32
Percent voltage unbalance = 100 x
NEMA states that poly-phase motors shall operate successfully under running conditions at rated load when voltage unbalance at
the motor terminals does not exceed 1%. Furthermore, operation of a motor with over 5% unbalance is not recommended for it
probably results in motor damage.
Unbalanced voltages at motor terminals cause phase current unbalance ranging from 6 to 10 times the percent of voltage unbalance
for a fully loaded motor. This causes motor over current resulting in excessive heat that shortens motor life, and hence, eventual
motor burnout. If the voltage unbalance is great enough, the reduced torque capability might not be adequate for the application
and the motor will not attain rated speed.
Some of the more common causes of unbalance voltages are:
●
Unbalanced incoming utility supply
●
Open delta connected transformer banks
●
Large single phase distribution transformer in the system
●
Open phase on the primary 3-phase transformer in the distribution system
●
Blow fuse on 3 phase bank of power factor improvement capacitors
●
Unequal impedance in conductors of power supply wiring
●
Unbalanced distribution of single phase loads such as lighting
●
Unequal transformer tap settings
●
Faults or grounds in power transformer
●
Heavy reactive single phase loads such as welders
Summary of Contents for ExP Series
Page 1: ...1 ...
Page 6: ...6 1 4 ATEX code description II 2 G c IIB T4 II 2 G Ex d IIB T4 Gb ...
Page 8: ...8 RC2 180 ExP A B RC2 200 ExP A ...
Page 9: ...9 RC2 200 ExP B RC2 230 ExP A B UNIT SI mm Imperial in ...
Page 10: ...10 RC2 260 ExP A B SI mm Imperial in UNIT RC2 300 ExP 320 ExP A ...
Page 11: ...11 RC2 300 ExP 320 ExP B RC2 310 ExP A B ...
Page 12: ...12 RC2 340 ExP 370 ExP A RC2 340 ExP 370 ExP B ...
Page 13: ...13 RC2 410 ExP A B RC2 470 ExP A ...
Page 14: ...14 RC2 470 ExP B RC2 510 ExP A B ...
Page 15: ...15 RC2 550 ExP A RC2 550 ExP B ...
Page 16: ...16 RC2 580 ExP A B RC2 620 ExP A B ...
Page 17: ...17 RC2 710 ExP A RC2 710 ExP B ...
Page 18: ...18 RC2 790 ExP A RC2 790 ExP B SI mm Imperial in UNIT ...
Page 19: ...19 RC2 830 ExP A RC2 830 ExP B ...