should be controlled within 80% of the transformer capacity.
❒ The impact of voltage drop and countermeasures
For motors that start frequently, if the voltage drop exceeds 10% during startup, or for motors that start occasionally, if the voltage drop exceeds 15% during startup, reduced-voltage starting should be considered. This is because a significant voltage drop may interfere with the normal operation of other equipment.
To accurately control the voltage drop, it is recommended to use a dedicated voltmeter for measurement. In actual operation, adjustments can be made flexibly according to specific circumstances to ensure smooth motor startup while maintaining the stable operation of other equipment.
For motors that start frequently, if the voltage drop exceeds 10% during startup, or for motors that start occasionally, if the voltage drop exceeds 15% during startup, reduced-voltage starting should be considered. Excessive voltage drop may interfere with the stable operation of other equipment.
To accurately control voltage drop, a dedicated voltmeter can be used for measurement. In practical applications, adjustments need to be made flexibly according to specific circumstances to ensure smooth motor startup without affecting other equipment.
If the voltage drop of a motor that starts frequently exceeds 10%, or the voltage drop of a motor that starts occasionally exceeds 15%, reduced voltage starting should be considered to ensure normal equipment operation.
In conclusion, the decision to use reduced-voltage starting requires a comprehensive consideration of various factors, such as motor power, transformer capacity, and motor operating frequency. Through meticulous calculations and practical testing, the optimal solution can be found.
