What is a Soft Starter?

Induction motors are extensively used for a variety of industrial and domestic applications. For industrial applications, three-phase AC motors are the most popular due to their simple and robust construction, high-power output, low maintenance requirements, high efficiency, and suitability to function reliably in any working condition. However, at standstill, the windings of such motors are characterized by a low impedance. They thus draw huge amounts of current during start-up to accelerate to nominal speed. This makes them prone to input current surge known as “inrush current”.

A high inrush current is usually 5 to 8 times higher than the rated current of an induction motor at full load. And since induction motors are not designed to tolerate the high inrush, their windings can get quickly damaged which reduces the motor’s performance and lifespan. Also, high inrush currents are likely to cause a huge dip in the line voltage resulting in low levels of the supply voltage. This can be dangerous for other electric appliances connected to the same power line.

In order to avoid the high inrush current and its effects, three-phase induction motors require motor starters to limit the initial surge of current for a short duration. Once the motor speed gets to a given value, the motor starter allows resumption to the normal power supply. This results in a smooth and gradual start-up that protects the induction motor against overcurrent and low voltage.

In general motor, starters use various kinds of techniques (methods) to provide gradual motor start-up. These methods include:

  • Voltage Reduction Method: A motor starter using this technique reduces the supply voltage across the motor during start-up, thereby reducing the inrush current.  
  • Across the Line or Full Voltage Technique: In this case, the motor starter connects the induction motor to the full voltage of the supply line. This technique is mainly used for starting small induction motors. 
  • Multispeed Technique: This starter technique has the motor winding (pole) configuration designed to provide multiple pre-selected speeds. The gradual increase from one-speed setting to the next then limits the inrush current.  

What is a Soft Starter? 

An electrical soft starter is a type of motor starter, which uses the technique of voltage reduction to temporarily reduce the voltage supply across an induction motor during the start-up. That’s why soft starters are also referred to as Reduced Voltage Soft Starters (RVSS).

When an induction motor is starting, the soft starter provides a gradual increase in the voltage input thereby limiting inrush current. Limiting this inrush current is possible because the current input to an AC motor depends on its supply voltage. The gradual increase in voltage input across the motor enables it to slowly accelerate to full speed in a smooth manner. This prevents any mechanical jerking or tear of the motor’s windings due to the sudden supply of full voltage.

In addition, the soft starter can also be used to control the starting torque of a three-phase induction motor. Given that the starting torque of such motors is directly proportional to the square of the input current: Ts ∝ I2i, with Ts being the starting torque and Ii the current input to the motor. So, as the soft starter reduces the supply voltage to the motor it also controls the amount of starting torque required. Therefore, a soft starter can also be defined as an electrical device that reduces the starting torque of an induction motor and slowly increases it in a safe manner until the motor reaches full speed. Once the motor attains its rated full-speed, the soft starter will resume to the full voltage supply.

Basic Construction of a Soft Starter 

To better understand how a soft starter works, let’s first look at how it is constructed. The most construction of the most basic soft starter consists of two sections: (i) Power Unit; (ii) Control Unit. 

Power Circuit of a Soft Starter

Power Unit

This unit consists of semiconductor switches like Silicon Controlled Rectifiers (SCRs) that regulate the supply voltage. The most commonly used SCRs in soft starters are Thyristors which have three terminals. A thyristor is a fast-acting semiconductor switch, which at normal state is electrically isolated. But once a firing signal is applied to its control gate, it starts conducting allowing current and voltage to pass through it.

For a soft starter being used in a three-phase induction motor, two thyristors are connected in an anti-parallel configuration (back to back) along with each power phase of the motor-adding up to a total of six thyristors (SCRs). While the main components of the power unit are the thyristors, other components like a thermal Overload Relay (OLR), magnetic contactor, circuit breaker or fuse, and a Switch Fuse Unit (SFU) are used for overcurrent protection. Various soft starter versions consist of an integrated Electronic Overload Relay (EOL), hence, an external OLR is not needed with such models.

In addition, a heat dissipator consisting of a heat sink and fan is included in the power unit to dissipate any generated heat to the environment. A bypass contactor switch is also used for resuming full voltage supply after the induction motor attains its rated full-speed. A typical circuit diagram of the soft starter power unit is as shown to the right.

