This field level must be pre-set before the test and therefore requires the operator to have prior knowledge about the required force rating (lower field current means lower available force). Further, what happens if the required force changes during the test?
The difficulty in knowing the required force throughout a test means that the field often can’t be pre-set. Of course a pre-set level means that further energy savings could be missed if the actual test had a lower force requirement and therefore the possibility to set the field even lower.
These traditional methods to reduce the field current typically rely on a 3-phase controlled rectifier. As the field current is reduced, a 300Hz ripple appears on the field waveform and this will be reflected on to the armature. This limits this technique to a maximum reduction in the field current to around 70% of the nominal value and therefore limits any potential power saving.
For further information, please, contact us!