Would a 3 phase motor behave differently if it lost a leg completely from its supply vs blowing a single branch fuse? I know both cases are "single phasing", but is there a difference between single phasing caused by completely losing a phase and single phasing caused by one of the branch OCPD's opening up?
BLOWN FUSE VS LOSS OF PHASE
- Thread starter fifty60
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growler
Senior Member
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- Atlanta,GA
The motor should be the same but you may notice it more if the phase is lost at the service. When some of the lights go out or the A/C stops working or whatever.
meternerd
Senior Member
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- Athol, ID
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- retired water & electric utility electrician, meter/relay tech
The control power transformer (assuming 480/120) is single phase, so if either of those phase fuses blew, the starter would de-energize. If it wasn't one of those phases, the motor would continue to run and may or may not blow one of the other fuses.
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- San Francisco Bay Area, CA, USA
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If a branch OCPD fuse opens, that phase has no power on it. How would that be different from not having power from a loss of phase further up stream?
Rhetorical question. Here's how. If you lost a phase to a wide are in a facility, every 3phase motor that was already spinning in that facility is now acting as a rotary phase converter, trying to recreate that missing 3rd phase from the other two. So what you get is an incomplete voltage on the missing phase, as opposed to zero volts on the missing branch. So you get a severe voltage imbalance on every motor affected by the phase loss, but not a complete phase loss. That's why voltage based phase monitors are sometimes fooled, they see the phantom voltage from the spinning motors and fail to trip.
If you are talking about a small installation with only one 3phase motor, like a remote pumping plant or something, then there is no difference.
Assuming of course that the phase loss is NOT on one of the phases supplying control power, as meter nerd said.
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That is exactly what happened. Two 3 phase motors both dropped a fuse in the same leg, and a 3 phase heater also opened a fuse. One of the 3 phase motors actually lost 2 fuses. The fuses were sized not too far above the overloads so my guess is that the fuse went before the overload.
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
That sounds like bad fuse selection.
MichaelKelley
Member
It all depends on the nature of fuse, if it is single, then it will stop working properly, else it will run normally, but you should consider to fix it soon as it may affect motor.
weressl
Esteemed Member
Dependent on the motor size and the supply transformer's size ratio, and of course the 'stiffness' of the primary supply and impedance of the transformer, the loss of a phase will create a greater voltage imbalance in the remaining phases than just loosing a partial load on one phase.
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