Selection principle of motor using inverter as power supply

When the motor with the inverter is used as the power supply, the first question to be considered is how to select the type of motor and motor that should meet the speed regulation requirements. At the same time, we must consider the problem of matching the inverter to the motor. Therefore, there are also problems with the selection of the inverter. The choice between the two is good enough to achieve the best speed control.

1 basic principle of motor selection

1.1 Be aware of the load characteristics, torque characteristics, temperature rise and its limits, starting and starting currents, and vibration and noise of the selected motor. These characteristics should be provided by the motor manufacturer.

1.2 Torque characteristics of the load: This is determined by the load being dragged.

1.3 Motor power is determined. Because the high frequency harmonic content of the power supply is higher than that of the power frequency power supply, the power loss is higher than that of the power frequency power supply. The temperature rise is increased compared with the power frequency power supply. Therefore, the required motor power is usually calculated according to the power frequency power supply. After that, it is generally necessary to increase the power by 20% as the power of the motor when the inverter is powered.

This choice can not only solve the problem that the motor temperature rise does not exceed the temperature rise limit, but also solve the problem of insufficient torque when the motor is running at low speed.

1.4 The rated voltage and rated frequency of the motor are determined. Sometimes according to the actual situation of the speed control transmission, the standard voltage and frequency (such as 380V, 50Hz) cannot be selected. The rated frequency and voltage (such as 400V, 35Hz) can be re-determined. At this time, the motor will be Designed to the newly determined voltage and frequency.

1.5 The speed range of the motor must be confirmed to allow the highest frequency range. This means that the motor allows the maximum speed, which is limited by the following factors:

a. the limit speed of the bearing;

b. the strength of the fan;

c. The rotor allows the maximum speed;

d. Strength of other structural members.

When selecting the motor for this purpose, the manufacturer must provide the maximum allowable speed of the motor or special design and manufacture according to the requirements of the user.

2 different types of motor selection

2.1 Ordinary standard squirrel cage motor

This type of motor is a series of cage motor, which is combined with a general-purpose inverter. It can usually drive a load such as a fan, a pump or a compressor, or it can be used as a power shifting mechanism of a crane translation mechanism. Calculate the torque according to the selection principle mentioned above, and the current and temperature rise can meet the requirements. This type of motor is suitable for V/f control and is also suitable for vector control under certain conditions.

2.2 Vector control dedicated motor

The motor that adapts to vector control should be a special motor series with small moment of inertia, special design and high precision of parts processing, and good assembly precision. The matching frequency converter is also dedicated. Motor parameters such as stator-rotor resistance, stator-rotor reactance, and magnetizing reactance and current are all specific. How to meet the vector control needs, the supply and demand sides must negotiate and determine.

2.3 Motor with brake

At present, the brake motor is produced at home and abroad, and the brake is usually a disc brake, which is divided into an alternating current electromagnet type and a direct current electromagnet type. Pay attention to the following points in the selection:

A1 The brake power supply is separately supplied and must be connected to the input of the inverter.

B1 When braking at speed above the power frequency, there must be sufficient braking torque, usually the braking torque is proportional to the square of the speed.

A more appropriate method is that the inverter has a regenerative braking circuit or a braking unit, so that the speed is reduced to a low level and the braking effect is better, which is stable and reliable.

C1 Generally, the electric motor with brake adopts the power-off braking method. Therefore, when the motor cuts off the power supply and the brake is synchronously de-energized, the motor can be stopped immediately.

D1 The frequency converter should have the function of detecting whether the brake is open, and supply power to the motor when the brake is turned on, otherwise it will not be able to supply power.

2.4 pole-changing motor

The pole-changing motor is operated by a frequency converter and can be used within a wider range of speed regulation. But should pay attention to the following points:

A1 When changing the number of poles, be sure to do it after the motor has stopped;

B1 should pay attention to the choice of the inverter;

C1 takes into account the maximum speed allowed by the motor.

2.5 explosion-proof motor

All types of motors described above can be made explosion-proof, the selection principle is the same, the difference should consider the explosion-proof performance requirements. Therefore, the motor and the inverter must be made explosion-proof, which is a problem that the manufacturer has to solve. Now only mention the matters to be noted when selecting the type, that is, the surface temperature of the explosion-proof electrical equipment can not exceed the specified value. The TE time should also be considered for the increased safety motor. Therefore, when selecting the motor power, the motor heat should be calculated according to the load condition. There is a margin that is crucial.

2.6 Rotor is a winding type motor

In the design and selection, this type of motor can not be selected, but in the process of equipment transformation, in order to make use of the motor in use, it can be changed to be suitable for frequency control. The method of restructuring is to short the rotor winding into a squirrel cage type. The control method is to start at low frequency so that the starting current does not exceed 2 times the rated current of the motor, and then transition to the required speed.

When the inverter and the motor are combined for speed control, if you want to achieve the best condition, you must consider the mutual selection problem. For example, the power supply at the output of the inverter is as close as possible to the sine wave, and the loss is small, otherwise the motor vibration and noise are large and the temperature rises. The installation and wiring of the inverter should pay attention to anti-interference, and the interference will have a bad impact on the operation. Similar to these problems, not the motor itself can solve.

3 gear motor

When the inverter and the gear motor are combined for speed control, they should be selected according to the characteristics of the motor and gear. The selection of the motor part has been described above, and some points should be noted when considering the characteristics of the gear.

3.1 The maximum frequency allowed. If the speed is too high, the tooth surface temperature rises, the lubrication performance is lowered, and the gear is damaged. The seal is also limited by the high speed.

3.2 The minimum frequency allowed. When the speed is low, the gears cannot be fully lubricated and cannot be operated continuously at low speeds due to structural reasons.

3.3 It is best to choose a gear reduction ratio to increase the torque.

The selection of the number of motor poles and the number of reducers should be considered in the frequency control, the appropriate number of motor poles and operating frequency should be considered, if necessary, increase the reducer, the three considerations at the same time, in order to obtain satisfactory speed control transmission performance, the inverter The use of electric motors is advantageous. Increasing the speed reducer can also increase the drag torque and solve the problem of insufficient motor torque.

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