Figure 6 - Typical Torque/Speed Curves for Motor at Reduced Voltages
Torque of the motor in the case of
a direct-on-line starting
@Um (0) = 100% Ue
b starting with 3RW34
@Um (0) = 75% Ue
c starting with 3RW34
@Um (0) = 50% Ue
Relationship of voltage and
speed with respect to time
in the case of soft - starting.
Showing the effect of the
potentiometers Um and tr
located on the front panel of
the devices.
Siemens Energy & Automation, Inc.
8
Figure 5 - Wye and Delta Motor Connections
2.3.3 Controller-to-Motor Connections
Wye Motor. The controller can be used for either a three- lead
or nine-lead wye motor. Connecting the controller to a wye
motor inserts the SCR’s directly in the line wiring, referred to as
“In Line” wiring.
Delta Motor. The controller can be used for either 6 or 12 lead
delta motors. If the motor is hard wired as delta, the controller
must be connected and sized with “In Line” wiring as shown in
figure 5a.
Figure 5b shows the controller connected with the thyristors
inside the delta, referred to as “Inside Delta” wiring. For Inside
Delta wiring, the controller power rating may be increased (line
current = 1.73 phase current, figure 4) relative to the In Line
power rating.
2.4 AC Motor Starting and Stopping
Figure 6 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 accelera-
tion of the motor and its load.
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