Electrical Engineering ⇒ Topic : Starting of Slip-ring Induction Motors

Slip-ring Induction Motors Starting of

 Rotor rheostat. The slip-ring induction motors are practically always started with full line voltage applied across the stator terminals. The value of starting current is adjusted by introducing a variable resistance in the rotor circuit The controlling resistance is in the form of rheostat connected in star (Fig.(a)), the resistance being gradually cut out of the rotor circuit

                                                            Fig.(a) Starting of slip-ring induction motor

as the motor gathers speed, By increasing the rotor resistance, not only is the rotor (and hence stator) current reduced at starting but at the same time torque is also increased due to improvement in power factor.

The rheostat is either of stud or contractor type and may be hand operated or automatic.

As discussed earlier, the introduction of additional external resistance in the rotor circuit enables slip-ring motor to develop a high starting torque with reasonably moderate starting current. Hence such in motors can be started under load. When the motor runs under normal conditions the rings are short-circuited and brushes lifted from them.