Motor soft starter
Encyclopedia
A motor soft starter is a device used with AC electric motor
s to temporarily reduce the load and torque
in the powertrain
of the motor during startup. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network
, extending the lifespan of the system.
Motor soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutch
es and several types of coupling
s using a fluid
, magnetic forces, or steel shot
to transmit torque, similar to other forms of torque limiter
. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage
or current
input, or a device that temporarily alters how the motor is connected in the electric circuit.
Electrical soft starters can use solid state
devices to control the current
flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases
of the voltage applied to the induction motor with the best results achieved by three-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifier
s (thyristor
s), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode
.
Another way to limit motor starting current is a series reactor
. If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all type of 3 phase induction motor [ synchronous / asynchronous ] ranging from 25 KW 415 V to 30 MW 11 KV. Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc. Usually high starting torque applications do not use this method.
Torque surges entail high mechanical stress on the machine, which results in higher service costs and increased wear. High currents and current peaks lead to high fixed costs charged by the power supply companies (peak current calculation) and to increased mains and generator loads.
A soft starter continuously controls the three-phase motor’s voltage supply during the start-up phase. This way, the motor is adjusted to the machine’s load behavior. Mechanical operating equipment is accelerated in a gentle manner. Service life, operating behavior and work flows are positively influenced.
AC motor
An AC motor is an electric motor driven by an alternating current.It commonly consists of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the...
s to temporarily reduce the load and torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
in the powertrain
Powertrain
In a motor vehicle, the term powertrain or powerplant refers to the group of components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, drive shafts, differentials, and the final drive...
of the motor during startup. This reduces the mechanical stress on the motor and shaft, as well as the electrodynamic stresses on the attached power cables and electrical distribution network
Electricity distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...
, extending the lifespan of the system.
Motor soft starters can consist of mechanical or electrical devices, or a combination of both. Mechanical soft starters include clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
es and several types of coupling
Coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.The...
s using a fluid
Fluid coupling
A fluid coupling is a hydrodynamic device used to transmit rotating mechanical power. It has been used in automobile transmissions as an alternative to a mechanical clutch...
, magnetic forces, or steel shot
Lead shot
Lead shot is a collective term for small balls of lead. These were the original projectiles for muskets and early rifles, but today lead shot is fired primarily from shotguns. It is also used for a variety of other purposes...
to transmit torque, similar to other forms of torque limiter
Torque limiter
A torque limiter is an automatic device that protects mechanical equipment, or its work, from damage by mechanical overload. A torque limiter may limit the torque by slipping , or uncouple the load entirely...
. Electrical soft starters can be any control system that reduces the torque by temporarily reducing the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
or current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
input, or a device that temporarily alters how the motor is connected in the electric circuit.
Electrical soft starters can use solid state
Solid state (electronics)
Solid-state electronics are those circuits or devices built entirely from solid materials and in which the electrons, or other charge carriers, are confined entirely within the solid material...
devices to control the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flow and therefore the voltage applied to the motor. They can be connected in series with the line voltage applied to the motor, or can be connected inside the delta (Δ) loop of a delta-connected motor, controlling the voltage applied to each winding. Solid state soft starters can control one or more phases
Three-phase electric power
Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...
of the voltage applied to the induction motor with the best results achieved by three-phase control. Typically, the voltage is controlled by reverse-parallel-connected silicon-controlled rectifier
Silicon-controlled rectifier
A silicon-controlled rectifier is a four-layer solid state device that controls current. The name "silicon controlled rectifier" or SCR is General Electric's trade name for a type of thyristor. The SCR was developed by a team of power engineers led by Gordon Hall and commercialized by Frank W...
s (thyristor
Thyristor
A thyristor is a solid-state semiconductor device with four layers of alternating N and P-type material. They act as bistable switches, conducting when their gate receives a current trigger, and continue to conduct while they are forward biased .Some sources define silicon controlled rectifiers and...
s), but in some circumstances with three-phase control, the control elements can be a reverse-parallel-connected SCR and diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
.
