FLUID COUPLING FITTED ON ELECTRIC MOTORS
3
TRANSFLUID COUPLING FITTED ON ELECTRIC MOTORS
Three phase synchronous squirrel cage motors are able to supply
maximum torque only near 100% synchronous speed. Direct
starting the system utilized the most current. Figure 1 illustrates the
relationship between torque and current. It can be seen that the
absorbed current is proportional to the torque only between 85%
and 100% of the synchronous speed. With a motor connected
directly to the load, there are the following disadvantages:
– The difference between available torque and the torque required
by the load is very low until the rotor has accelerated to between
80-85% of the synchronous speed
– The absorbed current is high (up to 6 times the nominal current)
throughout the star ting phase causing overheating of the
windings, overloads in the electrical lines and, in cases of
frequent starts, major production costs.
– Oversized motors are required by the limitations indicated above.
To limit the absorbed current of the motor during the acceleration
of the load, a Y-D (wye – delta) starting system is frequently used
which reduces the absorbed current by about 1/3 during starting.
Unfortunately, during operation of the motor under the delta
configuration, the available torque is also reduced by 1/3 and for
machines with high inertias to accelerate, oversizing of the motor is
still required. Finally, this system does not eliminate current peaks
originating from the insertion or the commutation of the device.
Any drive system using a Transfluid fluid coupling has the
advantage of the motor starting without load. Figure 2 compares
the current demands of an electric motor when the load is directly
attached verses the demand when a fluid coupling is mounted
between the motor and load. The colored area shows the energy
that is lost, as heat, during start-up when a fluid coupling is not
used. A Transfluid fluid coupling reduces the motor’s current draw
during start-up thus reducing peak current demands. This not only
reduces power costs but also reduces brown outs in the power grid
and extends the life of the motor. Also at start-up, a fluid coupling
allows more torque to pass to the load for acceleration than in drive
systems without a fluid coupling.
Figure 3 shows two curves for a single fluid coupling and a
characteristic curve of an electric motor. It is obvious from the stall
curve of the fluid coupling (s=100%) and the available motor
torque, how much torque is available to accelerate the rotor of the
motor (colored area). In about 1 second, the rotor of the motor
accelerates passing from point A to point B. The acceleration of the
load, however, is made gradually by the fluid coupling, utilizing the
motor in optimal conditions, along the part of the curve between
point B, 100% and point C, 2-5%. Point C is the typical point of
operation during normal running.
Fig. 1
Fig. 2
Fig. 3
% motor speed
% start-up time
% motor current
% torque
% motor speed
without fluid coupling
with fluid coupling
Summary of Contents for CCKSD-QD
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