What is a Soft Starter?

Soft starter is a electrical device specializes for controlling AC motor in soft start, soft stop, light-load energy saving and much more protection features. The soft starter's main components are the three phase reverse parallel thyristors between power source and the AC motor and related control circuits. Controlling the conduction angle of the three phase reverse parallel thyristors by different ways, to control the motor by the changeable input voltage on it.

How is soft starter protects motor?

  1. Overheating protection: the soft starter detects the thyristors internal radiator's temperature by its thermal relay, automatic cut down and send alarm signal once the temperature exceeds allowable value.
  2. Phase loss protection: the soft starter detects the changes in three-phase line current, to make phase loss protection response once the current off.
  3. Overload protection: the soft starter has current control loop to track and detect the changes of the AC motor current, to cut down the thyristor and send alarm signals when motor is overload.
  4. Other functions: achieve lots of mixed protection features by combining electronic circuits.

What's new?

API considers 2 starts in succession from cold condition (based on the rotor and stator being at ambient temperature) or 1 hot start (with the rotor and stator being at full load, running temperature). After the second of a two start attempt, it could well be 6 or 8 hours before the one hot attempt could be made.
Don't forget that the answers also change if a variable frequency drive or soft starter is being used.
There are several methods available to soft start a motor. The more traditional methods are Direct-on-line, Star/Delta, Auto transformer and Primary resistance starters. The more sophisticated methods also provide a soft stop and include Soft Starters and Variable Frequency Drives. Variable frequency drives can provide a better soft start with significantly lower starting currents and many other benefits including energy savings. However, a Soft Starter may provide a more economical solution to motor starting over the life of the motor.
The soft starter can be configured to operate in 6 wire mode. 6 wire mode may be the preferred method due to:
  • Possible reduction in the size of starter required, saving on space and /or cost.
  • Wiring may already be present if a start/delta type starter was previously used.
  • Reduced motor cable size.
The major advantage is that the current in the SCR is 58% less than it would be for the same motor connected in 3 wire.
Soft Starter Basic OperationSoft starter is a reduced voltage controller designed for starting standard 3 Phase induction motors. The unit is solid state, using a microprocessor to control inverse parallel (back to back) pairs of SCR's. An SCR/thyrisor is a semiconductor device that latches when triggered. Once triggered it allows current to flow in one direction only and turns off at zero current.
Soft starters are solid-state devices that protect AC electric motors from damage caused by sudden influxes of power by limiting the large initial inrush of current associated with motor start-up. They provide a gentle ramp-up to full speed and are used only at start-up (and stop if equipped). Ramping up the initial voltage to the motor produces this gradual start. Soft starters are also known as reduced voltage soft starters (RVSSs).
This is a unique feature of the Soft starter. Its effect is to prevent an undesirably sudden deceleration of the load. It is useful in various mechanical handling and conveyor systems and in many hydraulic pumping operations where a sudden removal of the drive input can induce undesirable effects, such as the fluid hammer in the pipelines.
The AC induction motor with a permanently coupled load, when it is operated without the benefit of a soft starter draws a high starting current. Typically, more than 7 times it’s rated full load current. “Reduced voltage-starting decreases the demand”. A Soft starter makes use of this principle.
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.
Reactors will not really help and will increase the throughput losses in the soft starter, I would not waste time on that. Starting a spinning motor is not an issue with soft starter either. Both of these are potential issues with variable frequency drive, totally different animal.
Soft starter - an electronic component which helps in reducing the inrush current during acceleration of a motor from zero speed to rated speed. Once the motor reaches its rated speed, it doesn't play much role.
If there is no need for a variation of speed, the Soft Starter is the ideal solution. Soft Starting solutions will enable a reduction in equipment size, reduced demand on the electrical network, zero harmonics (after starting), reduced maintenance on equipment, user friendly configuration and set up.... and a great commercial offer. Although the starting conditions can be very harsh (High Inertia), from units in new projects, through replacement of DOL starting and gears to replacement of slip ring motor starting with Soft Starters.
In many situations we deliberately reduce the starting voltage in order to better control the actual motor and machinery start, and there is rarely any legislation telling us how to achieve this. This could be by series-reactors, auto-transformers, or the popular, but expensive (virtual reduced voltage) star/delta start, so in some cases you can simply rely on the built-in series impedance of the supply cable.
Whether the differential protection is provided for the Feeder or for the Motor windings or both. In all cases usage of differential protection or not will not result increase/reduction in the size of the cable and/or the load current that have to pass through.
The control core of Gozuk HV/MV solid soft starter cabinet is composed of a HV motor soft starter integrated controller, a thyristor dynamic safety monitoring and protection unit, a thyristor firing unit, HP constant current source and so on.
