Phase monitor

Phase monitor for a short-circuited asynchronous three-phase motor, said phase monitor comprising a sensing system (14, 15), operably coupleable to a supply voltage line (11) for the motor (10) in order to sense voltage conditions on said line (11), a control switch (18), controlled by said sensing system (14, 15) and operably coupleable in a control circuit (19) for a supply voltage switch (20) for the motor (10) in order to close and open said control circuit (19) in dependence upon the sensed voltage conditions on said line (11), said supply voltage switch (20) being connected in the supply voltage line (11) at a location between a connection point (16) of said sensing system (14, 15) with said line (11) and the motor (10), and delay means (21) for delaying the switching-on and switching-off operations of the control switch (18),

• wherein said sensing system (14, 15) comprises:
• (a) a first sensing unit (14) which is operatively connected to the control switch (18) without the intermediary of any delay means and which is arranged to cause an immediate switching-on operation of the control switch (18) when the supply voltage line (11) is connected to a supply voltage source (12), if the voltage conditions on the supply voltage line (11) sensed by said first sensing unit (14) at the connection moment are acceptable, and
• (b) a second sensing unit (15) which is operatively connected to the control switch (18) through the intermediary of said delay means (21) and which is arranged, upon a preceding switching-on operation of the control switch (18) caused by said first sensing unit (14), to cause a delayed control of the state of the control switch (18) in dependence upon the voltage conditions on the supply voltage line (11) sensed by said second sensing unit (15).