Operating an electric motor at reduced voltage
What are the consequences of operating an electric motor,
say 4.0 MW 11 kV delta connected, at 6.6 kV keeping the same
delta configuration? It is understood that the 4.0 MW motor
is not provided with six leads at the terminal box.
It is also understood that as long that the I_max of the
motor is not exceeded, there is no risk of damaging the
motor, and also that the shaft power will only be one third
of 4.0 MW .
It will be simpler if the motor was 11 kV star connected.
Should the main concern be about the fluxing of the motor –
will the reduced voltage sufficient to provide the required
magnetic flux ?
Some ideas:
Operating the motor at a lower voltage will reduce the
magnetic flux in the gap and thereby reduce the torque
capacity of the motor. The should be no other problems
provided that the shaft power is reduced. The operating
efficiency will not be optimum as the iron loss will be less
but the copper loss will be higher than for a motor wound
for that voltage.
One must still be careful that the motor does not operate at
increased slip as this will damage the rotor.
So, one should be clear that the output torque of the motor
will be roughly 1/3 of normal, throughout the torque speed
curve. A hidden consequence of that may be recovery after a
load change. If the intended application has any kind of
step change in load, you will not have the full benefit of
all the original breakdown torque available to re-accelerate
it. Hence, the engineer may want to consider using a lower
overload setting as well just to better protect the motor.
As far as the connected load does not exceeds the nameplate
full-load current for 11 kV, the windings are safe. It is
assumed assume the engineer will keep the same over-current
protection as that for the original 11 kV condition.
The flux will drop proportional to the voltage reduction and
the torque will be reduced with the squared ratio of voltage
reduction (6.6/11)^2*100 = 36%
There are also variable frequency drive (VFD) and
auto-transformer controllers that do this for industrial
motors that are oversized for some reason. Reasons can be
extra starting torque using a standard design B motor rather
than a harder-to-obtain and less-efficient design D motor.
However, most of these only do perform reduction to 75% to
90% of full voltage as a lower voltage increases copper
losses and impairs recover torque.
Other reasons can be for worst-case load such a vibratory
finishing using steel shot (which uses more power than
stones or corn-cob meal) or the largest possible die that
can fit into a punch press. When operating at a lower load,
reduced voltage running can make a 20 horsepower motor act
as a 10 horsepower motor if a machine is that “overmotored”
for that much — if the load can span 7 to 20 horsepower for
example than reduced voltage running is feasible if you
already are paying for a VFD or autotransformer starter.
On the other hand, you can’t do that with a *typical*
autotransformer starter. The transformers are rated for
maybe 15 seconds of operation, not continuous. As a
custom-designed system where all factors are considered,
maybe. This needs to be made clear to someone unfamiliar
with how specialized that is might try it with an
off-the-shelf autotransformer starter.
A VFD may be a bit impractical since one for a 4MW 11kV
motor would cost $750,000.00+ USD.
One final question:
Since reducing the voltage from 11 to 6.6 kV will reduce the
torque by three. What about the output shaft power?
Since the torque is one third, and the speed is the same,
the power must also be one third. Power = torque x speed.