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SMOOTH OPERATION
New Soft-Starters For Controlled Electric Motor Start-Up
By Soren Kling
Skiers know the problem only too well - the sudden, abrupt jerk that signals "you're off" as the chair lift starts its climb to the slopes. The same problem, but in another setting, costs industry millions of dollars every year: countless AC machines - used to drive fans, crushers, agitators, pumps, conveyors, etc, - are unnecessarily stressed by unwanted load peaks day in and day out in production plants all over the world. This sudden, violent starting of AC motors takes its toll in a variety of ways. Among them are:
- Electrical problems due to voltage and current transients arising from direct online or star/delta starts. Such transients may overload the local supply network and cause unacceptable voltage variations that interfere with other electrical equipment connected to the same network.
- Mechanical problems that subject the entire drive chain, from motor to driven equipment, to severe stress.
- Operational problems such as pressure knocks in pipelines, damage to products on conveyor belts, and uncomfortable escalator rides.
The financial consequences are considerable: every technical problem, every breakdown, costs money - in the form of repairs as well as lost production. In industry, it is usually the cost of the latter that dominates.
A Problem With a Long History
Since the problem has been around for a long time, several solutions have been put forward and tried.
Star/Delta Starter
An early solution to the problem was the star/delta starter. During start-up, this connects the stator windings of the motor in a star configuration between the phase and neutral conductors of the supply network, thereby reducing the motor voltage, and consequently the motor current, by a factor of 1/v --- 3. As soon as the main moment of inertia has been overcome, the motor windings are connected in a delta configuration between the phase conductors of the network to obtain the full motor voltage and power. However, this starter does not eliminate the undesirable mechanical and electrical transients; it only reduces them slightly and distributes them across two points along the time axis - the original switch-on and the subsequent star/delta changeover. The above applies under normal conditions; unfortunately, in other circumstances change over from star to delta mode can sometimes be worse than direct on-line start-up. Because of this, the star/delta starter is a simple but rather limited solution to the problem.
Slipring Motor
Another early solution was the slipring motor. This motor is started with a starting rheostat connected to the rotor circuit by means of a slipring device. In this way, the starting current can be reduced, although the motor torque remains at the level necessary to start up the load.
As the start-up progresses and the motor gains speed, the rotor resistance is gradually reduced. Once the starting rheostat is completely out of circuit, the motor can be run up to its full speed. At this point, the rotor windings are short-circuited, so that the motor operates from here on as an ordinary squirrel-cage motor.
The advantage of the slipring motor is that a high torque can be obtained with a limited starting current. This solution is suitable mainly for applications in which there is a heavy load from the start, such as crushers and mills. Its disadvantage is its greater electromechanical complexity, in the form of brushes, sliprings, resistors and contactors, which increases costs (including maintenance) and reduces reliability.
Frequency Converter
The frequency converter is technically superior to the two previous solutions since it allows precise control of all the relevant variables, such as speed, torque and power, during every phase of the operating cycle of the motor, from start through normal operation to shut-down. Another important advantage is that the control equipment is static (there are no moving parts). Reliability is therefore high and only little maintenance is needed.
However, the frequency converter has the disadvantage of requiring a relatively large up-front investment, and this disqualifies it for many applications, especially those where regulatory control is not actually required during normal operation. Nevertheless, frequency converters have gained ground as the technology has been refined and prices have fallen. Today, they have practically supplanted the slipring motor.
Soft Starter
The soft starter, which reached the market sometime between the end of the 1970s and the early 1980s, resembles the frequency converter in that it is also electronic and based on thyristors. It could be said that it fills the gap in functionality and price between the star/delta starters and frequency converters. With soft starters it is possible to control the motor voltage such that it increases gradually during start-up, which naturally limits the starting current. This means that the motor starts smoothly and the mechanical and electrical stresses are reduced to a minimum. As a "bonus", the device can also be used for "soft" stopping.
Since the circuitry of the starter is electronic, it is comparatively easy to supplement its basic function with different safety and fault indications to improve motor protection and simplify fault finding. Examples are: protection against phase-loss, over-current and over-temperature, as well as indications of normal operation, full motor voltage and certain faults. All settings, for example of the ramp slope and initial voltage, can easily be made on the front of the unit.
Consequently, the soft starter amply satisfies the primary requirement, which is smooth starting of electric motors, while offering further advantages, such as high reliability, a much-reduced need for maintenance, good motor protection and simple settings.
