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  0.27369
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