Danfoss VLT Pocket Guide

38 VLT
®
Soft Starter Pocket Guide
Soft starters are suitable for use with slip-ring motors provided that the
motor can still deliver the torque required to accelerate the load. Soft
starters are not suitable if the load requires extremely high start torque,
or if the slip-ring motor is intended to provide speed control. When
considering a soft starter for slip-ring applications, a trial should be
conducted to verify the performance.
To develop starting torque, some resistance must remain in the rotor
circuit during motor starting. This resistance must be bridged out using
a contactor (AC2 rated for rotor current) once the motor is running
close to full speed.
Rotor resistance (R) can be sized using the following formula:
Where V
R
= open circuit rotor voltage
I
R
= full load rotor current
Slip-Ring Motors: Are soft starters suitable for use
with slip-ring motors?

Danfoss VLT Specifications

General

Frequency	50/60 Hz
Enclosure	IP20, IP21, IP55, IP66
Series	VLT
Control Technique	Flux Vector Control
Output Voltage	0-Vinput
Control Method	Fieldbus
Communication Interface	PROFIBUS, CANopen
Protection Features	Overload, Overvoltage, Undervoltage, Short Circuit, Ground Fault
Cooling Method	Liquid Cooled
Storage Temperature	-25°C to +70°C
Relative Humidity	5-95% non-condensing
Altitude	Up to 1000m (3280ft) without derating, up to 4000m (13120ft) with derating
Operating Temperature	-10°C to +50°C (14°F to 122°F)