Reduce associated voltage drop when starting large inline motors

Standard induction motors during Direct Online (DOL) start-up can draw as much as seven times the nominal operation current. For larger MV motors, these currents can be quite significant. A motor operated without any control devices, eg VSDs (Variable Speed Drives) functions at approximately 0,87 lag power factor while operating at full load. Also during DOL start up, the power factor can drop as low as 0,2 lag. While VSDs are a tried and trusted solution, what happens when clients request that they only need motor start up and don’t require speed control during operation? Nor do they want to pay to replace any existing equipment.


Reactive power compensation

A solution could be the use of fast switching reactive power compensation.


  • Ability to drastically reduce the starting currents and help reduce associated voltage drops.
  • Can connect large groups of reactive compensation within the first voltage cycle (20 msec), which helps raise the starting power factor close to the unity and thus reduce the starting current.
  • Ability to minimise the amount of MV switchgear required, installation and equipment costs.
  • All capacitor switching is done at ‘zero crossing’ of the voltage waveform – therefore there are no associated voltage or current transients associated with capacitor switching.
  • All capacitors used in this system are low voltage (690 V) and the reactive power generated is supplied to MV levels using a step up transformer.
  • All components are low voltage and, as all capacitor switching is done at zero crossing, the electrical stresses on the components are reduced, extending their lifecycle and lessening long-term maintenance costs.

Take note

  • High current draw, especially during motor start-up, can result in unacceptable volt drops on the network.
  • There are a number of alternatives to direct online (DOL) starting.
  • Fast switching reactive power compensators can provide a cost effective solution for starting large motors.Read the article


Published as: Alternative motor starting solution,

By D Kamat, Siemens Australia, Electricity+Control: January 2012 (pgs 12 – 14)