Rockwell Automation 20B PowerFlex 70, PowerFlex 700 Reference Manual - Manual (Page 118)

Motor Overload
118 Rockwell Automation Publication PFLEX-RM001H-EN-P - June 2013
3. [Motor OL Hertz] is used to further protect motors with limited speed
ranges. Since some motors may not have sufficient cooling ability at lower
speeds, the Overload feature can be programmed to increase protection in the
lower speed areas. This parameter defines the frequency where derating the
motor overload capacity should begin. As shown here, the motor overload
capacity is reduced when operating below the motor overload Hz. For all
settings of overload Hz other than zero, the overload capacity is reduced to
70% when output frequency is zero. During DC injection the motor current
may exceed 70% of FLA, but this will cause the Motor Thermal Overload to
trip sooner than when operating at base speed. At low frequencies, the
limiting factor may be the Drive Thermal Overload.
Duty Cycle for the Motor Thermal Overload
When the motor is cold motor thermal overload will allow 3 minutes at 150%.
When the motor is hot motor thermal overload will allow 1 minute at 150%. A
continuous load of 102% will not trip. The duty cycle of the motor thermal
overload is defined as follows. If operating continuous at 100% FLA, and the load
increases to 150% FLA for 59 seconds and then returns to 100%FLA, the load
must remain at 100% FLA for 20 minutes to reach steady state.
Changing Overload Factor
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80 90 100
% of Base Speed
Contin
uous Rating OL % = 1.20
OL % = 1.00
OL % = 0.80
Changing Overload Hz
20
40
60
80
100
120
0 10 20 30 40 50 60 70 80 90 100
% of Base Speed
Contin
uous Rating
OL Hz = 10
OL Hz = 25
OL Hz = 50