Motor Overload - Siemens 6SR41 series Product User Manual

Application and Operation

6.12 Motor Overload

6.12 Motor Overload
NXG Perfect Harmony control provides Motor Thermal Overload (TOL) protection to prevent
the motor from being subjected to excessive temperatures. TOL protection of the motor can
be set up using the NXG control menu system. The "overload select" parameter allows one
of three options to be selected for motor protection. The first model, which is called
"constant," is based on the current flowing into the motor. A Motor Thermal Overload Alarm 1
is issued as a warning to the user (of an impending overload fault) when the motor current
exceeds the "overload pending" parameter. When the drive current exceeds the "overload"
setting, Motor Thermal Overload Alarm 2 is issued and a thermal trip timer is started. If this
condition is present for a period greater than the time set in the "overload timeout"
parameter, the drive will trip and annunciate the event as Motor Thermal Overload Fault. It
should be noted that both the Alarms 1 and 2 have to be enabled through the SOP for the
drive to display these conditions.
The second and third thermal models, which are called "straight inverse time" and "inverse
time with thresholds," use a software motor thermal model to determine motor temperature
(see Figure "Block Diagram of Motor Thermal Model"). For these options, the "overload
pending" and "overload" settings represent the motor temperature limits (in percent of rated
motor temperature) at which the overload warning and trip are generated. A brief description
of the thermal model follows.
Figure 6-8
The motor model estimates motor temperature based on the net heat generated in the motor
and its thermal mass. A block diagram of the implementation is shown in Figure "Block
Diagram of Motor Thermal Model". The heat generated in the motor is estimated from the
stator voltages, currents, and motor parameters, while an estimate of the heat transferred
from the motor (due to motor cooling) is made from the allowable motor current (explained in
the following paragraph). The motor loss calculation also includes the losses generated with
Dual-Frequency Braking. The thermal mass (shown as MTH) of the motor (or its heat
capacity) is determined from the maximum load inertia listed in Table 20-1 of
Standard MG-1 1993 Part 20.42
load inertia as well (which can be obtained from the manufacturer).
If "straight inverse time" protection is chosen, then it is assumed that the motor has an
allowable current level of 100% (for example, when the motor is equipped with a constant-
speed cooling fan). With "inverse time with speed-derating," the allowable current level is
determined from the speed-derating curve entered through the keypad. This curve requires
the user to enter allowable motor load for various speed breakpoints. The default-derating
curve provides breakpoints for a quadratic cooling curve (and is shown in Figure "Default
Speed Derating Curve showing Maximum Motor Load as Function of Speed"). The motor
134
Block Diagram of Motor Thermal Model
. The user has the option of entering a known value of max
Operating Instructions, Version AE 12/2009, A5E01454341C
NEMA
Product User Manual