Motor is running as a single phase motor with lower speed
and creating noise. In this situation it will drawing higher
current. If it runs for long time then insulation may get
damage and ultimately burnout the winding.
when three 3 phase motor is suddenly single phased .
it will giving sound and its speed lower down. it
may also burn . and mcb or proction will come in
action. because current will rise very high in this
If a three phase induction motor is suddenly single-phased
while running, no distinct change will happen if it is
running under noload condition. You will just hear a
hunting sound and the the motor will continue to run with a
slightly lowered speed and increased current, but if the
motor is running under loaded condition it will stop down
and take excessive current due to single phasing and the
protection circuit will trip, if protection ckt is not
installed the motor will be ultimately burnt due to
excessive locked rotor current.
And note: This experiment is tested.
If protection is proper then it will operate and stops the
motor. If there is no protection which is the normal case
in LT motors, then winding will burn there are two diffrent
pattern which will apear depending on the type of winding
connection i.e. delta or star.
You will find most of the winding failure in LT motors
happens due to single phasing.
For a Delta connected motor two of the phases (coil sets)
will see normal curent and the other phase will see 200%
current. The supply current in the two remaining phases
will be 173% of normal current. In this situation one coil
is across AC power and the other two coils in series with
each other are in parallel connection with the first.
For a Star connected motor tow coils will carry %173 of
normal current while the other leg has no power. The two
active supply lines will have %173 of normal current.
If the motor is running at %70 load the current may not be
high enough to trip the overload protection and the motor
Slow blow overload protection should be sized to trip in
A three phase motor will continue running if a phase is
lost. Most three phase overload relays are not intended to
provide true single phase protection, so the most likely
scenario is the motor will run at reduced speed for some
time and will eventually overheat and fail. A three phase
motor will not start, on its own, with only single phase.
However, if the motor shaft is "manually" spun it may begin
to turn and accelerate slightly. If the motor shaft is
turned backwards then the motor would start in reverse.
Usually when talking about "single-phased" motor, it means
that two of three phases are connected. In these cases, the
motor will slow down, and the phase currents for the
remaining phases will be increased. I don't know how much. I
am also not sure if the overload device would cut-off the
motor from power supply. Unless problem is not noticed, the
motor will most likely burn. When the motor is opened, you
should be able to see that 2 of the 3 phase windings are
black, and one is good looking (if motor is Y-connected)