Rockwell Automation 150 MNL/SMC DIALOG+/APP-PROD GUIDE - Manual (Page 154)

Chapter
8
Reduced Voltage Starting
Introduction to Reduced
Voltage Starting There are two primary reasons for using reduced voltage when
starting a motor:
• Limit line disturbances
• Reduce excessive torque to the driven equipment
The reasons for avoiding these problems will not be described.
However, different methods of reduced voltage starting of motors will
be explored.
When starting a motor at full voltage, the current drawn from the
power line is typically 600% of normal full load current. This high
current flows until the motor is almost up to speed and then decreases,
as shown in Figure 8.1. This could cause line voltage dips and
brown-outs.
Figure 8.1 Full Load Current vs. Speed
In addition to high starting currents, the motor also produces starting
torques that are higher than full load torque. The magnitude of th
starting torque depends on the motor design. NEMA publishes
standards for torques and currents for motor manufacturers to follow.
Typically, a NEMA Design B motor will have a locked rotor or
starting torque in the area of 180% of full load torque.
In many applications, this starting torque can cause excessive
mechanical damage such as belt, chain, or coupling breakage.
600
500
400
300
200
100
0 % Speed 100
%
Full
Load
Curren