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A soft starter is device which controls the acceleration of an electric motor by  controlling the applied voltage

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In technical terms, a soft starter is any device which reduces the torque applied to the electric motor. It generally consists of solid state devices like thyristors to control the application of supply voltage to the motor. The starter works on the fact that the torque is proportional to the square of the starting current, which in turn is proportional to the applied voltage. Thus the torque and the current can be adjusted by reducing the voltage at the time of starting the motor.

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- Controlled Startup
- Low Cost and Size.

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-  unsuitable for applications requiring speed control
-  unable to control the speed

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Steel industries (Rolling mills and processing lines)

Cement industries

Sugar plants

Paper and pulp

Rubber and plastic

Textile industries

Machine tool applications

Power sector

Water supply scheme

Baby Aiman (Toy Product)

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A frequency inverter changes output voltage frequency and magnitude to vary the speed, power, and torque of a connected induction motor to meet load conditions.

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1. Powering up an old radio or amplifier.
To power up an old radio or amplifier that has not been used for a long time, reduced voltage power is applied to the unit in stages in order to re-condition the electrolytic capacitors.

2.  Adjusting the voltage of an AC power supply.
The output of an AC power supply, such as an electric train transformer, can be adjusted with an auto transformer.

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Auto Transformer is a kind of transformer where primary and secondary shares same common single winding. So it is basically it’s a one winding transformer.

How does it work: The auto transformer makes it possible to start induction motor with reduced starting current and does not reduce too much starting torque.

Main Component: Main contact, fuses, wiring, push button, lamp indicator, timer and TOR.

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Before solid-state devices became available, variable-frequency drives used rotary machines and the General Electric Company obtained several patents for these in the early 20th century. Older inverter designs typically used silicon controlled rectifiers (SCR) or bipolar junction transistors (BJT) as the switching components.

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there are 3 main section in Frequency Inverter

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AuCom was established in 1978 to design and manufacture a top end range of stereo amplifiers. The name AuCom originates from the combination 'Au' for audio, and 'Com' for communication. The two founders were brought together by their mutual interest in amateur radio and appreciation of music.  Mark Empson was the electronics design expert and Ray Archer the business partner with entrepreneurial skills.

As well as amplifiers, AuCom was also involved in the design of power supplies for radio broadcast systems, taxi alarms and energy saving devices for single phase and three phase ac induction motors. These energy saving devices were manufactured under a license agreement with the American space agency, NASA.  From here the electronic soft starter technology was born and this became the main focus of the company who produced the world’s first range of commercially viable soft starters.

Now AuCom expands the range to include a wider range of industrial electronics, motor control centres and switchgear to create a complete solution.

Throughout AuCom's history, AuCom has always been dedicated to innovation and research. Through a mix of agility and hard work AuCom keeps at the forefront of industrial electronic technology.

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1. The rectifier section consists of an array of fast-acting switches that convert an incoming ac voltage supply to a pulsating dc voltage.

2. DC bus

~ To stabilize and smooth the pulsating rectifier output.

~The dc bus voltage is roughly 1.414 times greater than the incoming ac supply voltage.

~ This DC bus voltage is made available to the inverter section, which synthesizes an ac sine wave voltage output from the dc bus voltage.

3. Inverter

change DC Current to Ac current

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1. Speed Control

It can adjust the speed of electric motors.

Basic Command frequency is from 0 to 50 Hz.

At low frequency, there must be enough cooling produced by the fan

High frequency may lead to mechanical failure because of the mechanical design of the motor bearing which rated at speed 2, 4 and 6.

It may also cause torque loss.

2. Torque Control

Monitors the motor current.

Adjust the voltage to perform torque control.

If the current of the motor is not different , it may produce different result.

3.Ability to interface with other intelligent control systems

Allows management to see if they could increase production safety.

Normally done with serial interface .

It is also possible to integrate into complex network system.

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Star Delta Starter, which is sometimes called as Y – Δ or Wye – Δ Starter, is a common type of Reduced Voltage Starter. Star Delta Starter can reduce the starting current without the need of any external devices.

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The starting sequence is :-

The Star switch is open

The Start switch is closed to energise the autotransformer

The motor is connected at a selected reduced voltage tap on the autotransformer and starts to turn and accelerate

After a predetermined period the Star switch will open

After a milli-second delay the Run switch will close, connecting full line voltage to the motor

The Start switch will then open and the motor will be at operational speed

Since 1920, the autotransformer starter has been the most popular device for reducing the starting current inrush for induction motors; it provides maximum starting torque with minimum line current.

Starting Torque /Current with 3 Reduced Voltage taps

Voltage | 50%tap | 65%tap | 80%tap
Torque | 25% | 42% | 64%
Current | 50% | 65% | 80%

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The Star Delta Starter is a very common type of starter and is used extensively as compared to the other type of starting methods of the induction motor. A star delta is used for a cage motor designed to run normally on the delta connected stator winding.

