Handbook of Electrical Installation Practice - BeKnowledge

398 Handbook of Electrical Installation Practice
Table 15.2 Nominal starting currents and torques.
Soft starters are becoming a common method of starting induction motors. They
are thyristor-controlled a.c. voltage regulators which ramp up the voltage applied
to the motor during starting. The ramp rate or starting time can be adjusted while
at the same time a current-limiting circuit limits the maximum current drawn by the
motor during starting.
As with other forms of reduced-voltage starting the starting current is restricted
at the expense of staring torque. Soft starters therefore are particularly useful for
motors driving centrifugal loads and, because of their infinitely variable voltage
range between zero and system voltage, for minimising the starting current depending
on the motor speed torque characteristic and load requirements.
Slip-ring induction motors
Much less widely used are slip-ring induction motors where the cage rotor is
replaced by a distributed wound rotor, the three-phase rotor winding being connected
to slip rings mounted on the shaft. Except during starting the slip rings are
short-circuited and the rotor behaves exactly as a cage rotor. The advantage of the
wound rotor slip-ring machine lies in the possibility of inserting external rotor
circuit resistances during the starting period.
The rotor control gear can be arranged to be switched sequentially, automatically
reducing the values of resistance in the rotor circuit as the motor runs up to speed.
A family of speed/torque characteristics, one for each resistance value, is generated
as shown in Fig. 15.3. Note that changing the value of externally connected rotor
resistance does not change the value of maximum torque – only the speed or slip
at which it occurs.
The number of resistance steps can be chosen appropriate to the starting
torque requirements and starting current limitations. The external resistors usually
take the form of metallic grids and if appropriately rated, allow the motor to
run for long periods at reduced speeds. Very large slip-ring motors may still from
time to time employ liquid rotor resistors for starting. These are better able to
dissipate the heat generated, and the speed variation during start up is stepless.
The high losses appearing as heat in the rotor circuit resistances prevent this
system being more widely used for economic continuous reduced or variable speed
running.