Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 21)

Asynchronous motors behave electrically like transformers. The secondary winding is the rotor
and the mechanical power output of the motor acts on the primary side like a – variable – load
resistance. If no mechanical power output is produced at rest (on initiation of start-up), this load
resistance is zero, i.e. the transformer is in effect secondarily shorted. This leads – depending
on the rotor-internal resistance – to a high or very high current consumption of the motor during
starting. In the case of slip-ring motors, the current consumption is reduced by connecting
external resistors and hence the torque characteristic is adapted to the driven machine. With
squirrel-cage induction motors (see section
1.7.1.2 ) the current consumption and hence the
torque characteristic are influenced by the design of the rotor cage.
1.7.1.1 Slip-ring motors
With slip-ring motors, the rotor winding is connected to slip rings and terminated with external
resistances. The resistance of the external resistors influences the current flowing through the
rotor and the speed-torque characteristic.
M
3~
Fig. 1.7-3
Principal diagram of a slip-ring motor with external rotor resistances
Slip-ring motors represent the conventional method of controlling starting torque (and the
current consumption) by selection of the rotor resistances. The highest attainable starting torque
corresponds to the breakdown torque of the motor. This is independent of the magnitude of the
rotor resistance. The primary current consumption of slip-ring motors is proportional to the rotor
current. Thanks to these characteristics, slip-ring motors can achieve a high starting torque with
relatively low current consumption.
The external resistors are usually changed in steps during motor startup. The rotor windings are
shorted in normal continuous duty. By designing the rotor resistances for continuous duty, it is
even possible to continuously influence the speed, albeit at the cost of high heat dissipation.
LVSAM-WP001A-EN-P - April 2009 1-11