ECE 4412 Impedance Z is either an Inductor or a Resistor to limit the current during start-up
ECE 4413 Series-Impedance Starting continued Select the resistor or inductor (reactor) to provide about 70% rated voltage. Simplest method for starting
ECE 4414 Example 5.20 A 30-hp, wye-connected, three-phase, 230-V, 60-Hz, 78-A, 1748 r/min, design B motor has a locked-rotor impedance per phase of 0.273 /69°Ω at rated temperature and frequency. The motor is to be started using series resistors in each line. Determine a) the resistance of the resistors required to limit the locked-rotor current to three times rated current
ECE 4415 Example 5.20 continued The circuit configuration is shown here
7 Example 5.20 continued Apply Ohm’s Law to one branch
ECE 4418 Example 5.20 continued b) the stator voltage/phase at locked-rotor
ECE 4419 Example 5.20 continued c) The expected minimum locked-rotor torque at rated when starting as a percent of rated torque. –Minimum locked-rotor torque at rated voltage and frequency is 150% rated torque (Table 5.1)
ECE 44110 Example 5.20 continued Note – Calculations must be made on the actual voltage applied to the stator.