Leaving a margin can be right if it is clear when and by how much the increase will come and if the machine itself is suited to the increase. If the increase is only a possibility, choosing a motor suited to today's load and replacing it when needed is generally less costly, because until then the larger motor runs at low load.

DRG-160L-6 11 kW 1000 rpm cast iron frame DRG three-phase motor – motor power margin for capacity increase

First look at the machine's limit

A capacity increase rarely comes from motor power alone. The pump impeller, fan blades, gearbox, belt and shaft must also be able to carry the extra load. If the machine cannot handle the new capacity, the larger motor merely strains the drive train harder at start-up.

When leaving a margin makes sense

  • If the increase is dated and approved in the project plan
  • If replacing the motor is in a hard-to-reach place and requires a lot of labour
  • If the frame difference between the two power steps causes no problem in fitting the machine
  • If the panel, cables and protective devices are to be installed for the higher power anyway

The price of leaving a margin

A larger motor requires more investment today, its power factor drops at low load and its starting current is higher. These losses are incurred every year until the increase happens. If the increase never comes, they become permanent.

A middle way

Choosing the motor suited to the current need while installing the panel, cables and base ready for one rating higher means that later only the motor has to be changed. The motor that is replaced can then be used on another machine on site or kept as a spare. If capacity on a fan or pump is to be increased by raising speed, remember that the power required rises rapidly with speed.

You can determine options for today's and the target power with the motor selection wizard and compare the dimensions of the two power steps on the model pages in the shop.