Medium Voltage Application Guide

SOFT STARTERS
Example
Calculate the acceleration time of a 2000 kW motor driving a pump load from 40% to 50% full speed. The full
load speed is 2990 rpm. Average acceleration torque from 40% to 50% full speed is 20% motor FLT. The motor
shaft inertia is 60 kg·m 2 and the pump inertia is 12 kg·m 2 .
Total inertia is motor plus load inertia:
J T = 60 + 12kgm 2
= 72 kgm 2
Speed difference is 10% of full load speed:
n = n2 n1 × full load speed
= 50% 40% × 2990 rpm
= 299 rpm
Average acceleration torque is 20% of motor FLT.
kW × 9550
FLT =
n
2000 × 9550
=
2990
= 6388 Nm
T av = 6388×
20
100
= 1278 Nm
Calculate the acceleration time from 40% to 50% full load speed:
JT
× n
t acel =
9.55 × T
av
72kgm
=
2 × 299 rpm
9.55 × 1278 Nm
21528
=
12205
= 1.76 sec
Page 58 Medium Voltage Application Guide 710-12280-00A