Published On: Sat, Dec 4th, 2021

Illustrated “alphabet” of soft starters for everyone – Part 1

Illustrated “alphabet” of soft starters for everyone Part 1

Author: Radoje Jankovic

An unconventional concise explanation

Why the alphabet?

Simply because if you don’t know the alphabet you don’t know how to read, you are illiterate.

This is also the case with this new technology of modern electric motor drives, which is increasingly replacing the classic ways of starting and controlling electric motors and electric motor drives.

If you do not know and do not know electrotechnical graphic and schematic symbols and signs in this area, you are practically, technically illiterate and cannot do anything with soft starters. You do not know how to read electrical diagrams of all types, you do not know how to follow the main contracting electrical projects of electrical installations and plants during installation and later during regular maintenance of electrical equipment of this type.

I don’t know about you, but I have never met in school and university literature that professors-textbook writers and in regular lectures to their students tried to explain this and introduce them to this technology of modern starting and control of electric motors, as well as technology  frequency control of electric motor drives (Variable Frequency Drives).

I have tried to cover this electrotechnical “alphabet” in detail on these 100+ A4 pages, which is used in practice by most of the world’s leading manufacturers of soft starters for low and medium voltage electric motor drives from electric motors of the lowest power <1 kW to > 5000 kW.

I did not have any practical example of how to do that, but in my humble opinion I succeeded well. All graphic and schematic symbols are fully in accordance with IEC standards and the established practice of making electrical projects of this type that I had at my disposal to study. The original documentation of the world’s largest manufacturers of this equipment also helped me a lot. Of course, colleagues around the world who deal with this issue will have their say in the end.

Thus!

Let’s learn this modern technical alphabet.

Dear colleagues around the world, do not be illiterate!

You are free to criticize me, point out mistakes if I made a mistake somewhere, scold and of course praise if you want. I am not one of those who are vain, selfish, arrogant and think that they are God-given to know everything and the only competent ones that we must listen to them and obey their wishes.

As for me, I know that I succeeded quite well, if not excellently, in this task that I set myself.

Thanks.

Note:

I will continue to have a lot of new original practical-theoretical works for each of you, as I have done so far.

1. Marking of soft starter connection terminals

For marking the connection terminals (power voltage) of soft starters on the input side and on the output side, it is done in accordance with IEC standards and according to some other standards that are widespread in everyday practice in many countries, capital letters and a combination of capital letters and numbers are used. . In the practice, during installing and wiring a soft starter you must follow the Technical installation manuals of manufacturers of the starters. The soft starters are mounted in the vertical position.

In Figure A1. Letter, numerical and alphanumeric designations of the input and output phase terminals of the power voltage at the soft starter.

In Figure A2. Typical terminological names used in everyday practice and in the technical literature for the upper and lower side of soft starters.

Fig. A1.

Main power supply

Line side

Input side

Incoming side

Power supply terminals

External bypass terminals.

Fig. A2.

Terminals on the top of the soft starter:

A, B, C;

R, S, T;

L1, L2, L3;

1, 2, 3;

1 / L1, 3 / L2, 5 / L3;

1L1, 3L2, 5L3;

L1, L2, L3;

L1 / 1, L2 / 3, L3 / 5;

R / L1, S / L2, T / L3.

Motor connecting terminals

External bypass connecting terminals.

The following groups of terminal blocks on the underside of the soft starter correspond to each of these input terminal groups:

U, V, W;

U, V, W;

T1, T2, T3;

2, 4, 6;

2 / L1, 4 / L2, 6 / L3;

2 / T1, 4 / T2, 6 / T3;

U1, V1, W1;

T1 / 2, T2 / 4, T3 / 6;

U / T1, V / T2, W / T3.

When drawing three-pole schemes of power circuits of three-phase electric motors using soft starters, I used more or less all these ways of marking the terminals of electric circuits that are almost completely in line with IEC standards because leading manufacturers of this equipment always adjust their labels to keep their customers and large users of soft starters in modern electric motor drives around the world.

Fig. A1.
Fig. A2,

2. Switching and protection devices for electric motor installations with soft starters –

Graphic schematic symbols

The technology of soft or smooth (as some call it) starting three-phase asynchronous electric motors with cage or short-circuited rotor and even asynchronous motors with coiled rotor – sliding motor motors is a newer branch of electric motor drives, which every day always prevail in practice.