Soft start is used for three main reasons: First, with a very fast output voltage rise time the charging current of the output capacitors (C × dV/dt) can trip your overcurrent circuit or at the very least, cause excessive component stress.
In soft starter, the thyristors and microprocessors are used that can cause emission during operation. This emission can be reduced by using a by-pass contactor that ensures the system fulfills the compatibility requirement. Another advantage by using a by -pass contactor is that the total power loss is reduced, and thus it is not needed to have extra fans for cooling if the soft starter is enclosed.
It depends on load, every electrical/electronic hardware has capacitance inductance, to transfer the current from power supply to another machine need these parasitic capacitances to charge which then acts as local load as it provides a discharge path for those charges accumulated in those contact capacitances & inductance. I told this because the delay very much depends on current & parasitic reactances offered by them which ultimately decides the charging time & discharging time.
The soft starter can meet such requirements. In pumping station, soft stop technology can avoid the pump door damaged of the pumping station, to reduce maintenance costs and maintenance works. The soft stop function in soft starter is, when the thyristor gets stop instruction, decrease conduction angle gradually from full conduction, and achieve full closed after a certain time. Stopping time can be adjusted according to actual requirements within 0 - 120s.
When the motor load is light, the soft starter working at energy-saving conditions, PF switch to Y position, under the current feedback action, the soft starter reduces the motor voltage automatically, to reduce excitation component of the motor current. Thereby improving the power factor of the motor (COS∮). If the contactor in bypass state, this feature cannot works. TPF switch provides energy saving features with two reaction times: normal speed and slow speed. The soft starter operation in energy saving state automatic (In normal and slow speed), saving 40% energy in no-load and 5% with load.
When use soft stop mode to stop the motor, the power supply of motor will be transferred from the bypass contactor to the controlled silicon of soft starter, and the output voltage of starter will be reduced gradually so that the running speed of motor can be cut down smoothly avoid mechanical shock. The output ending voltage is the same as the starting initial voltage. Soft stop mode can reduce or remove the surge of the loading equipment such as the water pump.
We make our motor soft starters have all kinds of protection functions to protect the safety of the soft starter and the motor. You can choose the correct protection Class and parameters according to your application conditions.
Gozuk soft starters have five working states: Ready, Run, Fault, Start and Stop. The control panel will show the current value of the motor in start/stop, and it will show the set and help menu at other states.
Stone Crushing Machine Motors required high starting torque at the time of starting the motor. It is not required all the time this motors are the short time duty cycle, so Soft starter only limit the starting current to motor. The torque Compensation or the Torque Boosting function is absent in the Soft Starter.
It depend on your load characteristic for example you can use soft starter for centrifugal pumps, fan...where Moment (N.m) directly proportional to speed (rpm). Soft starter will reduce your start up current by reduce stator voltage when start up motor. Typically, it start with 60% nominal voltage. So it should not use for flit, lathe...which need more/most moment when start-up.
A soft starter is incapable of speed-control. It only restricts starting current by applying the voltage in a ramp. The consequence is that for much of the ramp, the torque is severely compromised, as torque is proportional to the square of voltage! For 20%, 50%, 80% voltage the corresponding torques available are just 4%, 25% & 64% of full load torque! But this is OK for fans and centrifugal pumps whose torque requirements are just 0.08%, 12.5% & 50% at these speeds, as these are proportional to the CUBE of their speed.
The way a soft starter works is NOT AT ALL like an inverter. All it is doing is retarding the gate (turn-on) time of the SCR so that only a portion of the AC sine wave gets through. Inverters create pseudo sine waves by PWM firing of DC with high speed transistors. Totally different and there are no "pulses" with Soft Starting, so there are no additional shaft currents created. The soft starter is NOT the cause of your fluting.
Soft starter is an AC motor controller that ramps the motor up to full speed slowly and gently with less starting current over a longer (albeit limited) time, but it always results in full speed operation of the motor.
Breaker is an on-off device that protects electrical circuits from short circuits and possibly over current conditions, depending on the type of breaker that you select.
VFD is an AC motor controller that does everything that a soft starter does, but also facilitates operation at any speed between zero and maximum for the motor, for an indefinite amount of time.
A Soft Starter is a power electronics variant on a reduced voltage starter. The key difference is the soft start is gated on and ramped up in voltage until you reach full speed, at which point it can be bypassed to save the energy loss in the power electronics; it thus can be described as a 'bumpless' transition to full speed since you really don't get a momentary inrush 'bump' as the standard reactor, auto-transformer or delta-wye starter gives when it is transitioning from start to run configuration.
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