Published On: Sun, Mar 14th, 2021

VFD Scheme – Modern electric motor drives

VFD Scheme – Modern electric motor drives

Draws and writes: Radoje Janković.

In this group of schemes, I will present mostly practical schemes of modern electric motor drives. In school and university literature, as well as other electrical engineering literature, one can find few modern practical schemes for connecting three-phase electric motors in relation to classical schemes of electric motor drives.

In a separate article, I will publish a number of illustrated test questions in this area, both with and without answers. Unanswered questions will be simpler because this is a more difficult area for many because they rarely encounter it in their daily practice of both assembly, connection and technical maintenance. As for technical maintenance, it is practically minimal, because these equipment are very reliable in operation and very rarely there are some failures where experts dealing with technical and regular maintenance would have the opportunity to intervene.

Scheme 1.

Single-line scheme of the complete high-voltage cubicle of the 3×20 kV / 0.7 / 0.7 kV converter transformer. The primary winding is in connection to the triangle. Two secondary windings; secondary S1 is a connection to a star and secondary S2 to a triangle. If there are several converter transformers in an electrical switchgear, then in the schemes these windings are marked with T1.S1, T1.S2, for others, T2.S1, T2.S2, etc. Other ways of marking can be adopted. Two secondary windings of this transformer supply 12-pulse rectifier units (modules as they are also called) in the converter equipment. The rectifier units work in parallel and each rectifier unit at its outputs separately gives 690 V DC voltage that goes to the common DC buses. 2-3-… or more rectifier units are often used in practice, depending on the installed power of the electric motor drives.

Scheme 1.

Scheme 3.

One-line scheme of one high-voltage cubicle (in this case 20 kV, and it can be any medium voltage) with three all-voltage cubicles intended for frequency-regulated electric motor drives. The first cell is supply – 20 kV power supply, the second and third cells are output cubicles for power supply of converter transformers. Depending on the need, there is usually a fourth high-voltage cell for the so-called. “Home” transformer from which consumers are supplied with voltage 3×400 VAC or 3×400 / 3×230 VAC. It should be noted here that, depending on national standards, the values ​​of medium high voltages are in a wider range. Also low voltages that serve different needs in the company’s plants.

The T1 converter transformer is three-winding, 20/069/069 kV, in this case 1,000 kVA. It supplies about 10 three-phase asynchronous motors through its own converter units. From the busbars T1.S1 and T1.S2, special cables (usually fine-wire single-core cables) are fed to the input units of the frequency converter cubicle at the “end” from the DC link through the output inverters to power each motor separately.

You should not be bothered that the leads are marked M1 .. M5 .. Mn, these voltages go directly to the input units of the rectifier units or modules that convert it to DC voltage.

Scheme 3.

Scheme 4.

One-line electrical scheme of VFD three-winding transformer for power supply with three-phase low voltage 3×690 V of two groups of electric motor drives. A high-voltage power switch with motor drive was applied in the high-voltage cubicle of the transformer. The first group of motors is powered from a low voltage switchgear + 1N1. From this switchgear, a low-voltage cable outlet goes through a slip-rings + 1W (sliding rings) to another low-voltage switchgear + 1N2 from which the second group of electric motor drives is supplied through the frequency control units of each individual electric motor drive.

Scheme 4.

Scheme 5.

One-line electrical scheme of high-voltage and low-voltage AC cubicle for power supply of frequency regulator unit for electric motor drives.

As can be seen from the diagram, the converter transformer T3 is connected in one of the cubicle of the high voltage switchgear 35 kV via a power switch Q0. From the power switch the single-core cables go to the high voltage slip rings + W1 with slip rings, further, the single-core cables go to the power switch Q1 in front of the converter transformer T3. Low voltages with two second converter transformer cables go to 2 x 3×690 VAC busbars in a low voltage + 1N1 switchgear. The switchgear of frequency regulators for a large number of electric motors is connected to these buses. A special feeder from this low voltage switchgear 1N1 goes via single-core cables to another low voltage distribution plant + 1N2 via a sliding sliprings + W2.

You should not be bothered that the terminals on + 1N1 and + 1N2 are marked with M1, M2,… instead of the number of the frequency regulator, because it is done and it is assumed that these are individual motors of higher and higher power that drive the appropriate devices.

Scheme 5.

Scheme 6.

A typical single-line scheme of a part of a high-voltage switchgear (in this case, 20 kV) with two transformer cubicles from which two converter transformers T1 and T2 are supplied. The transformer is supplied via a disconnector with earthing blades Q0 and power switches Q1. In each phase, both feeders are current transformers for appropriate needs. The transformers are powered the high-power frequency regulators, which we will see in the following connection diagrams. Secondary connections are made using fine-wire single-core cables of appropriate cross-section and number of cables per phase. Each single-core cable has its own number, starting and ending position as well as the equipment to which it goes. Transformer secondary voltages are marked for the first transformer (T1) with T1S1 & T1S2, and for the second (T2) with T2S1 & T2S2 to avoid confusion.

Scheme 6.

Scheme 7.

