CG Drives & Automation 01-5980-01r2 Softstarter theory 147
10.3 Other starting methods
In contrast to the preceding sections of this chapter, which
focused on squirrel-cage motors, slip-ring motors are dealt
with later on. A slip-ring motor is equipped with a wound
rotor; one end of each rotor winding is available for external
connection via slip-rings. These motors are often optimised
for rotor resistance starting, i.e. with short-circuited rotor
windings they develop a very low torque at an extremely
high current. For starting, external resistances are connected
to the rotor windings. During the start, the resistance value
is decreased in several steps until the rotor windings are
short-circuited at nominal speed. The following figure
shows typical torque and current characteristics for a slip-
ring motor during the start with an external rotor-resistance
starter.
Fig. 91 Rotor-resistance starting
Because of the low starting torque it is often not possible to
short-circuit the rotor windings and replace the rotor-
resistance starter with a softstarter. However, it is always
possible to use a frequency inverter instead. The following
illustration shows how the torque and current characteristics
are affected when the stator frequency is changed.
Fig. 92 Voltage/frequency regulation
Thus, such a motor can be started with a quite simple
frequency inverter with voltage-frequency regulation. This
solution is also valid for all other applications, which for
some reason (high load torque compared to motor torque
etc.) cannot be started by a softstarter.
I/I
n
n/n
s
Torque
T/T
n
n/n
s
Current
I/I
n
n/n
s
Torque
T/T
n
n/n
s
Current
f
3
<f
2
f
2
<f
n
f
n
f
3
<f
2
f
2
<f
n
f
n
To Next Page
Loading...
Need help?
General
