Gingoog Electric Trading (G.E.T.) ™

April 2, 2009

Why Softstarters?

Filed under: GET Softstarters — Ralf @ 10:45 AM

Softstarters and their benefits

The word Softstarter originated of course from the idea to start something in a soft and smooth way.

Soft start is an electronics term describing any circuit which reduces excess current during initial power-up.

Electric motors for example will consume a large initial current, also known as the starting current or inrush current when first turned on. That can cause voltage fluctuations and affect the performance of other circuits connected to a common power supply.

High initial current can cause damage to other components such as semiconductors or other electronic and electric devices if they are not rated for the initial high current of loads such as but not limited to filament lamps, electric motors itself or capacitors.

The current drawn by an electric motor during a start can be 2 to 10 times in extreme situations but usually averages in the most common applications to 5 or 6 times the normal operating current, and this can quickly exceed the supply’s ratings if not controlled.

The solution we want to highlight here is a Softstarter.

A Softstarter is a device used to ramp up or down AC electric motors by temporarily reducing the load in the power train of the motor during startup or stopdown. This reduces the mechanical stress on the motor and shaft, as well as the electro dynamic stresses on the attached power cables and electrical distribution network, extending the lifespan of the system quite significantly.

Let us go now a little bit deeper into all the benefits that come along with Softstarters:

As pointed out already, Softstarters save costs not only on the electrical side but also or even much more so on a variety of mechanical aspects.

Statistics show that about 90% of the motors in industrial use have no form of control, other than simple electromechanical switching. This results in huge industry costs in maintenance and equipment replacement. Fitting Softstarters could reduce these costs dramatically but where should the initiative come from, the electrical engineer or the mechanical engineer? If this seems a strange question, consider for a moment where the main benefits of fitting soft starters accrue.

The electrical engineer benefits in two ways.

By avoiding the dips in mains voltages that occur due to current peaks inherent in “Across the Line Starting”.

By avoiding the considerable stresses on the motor windings, and the iron cores of the stator and rotor, which result in reduced motor life, especially important in larger horsepower motors.

Although these benefits are considerable, the benefits to the mechanical engineer are greater still. This is because the sudden impact at startup of uncontrolled starting, followed by the rapid acceleration to full speed, causes problems across a wider range of equipment types.

Sudden torque stresses cause excessive wear on belts, pulleys, gears, chains, couplings and bearings, and also cavitations in pumps, which reduces their efficiency and life. Similarly, shock waves can be generated and transmitted along hydraulic pipe work, weakening joints in pumping systems. In conveyor systems, too, loads may be displaced or damaged on startup, and products may become contaminated.

Clearly then, mechanical engineers have more reasons to press for the fitting of Softstarters in fixed speed motor applications. Especially so, as the cost savings resulting from reduced downtime and from not having to replace bearings, gears, pulleys and bearings so frequently, will ensure quick payback on any soft starter unit.

A further argument for the mechanical engineer in fitting soft starts is that equipment previously left running, due to concerns about it restarting and also concerns regarding belts breaking, shafts shearing and bearings failing can now be switched off, enabling real savings in energy to be achieved.

Similar savings can also result if there is a breakdown situation. Taking the example of a blockage in a mill, historically all other motors have been left running when this situation occurred, even though the time required to remove the blockage was considerable. Today, this cost can be avoided simply by employing soft starters on the motors, enabling them to be switched off with no negative consequences for restarting.

This example deals with a breakdown situation. Prevention, however, is always better than cure. Modern manufacturing processes employ large numbers of pumps to convey everything from water to hazardous fluids. In many applications these pumps are driven by motors, which have no form of control during their starting and stopping.

As a result, pressure surges and water hammer occur, which can damage pipe work and equipment. Soft starters provide a solution to this problem with a feature known as “ramp down”. Intended for use where heavy dynamic loads are encountered, ramp down is particularly useful in pumping applications where it ensures that pump motors stop progressively, thus minimizing hammer or so called fluid shocks.

Another soft starter feature especially relevant to mechanical engineers is the “Electronic Shear Pin”. The Electronic Shear Pin enables the soft starter to cater for situations where loads are likely to jam suddenly, such as in wood sawing, rock crushing, wood chipping, etc. The traditional method of achieving this protection was via a mechanical shear pin that consisted of a pin of a deliberately weak material inserted into two concentric shafts at a convenient point in the drive train to the load. If the load became jammed, the sudden rise in torque would cause the pin to shear so that the two shafts could then rotate independently, thereby disconnecting the motor from its load. Before the load could be restarted, the old pin would need to be removed and a new one inserted an obviously inconvenient and time consuming process. The Electronic Shear Pin facility eliminates the need for a mechanical shear pin entirely because the speed and extent of a sudden and rapid rise in motor torque is immediately detected by the Softstarter which will then decide on a course of actions ranging from instantaneous shutdown to monitoring for recurrences if the blockage is released rapidly.

