Series-Impedance Starting ECE 441 1 Impedance Z is either an Inductor or a Resistor to limit the current during start-up ECE 441 2 Series-Impedance Starting continued • Select the resistor or inductor (reactor) to provide about 70% rated voltage. • Simplest method for starting ECE 441 3 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 441 4 Example 5.20 continued • The circuit configuration is shown here ECE 441 5 Z Rex Z lr Rex 0.27369 Z Rex 0.0978 j 0.2549 Vline to neutral 230 132.79V 3 I lr 3 78 234 A ECE 441 6 Example 5.20 continued • Apply Ohm’s Law to one branch Vbranch 132.79 I lr 234 I branch ( Rex 0.0978) 2 (0.2549) 2 ... Rex 0.4903 ECE 441 7 Example 5.20 continued • b) the stator voltage/phase at locked-rotor VT 1 N Ilr Zlr 234 0.273 63.88V ECE 441 8 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) Tlr 1.5Trated ECE 441 9 Example 5.20 continued • Note – Calculations must be made on the actual voltage applied to the stator. 2 1.5Trated 132.79 T63.88 63.88 ... T63.88 0.347Trated 34%Trated ECE 441 10