The permissible current of the inverter is inversely time-bound with the process time

In the process of motor starting (acceleration), the motor should not only bear the load torque of steady speed operation, but also bear the acceleration torque, if the production machinery has no special requirements for the starting (acceleration) time, the starting (acceleration) time can be appropriately extended to avoid the peak current. If the production machinery has certain requirements for the starting (acceleration) time, it should be carefully considered.

With the rapid development of power electronics, microelectronics, computer technology and control theory, the AC drive system has also achieved a performance comparable to DC speed regulation in terms of wide speed regulation, high precision, fast response and four-quadrant operation. In particular, the frequency conversion speed regulation with the inverter as the core is recognized as the most promising speed regulation method because of its excellent speed regulation performance. At present, the inverter has entered the stage of high performance, multi-function, miniaturization and cheapness. For the rational use of frequency converters, this article will briefly discuss the process of selecting the capacity of frequency converters.

Selection of frequency converter capacity

The selection of inverter capacity is an important and complex problem, to consider the matching of inverter capacity and motor capacity, the small capacity will affect the output of the effective torque of the motor, affect the normal operation of the system, and even damage the device, and the capacity is too large to increase the harmonic component of the current, but also increase the equipment investment.

(1) Three steps of inverter capacity selection

1. Understand the nature and change law of the load, calculate the size of the load current or make the load current diagram I=f(t);

2. Pre-select the capacity of the inverter;

3. Verify the pre-selected inverter, and verify the overload capacity and starting ability if necessary. If all pass, the pre-selected inverter capacity is selected; Otherwise, start again from 2 until you pass.

On the premise of meeting the requirements of production machinery, the smaller the capacity of the inverter, the more economical.

(2) Inverter capacity selection based on different motor load currents

There are three ways to express the capacity of a frequency converter

In general, there are three ways to express the capacity of a frequency converter:

Rated current;

Adapt to the rated power of the motor;

Rated apparent power.

2. The overload capacity of the inverter

No matter which way it is expressed, in the final analysis, it is the choice of the rated current of the inverter, which should be determined according to the current that the motor may absorb to the inverter in combination with the actual situation. In general, there are two types of overload capacity of the inverter:

1.2 times the rated current, which can last for 1min;

1.5 times the rated current, which can last for 1min.

Moreover, the allowable current of the inverter is inversely related to the process time. For example, 1.2 (1.5) times of the rated current can last for 1min, and 1.8 (2.0) times of the rated current can last for 0.5min. This means: the duration of the allowable overload current provided to the motor at any time must be within a limited range; the index of overload capacity is meaningful for the motor only in the process of starting (accelerating), and in the process of operation, it is actually equivalent to not allowing overload.

How to choose the capacity of the inverter according to the load current of the motor

(1) One inverter is only used for one motor (i.e., one to one)

After calculating the load current, three factors should also be considered: first, when the inverter is used for power supply, the pulse of the motor current is larger than that of the power frequency power supply; The second is the starting requirement of the motor, that is, whether it is started by low frequency and low voltage, or directly started at rated voltage and rated frequency; Third, the relevant data in the inverter's instruction manual are tested with the company's standard motor.

It should be noted that there may be some differences in the performance of the motor produced according to the conventional design, so it is necessary to leave an appropriate margin when calculating the capacity of the inverter.

1. Calculation of inverter capacity when constant load is running continuously

When starting from low frequency and low voltage, when the frequency converter is used to complete the frequency conversion speed regulation, the rated current of the frequency converter is required to be slightly greater than the rated current of the motor:

IFN1.1IMN

where: IFN inverter rated current;

IMN motor current rating.

When the rated voltage and rated frequency are started directly, for the three-phase motor, it can be seen from the rated data of the motor that the starting current is 5-7 times of the rated current. Therefore, the following formula must be used to calculate the frequency current IFN of the inverter.

IFNIMst/KFg

where: the starting current of the IMst motor at the rated voltage and rated frequency;

Overload multiplier of KFg inverter.

2. Calculation of inverter capacity when the load is constantly changing

In many cases, the load of the motor is characterized by periodic changes. Obviously, in this case, selecting the capacity of the inverter according to the small load will be overloaded, and it will be uneconomical to select the large load. It can be inferred that the capacity of the inverter can be appropriately selected between large and small loads, so that the inverter can be fully utilized without overloading.

Firstly, the motor load current diagram n=(t) and I=f(t) are made, and then the average load current Iav is calculated and then the capacity of the inverter is pre-selected, and the following formula is used to calculate Iav:

Iav=（I1t1+I2t2++Ijtj+）（t1+t2++tj+）

Considering that during the transition process, the motor draws a larger current from the inverter than during stable operation, and the above IAV does not reflect the situation during the transition process. Therefore, the capacity of the inverter is pre-selected according to the IFN(1.1-1.2)Iav correction (where Ij is the average current in the j-section operating state, tj is the corresponding time in the j-section operating state), and if the transition process accounts for a large proportion in the whole working process, the coefficient (1.1-1.2) is selected as a larger value.

3. Calculation of inverter capacity during continuous operation of non-periodic change load

In this case, it is generally difficult to make a load current diagram, and the rated current of the inverter can be calculated according to the current of the motor when the motor outputs a large torque, which can be determined by the following formula:

IFNIM（max）/KFg

Where: IM (max) is the current of the motor when it outputs a large torque.

(2) One inverter is used by multiple motors at the same time (i.e., one to many)

In addition to considering several situations in which one drags one on the other, it can also be treated differently according to the following situations.

1. Each motor is started by low-frequency and low-voltage, and one of the motors that is shut down due to failure is not required to start directly after normal operation, and the capacity of the inverter is:

IFNIM（max）+IMN

where: IMN is the sum of the rated currents of the other motors;

IM (max) is the starting current of the largest motor.

2. One part of the motor starts directly, and the other part of the motor starts by low frequency and low voltage. In addition to keeping the total current of the motor operating from not exceeding the rated output current of the inverter, the starting current of all direct-start motors should be considered, namely:

IFN（IMst+IMN）/KFg

where: IMst is the sum of the starting current of all direct starting motors at rated voltage and rated frequency;

IMN is the sum of the rated currents of all motors.

The above are the general principles and steps for the selection of drive capacity. In practice, it is also necessary to deal with the special requirements of specific production machinery flexibly, and in many cases, some more practical methods can also be adopted according to experience or suggestions provided by suppliers.

Verification of the starting acceleration ability of the inverter

In the process of motor starting (acceleration), the motor should not only bear the load torque of steady speed operation, but also bear the acceleration torque, if the production machinery has no special requirements for the starting (acceleration) time, the starting (acceleration) time can be appropriately extended to avoid the peak current. If the production machinery has certain requirements for the starting (acceleration) time, it should be carefully considered.

As mentioned earlier, the allowable current of the inverter is inversely related to the process time. If when the motor starts (accelerates), its current is less than the overload capacity of the inverter, then the pre-selected capacity passes, if the motor starts (accelerates), its current has reached the overload capacity of the inverter, and the required acceleration time conflicts with the time limit specified in the inverter's overload capacity, at this time, the capacity of the inverter should be increased on the basis of the pre-selected capacity.
The permissible current of the inverter is inversely time-bound with the process time

The permissible current of the inverter is inversely time-bound with the process time

The permissible current of the inverter is inversely time-bound with the process time

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