A limitation of the soft starter, however, is that it cannot always be used in motor drive applications which require a high torque from the beginning. This limitation is due to the fact that the device does its job by ramping up the motor voltage to its maximum (and during shut-down ramping it down to the set switch-off level).
As the torque is proportional to the square of the voltage, the connected motor will not be able to reach full torque from the beginning.
Because of this, the soft starter is more suitable for lighter, more easily started motor drives, such as for pumps, fans, conveyors and elevators.
The New ABB Range of Soft Starters
ABB has been making soft starters since the beginning of the 1980s, and the valuable experience gained in the meantime has gone into the design of today's product range. The latest series, named PS S, has been significantly improved in many respects. It covers motor currents from three to 515A and supply voltages from 208 to 690V. The new product range has several important attributes:
- Compact: a result, there is room for more soft starters on a given mounting surface.
- Easy to install: the device can be screwed onto a mounting plate (only four holes are needed) or, alternatively, snapped onto a mounting rail; the cables are easy to connect in either case. Clear instructions are provided on the front.
- Easy to set up: With just three settings - slope of the start-up ramp, slope of the shut down ramp and initial voltage - it is possible to adjust the soft starter for a wide range of applications. These settings are easily made with the clearly labeled rotary switches on the front of the unit.
- Solid-state electrical circuit: This ensures the highest reliability and reduces maintenance to a minimum, even in applications with frequent starts and stops.
"Inside-Delta" Connection
The larger units in the family can be connected into the delta circuit of the motor just like a star/delta starter. This means that the current loading of the soft starter is reduced by a factor of 1/v--- 3, which extends the current handling range as far as 515A.
The inside-delta connection makes it possible to satisfy the requirements of any given application with a smaller device than would otherwise be possible, saving users both space and money.
Safe and Rugged Design
The units have sturdy enclosures and feature good insulation of all live parts. Thus, they are able to withstand rough handling without exposing personnel to danger. And the electrical circuit is largely fail-safe; in the unlikely event of an internal fault, the unit shuts down to protect the connected equipment.
PS S 03...25 - Compact Units For Three to 25A
Soft starters in this series are designed for smaller motors with rated currents in the range of three to 25A and mains voltages of 230, 400, 500 and 600V. They are intended for installation side by side on DIN rails. The units are equipped with bypass contacts in the main circuit, which relieve the thyristors during normal operation and reduce heat generation. Thus, very compact starter solutions can be built.
Every unit can be wired for control voltages in the ranges 24 to 110V AC/DC or 110 to 480V AC, which simplifies their incorporation in existing control systems and reduces the number of different versions required.
PS S 18/30...300/515 Flexible Units For 18 to 515A
Units in this series are intended for larger motors and offer greater flexibility with regard to installation and use. The series covers rated motor currents in the range 18 to 300A, but as they can be connected into the delta circuit of the motor like a star/delta starter, they can manage currents as high as 515A. This capability makes it easier than ever to replace existing star/delta starters to achieve smoother starts (and stops).
The solid-state circuit design (there are no electro-mechanical contacts in the main circuit) makes the devices particularly well suited for motor drives, which have to be started and stopped frequently.
All the units in this series can be equipped with a separate current limitation circuit, making it possible to set a maximum starting current that will be adhered to whatever ramp time is set. This function simplifies set-up, especially of drives with high moments of inertia, which often need long start-up times. The devices are equipped with four LEDs indicating "On", "At full voltage", "External fault" and "General fault", as well as a built-in master fault indication relay. These diagnostics functions simplify supervision and failure identification.
The units in the series are designed for a utilization factor of 110 to 115 per cent, which means that they are able to handle the corresponding over-current of the connected motors.
The built-in bypass signal relay can be used to operate an external contactor that bypasses the thyristors during continuous operation or when starting several motors sequentially using one soft starter.
The Problem Solved
The benefits of soft starting are felt right across industry. Hardly a production area exists that does not depend to some extent on motor drives, and which therefore would not profit from having ABB soft starters installed. Comments by numerous plant operators testify to this. Their functionality and design attributes put soft starters among the essential "must have" devices for a whole range of applications in modern industrial plants.
Soren Kling works for ABB Automation Technology Products in Sweden. He can be reached via e-mail at soren.kling@se.abb.com. ET
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