Danfoss VLT® Soft Starter - Applications like pumps, conveyers, centrifuges and bandsaws must be started slowly, and sometimes stopped slowly, to prevent mechanical shocks such as water hammer, and strains on bands, couplings and shafts
Danfoss VLT® Soft Starter – Applications like pumps, conveyers, centrifuges and bandsaws must be started slowly, and sometimes stopped slowly, to prevent mechanical shocks such as water hammer, and strains on bands, couplings and shafts

Introduction to Soft Starter

An induction motor connected directly to the power supply draws a constant magnetizing current at full speed, no matter what the load is. At anything less than full load, power factor of the induction motor is less than its rated power factor because the torque producing current decreases as the load decreases, but the magnetizing current does not change. The lighter the load, worse the power factor.

Motors selected from a standard range are almost always chosen with a rated power in excess of maximum load demand, with the result that in any installation the motors seldom operate at their full rated load. Consequently, they can never achieve their rated power factor even at a maximum load demand. More over, if the load is variable.

The wastage of energy is worse Electrical energy supply tariffs for industrial users almost invariably carry a heavy penalty –i.e.; higher cost per unit- for consumption at low power factors. Further, the cost charged is increased if demand exceeds a maximum limit.

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.

The mechanical shock delivered to the rotor or to couplings or to any intermediate gearing and the driven load by a high starting current is most severe when the motor is started directon-line. Even reduced voltage starting devices like the star delta starter or Auto transformer starters still impose shock loads because of the very current peaks which cause severe transient torques at the moments of intermediate switching.

The effect of such repeated shocks is to decrease the life of the motor and increase the cost of maintenance.

Soft Starter Construction

The Flux compensated magnetic amplifier (FCMA) Soft starter essentially consists of predominantly inductive impedance in series with the motor windings to control the motor current through a potential divider.

Inductive impedance in series with the motor windings
Inductive impedance in series with the motor windings

Finally when the motor has reached full speed and the motor terminal voltage has reached around 90-96% a shorting device cuts the Mag Amps out of the motor circuit and the motor runs under normal operating conditions.

Inductive impedance in series with the motor windings with shorting device
Inductive impedance in series with the motor windings with shorting device

Constructionally the Flux compensated magnetic amplifier is an extremely rugged device comprising of magnetic core and amply sized power windings. It should be emphasized that there is no external control loop on the system and impedance variation is the natural characteristic of the system achieved through suitable winding design and geometry.

This leads to 100% reliability of the operation.

Working principle

Conventionally the magnetic amplifiers have been equated with saturable core reactors, which work on the principle of superimposition of magnetic fluxes in the additive mode thereby saturating the magnetic core & varying the impedance .These have found limited usage in alternating current circuits because they require D.C. excitation & generate a heavy percentage of harmonics in the current waveform due to core saturation.

The Flux compensated magnetic amplifier (FCMA) is an antithesis of the saturable core reactor. The FCMA works on the principle of flux opposition instead of flux addition & hence operate always in the linear nonsaturable zone of the magnetic circuit. Oppposing flux linkages are impressed on the same magnetic circuit, thus reducing total flux & hence reducing the impedance.

Total flux linkage = main flux linkage – opposition flux linkage

N I = N1*I1 – N2*I2

The system is designed to provide a predetermined constant current in the motor windings in the starting zone. The main flux is generated by the current in the motor winding and the opposing flux is generated by the counter emf of the motor as the motor speed increases. The net flux thus reduces, allowing the motor voltage to rise gradually form as low as 50% to 96% while keeping the current within the prefixed limit, thus efforting a smooth start. In fact the reduced start current allows closer calibration of the protective relays.

The FCMA Soft starter achieves this through a unique system of flux compensated non saturated magnetic amplifier for controlling the motor current and hence the motor torque during acceleration.