The single-line scheme of 690 VAC power supply cells in this figure shows the cable connection of one converter transformer for the frequency regulation of electric motors of higher power. In such higher power switchgears, each VFD has a separate low voltage two-units  – input unit IN.AC1 for the first secondary and IN.AC2 for the second secondary, from a transformer equipped with the necessary electrical devices as seen in this diagram. This is a single-line scheme, with the difference that all single-core power cables are shown, of which in this case there are three for each phase. It should be emphasized here that the required number, type of cable and cross section are defined by the manufacturer of frequency regulators of electric motor drives. Phase lines with T1.S1 and T2.S2 are marked as follows; 1L1, 1L2 & 1L3 for the first secondary, or 2L.1, 2L.2 & 2L.3 for the second secondary, as well as 2PE for protective conductor. The same scheme is for the second converter transformer T2. From this scheme, a group is fed in parallel connection of diode-thyristor rectifier modules 2-3 and even more, depending on the requirements.

Scheme 7.

Scheme 8.

A typical single-line diode rectifier cell circuit for high power frequency regulators (VFDs). Power is supplied from input, IN.AC1 and IN.AC2 low voltage units 2 x 3 x 690 VAC, via the required number of single-core cables of appropriate cross-section AC1 and AC2. The DC voltage from the rectifier units U11.1 to U11.3 goes directly to the common 690VDC buses from which the DC / AC inverters, 690 VDC / 3×690 VAC regulated frequencies are further supplied, which is shown in the following diagram.

Scheme 8.

Scheme 9.

Typical single-line circuit diagram of main busbars of DC voltage with several feeders, for “soft” motor starters and inverters with frequency control of three-phase electric motors. Chopper units with resistors for dynamic braking are also supplied from these main busbars.

Typical single-line circuit diagram of a DC power voltage switchgear.

Scheme 9.

Scheme 10.

A typical single-line-three-line connection scheme of a three-phase asynchronous electric motor of higher power (> 500 kW) with a change of direction of rotation, supplied via frequency converters of direct voltage to three-phase sinusoidal voltage of 3×690 VAC, U1.1 & U1.2. From the main DC buses 690 VSC inverters are connected via power switch Q1. As we can see from the diagram, two parallel connected modules for a higher power motor are applied here. For lower power motors where there is no need to change the direction of rotation, only one module is used.

This three-line motor connection scheme is also important because it clearly shows the correct connection of the PE protective conductor to the braid or cable sheath as well as the direct conduction of the PE conductor to the external connection screw at the motor housing feet behind the protective conductor. In my photo archive, I have my original photographs of the details of the PE protective conductor.

Scheme 10.

Scheme 11.

Typical single-line-three-line connection scheme of a three-phase asynchronous electric motor of higher power (> 100 kW) for “soft” operation on one DC / 3x AC inverter unit from common main direct current busbars obtained from high power rectifiers or more in parallel connection to a large number of electric motors in the electrical installation of an electric motor drive.

Instead of the DC switch Q1, a fuse switch can be used in the input unit as seen on the right. Switch Q 1 is usually of the manual type in larger and large (VFD) frequency regulators.

Scheme 11.

Scheme 12.

The figure below shows a single-line-three-line connection of a three-phase asynchronous electric motor for “soft” start (soft starter) to the inverter cabinet for multi-motor drives, or to the common main busbars of 690 VDC in multi-motor frequency control switchgears.


Scheme 12.

Scheme 13.

A typical single-line-three-line connection scheme for a three-phase asynchronous electric motor for “soft” starting with chokes in each phase at the output of the inverter.

Scheme 13.

Scheme 14.

One-line-three-line connection scheme (cubicle) of the brake control cubicle and the braking resistor R1 in one multi-motor drive of frequency control of the regulators of individual motors (VFD multidirve), to the common, main busbars of direct voltage 690 VAC.

Scheme 14.

Scheme 15.

A typical three-line connection diagram of a power supply of one FR power supply unit with three-phase voltage from 3×690 VAC main buses. The same scheme applies to other FR units if there are more than one. For FR1; 1L1, 1L2 & 1L3> FR1 power buses. For FR2; 2L1, 2L2 & 2L3, which we will see in the following scheme.

Scheme 15.

Scheme 16.

Typical three-line main power circuit of double DC power supply units for frequency converters.

A typical three-line power circuit diagram of double DC power supply units (modules). We see that the two units are connected in parallel to the main DC busbars. Among other things, these units are equipped with inlet chokes. These buses supply high-power DC / AC converters to drive one or more large three-phase asynchronous electric motors. The DC voltage from the main buses goes to the inverter unit – INU. Two separate units of this type are used to drive two electric motors in parallel operation.

These two rectifier units are powered by a three-winding converter transformer. The first rectifier unit with T1.1, (T1.S1 – first secondary) is supplied with alternating voltage 3×690 VAC from the first secondary, and the second rectifier unit with T1.2 (T1.S2 – second secondary) with alternating voltage 3×690 VAC. The same applies to the second group of rectifier units that are powered from another converter transformer T2, where T2.1 (T2.S1), T2.2 (T2.S2), etc.

