The choice of inverter capacity is an important and complex problem, to consider the match ingress with the capacity of the inverter and the motor capacity, the small capacity will affect the output of the motor has the effective moment, affect the normal operation of the system, and even damage the device, and the capacity is large reverberating the harmonic component of the current will increase, but also increase the equipment investment.

**Principles for the choice of inverter capacity**

The basic principles for the choice of inverter capacity are as follows.

**1, matching principle**

The selection of the drive should match the load. The performance is as follows.

(1) Power matching: the rated power of the drive is consistent with the load rating power, it should be noted that the load of the motor is different and its power requirements are different. For example, motors of the same power vary in capacity of the inverter sinter edgy depending on the load. The inverter capacity required for the square torque load (fan) is lower than that required for the constant torque load. In general, the inverter product specification directly gives the rated power suitable for the driving motor or its visual power, therefore, the wind turbine, pump class and other square torque load, can be selected according to the motor power of the corresponding inverter.

(2) Voltage matching: the voltage rated by the drive corresponds to the load rating voltage.

(3) Current matching: ordinary centrifugal pump, the choice of inverter rated current and motor rated current match, special load, for example, deep water pump, need to consider the motor performance parameters, with the maximum current to determine the inverter current and overload capacity.

(4) Torque matching: in the constant torque load or when there is a deceleration device to consider.

**2, economic principles**

Technical analysis and economic analysis should be carried out, and control schemes that meet the application requirements and have a high performance price ratio should be selected.

3, the specific analysis principle

The different applications should be analyzed and the inverter capacity determined.

(1) According to the inverter product specification to use the choice of motor capacity: the following circumstances can be according to the inverter product specification with motor capacity to select the inverter capacity.

(2) When selecting the inverter capacity, you need to select one or two blocks according to the specification capacity: the following circumstances need to choose a high one or two stops.

(3) When using the inverter rated power as the inverter capacity index to select the inverter, because the influence of the motor pole and the motor rating current is not taken into account, the reforest may not meet the motor rating current requirements when selecting the inverter according to the rated power of the inverter.

(4) When using the motor rated current as the inverter capacity selection, because the choice of motor capacity is not considered, there is usually a certain amount of margin, especially the inverter transformation project, because the motor selection of the residual amount is generally 40% to 50%, therefore, According to the motor's rated current selection of inverter capacity may be too large, so that the inverter under low load operation, resulting in waste of resources.

In addition, the maximum operating current of the motor affects the heat and temperature rise of the motor, for short-term overload, the general inverter has 150%, 1min overload capacity, therefore, for the application of large load fluctuations, the choice of the inverter should understand the maximum operating current and overload time of the motor, so that the inverter in the maximum operating current, Still does not exceed its rated current, or overload current is less than 150% rated current when the overload time is less than 1min.

(5) A suitable match between the drive and the motor means that the rated current (or maximum operating current) of the motor shall be less than the rated current of the inverter under the same power. For example, as can be seen from the table, the original 8 poles 18.

5kW motor, the choice of the same power inverter, due to consider the load fluctuations, so, choose ABB's ACS800 series of products.

(6) When selecting the inverter capacity to decrease, pay attention to the impact of the motor start ingress current and acceleration current. To this end, it can be considered to add output reactor between the inverter and the motor, the impact current, acceleration current filter smoothing, reduce the impact of impact current, acceleration current impact, in order to meet the production process acceleration and deceleration premise, the acceleration and deceleration time set longer, the start-up U/f preset value set less.

**Selection steps for drive capacity (method)**

**1, the drive capacity selection steps**

(1) to understand the nature of the load and the law of change, calculate the size of the load current or make a load current diagram I -f (t);

(2) Preselected inverter capacity;

(3) Check pre-selected inverter, if necessary, the load capacity and starting capacity of the verification.If all passes, the preselected drive capacity is selected; The smaller the capacity of the inverter is more economical under the premise of meeting the requirements of production machinery.

**2, the choice of inverter capacity method **

There are many misunderstandings in the actual operation of the capacity selection of the drive, and here are three basic capacity selection methods, which complement each other.

(1) **From the angle of the current **

Most drive capacity can be expressed from three angles: rated current, available motor power, and rated capacity.

One of the last two, inverter manufacturers by the domestic or the company's production of standard electric motors, or with the inverter output voltage and reduce, it is difficult to accurately express the capabilities of the inverter. When selecting a drive, only the current rating of the drive is a critical quantity that reflects the load capacity of the semiconductor drive device. The load current does not exceed the drive rating current is the basic principle for selecting the drive capacity. It should be stressed that before determining the capacity of the inverter, we should carefully understand the process of the equipment and motor parameters, such as submersible electric pumps, winding rotor motor rated current is greater than the normal cage asynchronous motor rated current, the metallurgical industry commonly used roller electric motor not only rated much higher, but also allows short-term in the blocking operation state, And roller drive is mostly multi-motor transmission, should ensure that the total load current in a fault-free state is not allowed to exceed the rated current of the drive.

(2) **From the point of view of efficiency **

The system efficiency is equal to the product of the efficiency of the inverter and the efficiency of the motor, and only when both work under higher efficiency, the system efficiency is higher.

From the point of view of efficiency, when selecting inverter power, we should pay attention to the following points:

1) The power value of the inverter is the most suitable when the power value of the motor is equivalent, so as to benefit the drive to operate under the high efficiency value;

2) When the power rating of the drive is not the same as that of the motor, the power of the drive should be as close as possible to the power of the motor, but it should be slightly greater than the power of the motor;

3) When the motor is frequently started, brake work or in heavy-duty start-up and more frequent work, you can choose a large rife to facilitate the long-term, safe operation of the inverter;

4) After testing, the actual power of the motor does have surplus, you can consider the choice of power is less than the motor power of the inverter, but pay attention to whether the instantaneous peak current will cause overcurrent protection action;

5) When the drive and the motor power is not the same, the setting of the energy-saving program must be adjusted accordingly, in order to achieve a high energy-saving effect.

**(3) From the angle of calculating power **

For continuously running drives, the following three calculation formulas must be met at the same time.

1) Meet the load output: PCN≥PM/η。

2) Meet the motor capacity: PCN≥√3kUeIe&TImes;10-3。

3) Meet the motor current: ICN≥kIe。

PCN is the inverter capacity (in kVA) ;PM is the output power of the motor shaft required by the load (in kW), Ue is the motor rated voltage (in V), Ie is the motor rated current (in A), and the motor efficiency (usually about 0. 85); cos is the motor power factor (usually about 0.75), and k is the current waveform compensation factor (since the output waveform of the drive is not a complete sine wave and also contains a component of high harmonics, the current should be increased, usually k is 1. 05 to 1. 1).