PART I - Electric Motors, Drives and Energy Saving
Chapter 4. Starting of Squirrel Cage Induction Motors
• Direct on-line starting (DOL)
• Reduced voltage starting
Direct on-line starting (DOL)
This is an ideal type of starting, simple and economical. In this type the full voltage is applied across the stator windings. The torque developed by the motor is maximum. The acceleration is fast and the heat of starting is low (Figure 4.1(a)). For heavy rotating masses, with large moments of inertia, this is an ideal switching method. The only limitation is the initial heavy inrush current, which may cause severe voltage disturbances to nearby feeders (due to a large Ist · Z drop). With this in mind, even local electricity authorities sometimes restrict the use of DOL starting beyond a certain rating, say, 10 h.p., for small installations. For large installations, this condition may be of little significance as, in most cases, the power from the electricity authorities may be available on MV 3.3–33 kV. When it is so, a transformer is provided at the receiving end for the distribution of power on the LV side thus eliminating the cause of a line disturbance due to a voltage dip. On the MV side, the effect of a voltage dip, caused by small motors, is of little significance. However, the method of DOL switching for larger motors is recommended only where the supply source has enough capacity to feed the starting kVA of the motor, with a voltage dip of not more than 5% on the LV side.