For installation, connection, maintenance and design of these electric motor drives, it is necessary to know how to read their electrical diagrams. And to know that, you need to know the graphic symbols of the new equipment. In these schemes, graphic symbols are quite different and more complex compared to classical electrotechnical graphic and letter symbols.

These are practically schematic graphic symbols of power electronics (schematic three- and single-line circuit diagrams of power electronics), ie three-pole and single-pole circuit diagrams of power control circuits of three-phase electric motors using soft starters.

At the same time, I included all switching and protective devices of electric power circuits, ie. excerpts from low and medium voltage busbars for electric motor drives. For other, auxiliary, signal and control (command) circuits, the standard graphic symbols used in practice apply. I also included graphic symbols of electric motors with different connections – stator winding connections used for this purpose.

In this paper, I have tried to collect almost all graphic symbols with complete schematic graphic symbols used in practice today, which are in accordance with IEC standards. Less, more, they are the same with all renowned world manufacturers of soft starters.

With each graphic or actual schematic symbol I have given a shorter or longer description. Therefore, this paper can serve as a kind of technical manual for all who deal with these electric motor drives. I also used all alphanumeric, numeric or alphanumeric terminals of the soft starter connection terminals, ie as marked by the world’s largest manufacturers of soft starters and related equipment for them, and they are more or less in accordance with IEC standards.

2.1. Fuses

We start with fuses as the most common short-circuit devices for short-circuit conductors of all types and devices located at the end of the conductor.

Fig. 1.1. Line fuses. In three-line (a) and single-line (b and c) electrical  power circuit diagrams. The same symbols are in all other types of electrical and electronic circuits. Their purpose is to protect all types of live conductors in the event of any short circuit of conductor or to / in an electrical device.

Fig.. 1.2. Line high-speed semiconductor fuses in three-line (a) and single-line (b and c) in the electrical power circuit diagrams. This method of protection of thyristor phase branches in soft starters when only the main-line contactor in front of the soft starter is used for the drive motor, which serves for complete isolation of the starter from the mains voltage. It is also used behind the line contactor after the output conductors for the external thyristor bypassing contactor, which will be shown in the electrical diagrams from practice. This type of graphic symbol is used for the reason that a high-speed semiconductor fuse does not mix with classic, fusible fuses.

Sl. 1.3.Line manual fuse disconnectors in three-line (a) and single-line (b & c) in the electrical power circuit diagrams. Fuses are connected directly to the busbars.

2. 2. Motor switching devices

Fig. 2.1. Three-line (a) and single-line (b) circuit diagram in the electrical power circuit diagrams, of a three-pole circuit breaker with bimetallic and fast overcurrent (electromagnetic) trigger.

Fig. 2.2. Three-line (a) and single-line (b) circuit in electric power circuit diagrams of  a three-pole circuit breaker with thermomagnetic trigger – switches in case of failure, overload or short circuit.

Fig. 2.3. Standard three-line (a) and single-line (b) scheme in the electric power circuit diagrams  of three-pole manual motor protection switch for control of small single-phase electric motors of alternating current by means of soft starter.

Control of soft starters with small single-phase motors up to a maximum of 10 kW of rated power and operating-rated voltage 220 V, 230 V, 240 V, 50/60 Hz.

Of course, the same connection applies to the classic starting of small single-phase electric motors.

Fig. 2. 4. Standard drawing in three-line (a) and single-line (b) schemes in the power electric circuit diagrams of three-pole contactor with bimetallic relay for control of small single-phase AC motors up to max. 10 kW, using a three-phase soft starter, voltages 220 V, 230 V, 240 V, 50/60 Hz.

Of course, the same connection applies to the classic starting of small single-phase motors, with the difference that a fuse or automatic short-circuit protection fuse must be placed in front of the contactor.

Fig. 2. 5. Standard drawing of a line contactor in three-line (a) and single-line (b&c)in the  power circuit diagrams for three-phase soft starters.

Behind the contactor are usually separate wires to the input terminals of the contactor for external bypassing (if any) of the phase thyristor branches in the soft starter. The conductors also go to three high-speed semiconductor fuses in front of the soft starter input terminals. Examples of applications will be shown in the schemes below.

