144 Softstarter theory CG Drives & Automation 01-5980-01r2
Typical applications with constant load are elevators, cranes
and conveyors. Linear load characteristics are typical for
calender rollers and various smoothing machines, whereas
quadratic correlation between speed and torque is typical for
pumps and fans. Some applications like conveyors or screws
may need an initial torque boost. However, for many
applications it can be seen that the torque needed is much
lower than the torque delivered by the induction motor in a
DOL start.
A common method to reduce both starting torque and
current is to decrease the motor voltage during starting. The
following figure shows how the motor’s torque and current
characteristics are changed when the supply voltage is
reduced.
Fig. 86 Reduced voltage start
A general rule of thumb is that the torque at each operating
point is roughly proportional to the square of the current.
This means when the motor current is decreased by a factor
of two by means of reducing the supply voltage, the torque
delivered by the motor will be decreased by a factor of four
(approximately).
This relationship is the base for any starting method using
reduced voltage. It can be seen that the possibility of
reducing the starting current depends on the correlation
between the motor’s and the load’s torque characteristic. For
the combination of an application with very low starting
load and a motor with very high starting torque, the starting
current may be reduced significantly by means of decreasing
the voltage during start. However, for applications with high
starting load it may – depending on the actual motor – not
be possible to reduce the starting current at all.
Torque
T/T
n
n/n
s
Current
I/I
n
n/n
s
U
n
U
2
<U
n
U
3
<U
2
U
n
U
2
<U
n
U
3
<U
2
T ~ I
2
I
LV
= 1/2 I
DOL
→ T
LV
≈ 1/4 T
DOL
I
LV
= 1/3 I
DOL
→ T
LV
≈ 1/9 T
DOL
LV=low voltage
DOL=Direct on line
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