Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 22)

0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0.0 0.2 0.4 0.6 0.8 1.0
n /n
s
T
/T
e
T
e
≈ T
L
T
av-acc
T
4
T
3
T
2
T
1
T
0
Fig. 1.7-4
Torque characteristic of a slip-ring motor with full-load start-up and stepped change of the rotor resistance
during start-up
T
4
… T
1
motor torque with series-connected resistance stages (R
4
>R
3
>R
2
>R
1
)
T
0
motor torque with shorted rotor windings
T
av-acc
average starting torque
T
e
≈ T
L
rated torque corresponds to load torque
1.7.1.2 Squirrel-cage induction motors
In the case of asynchronous machines with squirrel-cage induction rotors, the rotor consists of a
grooved cylindrical laminated rotor package with rods of highly conductive metal (preferably
aluminum), that is joined on the face side by rings to form a closed cage. The cage – at least in
the case of small motors – is usually cast into the rotor.
To reduce the starting current and influence the starting torque characteristic, the coil rods are
specially designed so that they create a high rotor resistance at rest and at low speeds by
current displacement. They are usually placed crosswise at an angle to the axis of rotation to
avoid variations in torque and to ensure smooth running characteristics.
Fig. 1.7-5 shows the typical characteristic of the torque and of the current in a cage induction
motor in the speed range from rest to synchronous speed. Material and design form of the cage
influence the shape of the characteristic curves.
0.00
1.00
2.00
3.00
4.00
5.00
6.00
I/
I e
0 20 40 60 80 100
n/n
s
[%]
0.00
0.50
1.00
1.50
2.00
2.50
T
/T
e
I Δ
T Δ
T
L
Fig. 1.7-5
Typical current and torque characteristic of a squirrel-cage induction motor between rest and synchro-
nous speed.
I
Δ
current characteristic with delta-connected windings
T
Δ
torque characteristic with delta-connected windings
T
L
load torque (example)
LVSAM-WP001A-EN-P - April 2009 1-12