Fig. 2. 5. Standard drawing in three-line (a) and single-line (b)  in the power circuit diagrams of the motor protection circuit breaker with one “START” and “STOP” button for switching the three-phase electric motor on and off.

The switch has complete protection against overload and short circuit whose current can be adjusted within appropriate limits.

The switch is completely sufficient with a soft starter to control the electric motor.

2. 3. Several practical schemes for marking electric motors

Fig.3. 1. These are the two most common ways of drawing a three-phase asynchronous electric motor with a short-circuited rotor (a and b) in three-line and (c) in single-line power circuit diagrams in accordance with IEC standards.

 Fig.3. 2. Standard drawing in three-line (a) and single-line (b) in the power circuit diagrams of a three-phase asynchronous electric motor with stator windings with 6 leads for connecting soft starters to phase windings of a triangle connection (in-the-delta connection).

Fig.3. 3. Another way of practical drawing in three-line (a) and single-line schemes (b) in the  power circuit diagrams of three-phase asynchronous electric motor with windings connected in a star. It also complies with IEC standards. Often, these types of symbols are used by designers when they want to present in more detail a three-line or single-line power circuit diagrams of an electric motor drive.

Fig.3. 4. Practical drawing in three-line schemes (a) and single-lines schemes (b) of the power circuit diagrams of an asynchronous electric motor with windings connected in a triangle. The rest is valid as in Fig. 3. 3.

Fig. 3. 5. Three-line circuit (a) and single-line circuit (b) in the power circuit diagrams of a three-phase asynchronous electric motor for connecting the phase branches of the thyristor to the branches of the motor winding with 6 terminals for motor operation in a triangular connection. (Inside delta soft starting motor). In the case of higher power motors, the input ends of the stator phase windings (ends) are connected, as a rule, by single-core cables as follows;

U1 to the soft starter output terminal 2T1, V1 to the soft starter output terminal 4T2 and W1 to the soft starter output terminal 6T3.

The output ends of the motor motor phase windings are connected by single core cables as follows;

W2 to phase L1, U2 to phase L2, V2 to phase L3 directly in front of the soft starter input terminals.

This connection ensures the operation of the motor in the connection of the windings in the triangle on the three-phase network. Practical diagrams give examples of a complete soft starter with accompanying equipment for controlling a three-phase motor whose stator windings are connected in a triangle for direct connection to the mains voltage.

In any case, it is necessary to strictly adhere to the Technical Instructions for installation, connection, testing and commissioning of the motors with a soft starter.

2. 4. Main low voltage busbars – graphic symbols for drawing in electrical diagrams

For drawing three-line and single-line schemes of electric circuits of power of low-voltage switchboards, distribution cabinets and also high-voltage switchboards, the standards define drawing methods. Today, schemes are drawn in the vertical, ie. in the classical way and in recent years in the horizontal way of drawing. All these ways can be seen in the practical schemes of this work and certainly many have had the opportunity to see in various electrical projects in practice. Here are some ways to draw low voltage busbars. Fully understandable and comprehensible descriptions will be presented in detail with each image.

Fig.4. 1. Two ways of drawing and marking the number of main busbars in single-line power circuit diagrams which are located in the upper part of the distribution cabinet.

(a, a1) Left end of main or auxiliary buses. The arrow in the right direction shows the direction of the busbars – departure to the next terminal, the arrow to the right in front of terminal (b) indicates the direction from which the three-phase voltage comes, and the arrow behind the terminal to the right, indicates that three-phase voltage goes to the next terminal.

(b, b1) main or auxiliary buses with excerpts below (within one drawing) of the scheme from one page of the drawing to another page of the drawing (if the whole single-line scheme cannot fit on one page of the drawing – usually A3 format). schemes of so-called “sheets” on the extended A4 format. They were more than one meter long.)

(c, c1) Right end, end of main busbars.

The number of busbars is marked with three slashes or one slash and the number next to it, which is clearly seen in the picture. power and control panels, which are usually located in the lower part of the switchboard.

It should be noted here that there are versions of distribution cabinets for powering consumers from the top of the cabinet, in which the main horizontal busbars in the lower part of the distribution cabinet with a neutral conductor bus and protective conductor (PE). As a rule, the PE conductor-bus is always in the lower part of the distribution cabinet and there are often versions with vertical N and PE bus, all of which are in accordance with IEC or other national standards.

