Phase loss on a lightly loaded motor
Phase loss on a lightly loaded motor
(OP)
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!
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!





RE: Phase loss on a lightly loaded motor
Keith Cress
kcress - http://www.flaminsystems.com
RE: Phase loss on a lightly loaded motor
Remember that this was SOP for the old phase converters built from a three phase induction motor and a box of capacitors.
Most in service motors will have more load than that and will burn out. The ones that survive, you and I won't be called to look at them. That may explain why we don't see them Keith.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Phase loss on a lightly loaded motor
RE: Phase loss on a lightly loaded motor
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Phase loss on a lightly loaded motor
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Phase loss on a lightly loaded motor
RE: Phase loss on a lightly loaded motor
RE: Phase loss on a lightly loaded motor
The energized phase induces a magnetic field in the rotor. As well as inducing the back EMF in the active phase, this field also induces an EMF in the other windings. With no load connected to the open phase lead, the EMF will be close to line voltage.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Phase loss on a lightly loaded motor
When you mention "no load" being connected to the open phase lead are you referring to a load such as something other than a digital meter that would impose a small load?
RE: Phase loss on a lightly loaded motor
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Phase loss on a lightly loaded motor
RE: Phase loss on a lightly loaded motor
Once a 3 phase motor is already turning and then the phase is lost, it might keep going as long as the load is light enough and/or there is another larger motor still spinning in the system. But that largest motor now is acting as a rotary phase converter for all the rest of the loads and will likely overload quickly. Once it does, the remaining units will eventually trip off line too.
By the way, this is a similar scenario to what accounts for "load contributions" to a fault.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Phase loss on a lightly loaded motor
I know I have often used a 3 phase starter to supply a single phase motor and had to connect two overloads in series to prevent the Out of Balance trip.
Roy
RE: Phase loss on a lightly loaded motor
It depends!
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Phase loss on a lightly loaded motor
Slightly off Topic
I used to see the odd two phase motor installed in shearing sheds (NZ), I think the idea was to save on copper feeding remote farms.
Roy
RE: Phase loss on a lightly loaded motor
I would never make that assumption without having a lot more data.
First, I would not assume a lightly-load motor will not overheat when single-phased. This will depend on the type of motor and the rotor construction.
Second, old-style thermal overload relays will not reliably protect a motor from destruction via single-phasing. They just won't.
I once had the pleasure of visiting a sawmill where virtually every three-phase motor was destroyed in about 30 minutes when one phase of the utility supply was lost. AFAIK, not one tripped their overloads prior to failure.
David Castor
www.cvoes.com
RE: Phase loss on a lightly loaded motor
htt
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Phase loss on a lightly loaded motor
That said, there is probably a semantics issue at play here. In the US we at one time had true 2-phase power; 2 separate sets of phase windings that were 180 degrees out from each other. Elsewhere in the world, they refer to "2-out-of-3 phases" as "2 phase" power where we refer to that as "single phase" power. It is 3 phase power, with the phases 120 degrees out from each other, but with only 2 of them used. The MOTOR however would be called a single phase motor here in the US.
Semantics semantics semantics...
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: Phase loss on a lightly loaded motor
In a 2 phase system the two are not 180 out of phase but are in quadrature, or 90 degrees apart. 180 wouldn't get you a rotating field.
RE: Phase loss on a lightly loaded motor
I give up.
How about "they are different"?
RE: Phase loss on a lightly loaded motor
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Phase loss on a lightly loaded motor
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?
RE: Phase loss on a lightly loaded motor
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