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2.3.1 Controller-to-Motor Connections
Wye Motor. The controller can be used for all wye motors.
Connecting the controller to a wye motor inserts the thyristors
directly in the line wiring, referred to as "In Line" wiring.
Delta Motor. The controller can be used for all Delta motors.
The motor is hard wired as delta and the controller must be con-
nected and sized with "In Line" wiring as shown in figure 3.
Figure 3 - Wye and Delta Motor Connections
2.4 AC Motor Starting and Stopping
Figure 4 shows three torque/speed curves (a, b, c) for a typical
induction motor.
a This curve shows the torque/speed relation when the motor
starting voltage, U, is 100% of line voltage (Ue).
b This curve shows the relation when the controller voltage
potentiometer, U (initial soft start voltage setting) is set for
75% of line voltage.
c
This curve shows the relation when the controller voltage
potentiometer is set for 50% of line voltage.
The curve a motor (without soft starting) produces a very high
torque across most of the speed range, whereas, the curves b
and c motors (with soft starting) produce a much lower and
adjustable torque. This allows slower and smoother acceleration
of the motor and its load.
Figure 4 - Typical Torque/Speed Curves for Motor at Reduced Voltages
Siemens Energy & Automation, Inc.
Torque of the motor in the
case of
a
direct-on-line starting
@Um (0) = 100% Ue
b
starting with 3RW35
@Um (0) = 75% Ue
c
startingwith 3RW35
@Um (0) = 50% Ue
2.4.1 Soft Start with Coast to Stop
Figure 5 shows the relationship of voltage and speed with
respect to time when a soft start is used with coast to rest. The
controller potentiometers have been set as follows.
U The initial voltage is set at approximately 30%.
T1 The start time setting is greater than 0.
T2 The stop time is set at 0 which allows the motor to coast to
a stop.
On the voltage/time graph, the voltage starts at U when the run
coil is energized and increases to 100% within the T1 time set-
ting. The voltage immediately drops to zero when the run coil is
de-energized.
The "speed/time" graph shows the motor accelerating from 0,
when the run coil is energized, to operating speed. The time
required to accelerate may be more or less than the T1 setting
depending on the connected inertia. The motor speed coasts to
zero when the motor is de-energized.
Figure 5 - Voltage, Speed, & Coil Input Curves
for Soft Start with Coast to Stop.
7
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