The choice of the frequency converter for current, power and other parameters

Changing the speed and direction of rotation of an induction motor is a problem that has to be solved in a number of problems. You can use a frequency converter for this. This is a power converter, to which asynchronous motors are connected, as a result of changing the frequency of the output voltage, the rotation speed of the motor rotor also changes. Proper control of the electric drive allows you to increase the efficiency of its application. In this article we will tell you how to choose a frequency converter for an electric motor in terms of power, current and other parameters.

What parameters to pay attention to

It is immediately worth noting that with the help of a frequency converter you can connect an asynchronous three-phase motor to a single-phase network without capacitors, respectively, and without loss of power.

Frequency converters

To understand how to choose the right frequency converter, let's look at a number of basic parameters:

  1. Power. Pick up more than the full power of the engine, which will be connected to it. For a 2.5 kW engine, if it works with occasional slight overloads or in the nominal value, the frequency converter selects the closest upward one from the model range, say 3 kW.
  2. The number of supply phases and voltage are single-phase and three-phase. It connects to a single-phase input at 220V, and at the output we get 3 phases with a linear voltage of 220V or 380V (specify what output voltage when buying, this is important for the correct connection of the motor windings). Three phases are connected to powerful three-phase devices, respectively.
  3. Type of control - vector and scalar. Frequency inverters with scalar control do not provide precise adjustment over a wide range; at too low or too high frequencies, the motor parameters can change (torque drops). The moment itself is supported by the so-called VCHF (function U / f = const), where the output voltage depends on the frequency. Feedback loops are used for chastotniks with vector control, with their help stability is maintained in a wide range of frequencies. And also, when the load on the motor changes at a constant frequency, such frequency converters more accurately maintain the torque on the shaft, thereby reducing the reactive power of the motor. In practice, frequency converters with scalar control are more common, for example, for pumps, fans, compressors and others. However, with an increase in frequency higher than in the network (50 Hz), the moment begins to decrease, in simple terms - there is nowhere to increase the voltage with increasing speed. Vector control models are more expensive, their main task is to maintain a high moment on the shaft, regardless of the load, which can be useful for a lathe or milling machine, to maintain stable spindle speeds.
  4. Range of regulation.This parameter is important when you need to adjust the drive over a wide range. If, for example, you need to adjust the pump performance, the adjustment will occur within 10% of the nominal value.
  5. Functional features. For example, to control the pump, it would be good if the inverter has a dry running tracking function.
  6. Performance and moisture resistance. This parameter determines where the chastotnik can be installed. To make the right choice, decide where you install it, if it is a damp room - a basement, for example, then it is better to place the device in a shield with protection class IP55 or close to it.
  7. Shaft braking method. Inertial braking occurs when the power is simply disconnected from the engine. For sharp acceleration and braking, regenerative or dynamic braking is used, due to the reverse rotation of the electromagnetic field in the stator, or a quick decrease in frequency using a converter.
  8. The method of heat dissipation. During operation, semiconductor switches emit a fairly large amount of heat. In this regard, they are installed on radiators for cooling. Powerful models use an active cooling system (using coolers), which reduces the size and weight of the radiators. This must be taken into account before purchase, before you decide to choose one or another model. First determine where and how the installation will be carried out. If it is installed in a cabinet, it should be noted that with a small amount of space around the device, cooling will be difficult.

Frequency converters are often selected for a submersible pump. It is needed to regulate pump performance and maintain constant pressure, smooth start-up, dry-running control and energy saving. For this, there are special devices that differ from the general purpose chastotniks.

Well pump control unit

How to calculate a chastotnik under the engine

There are several calculation methods for selecting a frequency converter. Consider them.

Current selection:

The frequency converter current must be equal to or greater than the current for a three-phase electric motor consumed at full load.


Let's say there is an induction motor with the characteristics:

  • P = 7.5 kW;
  • U = 3x400 V;
  • I = 14.73 A.

This means that the continuous output current of the frequency must be equal to or greater than 14.73A. The calculation shows that this equals 9.6 kVA with a constant or quadratic torque characteristic. Such requirements with a small margin correspond to the model: Danfoss VLT Micro Drive FC 51 11 kW / 3ph, which it would be quite reasonable to choose.

 Danfoss VLT FC 51 11

Full power selection:

Let's say there is an AIR 80A2 engine, the nameplate of which indicates (for a triangle):

  • P = 1.5 kW;
  • U = 220 V;
  • I = 6 A.

Calculate S:

S = 3 * 220 * (6 / 1.73) = 2283 W = 2.3 kW


We select a frequency converter with a good margin, while we will connect it to a single-phase network and use it to control the rotation of the spindle of a lathe. The nearest model that is suitable for this: CFM210 3.3 kW.


It is worth noting that the model range of most manufacturers corresponds to the standard range of capacities of asynchronous motors, which will make it possible to choose a frequency inverter with the corresponding power (not exceeding). If you use a obviously more powerful motor and do not load it completely, you can measure the actual current consumption and select a frequency converter based on these data. In general, when calculating the frequency response for the engine, consider:

  1. The maximum current consumption.
  2. Overload capacity of the converter.
  3. Type of load.
  4. How often and for how long congestion can occur.

Now you know how to choose a frequency converter for an electric motor and what to look for when choosing this type of device. We hope the provided tips have helped you find the right model for your own conditions!

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