Question.

Describe the possible causes and effect of running a 3phase motor with one phase open circuited.

The less of current through 1phase un a 3 phase supply is known as single phasing.

Causes

Blown off fuse in 1phase.

Faulty contact

Broken wire either in R/S/T cables.

Very lose connection in one terminal.

Winding of motor gets open.

Effect. Single phasing cause unequal distribution of current

The other 2 lines would carry as high as 2-4 times the normal current.

The other 2 winding of motor would carry excess current and would rise in proportion.

A single phased motor would continue to run and if the overload relay doses not act, consequent overheating would cause the burnout of overload coils.

A single phase motor will not be able to start. Running – continuous to run.