Lost a phase.

[farmerspark]

So today, I get a phone call from one of the bosses customers. No power or lights in workshop. (3 phase DB feeds single phase DB).

Says the lights flicker on when 3 phase compressor tries to run. My guess over phone was a lost phase, or neutral problem

The boss went out later and confirmed my thoughts. One of the main fuses had blown.

Can any of you more intelligent people explain the flickering lights?

 

[telectrix]

maybe a momentary voltage across the dead phase when the 3 phase compressor tries to kick in on 2 phases.

 

[pc1966]

As Telectrix said, most likely the motor acted as a generator when running with 2 of the 3 phases. Not going to end well for the motor mind...

 

[farmerspark]

That was kind of my thoughts. I did recommend he didn't try starting it again

 

[Lucien Nunes]

A running 3-phase motor that loses one line will attempt to regenerate the missing phase and supply any connected loads. Its torque will be severely reduced and the current in the remaning two lines will shoot up, so it is likely to trip or stall very quickly. However, on a motor running mechanically light with minimal load on the missing phase, the singlephasing condition might continue for some time unless the overload relay is designed to detect it.

However, a 3-phase motor will not generally start from a standstill with one line missing even when lightly loaded. The remaining two lines only make one phase out of the three, so there is no rotating component of stator flux to give it an impetus in one direction or the other, just like a single-phase motor with its start winding or capacitor winding disconnected. However, it will still act like a (bad) transformer and deliver some voltage into the missing line. If the load is heavy enough, i.e. there is a low enough resistance to neutral, it will function as an assymmetric 3rd phase and generate some torque, which might be enough to spin-up a very light load, but by then one would hope the O/L will intervene.