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Home / Technical Articles / Selection of a suitable softstarter for application with conveyor belts

Comparison of starting methods

Conveyor belts can have a lot of different looks and directions of use. It is a typical constant torque load with low to high braking torque depending on how heavy it is loaded. Let’s take a look at the most common motor starting methods and then select the most suitable softstarter for this kind of application.

Selection of a suitable softstarter for application with conveyor belts
Selection of a suitable softstarter for application with conveyor belts (photo credit: ABB)

Table of contents:

Direct-on-line start (D.O.L)

Conveyor belts often need a starting torque very near or just above the rated torque of the motor. A direct-on-line start with a normal squirrel cage motor gives approx. 1.5 to 2.5 times rated torque of the motor depending on motor size, type etc.

When making a direct-on-line start there is a very high risk of slipping between the belt and the driving role depending on this high starting torque.

Direct-on-line motor starter
Direct-on-line motor starter

Gearboxes and couplings are also exposed to high mechanical stresses. This result is considerable wear and tear and often high maintenance costs. Sometimes fluid couplings are used to reduce the transferred torque.

This method is expensive and requires a lot of maintenance.

Low braking and high braking torque of Direct-on-line start (D.O.L)
Low braking and high braking torque of Direct-on-line start (D.O.L)

Go back to Methods ↑

Star-delta start

Star-delta motor starter
Star-delta motor starter

This is a starting method that reduces the starting current and starting torque. The device normally consists of three contactors, an overload relay and a timer for setting the time in the star-position (starting position). The motor must be delta connected during a normal run, in order to be able to use this starting method.

It is not possible to use this starting method when the load torque is close to the rated motor torque during start (see figure below, High braking torque).

Instead we use star-delta motor start.

Low braking and high braking torque of Star-Delta start
Low braking and high braking torque of Star-Delta start

Go back to Methods ↑


By using the softstarter, starting torque can be reduced to a minimum value still able to start up the conveyor belt. The setting possibility of the softstarter makes it possible to adjust the torque to exactly the level that is necessary for the start.

The result is the least possible stress on gearboxes and couplings and no slipping belts during start. This will reduce the maintenance cost to a minimum.

When using a softstarter you will receive approx. 3 to 4 times rated motor current during start.

Low braking and high braking torque of
Low braking and high braking torque of

Selection of a suitable softstarter


Normal start

A start of short and light loaded conveyor belt is a typical normal start. For conveyors with D.O.L starting time less than 5 seconds, select a softstarter according to the rated motor power.

Heavy duty start

Conveyor belts can in some cases be very long and if the belt is fully loaded during start the starting time can be very long. For such applications select a softstarter designed for heavy duty start. It is also possible to select a softstarter for normal start if the softstarter is chosen one size larger than the rated motor power and use an overload relay class 30.

Recommended basic settings:

  • Start ramp: 10 sec.
  • Stop ramp: 0 sec.
    (If fragile material use 10 seconds)
  • Initial voltage: 40 %

Go back to Methods ↑

Electric Motor Soft Start 60HP

Reference: Softstarter handbook – ABB (Download)

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More Information

Edvard Csanyi

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry facilities. Professional in AutoCAD programming.


  1. David Nowicki
    Aug 07, 2017

    Great article and a very true about reducing mechanical stress. We generally found vfd’s to be a better economic option. We generally found having the ability to speed control the line gave us the ability to synchronize starting and line speed control to be advantageous to throughput.

  2. Eshu Verma
    Feb 14, 2016

    Soft starter is very good at reducing mechanical stress during starting but a question arrived that soft starter is energy efficient and is economical along with power factor improvement for high rating motor.

  3. Golden
    Sep 13, 2015

    Thank you Edvard for this informative site. I have found quite relevant information on the site.

    Can I use a soft starter on compressors?

  4. Anil mishra
    Sep 12, 2015

    What is difference between normal starter(DOL) and soft starter?

  5. elian electric
    Sep 07, 2015

    I have found eep to be one of the most valuable websites for every day information. Thank you. George ELian

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