A pump is run by a three phase motor, after a voltage fluctuation suddenly the motor starts moving in the reverse direction. However, the motor is connected with a single magnetic conductor for rotation in one direction. How could the motor rotate in the opposite direction? How do I prevent this from happening?

  • \$\begingroup\$ You probably don't have all three phases connected correctly - maybe one has an intermittent or broken connection. \$\endgroup\$ Nov 1, 2017 at 15:38
  • 2
    \$\begingroup\$ Please explain magnetic conductor in this question. \$\endgroup\$
    – Decapod
    Nov 1, 2017 at 16:01
  • \$\begingroup\$ Wild guess: each phase has one fuse or breaker, and one of them is blown. (that arrangement would suck, but I've seen it) \$\endgroup\$
    – bobflux
    Nov 1, 2017 at 16:07
  • \$\begingroup\$ Is the power source the utility power line or some type of electronic drive (VFD or inverter)? \$\endgroup\$
    – user57037
    Nov 1, 2017 at 18:30
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    \$\begingroup\$ Add schematic and possibly photo. \$\endgroup\$
    – winny
    Nov 2, 2017 at 14:48

2 Answers 2


You blew a fuse somewhere ahead of this pump and are now giving it single phase power. A 3 phase motor will not start with single phase power, but it will run if started some other way. Your pump likely does not have a check valve to prevent water from flowing backward, so when it is off, there is a slight back-flow spinning it backward, even if only slightly and when you energize it with the single phase power, it is already turning the wrong direction, so it continues to do so.

Find and replace the blown fuse.

  • 3
    \$\begingroup\$ ...and if this is indeed the case, replacing the fuses with a proper 3-phase breaker would be a good idea... \$\endgroup\$
    – bobflux
    Nov 3, 2017 at 16:50
  • \$\begingroup\$ Right, this sort of situation is exactly why overcurrent protection should interrupt all phases. Fuses can't do that, so breaker. \$\endgroup\$ Oct 8, 2021 at 22:51

One of the wires in the conduit could be shorting out causing the reverse I have seen this happen or internally in the motor though rare it's happens.


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