Usually the main power supply is in the first left field of the distribution cabinet.

I deliberately use both methods when drawing my schemes.

Fig. 4. 2. Two ways of drawing and marking the number of main busbars in single-line power circuit diagrams with horizontal feeders for consumers.

(A) Main horizontal busbars in the upper part of a distribution cabinet or distribution switchboars. The arrow in the right direction shows the direction of the busbars – departure to the next terminal, the left arrow in front of the terminal (b, c) indicates the direction from which the three-phase voltage comes, and the arrow behind the terminal to the right indicates that the three-phase voltage goes to the next terminal.

Vertical busbars (sub-busbars) with horizontal feeders for consumers.

(a) Left end (start) of main busbars,

(b, c) subfeeders from the main horizontal buses on the continuation of the drawing if the whole system cannot fit into one drawing.

Each subfeeder (m) goes from vertical subfeeder busbars which starts from the main horizontal busbars (m – number of feeders from subfeeder n). As a rule, these fields (cubicles) in the system of switchboards (or motor control centers – MCC) are typed and very often they are adjusted to the requirements of the customer.

Depending on the height of the distribution cabinet, which is standardized, the number of required horizontal feeders-cells and their widths from which the consumer or load is supplied is determined.

Fig. 4. 3. Two ways of drawing and marking in single-line schemes of low-voltage switchyards for electric motor drives and other consumers in various branches of industry and energy.

(A) Main horizontal busbars in the upper part of the low voltage plant. The start of the bus is always on the left.

Vertical busbars (sub-feeders) with a certain number of horizontal feeders (m) for connecting the intended consumers and loads.

All vertical bus sub-feeders they are usually in separate busbar parts of the cabinet (cell). Special terminals from them (m) can be bus or cable with switches, depending on the load for which they are intended. In these parts of each cabinet cell, all other switching-control-signaling equipment is located. Number of these fields (m) per horizontal feeders in one low voltage switchboard can be unlimited, which you probably, many of you have had the opportunity to see in your practice and career and even to do installation work and later maintenance.

Fig. 4. 4 Two ways of drawing and marking the number of main busbars in single-line power circuit diagrams which are located in the lower part of the distribution cabinet.

(A) Left end (start) of lower main buses. An arrow in the right direction shows the direction of the busbars – departure to the next terminal, an arrow to the right in front of terminal (b) indicates the direction from which the three-phase voltage comes, and an arrow behind the terminal to the right indicates that three-phase voltage goes to the next terminal.

Right end of the lower main buses

(a, a1) Left end (beginning) of main busbars,

(b, b1) busbars with feeders

Each designer initially decides how to mark the number of buses in order to unify the graphic documentation.

Depending on the standard dimensions of distribution cabinets as well as the dimensions of each piece of equipment that is installed in the output cells, the number of electrical devices that can be accommodated in one cell is determined. Many designers put too many electrical parts in distribution cabinets and cells, that is. high-density layout, while American designers and constructors of this electrical equipment prefer greater free space, which is a great advantage in visibility, monitoring wiring connections, which is also a great advantage and relief during installation and later in regular maintenance during service life.

Fig. 4. 5 Two ways of drawing and marking the number of main busbars in single-line power  circuit diagrams of low-voltage power switchboards from which consumers and drives are powered in a building. The main horizontal busbars are located in the lower part of the switchboard. They are always accompanied by two other busbars, one for the neutral conductor  and another for protective conductor (PE).

(A) Main horizontal busbars in the lower part of a low voltage switchboard. The start of the bus is always on the left, while the end of the bus is on the right. An arrow in the right direction shows the direction of the busbars – departure to the next terminal, an arrow to the right in front of terminal (b) indicates the direction from which the three-phase voltage comes, and an arrow behind the terminal to the right indicates that three-phase voltage goes to the next terminal.

Vertical busbars ie bus sub-feeders in low voltage switchboars. Each, this sub-bus has a number of feeders for connecting intended consumers or smaller distribution cabinets.

(a) Left end, beginning of the main horizontal busbars with the first busbar subdivision. The first output (1 + n) is almost always the incoming low voltage power supply cell of the entire distribution cabinet composed of a whole series of special cells-panels (cubicles).

