Published On: Sun, Mar 7th, 2021

FR Drives – Practical schemes of frequency speed regulation of asynchronous motors using high power frequency regulators

Practical schemes of frequency speed regulation of asynchronous motors using high power frequency regulators

Drawing and text by author Radoje Jankovic

1. High voltage cubicle of the frequency regulation motor speed

In this series of electrical circuits from practice, I will show the complete sequence of connecting larger asynchronous motors on the example of a 500 kW motor (100 – 1500 kW sinusoidal powers are almost exactly the same). The sequence starts from the high-voltage power supply cubicle of the power converter (typical medium voltages are 10, 20 and 35 kV) through all modules or units of the rectifier-converter system to the motor connection itself.

Only power circuit diagrams are shown here. Control schemes range from moderately complex to very complex, depending on the plants that electric motors have to drive (it often happens that one drive must drive two or more high-power drive motors) and contain from one to more industrial computer control systems, usually via PLC devices. I won’t even talk about the number of analog and digital encoders, sensors, switches.

In Fig.A. A typical “extended” single-line circuit of a Dy11d0 group connection of transformer, 3 MVA or more, is shown, which has two secondary windings  for two alternating voltages of 2 x 3 x 690 VAC. The first secondary is related to the stars Y11 and the second secondary is related to D0. It should be noted here that the manufacturers of frequency regulators, with their detailed technical instructions, precisely define the necessary cross-sections and types of conductors – cables for connecting transformers and rectifier-converter units, as well as details of processing cable terminations. Of course, the designer of the electrical plant must check the cross section of each cable. Due to the high reliability of the operation of these expensive switchgears, the cables must have a “reserve” in cross section, regardless of the fact that during the calculation all the coefficients required by the regulations on cable laying were taken (which I will not talk about here).

Due to very strict technical regulations and requirements for such electric motor drives as well as additional user requirements, in practice there are practically no serious breakdowns, faults and damage from the high voltage unit to the rectifier-converter unit to the electric drive motor itself and deeds e.g. gear through which drives the machine assembly-plant.

Secondary 2x3x690 VAC, power transformer is connected by multiple single-core fine-wire cables to LV buses, which can be seen in the following Fig. 1.

Here I will show in detail the complete three-pole circuit of the power circuit for controlling one asynchronous motor of higher power (500 kW, 3×690 VAC) using a frequency motor speed regulator.

Fig. 1.a.

2. Low voltage buses 690 VAC

In Fig.1. Shows a three-line circuit diagram of a low AC power circuit 3×690 V. This diagram shows the connection of the first (Y11 – star) secondary to the LV buses and the output for the rectifier module (BSU – Basic Supply Module) with the input filter. For the second secondary of the power transformer with D0 connection – the triangle scheme is exactly the same. Low voltage busbars are usually of the rectangular section for the appropriate load current. Of course, they are calculated for all the “stresses” that may occur during operation. Everything said in the first part of this series also applies to this part of the electric motor drive with frequency regulators of higher and higher powers.

Fig. 1.

3. Basic line power supply module with rectifier

In Fig. 2.  shows a three-line circuit diagram of the power circuit of the basic line unit with the rectifier module of the first secondary of the power transformer. From the module via its connection terminals C and D, and the main fuses of the fusible type, the DC voltage is conducted to the main DC buses (Fig.3) from which the inverter modules for driving the electric motor are supplied.

From this module, the braking module with the chopper is supplied via the DCP and PCN terminals.

For the second secondary of the transformer, the power of the line power supply module with the rectifier is exactly the same.

Fig. 2.

DC bus diagram of higher power frequency regulators

In F.3.  shows a diagram of DC busbars of voltage 690 V obtained from two rectifier units connected to one power transformer from Fig. A.

The motor module for driving the motor from Fig. 1 is supplied from these buses. 5.

Motor module

Motor module Fig. 4. is a converter circuit for converting DC power voltage into three-phase AC voltage of 3 x 690 VAC.

This module supplies the motor, filter and chopper as the last power supply element of the induction motor for speed control via the frequency converter.

Connecting a three-phase electric motor to a converter

Lastly, there is the connection of a three-phase asynchronous motor to the converter as shown in Fig. 5 via DU / DT filter. The scheme is very clear that no more detailed comment is needed.

Eventually we came up with a complete scheme from high voltage to motor connection. I made this scheme from parts – blocks because it is impossible to draw the whole drawing paper on A4 format. You can do it on a computer and print it in an extended vertical A3 format so that it will be even clearer for practice.

I drew all the schemes in vertical form because it is easier to follow it. Today, some authors and designers work in the horizontal form of A3 format, also in parts.

Protection of three-phase motor against overheating

Figure 6 below shows the internal diagram of the motor terminal box.

Per one Pt100 sensor is installed in each phase winding of the motor. Pt100 sensors are also built into both engine bearings. As a rule, temperature signals go to PLC devices as well as signals of all other sensors, encoders or transducers of mechanical and electrical quantities and the reaction of other important electrical components that are part of a complex electric motor drive.

In one of the found series of schemes from the practice of this type, I will also show group drives that use frequency regulators of motor speed.

Brake module

As the last part of the scheme of switching and control of a three-phase motor via frequency regulators is the brake module for dynamic motor braking, which consists of two parts; chopper and brake resistor as shown in Fig. 7. Protection of resistors against overheating is performed by means of one thermal element set for a certain temperature.

Note: All material is from Jankovic’c documentation.

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FR Drives – Practical schemes of frequency speed regulation of asynchronous motors using high power frequency regulators