Motor Overload - Allen-Bradley PowerFlex Reference Manual

Motor Overload

The motor thermal overload uses an IT algorithm to model the temperature
of the motor. The curve is modeled after a Class 10 protection thermal
overload relay that produces a theoretical trip at 600% motor current in ten
(10) seconds and continuously operates at full motor current. Motor
nameplate FLA programming is used to set the overload feature. This
parameter, set in the start up procedure, is adjustable from 0 - 200% of drive
rating and should be set for the actual motor FLA rating.
Setting the correct bit in the Fault Config parameter to zero disables the
motor thermal overload. Since the MTO cannot distinguish individual
currents in a multimotor application, it is suggested that the MTO be
disabled.
The operation of the overload is actually based on three parameters, Motor
Nameplate Full Load Amps, Motor Overload Factor, and Motor Overload
Hz. Motor nameplate full load amps is multiplied by the motor overload
factor to allow the user to move the motor overload protection into the drive
overload area (simulating a higher motor service factor) by defining the
continuous level of current allowed by the MTO.
Motor Overload Hz is used to allow the user to adjust the response of the
MTO to lower motor speeds (lower output frequencies) where a higher
degree of protection may be required due to decreased motor cooling.
100000
10000
1000
100
10
100 125
[Motor OL Hz]
[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.
Motor Overload Curve
150 175 200 225
Full Load Amps (%)
Motor Overload 2-97
Cold
Hot
250