(b&c) Bus sub-feeders – correct drawing when the single-line scheme from the first page of the drawing continues on the next page, ie. a list, where the busbars must be properly marked; an incoming arrow is drawn in front of the point of separation on the buses, and from the point, an arrow showing that it continues … Each excerpt; 1, 2,… m towards consumers goes from the vertical bus sub-supply to the main switch intended for that output. The terminal blocks of each terminal (m) and also of all conductors for control, signaling, protection and other are usually in the upper part, although there may be exceptions.

Fig. 4. 6 Two ways of drawing and marking the number of main busbars-conductors in single-line power circuit diagrams of low-voltage power switchboards from which consumers and drives in a building are powered. The main horizontal busbars are located in the lower part of the plant. They are always accompanied by two other busbars, one for the neutral conductor  and another for protective conductor (PE).

(A) Main horizontal busbars in the lower part of the low voltage switchboard. The beginning of the busbars is on the left side of the switchboard and the end is on the right side.

Vertical buses (bus fields) in low-voltage switchboards with a certain number of horizontal feeders (m) for consumers and loads.

Fig 4. 7 Two ways of drawing and marking busbars-conductors in single-line power circuit diagrams of low-voltage power switchboards, combined, one main busbars with a transitional vertical busbar field from the upper horizontal to the lower horizontal busbars.

THB – Top Horizontal Busbars,

BHB – Bottom Horizontal Busbars ,

TVBC – Transitional Vertical Busbar Cubicle from upper horizontal busbars to lower horizontal busbars, in large low voltage electrical installations.

In each part of such a switchboard, the first field (cubicle) is the supply field of the main supply voltage. At the same time, in addition to the normal feeders for consumers, there may be an feeder field (Feeder Cubicle) for another similar but smaller electrical switchboard. Also, this vertical transition field can contain measurements of current, voltage, power and protection of a part of the switchboard on the lower busbars in its space. Such low-voltage switchboaeds can be supplied with main voltage on both sides, ie. one supply to the upper main busbars at the beginning (left end) and the other supply to the lower main busbars with the supply of the main voltage at the right end of the busbars.

2. 5. Typical thyristor configurations in soft starters

2.5.1. Graphic symbols of soft starters in single-pole power circuit diagrams

Almost all graphic symbols for soft starters that are used in practice in the manufacturer’s technical literature, technical and design documentation are presented here. All comply with IEC and ANSI standards.

Fig. 5. 1 Typical connection-configuration of power thyristors two anti-parallel connected for each phase in soft starters.

Fig. 5.2. Circuit diagram of one phase of soft starter with two anti-parallel connected power thyristors and RC circuit (snuber) for prevention of thyristor ignition (a) and the same (b) scheme of spft starter with internal contact for bypassing thyristors. This circuit provides the stability of the thyristor control and the level of overvoltage protection.

Sl. 5. 3. Thyristor phase of soft starter with RC circuit on both antiparallel connected thyristors.

Fig. 5. 4. Two general graphic symbols for three-phase soft starters, in single-line power circuit diagrams without the integrated devices in them.

a) A simple symbol.

b) Extended and slightly clearer graphic symbol with display of antiparallel connected thyristors in phases.

Fig. 5. 5. Block graphic symbols for soft starters in simplified single-line power circuit diagrams. They are also used in standard single-line power circuit diagrams as well as the symbols from the previous image. They meet IEC standards.

a) with integrated contacts for bypassing phase thyristors.

b) for reversing the electric motor and the reversing contact.

c) with the representation of the thyristor in the block symbol.

d, e, f) with letter marks for soft starters in the block symbol.

Fig. 5. 6. Graphic symbols for three-phase soft starters in single-line power circuit diagrams with integrated devices.

a, b) Soft starter with integrated electronic thermal overload protection; a) on the output side of the soft starter, b) on the input side of the soft starter.

c, d) Soft starter with integrated current transformers; c) on the output side of the soft starter, d) on the input side of the soft starter.

Fig. 5. 7. Simplified drawing of a three-phase soft starter in three-line power circuit diagrams for controlling electric motors.

Sl. 5. 8. Simplified drawing of a three-phase soft starter with integrated electronic thermal protection in three-line power circuit diagrams for electric motor control.

