SeriesImpedance Starting ECE 441 1 Impedance Z is

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

ECE 441 6

Example 5. 20 continued • Apply Ohm’s Law to one branch ECE 441 7

Example 5. 20 continued • b) the stator voltage/phase at locked-rotor 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) ECE 441 9

Example 5. 20 continued • Note – Calculations must be made on the actual voltage applied to the stator. ECE 441 10