Medium Voltage Application Guide

09678.B
Current
Torque (% motor full load torque)
09677.B
Current
Torque (% motor full load torque)
SOFT STARTERS
3.6 Calculations
What is the minimum start current with a soft starter?
Soft starters can limit start current to any desired level. However, the minimum level of start current for a successful
start depends on the motor and load.
To start successfully, the motor must produce more acceleration torque than the load requires, throughout the start.
Reducing the start current also reduces the torque produced by the motor. The start current can only be lowered
to the point where the torque output remains just greater than the load torque requirement.
The likely start current can be estimated from experience, but more precise predictions require analysis of motor
and load speed/torque curves.
Successful soft start
700
600
500
400
300
200
100
1
3
4
2
5
6
200
100
Full voltage start current
Current limit
Full voltage start torque
Torque output at current limit
Acceleration torque
Load torque
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Speed (%full speed)
Unsuccessful soft start
700
600
500
400
300
200
100
1
2
3
5
4
7
6
200
100
10% 20% 30% 40% 50% 60% 70% 80% 90% 100%
Speed (%full speed)
Full voltage start current
Current limit
Full voltage start torque
Torque output at current limit
Acceleration torque
Load torque
Stall
Calculating required start current for new or existing AC induction motor installations
A number of methods are available to estimate the level of start current a particular machine will require. These
methods range from generalisations producing approximations, through to advanced calculations which yield precise
predictions.
Typical Start Current Estimate
Where motor and load start characteristics are unknown and an estimate of typical start current is sufficiently
accurate, basic application information can allow experienced personnel to estimate the typical start current.
Required information:
Motor size (kW / HP)
Machine type and class (eg Compressor - Screw, Pump - Centrifugal)
Machine starting condition (eg Conveyor - Unloaded Start)
Page 54 Medium Voltage Application Guide 710-12280-00A