Main features of Soft Starter

These are the main features and benefits of soft starter:

  1. Fastest payback period
  2. Avoids fluid hammer
  3. Easy to install
  4. In built protection
  5. Designed for extreme operating conditions
  6. Programmable current and torque limits
  7. Programmable acceleration and deceleration ramps
  8. Two set point current limits
  9. Instantaneous protections

The soft starting sequence

A Soft starter normally operates in three distinct stages:

  • Ramp
  • Dwell period
  • Energy Optimizing

These operating stages are as shown in figure below:

Soft starter operating stages
Soft starter operating stages

The Ramp stage

A. For Open loop Configuration

From the instant that the start command is given, the system first imposes a fixed delay (3 to 300/500 ms) for self monitoring, and then causes a voltage to be applied to the motor terminals to achieve the break-away torque.

This is the pedestal voltage in the figure below. This voltage is then ramped up linearly providing a step less increase of motor terminal voltage from the pedestal upto the maximum voltage. At the end of the ramp period the motor terminal voltage is equal to the input power supply voltage. As a standard the pedestal voltage is 40% of the supply voltage. This can however be changed to suit desired applications.

The current drawn by any motor accelerating from the rest position in an open loop configuration depends upon the selected ramp. Shorter the ramp time, greater the peak starting current. Why choose one ramp time rather than another?

Pedestal voltage; Top: Long ramp; Bottom: Short ramp
Pedestal voltage; Top: Long ramp; Bottom: Short ramp

For every motor and driven load system, there is a natural ramp at which the acceleration of the load and the motor matches the rate of increase of the output voltage of the soft starter. Selecting a short time will increase the rate of acceleration and the starting current drawn; a longer time will prolong the time before the load reaches the full speed, and will reduce the starting current.

This may be an advantage depending upon the particular application and perhaps on what is convenient for matching with related process equipment.

B. Closed loop Configuration

This configuration of the Soft starter is used in applications where the driven load has a particularly high inertia else it is difficult to start. For example, in a stir application where the medium is glutinous and stiff at the beginning of the process.

The acceleration time of such loads is apt to lag behind the rate of rate increase of ramp voltage even if a long ramp time is chosen. With such loads it may however be necessary to select short ramp time to ensure sufficient starting torque. A short ramp will raise the voltage quickly but the nature of the load prevents the motor speed matching it. The result may be excess current demand.

This problem is over come by a current limit feature which stops the ramp when the current output reaches a pre-selected limit. The ramp is then held until the current falls naturally. The current limiting feature is active only during the ramp period of the soft start sequence. Closed loop control requires continuous measurement and feed back of the current delivered to the motor terminals by using a CT.

The Dwell period

The dwell period begins when the output voltage reaches maximum and this period is equal to the selected ramp time, approximately equal to 10 seconds.

The dwell period maintains a constant voltage at maximum to allow the time for the motor and its driven load to settle to a steady state after acceleration is complete and before the Soft starter automatically enters the energy optimizing mode.

The optimizing stage

The energy optimizing stage is the normal operating condition of the Soft starter when the motor is up to rated speed and driving the load at whatever torque output is demanded. The energy optimizing stage is continuous after the dwell period until a stop command is given.

During the ramp period the system computes a reference value of the power factor. For energy optimizing this value is continuously compared with the running power factor. From the comparator output the system continuously computes, adjusts and updates the firing points of the thryistors so that the total energy delivered to motor corresponds to the load torque demand without wasting energy in overfluxing the motor. The power factor at the supply terminals is in this way maintained at the highest possible value for every condition of the load demand.

Management of the power factor in no way detracts from the capability of the motor to respond to the load demand .The motor performance is unaffected. This feature of the Soft starter is a purely electric function which has the effect of ensuring that at all times the motor delivers the torque demanded, but is allowed to draw only the precise amount of magnetizing current required to support the torque output.

Without this feature, the motor would draw a maximum magnetizing current regardless of the load.

The effect of the energy optimizing function is a cumulative economy in energy consumption.

The Soft-Stop function

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 soft stop ramp operates to reduce the motor terminal voltage down to 40% of maximum, at which time, the thyristors are disabled. The motor and load will coast to standstill.

