Page 579 - Handbook of Electrical Engineering

572 HANDBOOK OF ELECTRICAL ENGINEERING

• MCC terminal voltage in percent.
No-load voltage = pre-disturbance voltage at the MCC.

Loaded voltage = voltage at the MCC at starting or running of the motor.
V lo − V ls
Volt-drop at starting% = × 100%
V lo
0.9936 − 0.7669
= × 100% = 22.82%
0.9936
V lo − V ln
Volt-drop at running% = × 100%
V lo
0.9936 − 0.9570
= × 100% = 3.68%
0.9936
• Generator and SWBD terminal voltage in percent.
No-load voltage = pre-disturbance voltage at the SWBD.
Loaded voltage = voltage at the SWBD at starting or running of the motor.

V go − V gs
Volt-drop at starting% = × 100%
V go
1.0 − 0.8152
= × 100% = 18.48%
1.0
V go − V gn
Volt-drop at running% = × 100%
V go
1.0 − 0.9695
= × 100% = 3.05%
1.0
o) Examine the actual volt-drops

Although the percentage volt-drops are now known, and they give an indication of the seriousness
of the volt-drop by simple inspection, what is important as far as each piece of equipment is
concerned is the actual voltage on its terminals in volts. This is especially important when the
rated voltage of the equipment is different from the nominal operating value as in the above
example. Consider each component.
• The motor.
Rated voltage = 4000.0 volts
Nominal operating system voltage = 4181.8 volts
Starting voltage received = 4181.8 × 0.7627 volts
= 3189.5 volts = 79.74% of the rated value
Running voltage received = 4181.8 × 0.9552 volts
= 3994.4 volts = 99.86% of the rated value
• The motor control centre.
Rated voltage = 4160.0 volts
Nominal operating system voltage = 4181.8 volts