Gingoog Electric Trading (G.E.T.) ™

April 3, 2009

Danfoss Softstarters

Filed under: GET Softstarters — Ralf @ 4:17 AM

Danfoss Softstarter intro and selection guide

Danfoss Softstarters protect gear, goods, equipment and environment

An AC electric motor switched directly on to the mains power supply will struggle to reach its nominal speed as quickly as possible.

This draws maximum current from the power supply and accelerates the application with its maximum torque. Depending on the exact application, this can cause different problems.

Applications like for example pumps, conveyers, centrifuges and band saws must be started slowly and sometimes stopped slowly to prevent mechanical shocks such as water hammer (also known as liquid hammer) and strains on bands, couplings and shafts.

A Softstarter is an electronic device that regulates the voltage to the motor and this provides a smooth transition from standstill to full speed operation of the particular application.

Danfoss VLT Softstarters all use the principle of phase angle control which means back to back coupled thyristors ramp up or down the motor voltage.

In some VLT Softstarters current transformers measure the actual motor current providing even more precise feedback for starting current control but also for numerous reliable motor and application protection functions.

Soft starting and soft stopping can be controlled in a number of ways, depending on the particular type of application.

Some applications require non linear voltage ramp up and the voltage ramp is therefore related to the actual current drawn. Conversely, a band saw usually requires a quick stop function provided by a DC brake.

Then again, a number of applications require a kick start torque for an instantaneous period of time followed by a soft ramp up acceleration. Danfoss VLT Softstarters do cover all of these applications and much more.

Softstarters are also an ideal upgrade to existing Star Delta, also known as Y-Delta and other conventional equipment such as Auto Transformers and the advantages are crystal clear.

Softstarters are indeed a far better alternative to Star Delta and Auto Transformer starters.

The Star Delta starter reduces start current at least somewhat but introduces still a damaging torque transient when switching from the Star to the Delta connection. Furthermore the Star Delta starting does not always allow selection of the best starting current level; it is in many cases not even a cheaper solution because Danfoss Softstarters don’t come with a heavy price tag. This sometimes means the load cannot accelerate to full speed in Star configuration, thereby making the Star Delta starter completely ineffective.

An addition a Star Delta starter does not impact at all the way the motor stops, thus leaving the voltage ramp down with no control.

The Auto Transformer on the other hand reduces start current as well but also allows some control over the level of start current. Auto Transformer starters do not however eliminate the risk of a jump in torque when shifting voltage, it lacks the often very necessary torque needed during ramp up and just like the Star Delta starter, the Auto Transformer leaves the stopping procedure completely without control.

We spoke already about quite a few general Softstarter benefits but here we would like to go just with a quick summary into some more application specific advantages in order to paint an even much clearer picture on what exactly we gain when using a Danfoss Softstarter.

Benefits on pumps:

Reduced hydraulic shocks in pipelines during start and stop

Minimized mechanical stress on motor shaft, bearings and many other mechanical components

Reduced starting current

Undercurrent protection preventing damages from a blocked pipe or low water situations (MCD 3000)

Phase rotation protection prevents damages from reverse pump rotation (MCD 202 and MCD 3000)

Instantaneous overload protection prevents damages from debris drawn into the pump (MCD 202 and MCD 3000)


Controlled start without mechanical shocks causing damages to both application and even to the products

Minimized belt stretch and reduced counter balance stress

Controlled stop without mechanical shocks

Optimum soft start even with varying starting loads like for example conveyors starting loaded or unloaded (MCD 202 and MCD 3000)

Extended mechanical life time


Reduced mechanical shocks extend the life of the compressor, couplings and motor

Limited start current enables large compressors to be started even when maximum power capacity is limited

Phase rotation protection prevents operation in reverse direction (MCD 202 and MCD 3000)

Instantaneous overload protection prevents possible damages from liquid ammonia or other refrigerants entering the compressor screw (MCD 202 and MCD 3000)


Smooth application of torque preventing mechanical stress

Reduced starting times over Star Delta starting


Smooth application of torque preventing mechanical stress

Reduced starting times over Star Delta starting

Reduced stopping times due to DC brake and soft braking (MCD 3000)

Band saws

Reduced time for saw band replacements since the soft braking function stops the motor quickly (MCD 3000)

Extended saw band life thanks to eliminated torque shocks during start

Easier saw band alignment, slow acceleration allows saw bands to be tracked without jogging

Maximum overload capability available to ride through of operating overloads (MCD-3000)

The motor thermal model (MCD 202 and MCD 3000) accounts for the actual overload capability of the connected motors and will trip only if absolutely necessary

OK, now that we know already that there is no way around, now that we know that we really need a Softstarter in our circuits, now which model is tailored for our needs?

The selection is quite easy. Danfoss offers three different models. To keep the things simple we are going to introduce them here with just a few comprehensive pointers but just as always we do include of course our downloadable links here for your further and more enhanced studies.

Danfoss Softstarter MCD 100

The smallest Danfoss Softstarter is our MCD 100 which is a cost effective and extremely compact Softstarter for AC motors up to 11.0KW due to a unique and robust semiconductor design. It is a true fit and forget product and selection can be simply made on the basis of the motor power, exactly as with traditional contactors. It comes with heavy ratings as standard, allows a unlimited number of starts per hour and is truly easy to install and to commission.

Please download here the MCD-100 Design Guide plus this very helpful Softstarter Pocket Book.

Danfoss Softstarter MCD 200

Due to extensive use of the latest technologies, such as for example the new semiconductor control algorithms and bypass design this new Softstarters are very compact and heat dissipation is very low. There is no need for extra ventilation or additional bypass contactors, this features are actually already built in which does not only lower the total cost but also simplifies the installation and keeps panel space at a minimum. It ranges up to 110KW, comes with advanced accessory modules, offers voltage ramps, current limit start and integrated motor protection when opting for the MCD-202 version.

Please download here the MCD-200 Brochure and also the MCD-200 Design Guide.

Danfoss Softstarter MCD 3000

This is a total motor starting solution. Current transformers measure the motor current and provide feedback for controlled motor voltage ramp up but also for numerous motor protection features. A numeric display and logic keypad buttons make programming easy and an operational status such as for example current and other data are showed via the display. Digitally fully featured and ranging up to 800KW, a very advanced tool, the ultimate solution, ready to perform.

Please download here the MCD-3000 Brochure and check out also all available parameters in the MCD-3000 Instruction Manual.

Another overview is also available at the download section of our Danfoss Softstarter Page.

Now NEW: The state of the art Danfoss Softstarter MCD-500

Do not forget to contact your friendly GET agent to get more answers to your pending questions. Simply use the GET Contact Form.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)


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