Browse Prior Art Database

Original Publication Date: 1990-Oct-31
Included in the Prior Art Database: 2004-Apr-05

Publishing Venue

Xerox Disclosure Journal


To cause a three phase motor to be unidirectional, a device is required to monitor the phase relations of the source lines and switch any two of them if incorrect. Figure 1 shows a three phase motor anti-reversal circuit 10 with trigger circuit 12 triggering a comparator latch combination 14. By triggering comparator 14 on the positive zero crossing of one of the phases, for example phase B, the polarity of phases A and C can be positively determined. Since phase B is near zero, the other two phases will be at approximately 90% of their peak positive and negative values. If the incoming phases are of the correct polarity at that time, comparator 14 will latch and no further action is required. If they are of the wrong relative polarity, comparator 14 will latch and output a signal to power line reversing relay 16 and switch the B and C phases to the motor or system. This switching need only take place once at the time the circuit is applied to the line. Circuit 10 can be powered by the line it is monitoring and be small enough to be built into a motor, making it selectably unidirectional by the end user (by adding a switch) and not sensitive to power line phase.