Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1999-03-02
2000-11-21
Martin, David
Electricity: motive power systems
Induction motor systems
Primary circuit control
361 23, 361 29, 361 33, 361 59, 361 86, 361 90, H02P 528, H02P 736
Patent
active
06150792&
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a phase monitor for a short-circuited asynchronous three-phase motor.
2. Description of the Related Art
More particularly, the invention relates to such a phase monitor of the kind comprising sensing means, intended to be connected to a supply voltage line to the motor in order to sense the voltage conditions on said line, a control switch, controlled by said sensing means and intended to be connected in a control circuit for a supply voltage switch for the motor in order to close and open said control circuit in dependence upon the sensed voltage conditions on said line, said supply voltage switch being connected in the supply voltage line at a location between the connection point of said sensing means and the motor, and delay means for delaying the switching-on and switching-off operations of the control switch.
Such phase monitors are commonly utilized to prevent damages and injuries caused by phase-sequence faults or phase voltage drop-outs in the supply voltage to the motor. Normally, phase-sequence faults do not occur in permanent installations, since, in the case of such installations, once the motor has been installed and connected in a correct manner to a power distribution network, the phase sequence will normally not be altered. However, in the case of portable equipments, such as those connected to the mains via plug contacts, there is a high risk of phase-sequence faults. Phase voltage drop-outs, on the other hand, may occur to the same extent at permanent installations as at portable equipments.
The delay means contained in the phase monitor have for their purpose to prevent normal voltage variations on the mains from causing alternating switching-on and switching-off operations of the control switch and hence also of the supply voltage switch for the motor.
However, when using phase monitors of the above kind previously known, the delay of the switching-on operation has been found to cause a serious problem. Especially in the case of portable equipments, said delay may very well result in that, when, as a consequence of said delay, the motor does not start immediately upon a connection of the supply voltage line to the mains, an operator may shift the phase sequence of the voltage applied on the supply voltage line and then leave the motor connected to a supply voltage having an incorrect phase sequence. The invention has for its purpose to provide an improved phase monitor of the kind initially specified which avoids the risk of such serios mistakes made by an operator.
BRIEF SUMMARY OF THE INVENTION
The phase monitor, according to the invention proposed for said purpose, is primarily characterized in that said sensing means comprise, on the one hand, a first sensing unit which is operatively connected to the control switch without the intermediary of any delay means and which is arranged to cause an immediate switching-on operation of the control switch when the supply voltage line is connected to a supply voltage source, if the voltage conditions on the supply voltage line sensed by said first sensing unit at the connection moment are acceptable, and on the other hand, a sensing unit which is operatively connected to the control switch through the intermediary of said delay means and which is arranged, upon a preceding switching-on operation of the control switch caused by said first sensing unit, to cause a delayed control of the state of the control switch in dependence upon the voltage conditions on the supply voltage line sensed by said second sensing unit.
The invention guarantees that, when the supply voltage line is connected in a correct manner with respect to the phase sequence to a mains on which any phase voltage drop-out does not exist, the motor will start rotating in the correct direction almost immediately, while, after a switching-off operation of the control switch caused by a later phase voltage drop-out, it will not be restarted until correct voltages have again existed on all th
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Martin David
Martin Edgardo San
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