Sl. 5. 9. Simplified drawing of a three-phase soft starter in three-line power circuit diagrams with current transformers on the input side of the soft starter. The other type of this scheme is the same, except that the current transformers are on the output side of the soft starter.

Sl. 5. 10. Simplified power circuit of a three-phase soft starter in three-line circuits. Thyristor phase arms are with RC circuit for protection against accidental thyristor ignition and with built-in contacts for internal bypassing of thyristor phase arms.

2. 6. Three-phase soft starters of incomplete type, with thyristor branches in one or two phases.

This type of soft starter construction is very common in practice. Manufacturers are happy to produce them because they are simpler and most often meet the needs of starting three-phase electric motors. They have their drawbacks, but they are cheaper for certain not very demanding needs of starting three-phase electric motors on the one hand, and they fully satisfy the safety, security and protective function of the motor on the other hand.

Fig. 6. 1. Complete practical schematic designation of power circuit in three-phase schemes of three-phase single-phase half-wave controlled soft starter. In the middle phase, the thyristor and the diode are connected in parallel.

In soft starters, such schemes are also called Power electronic circuit diagrams.

Sl. 6. 2. Complete practical schematic designation of the power circuit in three-phase schemes of a three-phase single-phase full-wave controlled soft starter. In the middle phase, two thyristors are connected in parallel.

Fig. 6. 3. Complete three-line schematic designation of the power circuit in three-line schemes of a three-phase two-phase half-wave controlled soft starter (anti-parallel connected thyristor and diodes). The phase arms of the thyristor are in the first (L1) and third (L3) phase.

Fig. 6. 4. Complete three-pole schematic designation of the power circuit in three-line  schemes of a three-phase two-phase full-wave controlled soft starter. Phase thyristor arms ara in the first L1 and third L3 phase.  

Sl. 6. 6. Full three-wave controlled three-phase soft starter circuit in three-phase soft starter schemes with two-phase full-wave controlled soft starter with anti-parallel connected two thyristors (full-wave controlled three-phase soft starter with thyristors in two-phases) and also with integrated contacts for internal bypassing. The phase arms of the thyristor are in the first (L1) and third (L2) phase

Sl. 6. 7. Three-line circuit diagram of the power of a three-phase two-phase controlled soft starter with a built-in electronic relay for overload protection at the input part of the starter in all three phases. In front of the thyristor phase branches, the connection terminals B1, B2, B3 for the contactor of the external bypass of the thyristor are made.

Fig. 6. 8. Complete three-line schematic symbol of the power circuit of a three-phase two-phase controlled soft starter with contacts for internal thyristor bypass and integrated electronic thermal overload protection on the input side of the soft starter.

Fig. 6. 9. Complete three-line schematic symbol of three-phase soft starter power circuit with thyristors in two phases, L1 and L3 and with contacts for internal thyristor bypassing and integrated electronic thermal protection in all three phases from overload on the output side of the soft starter.

Fig. 6. 10. Full three-line three-phase power circuit of two-phase controlled soft starter in phases L2 and L3, with built-in contacts for internal thyristor bypassing and RC circuit (snubber) which prevents possible ignition of the thyristor.

2.7. Complete three-phase soft starters

Here I use the name COMPLETE THREE PHASE SOFT STARTER because these soft starters have thyristor arms in all three phases. Of course, the main name for these control devices for electric motor drives is SOFT STARTER and it is regularly used in practice which is correct.

Fig. 7. 1. Complete practical three-line scheme in three-line power circuit diagrams of a three-phase half-wave controlled three-phase soft starter.

Fig. 7. 2. Complete practical three-line circuit in three-line power circuit diagrams of a three-phase fully controllable soft starter.

Fig. 7. 3. Complete graphic schematic symbol of three-phase soft starter with electronic thermal relays in all three phases for overload protection. The relays are in all three phases and are installed on the output side of the soft starter. Full-wave controlled starter.

Fig. 7. 4. Complete graphic schematic symbol of a three-phase soft starter with electronic thermal relay in all three phases for overload protection. The relays are in all three phases and are installed on the output side of the soft starter. Full-wave controlled soft starter.

Fig. 7. 5. Complete three-line scheme of three-phase soft starter with built-in current transformers in phase L1 and L3 on the input side of the starter.

Fig. 7. 6. Complete three-line scheme of three-phase soft starter with built-in current transformers in two phases L1 & L2 on the input side of the starter.

