Phase loss on a lightly loaded motor 1

[JPGQC]

Hi guys

I'm trying to learn more on motor protection. Topic of the moment: single-phasing protection.

I know about standard loss-of-phase and unbalance relays.

My question is: for a lightly loaded motor, say 50% of nominal HP, would a phase loss be dangerous for the motor?

Because of the light load, the current in the remaining two phases would not be high enough to be seen by the overload relays.

But what would be the impact of the negative sequence current in the rotor?

Thanks!

 

[jraef]

OK, granted. But it seems like last time I said it that way however someone corrected me and said that the PHASES were 180 out from each other, the SINE waves were 90 degrees out.

I give up.

How about "they are different"?

 

[waross]

How about "they are different"?

That does it for me, jraef.

Yours
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Nick84]

Hi everyone,

Can someone explain what happens if three phase motor losses a phase? There must be induction in coil that lost a phase. What is typical value of induced voltage at almost full load? What is the transition (induction) time for this voltage? Does it happen almost instantly?

 

[waross]

The back EMF is almost as great as the applied voltage. When a phase is lost, the motor terminal voltage on the lost phase will drop slightly from line voltage to the back EMF value.
jraef, the 180 degrees sounds like the description of the two windings of a 120:240V transformer as a phase shift of 180 degrees.
By the way, I don't buy that description. I don't see a reversed connection as a "shift". 180 degrees- Yes, shift- No.

Bill
--------------------
"Why not the best?"
Jimmy Carter