Another way to limit motor starting current is a series reactor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
. If an air core is used for the series reactor then a very efficient and reliable soft starter can be designed which is suitable for all type of 3 phase induction motor [ synchronous / asynchronous ] ranging from 25 KW 415 V to 30 MW 11 KV. Using an air core series reactor soft starter is very common practice for applications like pump, compressor, fan etc. Usually high starting torque applications do not use this method.
Applications
Soft starters can be set up to the requirements of the individual application. In pump applications, a soft start can avoid pressure surges. Conveyor belt systems can be smoothly started, avoiding jerk and stress on drive components. Fans or other systems with belt drives can be started slowly to avoid belt slipping. In all systems, a soft start limits the inrush current and so improves stability of the power supply and reduces transient voltage drops that may affect other loads.Motor and machine
Across-the line starting of induction motors is accompanied by inrush currents up to 7 times higher than running current, and starting torque up to 3 times higher than running torque. The increased torque results in sudden mechanical stress on the machine which leads to a reduced service life. Moreover, the high inrush current stresses the power supply, which may lead to voltage dips. As a result, the operability of sensitive consumers may be impaired.Motor start-up
A soft start-up eliminates the undesired side effects. Several types based on control of the supply voltage or mechanical devices such as slip clutches were developed. The list provides an overview of the various electric start-up types. The current and torque characteristic curves show the behavior of the respective starter solution.Direct on-line starting
- Three-phase motor with low to medium power rating
- 3 conductors to the motor
- High starting torque
- High current peak
- Voltage dip
- One simple switching device
Star-delta start-up
- Three-phase motor with low to high power rating
- Six conductors to the motor
- Reduced starting torque, 1/3 of the nominal torque
- High mains load due to current peak during switchover from Y to D
- High mechanical stress due to torque surge during switchover from Y to D
- Two or three switching devices, more maintenance
Soft start-up
- Three-phase motor with low to high power rating
- 3 conductors to the motor
- Variable starting torque
- No current peak
- No torque peaks
- Negligible voltage dip
- One simple switching device
- Optional: Guided soft stop, protective functions, etc.
- Zero maintenance
- Compared to contactor solutions, soft starters, sometimes also referred to as soft starting devices, offer considerable advantages.
Torque surges entail high mechanical stress on the machine, which results in higher service costs and increased wear. High currents and current peaks lead to high fixed costs charged by the power supply companies (peak current calculation) and to increased mains and generator loads.
A soft starter continuously controls the three-phase motor’s voltage supply during the start-up phase. This way, the motor is adjusted to the machine’s load behavior. Mechanical operating equipment is accelerated in a gentle manner. Service life, operating behavior and work flows are positively influenced.
See also
- Braking chopperBraking chopperBrake choppers are used in the DC voltage intermediate circuits of frequency converters to control voltage when the load feeds energy back to the intermediate circuit...
- Space Vector ModulationSpace vector modulationSpace vector modulation is an algorithm for the control of pulse width modulation . It is used for the creation of alternating current waveforms; most commonly to drive 3 phase AC powered motors at varying speeds from DC using multiple class-D amplifiers. There are various variations of SVM that...
- Variable speed air compressorVariable speed air compressorA variable-speed drive air compressor is an air compressor that takes advantage of variable-speed drive technology. This type of compressor uses a special drive to control the speed of the unit, which in turn saves energy compared to a fixed speed equivalent.The most common form of VSD...
- Vector control (motor)Vector control (motor)Vector control is one method used in variable frequency drives to control the torque of three-phase AC electric motors by controlling the current fed to the machine.-Method:...
- Korndorfer starterKorndorfer starterKorndorfer starter is a technique used for soft starting of induction motors. The circuit uses a three-phase autotransformer and three three-phase switches....
- Motor controllerMotor controllerA motor controller is a device or group of devices that serves to govern in some predetermined manner the performance of an electric motor. A motor controller might include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and...
- Adjustable-speed drive
- Electronic speed controlElectronic speed controlAn electronic speed control or ESC is an electronic circuit with the purpose to vary an electric motor's speed, its direction and possibly also to act as a dynamic brake...
- Variable-frequency drive
- Thyristor driveThyristor driveA thyristor drive is a motor drive circuit where AC supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor.-Applications:Thyristor drives are very simple and were first introduced in the 1960s...
- DC motor starter section of Electric motorElectric motorAn electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...