Control Unit

The components of the power unit particularly the thyristors are controlled using separate logic circuitry that makes the control unit. This logic circuitry can be a Microcontroller, PID controller, or a microprocessor-based controller like a Programmable Logic Controller (PLC) or Programmable Automation Controller (PAC). The controller logic regulates the application of gate voltage to the six thyristors (SCRs), i.e. to control their firing angle so that they can conduct current at the required part of the supply voltage cycle.

Note: The control unit (logic circuitry) of a soft starter is powered from the main AC power supply (i.e. the three-phase power line) using a transformer and rectifier circuit.

Thyristor Circuit Symbol

How Does a Soft Starter work? 

Adjusting the firing angle of each thyristor in the power unit, regulates the input voltage being supplied across an induction motor through them. Thus, the control unit of the soft starter can correctly adjust the variables of the firing signal, the current input, and starting torque of the motor as per the requirements of the connected load. So, the current input into the motor can be adjusted to the required minimum for accelerating the connected load, without any frequency changes.

The basic form of a Silicon Controlled Rectifier (SCR) or rather the thyristor consists of three terminals: (i) Anode (the positive terminal); (ii) Cathode (the negative terminal); (iii) Gate (control terminal). In soft starter circuit diagrams, the thyristor symbol used is more of an emphasis on its rectifier characteristics while also indicating the control gate. As a result, the standard symbol of a thyristor (SCR) is the conventional diode symbol that includes a control gate entering near the Anode-Cathode junction, as illustrated to the right.

To better adjust how thyristors control voltage input to a three-phase AC motor, let’s consider the following diagram.  In the diagram below, the White portion indicates Thyristor OFF while the Blue portion indicates Thyristor ON. To perform soft starting at motor start-up, the correct firing signal is sent to the thyristors through the gate connection. This firing signal allows only the last part (the Blue portion) of each half period of the sinusoidal voltage wave to pass through the thyristors, as shown above. After the motor is started, the sent firing signal continues to allow a bigger and bigger part of the supply voltage wave to pass through the thyristors.

Thyristor Firing Signal at Start-Up

Eventually, when the motor attains its rated full-speed, the firing signal sent to the gate connection after every zero-crossings allows 100% of the supply voltage to pass through the thyristors. So, during motor start-up, the soft starter power circuit (thyristors) initially allows a part of the input voltage to pass through it. Afterward, the portion of input voltage through it increases gradually as per the specified motor starting ramp-up time.

When the thyristor circuit reduces the supply voltage during motor starting, the input current and starting torque of the motor is also reduced. For example, a 50% decrease of the full voltage being supplied to the motor will result in a decrease of the input current by about 50% of the motor’s maximum current rating and the starting torque will also be reduced by about 25% of the rated maximum torque.

For the microprocessor-controlled soft starters, the control unit sends micro-processed commands to control the thyristors to adjust the voltage supply to the motor stator, without changing its frequency. The control circuit also provides the timing of the thyristor firing signals from the last zero crosses of both supply voltage and current sinusoidal waveforms.

When Should a Soft Starter be Used? 

You may consider using a soft starter if your motor related application requires: 

  • Speed and torque control during start-up. 
  • Reducing large amounts of inrush currents associated with larger motors. 
  • A mechanical system with a gentle start to eliminate torque spikes and mechanical tension during starting, such as in belt-driven systems, conveyors, and gears.  
  • Pumps to eliminate pressure surges within piping systems that are caused by rapid changes in the direction fluid flow. 

Benefits of Soft Starters  

A) Smooth Motor Startup: Unlike conventional motor starters, a soft starter provides a smooth and uniform motor starting, through a gradual increase of voltage and torque. This helps in preventing mechanical stresses or jerks, which can damage the induction motor.

B) Controlled Acceleration: A soft starter provides a fully adjustable acceleration of an induction AC motor. As varying the firing angle of the thyristors slowly or quickly can control the motor acceleration when it’s starting. This functionality is best leveraged in applications that require adjusting the start-up acceleration of a motor.

C) Eliminates Power Surges: Conventional motor starters allow full voltage to be supplied across a motor, and this is accompanied by a huge inrush current into the motor that causes a power surge in the motor circuit. But the soft starter provides a significant reduction in the current input during motor starting, along with limiting inrush current during acceleration. This prevents the occurrence of power surges.

