A phase monitor for a short-circuited asynchronous three-phase motor comprises a first and a second sensing unit, both of which are connected to a supply voltage line to the motor in order to sense the voltage conditions on the line, and a control switch which is controlled by the two sensing units and connected in a control circuit for a supply voltage switch for the motor, connected in the supply voltage line. When the supply voltage line is connected to a supply voltage source, the first sensing unit causes an immediate switching-on operation of the control circuit if the voltage conditions on the supply voltage line sensed by the first sensing unit at that moment are acceptable. After a switching-on operation of the control switch caused by the first sensing unit, the second sensing unit, which is operatively connected to the control switch via a delay unit, provides a delayed control of the state of the control switch in dependence upon the voltage conditions on the supply voltage line sensed by the second sensing unit.