How to choose a frequency converter for an electric motor

Expert of the "Question to the Electrician" section, author of articles.

Electrician for the repair and maintenance of electrical equipment, work experience over 5 years.

Changing the speed and direction of rotation of an induction motor is a problem that has to be solved in a number of tasks. A frequency converter can be used for this. This is a power converter to which induction motors are connected; as a result of a change in the frequency of the output voltage, the speed of rotation of the motor rotor also changes. Correct control of the electric drive improves the efficiency of its use. 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 should be noted right away 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.

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

1. Power. They select more than the full power of the engine that will be connected to it. For a 2.5 kW motor, if it works with rare insignificant overloads or in the nominal value, the frequency converter is chosen the closest upward from the model range, for example, by 3 kW.
2. The number of supply phases and voltage are single-phase and three-phase. It is connected to single-phase at the input to 220V, and at the output we get 3 phases with a line voltage of 220V or at 380V (specify what is the output voltage when buying, this is important for the correct connection of the windings engine). Three phases are connected to powerful three-phase devices, respectively.
3. Control type - vector and scalar. Frequency converters with scalar control do not provide accurate control in wide limits, at too low or too high frequencies the parameters of the motor can change (drops moment). The moment itself is maintained by the so-called HFC (function U / f = const), where the output voltage depends on the frequency. For frequency controllers with vector control, feedback loops are used, with their help, stability of operation in a wide frequency range is maintained. And also, when the load on the motor changes at a constant frequency, such frequency converters more accurately maintain the torque on the shaft, thus 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, when the frequency rises higher than in the network (50 Hz), the torque begins to decrease, in simple terms - there is nowhere to increase the voltage with an increase in speed. Vector control models are more expensive, their main task is to maintain a high torque on the shaft, regardless of from the load, which can be useful for a lathe or milling machine, to maintain stable rpm spindle.
4. Regulation range. This parameter is important when you need to control 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.
5. Functional features. For example, it will be good for pump control if the frequency converter has a dry-running monitoring function.
6. Execution and moisture resistance. This parameter determines where the frequency converter can be installed. To make the right choice, decide where you will install it, if it is a damp room - a basement, for example, it is better to place the device in a panel with protection class IP55 or close to it.
7. Shaft braking method. Inertial braking occurs by simply disconnecting power from the motor. For sharp acceleration and deceleration, regenerative or dynamic braking is used, due to reverse rotation of the electromagnetic field in the stator, or a rapid decrease in frequency using converter.
8. Heat dissipation method. During operation, semiconductor switches generate 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 allows to reduce the size and weight of the radiators. This must be taken into account even before buying, 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, then it should be taken into account 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, soft start, dry run control and energy saving. For this, there are special devices that differ from general-purpose frequency converters.

How to calculate a frequency converter for a motor

There are several calculation methods for choosing a frequency converter. Let's consider them.

Current selection:

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

Let's say there is an asynchronous motor with characteristics:

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

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

Full power selection:

Let's say there is an AIR 80A2 engine, on the plate of which it is indicated (for a triangle):

• P = 1.5 kW;
• U = 220 V;
• I = 6 A.

Let's calculate S:

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

We choose a frequency converter with a good margin, despite the fact that we will connect it to a single-phase network and use it to control the rotation of the lathe spindle. The closest model that suits this: CFM210 3.3 kW.

It should be noted that the lineup of most manufacturers corresponds to the standard range of capacities induction motors, which will allow you to choose a frequency converter with an appropriate power (not exceeding). If you use a known more powerful motor and do not load it completely, you can measure the actual current consumption and select the frequency converter based on this data. In general, when calculating a frequency drive for a motor, consider:

1. Maximum current consumption.
2. Overload capacity of the converter.
3. Load type.
4. How often and for how long overloads 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 advice provided has helped you find the right model for your own conditions!

Related materials:

• How to choose a thermal relay for an electric motor
• What is Automatic Frequency Unloading
• AC frequency measurement