Soft Starter: Principle of Working


As the industry has evolved, work tasks have become more complex, leading to partial or full automation of the process. Soft starters are used to set up functional processes, namely to automate and soft start electric motors, which not only makes the entire process more productive but also prevents the equipment from wearing out before its time.

The soft starter mainly works with asynchronous motors, which are operated from AC mains. For better improvement of the process, soft starters can be combined with frequency converters, which will allow making more accurate settings not only to start and stop the motor, but also automates the process, but for this, first of all, you need to make all the related settings, which will deal with trained professionals.

What is a soft start?

In today's industry, it is rare to find mechanical soft starters, so we will not go into them in detail in this article. By mechanical Soft Start, we mean various brake pads, clutches, magnetic interlocks, etc. This type of device blocks the "free" acceleration of the engine by creating counteraction. Thus, the engine needs more time to accelerate to its nominal values. Electronic soft starters consist of power thyristors and are available with phase or amplitude control. Phase Soft Start regulates the frequency characteristics of the phase current (analogous to frequency control), while the amplitude Soft Start regulates the voltage at the motor terminals.

How does a soft starter work

working principleSoft starter working principle: starters are of two types, which differ in the way they work and in the way they function. They are electric and mechanical Starters. The mechanical way of regulation is simpler because it is based on a simple action - it forcibly restrains the increasing speed of rotation at startup, in order to prevent the formation of too great a load. To do this, various parts and mechanisms have been used that act mechanically to help regulate the operation and obtain the desired parameters. Of course, each of these options has its own characteristics and characteristic principles of operation, but they perform the same task.

It is easy enough to imagine the method of mechanical action. If the rotating disk is touched by an object, the speed will slow down, because there will be a friction force directed in the opposite direction. It will take longer for the element to accelerate and this process will be smoother. This is how mechanical regulators work.

The electric regulators have a more complicated principle of operation, their work is carried out by special elements in the electrical circuit which limit the voltage supply. To understand how exactly and by what type Starters with this principle works, it is necessary to carefully study the process itself. This is the moment when electrical energy is converted into kinetic energy, due to which the motor starts to work. Also at this time the resistance increases, from small values to maximum values.

And the current level is at its maximum, as Ohm's law says. This means that the incoming energy at such a moment, according to the law, must be transferred at high speed. And if you connect the motor using special Starters, then the second formula of this law comes into play, the result is that the energy is still transferred at the same level, fast, but it comes out differently, slowly, because the voltage is limited. The lower the currently available, the longer the time it will take for the unit to accelerate, and it will be smooth.
However, the described devices have tangible disadvantages:

  • they cope only with low loads or no-load starting of the motor;
  • with increasing start-up time there is a risk of motor overheating, semiconductor elements of the SPP can also overheat and fail;
  • reduction of voltage entails the reduction of torque on the shaft.

More advanced devices are characterized by the absence of these disadvantages and are divided by the principle of operation into amplitude and frequency.

What Are Some Common Applications for Soft Starters?

Soft starters are used in many systems, especially those with high inertia, as well as in various mechanisms with a special load character, namely:

  • compressors and centrifuges;
  • fan machinery;
  • conveyors and conveyors;
  • piston compressors;
  • mills, crushers, and other equipment.

Being one of the most important components of the safe production process, soft starters have become an integral part of many industries. The most popular applications are in areas such as:

  • Factory - used by conveyors, machine tools, and other units;
  • Agriculture - using choppers and machines;
  • Ventilation systems - using CCI in smoke exhausters and fans;
  • The sphere of medium and small construction - using pumps, hoists, and other technical devices.

The most indispensable devices are considered to be soft starters just for pumps. Thanks to their use, pump systems have many new advantages, including:

  • uninterrupted operation of the motor for many years, thanks to the smooth operation;
  • the absence of under-or overload on the power line, as well as the possible occurrence of water hammer, is detrimental to the operation of pumping stations. This is especially important in oil production and many other industrial applications;
  • protection against sudden starts, which may cause not only a drop in line voltage but also considerable damage to mechanical parts of pumping devices. This applies to both domestic pumps and pumping stations of the same purpose.

What Is the Advantage of Using Soft Starters?

The main task of any soft starter is to automate the operation of drive mechanisms, their functional adjustment. Its function is to provide a smooth process in the increasing movement of the rotor, which occurs after starting any induction motor, with a further smooth stop of rotation of this motor.

The process of operation, in which the smooth start of the electric motor is carried out, is based on the principle of gradual increase or decrease of supply voltage on the motor winding, which must not exceed the rated load values. Such a device operation makes it possible to exclude the quick start, which leads to high values of starting current intensity. This in turn leads to premature wear and tear of the equipment.

Used to ensure the safety of electric motors of synchronous and asynchronous types at the moment of their start-up, as well as related mechanisms and systems, soft starters have received a huge list of advantages, due to which a growing number of enterprises use them as one of the most important basic elements of production and communication lines. Many companies produce soft starters of different parameters, allowing them to use them in all spheres where motors are used. The most famous in this case is soft starters Siemens, ABB, Schneider, Danfoss and others.

