2011-2012 bases maintain of motor was hold in china motor newspaper; all kinds of article about motor were all sorts of flowers. In the final activity, theory about protective circuit, proposed by an engineer of GuangDong M&C Electric Power Co., Ltd, which won the first prize and also got south zone champion. Here is a segment of article following.

  Motor in normal operation, if one fuse of the 3 phases wire was fused, power source would become single-phase from three-phase. When the power source lacks of one-phase , even it keeps moving, output torque will reduce a lot and current will be very large , both of them will lead to motor burning . So when open-phase was happened, the protector should stop the motor working. After combined in ways data ,the engineer of GuangDong M&C Electric Power Co.,Ltd said, in the villages open-phase accident which will made water pump burn ,even influence agricultural production because of submersible pump circuit has simply and mobile circuit board. The formal protectors are Voltage mode and Current mode. The Voltage mode protector has simply construction, but it will fail to protect motor as open-phase was raised between motor terminal and interior .while Voltage mode protector has complicate construction, high cost and unsteady. Based on above shortage , MC engineer designed a open-phase protector of a new current mode three –phase asynchronous which has simply construction ,reliable movement , convenient using ,low price and also free-maintain ,long-lifetime and small volume.

As the following circuit, the protector was made of diodes, relays, three-phase transformers ,after connected ,they will be put in a box which output 8 lines, contains al、a2，b1、b2，c1、c2，d1、d2 . Connecting as the circuit, then the protector can work.

1．Working principle
   When circuit works normal, turning on switch QS and pressing SB1 button, then coil KM1 gets electricity, main contact tip closed and self-locked, then motor keeps on forward direction. Meanwhile through potential transformer first winding and transformer T secondary windings, so three-phase current of three-phase motor will product the response voltage. After connecting secondary windings well as the fowling circuit, three- phase of closed circuit sum is zero. So no circular current was produced now. Both ends of KM2 relay voltage is zero, then relay didn’t work and the normally closed KM2 (d1, d2) keep quiescent, so motor will continue work. When one-phase of motor was out of work and transformer T secondary-side lack one-phase induced voltage ,the electromotive force of three-phase of closed circuit didn’t sum zero .So voltage are risen between A and B. The number of this voltage is the vector sum of the two-phase induced voltage. Both ends of relay produces direct voltage through diode rectification .With relay KM2 action , it normal through close contact (d1、d2) action switch off control circuit . Then no electricity for the coil (KM1) and KM1 main contact tip broke, then motor stop working.

2．Component selection
   Protector can circle 1～5 on iron core  with three-phase transformer T and low-voltage winding based on motor voltage size .This winding stall in series of main circuit and the secondary is high-tension winding, which should be induced voltage about 12V. The diodes D1、D2 are common diode 1N4007 and KM2 is direct current relay of 24V、3A.