Soft starters provide an electronic solution to mechanical problems at relatively low cost. They can extend the life of belts, chains, gearboxes, shafts, bearings and machine mountings.

In addition, Softstarters can achieve real savings in energy for users by providing the facility to switch motors off, without fears for their restarting. Added to these factors, traditional fit and forget reliability ensures security of operation even in the most critical of tasks.

Here we would like to refer to our Booster pump presentation which illustrates how to maintain a constant pressure in a booster pump application plus the Cascading pump presentation which explains how Softstarters can become a component in real automation projects and help a lot to save plenty of energy. The same principle could be used by the way also for compressors in an airconditioning or refrigeration circuit and similar applications wherein several motors are working together to achieve a certain performance.

New generation softstart technologies offer meanwhile even much more than just start and stop control of electric motors. They do perform not only complete motor control but also motor protection and interface systems.

A summary of the most significant benefits:

Elimination of water or fluid hammer

Softstart gradually increases pump speed and fluid velocity. This eliminates completely damaging pressure surges and reduces maintenance costs.

Increased start capability

Softstarters allow electric motors to be started and stopped very frequently to maintain optimum system performance without any fear of mechanical damages to the motor or its attached machine.

Reduced maintenance costs

Maintenance costs for systems using Softstart control are very much reduced because the damaging mechanical stresses of motor starting and stopping are equally reduced.

Prevent interference to electrical and electronic equipment

A Softstarter enables electric motors to be started and stopped in close proximity to other equipment without causing interferences such as for example disrupting computers, programmable logic controllers (PLC’s), etc, extinguishing high pressure discharge lighting or electrical disruption to domestic consumers.

Install equipment in areas with weak electrical supplies without the cost of upgrading

A Softstarter can be used to limit the starting current to the maximum possible level. This means large motors can often be installed in areas with weak or limited electrical supplies and without the expenses of installing a new, a better, an upgraded, a higher rated electrical supply system.

Gradually decelerate speed, hence fluid movement

Damage caused by water or fluid hammer can be entirely eliminated through the use of a soft stop which slows down for example a pump operation over an extended and adjustable, meaning customizable period of time so that fluid velocity is likewise gradually reduced, thus preventing potentially and literally damaging pressure surges in the system.

Phase sequence protection

Reverse pump rotation or any dangerous reverse motor rotation can be prevented by enabling the Softstarter’s phase sequence protection.

Electronic shearpin protection

Storm water, sewage or other pumps moving fluids may carry very damaging solid debris but they can be protected using the Softstarter’s Electronic Shear Pin feature. As explained already above there are many other application examples which may benefit from this feature. Basically it finds its use  wherever loads are likely to jam unexpectedly like for example in wood sawing, rock crushing, wood chipping, etc. Electronic Shear Pin operates immediately once it detects that a machine is about to stall and thereby prevents mechanical damages.

Undercurrent protection

Protects motors against damages caused by running dry through the use of undercurrent protection. The motor’s running current is monitored to determine that it is under load indeed. This is also known as the “Loss Of Prime” feature, so in the case of a pump application it would sense when the pump starts to run dry, meaning it doesn’t carry water anymore but is filled with air. The current falls and the starter stops the pump to prevent otherwise unavoidable damage.

Programmable motor thermal model

Closed loop Softstart units provide motor overcurrent protection as an integral function. The level of this motor protection varies from basic to extremely advanced programmable motor thermal modeling.

Phase imbalance

Motor damages caused by unbalanced phases can be prevented by Softstarters with the phase imbalance protection. This can be very important when equipment is installed in remote areas with poor supplies.

Thermistor protection

Motor thermistors may be interfaced directly with a Softstarter to provide additional thermal motor protection.

System interface

Softstarters can be easily interfaced with all manners of control options, including telemetry systems for remote controls.

Serial communication

For integration with advanced control systems Softstarters nowadays are provided with serial communication capabilities.

We will go more into the details of Danfoss Softstarters in particular when we write the next article but at least we hope that we were able to let you gain already a general overview about Softstarters and how powerful, yet still simple they actually really are.

Allow me to introduce here again this comprehensive presentation:

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)


Leave a Comment »

No comments yet.

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s

Create a free website or blog at

%d bloggers like this: