Squirrel cage induction motors starting

This is a vital relationship, which reveals that during start-up and until such speed, the reactance of the motor windings / 2> the rotor current will also remain almost the same as the starting current and will fall only at near the rated speed. (Refer to the current curves in Figures 1.5(a) and (b)). The initial inrush current in a squirrel cage induction motor is very high. In a slip-ring motor, however, it can be controlled to a desired level. (Refer to Section 5.2.1.)... 

The most recent development in the starting of squirrel-cage induction motors is the introduction of the electronic soft-start. This principle has been derived from variable-frequency speed controllers using switched Thyristor or power transistor bridges. The supply sine wave is chopped so that a reduced voltage and frequency is applied to the motor.These are gradually increased so that the motor speed rises in a controlled manner, with the starting current limited to any chosen value. 

When the motor first starts there is a sudden surge of current into the stator winding (inrush current). This is usually 6 to 6.5 times the amps the motor sees when it is running. This causes similar surge in the squirrel cage rotor hars, so great care must he taken to size all the hars, WMC uses copper hars, correctly. This magnetic circuit in the rotor induces a current thus, the name induction motors. 

Repulsion-Start Induction Motor. A repulsion-start induction motor is a single-pliasc motor having the same windings as a repulsion motor, but at a predetermined speed the rotor winding is short circuited or otherwise connected to give the equivalent of a squirrel-cage winding. This type of motor starts as a repulsion motor but operates as an induction motor w ith constant-speed characteristics. 

For starting, an auxiliary motor of another type may be used, or the synchronous motor may have a squirrel-cage winding in the pole face, so that it starts as an induction motor. 

These synchronous motors usually have pole-face squirrel-cage-type windings for both starting and damping. The motor is started as an induction motor with the dc field shorted, and at a predetermined speed the field is excited and the motor pulls into synchronism. 

Synchronous Motor Starting. This method is used for power-factor correction of heavy concentrations of induction motors. It is also used for constant-speed, slow-speed industrial drive applications and for maximum efficiency on continuous heavy loads in excess of 75 hp (55 kW). Three-phase ac power is connected to the stator and dc to the rotor (which has both a field and a squirrel-cage winding). 

---