Siemens bt300 Operator's Manual page 75

overload. This commonly happens at low frequencies. At low frequencies, the cooling effect of the motor is reduced
as well as its capacity. If the motor is equipped with an external fan, the load reduction at low speeds is small.
The motor thermal protection is based on a calculated model and uses the output current of the drive to determine
the load on the motor. The motor thermal protection can be adjusted with parameters. The thermal current IT
specifies the load current above which the motor is overloaded. This current limit is a function of the output
frequency.
The thermal stage of the motor can be monitored on the keypad display.
Motor thermal zero speed cooling (P3.9.8)
Defines the cooling factor at zero speed in relation to the point where the motor is running at nominal speed without
external cooling. See the Table
The default value is set assuming that there is no external fan cooling the motor. If an external fan is used this
parameter can be set to 90% (or even higher).
If you change Motor nominal current (P3.1.1.4), this parameter is automatically restored to the default value.
Setting this parameter does not affect the maximum output current of the drive which is determined by Current Limit
(P3.1.1.7) alone.
The corner frequency for the thermal protection is 70% of the Motor nominal frequency (P3.1.1.2).
Motor thermal time constant (P3.9.9)
The time constant is the time within which the calculated thermal stage has reached 63% of its final
value. The bigger the frame and/or the slower the speed of the motor, the longer the time constant.
The motor thermal time is specific to the motor design and it varies between different motor
manufacturers. The default value of the parameter varies from size to size.
If the motor's t6-time (t6 is the time in seconds the motor can safely operate at six times the rated current) is known
(given by the motor manufacturer), the time constant parameter can be set based on it. As a rule of thumb, the
motor thermal time constant in minutes is equal to 2*t6. If the drive is in stop stage, the time constant is internally
increased to three times the set parameter value. The cooling in stop stage is based on convection and the time
constant is increased. See Figure
Siemens Industry, Inc.
Building Technologies
NOTE:
The calculated model does not protect the motor if the airflow to the motor is reduced
by a blocked air intake grill. The model starts from zero if the control board is powered
off
Protections Settings
in the
Figure 18: Motor thermal current IT curve.
Motor Temperature Calculation
Siemens BT300 HVAC Drive
Chapter 4 - Parameters and Menu Structure
Protections (M3.9)
section.
.
Parameters (M3)
75 | 180
DPD01809
2016-06-07