D) Minimal Risk of Overheating: The large amounts of starting currents associated with a conventional motor startup can sometimes cause the motor windings to overheat. This overheating makes the motor keep shutting down temporarily and in extreme cases, it damages the motor permanently. However, a soft starter allows very small amounts of winding current during start-up, which greatly prevents the risk of overheating.

E) Multiple Motor Start-ups: There are applications that require multiple motors starting in a small period of time. Soft starters are well suited for such applications; as they can drastically increase the number of motor start-ups in a specific duration of time, without the risk of overheating due to high starting current.

F) Less Maintenance and Extended Service Life: An induction motor with a soft starter is less likely to have mechanical or electrical faults. This is because the soft starter provides smooth operation, and the absence of mechanical & electrical stress, resulting in fewer maintenance requirements and an improved lifetime of the motor.

In addition, by making operations start smoothly and in a controlled way, soft starters reduce the overall wear of motor-driven systems like pumps, conveyors, and cranes. Moreover, soft starters play significant roles in preventing electrical, mechanical, and thermal weakening of electrical equipment like transformers, motors, and switchgear. This makes such systems and equipment last longer with fewer maintenance requirements.

G) Improved Efficiency: Conventional motor starters are associated with high energy consumption, because they supply full voltage across the motor hence high inrush current. In contrast, by reducing the voltage supply to an AC motor the soft starter is able to significantly reduce the overall energy consumption. 

Also, the thyristors that make up the main component of the soft starter are controlled using a very low level of the supply voltage. Moreover, soft starters with microprocessor controllers have a software-controlled response at the rated full speed of the motor, which assists in monitoring energy consumption. These control techniques improve the overall efficiency of the induction motor.

H) Reduced Risks of Personnel Harm: Soft starters make the operation of large induction motors smoother and more predictable. This is an excellent way of reducing the risks of personal harm that are involved with using such machines.

Applications of Soft Starters 

You’ll often come across soft starters in the following systems: 

  • Motors Systems using Pulleys & Belts: Motors that drive loads through pulleys and belts cannot tolerate sudden mechanical jerks. As this would wear the belt coupling the motor to the load. Thus, soft starters are used in such applications to provide smooth motor start-up. 
  • Conveyor Belts: Soft starters are used in industrial conveyor belts that move heavy loads to provide smooth starting. This prevents sudden mechanical jerks which may misalign the belts, while in other cases the belts or the products placed on the conveyor may get damaged due to mechanical stress. 
  • Water Pumps: Any type of water pump driven by a motor requires smooth starting due to the sudden build-up of pressure inside the pipes. But a conventional motor starter can generate a lot of pressure during start-up which may break the piping system. Hence, a soft starter is used to provide a gradual increase of pressure in water pumps. 
  • Fans: Huge industrial fans are required to run at a constant speed. They thus need to be protected against overcurrent and low voltage conditions during starting. And the soft starter is highly suitable for use in such fans.  

Industries where applications of soft starters are most common include: 

  • Steel Plants (Processing lines and Rolling mills) 
  • Plastic and Rubber 
  • Sugar Plants 
  • Water Supply Schemes 
  • Cement Industries 
  • Textile Industries 
  • Machine Tool applications 

Disadvantages of Soft Starters 

A) No Motor Speed Regulation: Soft starters allow only the control of the voltage supplied across an induction motor i.e. from 0 Volts to the rated line voltage with a fixed-line frequency. So, if the supply frequency is constant, the motor speed will also be constant and can only be regulated by the connected load. However, the speed of an induction motor is regulated by adjusting the supply frequency to a higher or lower value than the line frequency as per the application requirements. This feature is only available with a Variable Frequency Drive (VFD).

B) Heat dissipation: The thyristors inside the power unit of the soft starter dissipate a considerable amount of energy in the form of heat. Thus, heat sinks are required for cooling those semiconductor switches.

C) Reduced Starting Torque: Soft starters reduce the voltage input across an induction motor which corresponds to its current input that is in turn directly proportional to the starting torque of that motor. This significantly reduces the motor’s starting torque. For this reason, soft starters are mainly used for applications that require low to medium starting torque.

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