Comparison: Soft Starter and Auto Transformer Starter

A. Operational differences

Soft StarterAuto Transformer Starter
Easy starting-by a push button.Similar method of starting.
Motor starts without any jerks i.e. smoothly and gradually picks up to the synchronous speed. The life of the motor is thus increased significantly and the maintenance problems are drastically reduced.Motor is subjected to heavy jerks during starting and a bit later because of the sudden inrush of current .This causes severe damages to the motor winding which ultimately burn off.
The valve on the delivery side need not be operated while switching the motor ON or OFF.This is mandatory on both occasions i.e. ON or OFF.
Soft starting and soft stopping avoids back thrust on the NRV or the pipe line which is therefore protected against the back thrust during switching OFF.Such a protection is not provided and therefore, proper operation of the valves is absolutely essential.
No electrical moving contacts and no oil hence no sparking and hazards.Circuitry consists of several moving parts and hence sparking is unavoidable.
No limitations on the No. of starts and No. of stops per hour.Limited No of starts and stops per hour because of: motor windings getting heated.
Acceleration time for motor to speed up from rest to rated full speed can be adjustable from 2 secs to 90 secs. This will facilitate selection of appropriate ramp times to suit the desired load/torque requirements.Such a facility is not available with conventional timers used in this method of starting.
In rush (starting) current restricted to 200% of rated full load current (FLC).Inrush current restricted to maximum of 400% of the FLC.

B. Protections

Soft StarterAuto Transformer Starter
Phase reversal, single phasing, overload, over voltage, under voltage and short circuit are available.Phase reversal and short circuit protections not available.
Current limit facility is available which limits the current drawn by the motor plus the load to a desired safe value.This facility is not available.

C. Economics

Soft StarterAuto Transformer Starter
Approximately Rs. 15000/- to Rs. 50000/- per annum, per equipment is saved by virtue of energy saving. The exact amount of energy saved depends upon the loading conditions.Energy saving not achieved.
By using this the power factor can be improved by .05 which will cost further saving and will also avoid KEB penalties.Provision for power factor improvement not available.
Reduction of maintenance costs on account of:

– No necessity of transformer oil.
– No moving parts present (like contactors, relays etc.).
– Motor fully protected and hence no rewinding costs.

This method of starting is prone to all these problems occurring very frequently and hence involves tremendous expenses on maintenance.
Minimum down time.Down time too large.

Advantages of Soft Starter

Soft starters are used on high tension motors for the following advantageous features:

  1. Smooth starting by torque control for gradual acceleration of the drive system thus preventing jerks and extending the life of mechanical components.
  2. Reduction in starting current to achieve break-away, and to hold back the current during acceleration, to prevent mechanical, electrical, thermal weakening of the electrical equipment such as motors, cables, transformers & switch gear.
  3. Enhancement of motor starting duty by reducing the temperature rise in stator windings and supply transformer.
  4. The microprocessor version of the Soft starter has a soft ware controlled response at full speed which economizes energy, what ever may be the load. Because of the tendency to over specify the motor rated power, this feature has benefits for most installations- not only those where load is variable.
  5. The power factor improvement is a self monitoring in built feature. When the motor is running at less than full load, the comparative reactive component of current drawn by the motor is unnecessarily high due to magnetizing and associated losses. Hence the voltage dependent losses are minimized with the load proportional active current component and as a result the power factor also improves simultaneously.

Areas of Soft Starter application

These are main applications of Soft starter:

  • 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
  • And various process control applications…


The starting performance of the squirrel cage induction motors using Soft starters provides valuable economics of electrical energy .Optimum benefits are gained when a motor duty involves frequent start or stop cycles but is still likely to be worth while in systems which are in continuous operation.

The word Soft starter has almost become synonymous to conventional starters.

Reference: Significance of soft starter in industrial applications by N.Mahesh and P.Vishnu (Aurora Technological and Researc Institute)

About Author //


Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

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