Medium Voltage Application Guide

Assessed Start Current Estimate
SOFT STARTERS
If information is available about the intended motor's locked rotor current (LRC) and locked rotor torque (LRT), the
motor's start performance can also be factored into the assessment. Even better estimates are obtained where
information on the machine start torque requirement are also known.
Required information:
Motor size (kW / HP)
Motor locked rotor current (LRC)
Motor locked rotor torque (LRT)
Machine starting torque (% FLT)
Calculation
Calculations use percentages of full load torque and full load current.
Minimum required start current
I
STR
= LRC ×
TSTR
LRT
Where:
I STR = minimum required start current (% motor FLC)
LRC = Motor locked rotor current (% motor FLC)
T STR = Minimum required start torque to accelerate machine from
standstill (% motor FLT)
LRT = Motor locked rotor torque (% motor FLT)
Example
A 1100 kW / 3.3 kV motor has a full load current of 235 A and a locked rotor current of 500% FLC. The motor
is required to start a pump with a minimum start current of 15% motor FLT. The motor's locked rotor torque is
150% FLT.
I STR = 500% FLC ×
= 500 × 0.316
= 158% FLC
158
= ×
100 235 A
= 371 A
15%FLT
150%FLT
Start Current and Torque Curve Analysis
To accurately calculate an application's start current requirements, torque and current curves for both the motor
and the load are required. Information from these curves is used to assess the minimum start current requirements.
With the following information, specific application software can accurately estimate the minimum required start
current (I STR ) and start time (t STR ):
Motor datasheet (including kW, full load speed and motor shaft inertia)
Motor speed/current curve
Motor speed/torque curve
Load speed/torque curve
Load inertia
710-12280-00A Medium Voltage Application Guide Page 55