To reiterate, to make it clearer:

The first converter three-winding transformer T1.

T1.1 or T1.S1 first secondary,

T1.2. or T1.S1 second secondary.

Bus codes: 1L1.1, 1L2.1 & 1L3.1 – first secondary, and

2L1, 2L2.1 & 1L2.1 second secondary.

Terminal connection terminals to rectifier unit T1.1 on busbars; U1: 1, V1: 1 & W1: 1.

The same is true for other converter three-winding transformer T2.

Scheme 16.

Scheme 17.

Typical three-line circuit diagram of three inverter units in parallel connection for the supplying 3-phase asynchronous electrical motor.

A typical three-line power circuit diagram of a high-power three-phase asynchronous motor via three inverter modules. The modules are powered by DC power from the main DC buses from two rectifier units (DSUs), one of which is powered by the first and the other by the second secondary of the converter transformer (discussed in the previous diagrams) within the VFD switchgear. and the previous scheme).

Scheme 17.

Scheme 18.

Three-line  connection scheme of a three-winding converter transformer 35 / 2×0.69 kV. Within large converter switchgears, there are several such transformers depending on the complexity of industrial plants. From this transformer and another completely the same one, high-power rectifier units (DSU) are supplied, which are located within the distribution cabinet of frequency regulation of electric motor drives.

Scheme 18.

Scheme 19.

Three-line  wiring diagram of the power circuit of one complete frequency regulation switchgear, three-phase high-power electric motors, one or two depending on the needs of the devices they drive.

This diagram shows the first part of the three-phase voltage supply from the transformer T1, the scheme of which we have shown in the previous figure, from the first secondary T1.S1 and the second secondary T1.S2. The other part of the power supply is from the second transformer T2 also from both of its secondary as seen in the diagram, T2.S1 & T2.S2.

Three-pole circuit breakers Q1 and Q2 are installed in front of the three-phase 3x690VAC / 690 VDC diode-thyristor rectifier (DSU) units (T1.1) and (T1.2). The DC terminals of the DSU units are directly connected to the common main DC buses (DC link) between the rectifier and the inverter part of the plant.

Inverter modules, in this case three, are directly connected to the main DC buses; T11.1, T11.2, & T11.3, as well as a chopper resistor unit for dynamic braking.

All three inverter units are connected in parallel to the common 3×690 VAC busbars, from which the drive electric motor is supplied by cable lines.

Scheme 19.

Scheme 20.

Complete three-line connection scheme of one three-phase asynchronous electric motor M1 of high power (in this case 1250 kWS or 1000 kWR) with a resistor for dynamic braking R1. Power is supplied from three inverter units in parallel operation from common AC buses from the distribution cabinet of the DC / AC frequency regulator.

Scheme 20.

Scheme 21.

Single-line high-voltage cubicle scheme with three-winding converter transformer T1 and they are used to power high-power frequency regulators from when two large three-phase motors are powered in parallel operation. Of course, this scheme can also be used for multi-motor frequency-regulated electric motor drives.

(1) High voltage indoor assembly cubicle,,

(2) External installation, in this case a three-winding converter transformer.

Scheme 21.

Scheme 22.

Three-line circuit diagram of the power cabinet of a high-power 12-pulse frequency regulator from which one large three-phase asynchronous electric motor with a power of 1250 kWS or 1000 kWR of power is supplied. Power supply with mains voltage 3×690 VAC is performed with both secondaries of the converter transformer, of which T1.S1 (first secondary) is connected to the star, and T1.S2 (second secondary) is connected to the triangle. It should be emphasized here that the converter transformer can be for any voltage level of medium voltages depending on national standards. In this case, the voltages can be 6 kV, 10 kV, 20 kV and 35 kV.

Scheme 22.

Scheme 23.

A complete single-line connection scheme of two large three-phase asynchronous electric motors operating in parallel, powered by high-power frequency regulators. The power of an individual FR is 1800 kVA. The switchgear is supplied from one three-winding converter transformer T1. It should also be emphasized that all connections are made with single-core fine-wire power cables of a certain cross-section and intended for this purpose. As a rule, the required cross-section and lengths of cables are determined by the equipment manufacturers who in their technical instructions give detailed instructions on how to properly perform all connections and which the contractor must strictly adhere to. Some manufacturers practice that all connections are made by their specially trained experts for this job.

It should be emphasized here that the motor power is 1250 kW sinusoidal while 1000 kW is regulated power.

Scheme 23.

Scheme 24.

Complete three-line scheme for connecting a three-phase asynchronous electric motor M1 powered by a frequency regulator as well as a resistor R1 for dynamic braking. For the second M2 engine running in parallel with the first, e.g. for the operation of one belt conveyor with a large transport capacity, the scheme is the same. The motors operate in opposite directions because their belt drive gears (RDs) are mounted on the left and right sides of the conveyor drive drum (PB) as shown in the sketch.

Scheme 24.

Original work by author.

About the Author

-

Leave a comment

XHTML: You can use these html tags: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>

VFD Scheme – Modern electric motor drives