How do I know that a hard start is occurring and that a soft starter is required for normal operation? This phenomenon usually appears in certain cases where it can be recognized by certain indications:

  1. Starting can be hard if the power supply being used is not performing as well as it should. In this case, the device requires a current from the mains that it can only deliver at a maximum voltage or is not capable of operating with such parameters in principle. When the start-up is performed, the circuit breakers and system elements will be disconnected, the indicator lights will start blinking and will also shut down, the generator will stop its operation.
  2. However, it is worth considering that in such a situation, the installation of the SPP will help only if this network is able to give 250 percent of the nominal available current value, then a particular engine will be able to work from it during a smooth start. If the power source is not capable of giving such readings, then there is no point in installing additional devices, they will not help the operation anyway, you need to connect to another network.
  3. During heavy starting, the running motor may not start directly, you can see that it does not begin to move or can not accelerate to the desired speed in the functioning, as a result of which the protection system is triggered. To solve the problem will not help the controller, but you can additionally try to use another device - a frequency converter, which will correct the situation.

In some cases, the startup may be good, but the circuit breaker is triggered. In this case, a soft starter can help, and the closer the particular received speed is direct to the nominal speed at the time when the protection is triggered, the higher the chances of success and successful resolution of this problem.

What Is the Difference Between a Soft Start and a VFD?

Soft starters come in a variety of configurations and applications. The simpler Soft Starts usually have several configurations: acceleration and deceleration times, start voltage, start/stop terminal, and a dry contact for alarm events. More expensive models have additional digital and analog inputs and outputs, as well as support for various industrial protocols (e.g., Modbus).

After Chinese manufacturers have improved the quality of their products to high performance, then compared to the European Soft Start, it became more profitable to install a frequency converter, as it is more functional and at the same time the cost is commensurate with the European soft start motor controls.

The soft-start electronics consist of counter-parallel thyristors in each phase. There is a motor soft starter circuit with counter-parallel thyristors included in two phases. Consider the thyristor control system, current and voltage sensor, shunt contacts, which will shunt the motor's contacts after the starter is running. Possibility of controlling the drive from remote points and integration of electric drives into APCS or ATS. Frequency converters and DCS are equipped with microcontrollers supporting one or more communication protocols.

Motor protection from overheating, unbalanced load, overloads, and other emergency modes. PDs and controllers have inputs for the connection of temperature sensors, alarm relays. They shut down the motors in case of faults or accidents in the network. The choice of a particular device depends on the requirements for the electric drive and industrial equipment and the peculiarities of the network. It is based on the analysis of production factors, engineering, and economic calculations.

Electronic Soft Start System

Working Example of Electronic Soft Start System for 3 phase induction motors

Three-phase motors are considered to be much more efficient and productive than single-phase motors designed for 220 V. Therefore if three phases are available, it is recommended that appropriate three-phase equipment be connected. As a result, connecting a three-phase motor to a three-phase network ensures not only economic but also the stable operation of the device.

The connection scheme does not require the addition of any starting devices, because immediately after starting the motor, a magnetic field is formed in its stator windings. The main condition for the normal operation of such devices is the correct execution of the connection and compliance with all recommendations. With a soft start motor controllers asynchronous motor it is possible to reduce the disadvantages of such electric machines and provide:

  • Reduced repair costs. Starting currents cause overheating of the winding, which significantly reduces the service life of the machines.
  • Absence of jerks. Abrupt starting of the motor leads to increased wear of the pinion gears, water hammer in the liquid supply network, and other undesirable consequences.
  • Reduced power consumption. Direct starting causes additional energy consumption. In addition, voltage sags in conditions of limited main power have a negative effect on all connected devices.

According to the method of control, the starters are divided into digital and analog. Digital devices are microprocessor-based. Such Starters have wider functionality and flexibility of electric motor control, convenience in adjustment and operation. Analog soft starters have limited possibilities and relatively low accuracy of signal processing, at the same time they are reliable and fast.

When selecting a soft starter, you must be guided primarily by the nature of the load. There are 3 types of load: normal, heavy, and very heavy. In normal operation, the inrush current can be up to 3 times the rated current. Due to their low cost and good quality, Soft Starts are gaining momentum in modern induction motor control systems. Soft starters that support industrial protocol Modbus RTU allow remote control, analysis in real-time of the current, voltage, power, frequency, etc., which is a big advantage in APCS systems of various industries. Soft Start is necessary to extend the service life of the asynchronous motors and also contributes to the reduction of emergency events in technological processes.

Also when choosing an ac soft starter you need to pay attention to the following parameters:

  1. Frequency of starts. Softstarter limits the maximum number of starts per hour;
  2. Number of control phases (two-phase and three-phase soft starters);
  3. Value of supply voltage;
  4. Functionality. The starter can perform a number of additional functions: protection of the motor from overloads, self-protection of the controller, the possibility of dynamic braking, bypassing. In case of parallel connection of several motors with synchronous start a bypass contactor for thyristor bypassing is obligatory;
  5. Softstarter operating conditions (ambient temperature, relative humidity, altitude, etc.).ws the connection of cheaper switching and protection equipment.

With a heavy load, the starting current can exceed the rating by up to 4.5 times, with a very heavy load by more than 6 times. Examples of heavy and very heavy starting: piston compressors, winches, mill crushers, vertical conveyors, centrifuges, band saws. Such equipment requires the installation of a soft starter one size larger than the electric motor (with a power reserve).