Sl. 7. 7. Complete three-line scheme of three-phase soft starter with built-in current transformers in phases L1 and L3 on the input side of the starter. Behind the transformers from the thyristor branches in phases L1 and L3, connections with terminals B1 and B3 are made for the connection of the contactor for external bypassin of the soft starter only through two phases

Fig. 7. 8. Complete three-line scheme of a three-phase soft starter with built-in current transformers in all three phases on the input side of the starter. Behind the transformers from the thyristor srms in phases are made connections with terminals L1B, L2B and L3B for the connection of the contactor for external bypassin of the soft starter only.

Fig. 7. 9. Complete three-line scheme of a three-phase soft starter with built-in current transformers in all three phases on the input side of the starter. Contacts for internal bypassing of thyristor arms are built into the starter.

Fig. 7. 10. Second view of the complete three-line scheme of the three-phase soft starter (previous picture) with built-in current transformers in all three phase.

Fig. 7. 11. Three-line soft starter scheme without internal bypassing. Current transformers are installed in each output phase of the soft starter 2/T1, 2T2, 2/T3. In front of the current transformers are the terminals for the connection terminals T1B, T2B, T3B of the contactor for the external bypassing of the thyristor arms in the starter.

Fig. 7. 12. Three-line soft starter scheme with contacts for internal bridging. Current transformers are installed in each output phase of the soft starter 2/T1, 2T2, 2/T3.

Fig. 7. 13. Complete three-line scheme of soft starter with contacts for internal bypassing the thyristors and RC circuit (snuber) in each phase group of thyristors.

Fig. 7. 14. Typical circuit of power circuits with two pairs of anti-parallel connected power thyristors with snuber circuits in medium voltage three-phase soft starters. RC snuber circuit, R2 graduation resistors, V1, V2, V3, V4 thyristors.

The snuber circuit ensures the stability of the thyristor control and the level of overvoltage protection.

In medium voltage soft starters, resistors for graduation (voltage distribution) are connected in parallel to each thyristor. The resistors are wired and of adequate power rating.

Fig. 7. 15. Typical electronic circuit of thyristor string power of one phase for medium voltage soft starters. Each medium voltage soft starter has three such series that are in separate housings, modules, cassettes and are of the withdrawable type, which enables easy installation and also replacement in case of failure. Here, each power section of one soft starter has 18 thyristors, 6 thyristors per phase. These starters are called cassette type soft starters.

Fig. 7. 16. A typical three-line power circuit diagram of a three-phase medium voltage soft starter. Each phase is equipped with one string with three groups of thyristors connected in parallel, two in an antiparallel connection. Each phase group of thyristors is an independent-modular-cassette unit and each can be replaced in case of failure or need.

Fig. 7. 17. A typical three-line circuit diagram of a three-phase medium voltage soft starter. Same as the previous one, with the difference that in this starter unit, current transformers CT1, CT2, CT3 are installed in each thyristor cassette on the input side. In front of the thyristor are three leads connected to the connection terminals B1, B2, B3 on the input side of the starter for external bypassing the thyristors.

Fig. 7. 18. A typical three-line power circuit diagram of a three-phase medium voltage soft starter. Same as the previous one, with the difference that in this starter unit, current transformers CT1, CT2, CT3 are installed in each thyristor cassette on the output side. Behind the thyristor are three leads that are connected to the connection terminals B1, B2, B3 on the output side for the external bypassing of the thyristors.

To be continued in the Part 2.

About the Author

- Radoje - Rade Jankovic Electrician, Electrical Technician, Electrical Engineer, PhD, Ecologist, Environmentalist, Designer, Educator, Investigator... Today PROFESSIONAL TECHNICAL WRITER AND DOCUMENTATOR Massive practical experience in almost all fields of electrical engineering over 40 years. My International education group in Facebook: EEEW - ELECTRICAL & ELECTRONIC EDUCATION WORLD where published more than 8000 small or bigger articles, lessons, technical advice, projects, technical calculations, test questions with and without answers, illustrated test questions with and without answers; all my original works and few thousands of may original photos from practice, drawings, circuit diagrams, environmental lessons and examples from everyday practice etc., etc.

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Illustrated “alphabet” of soft starters for everyone – Part 1