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The reduced voltage autotransformer starter or Korndorfer starter[4] was invented in 1908, by Max Korndörfer of Berlin. He filed the application with the U.S. Patent office in May 1908 and was granted the patent US 1,096,922 in May 1914. Max Korndörfer assigned his patent to the General Electric Company.

Max Korndörfer claimed four methods of using an autotransformer for reduced voltage motor starting. Three of the methods are not used by the industry and it is the 4th method of starting that has been in use for more than a hundred years

The fourth method is with the star switch closed and as the motor accelerates a change over is made from the reduced voltage tap on the autotransformer to direct-on-line.

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Video Description

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Advantages
1. Heavy

2. Cheaper than typical dual-winding transformers
3. An auto transformer has higher efficiency than two winding transformer. This is because of less ohmic loss and core loss due to reduction of transformer material.
4. Auto transformer has better voltage regulation as voltage drop in resistance and reactance of the single winding is less.

Disadvantages

1. Auto Transformer is direct electrical connection between the Primary and Secondary circuit

2. Disadvantage of the Auto Transformer is that it does not have the Primary to secondary winding Isolation of a conventional Double Wound Transformer.

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Star Delta starting is when the motor is connected (normally externally from the motor) in STAR during the starting sequence. When the motor has accelerated to close to the normal running speed (75% to 80% of full load speed), the motor is connected in DELTA.

These starters are normally set to a specific starting sequence, mostly using a time setting to switch between Star and Delta. There can be extensive protection on these starters, monitoring the starting time, current, Voltage, motor speed etc.

When the motor connected to Delta configuration the phase voltage increases by 173% and phase currents increase by the same ratio. The line current increases three times its value in star connection.

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Digital soft starter with motor protection

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Digital soft starter with motor protection

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Main Circuit

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Main & Control

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-Manual star delta with push button

-Semi-automatic star delta starter

-Automatic star delta starter (open circuit transition)

-Automatic star delta starter (closed circuit transition)

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Advantages:
- The operation of the star-delta method is simple and rugged.
- It is relatively cheap compared to other reduced voltage methods.
- Good Torque/Current Performance.

Disadvantages:
- star delta starter only work when the application in light load during start.
- The delta of motor is formed in starter and not on motor terminals.
- high transmission in current peak.

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Machine Tool
- Lathe Machine
- Drill Machine
- Milling Machine
- Grinding Machine
Pump
- Reciprocating Pump
- Rotary Pump

Motor Generator

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https://www.youtube.com/watch?v=km8MSWm39Z0

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The simplest form of motor starter for the induction motor is the Direct On Line starter. The Direct On Line Motor Starter (DOL) consist a MCCB or Circuit Breaker, Contactor and an overload relay for protection.

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To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.

As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed.

Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.

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1. Most Economical and Cheapest Starter

2. Simple to establish, operate and maintain

3. Simple Control Circuitry

4. Easy to understand and trouble‐shoot.

5. It provides 100% torque at the time of starting.

6. Only one set of cable is required from starter to motor.

7. Motor is connected in delta at motor terminals.

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1. It does not reduce the starting current of the motor.

2. High Starting Current: Very High Starting Current (Typically 6 to 8 times the FLC of the motor).

3. Mechanically Harsh: Thermal Stress on the motor, thereby reducing its life.

4. Voltage Dip: There is a big voltage dip in the electrical installation because of high in-rush current affecting other customers connected to the same lines and therefore not suitable for higher size squirrel cage motors

5. High starting Torque: Unnecessary high starting torque, even when not required by the load, thereby increased mechanical stress on the mechanical systems such as rotor shaft, bearings, gearbox, coupling, chain drive, connected equipments, etc. leading to premature failure and plant downtimes.

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1. Refrigeration system head     pressure control

Located behind the refrigerator where there will be the fan.

Used to operate at cooler weather.

By reducing the setting of the temperature point, it will reduce the ambient temperature.

2. Heat Pump System

It is actually a converted system from the heat pump loop.

Converted by a solenoid valve.

It is used to stop the flow of water through the local heat pump (HP), when the unit is not operating.

This allows the pump head to increase and the frequency inverter to modulate the pump speed to maintain a constant head (reduced flow).

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Comparison btw Soft starter & Frequency inverter

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Comparison btw Soft starter & Frequency inverter

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Frequency inverter:
Control the speed of motor by convert the input voltage into adjustable frequency

Soft Starter:
Control the acceleration of motor during startup by slowly increasing the speed of motor and to have a smooth stop. It also prevent the motor from jerking and risked to wear and tear

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https://www.youtube.com/watch?v=qsvHVIQgO_o

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1. Audible noise

The noise is unpleasant and irritatting.

Although it is not harmful, the sound is not acceptable.

To overcome this problem, it could increase at higher values.

It will eliminate noise problems  and also introduces harmonics.

2. Harmonics

Usually referred as supply current waveform.

Harmonic current cause harmonic voltage to experience by other equipment connected at same supply.

Harmonic voltage causes disturbance and stress to other electrical equipment.

3. RFI (Radio Frequency Inverter)

Very problematic by using faults on other equipment in close viscinity of installed unit.