15
If external fan cooling is provided, open ventilated motors can also achieve an
extended speed range at full rated torque. A box fan or blower with a minimum of
100 CFM per HP is recommended. Mount the fan or blower so the motor is
surrounded by the airflow. See Figure 4.
FIGURE 4
OPEN VENTILATED MOTOR WITH EXTERNAL FAN COOLING
4.4 ELECTRONIC MOTOR OVERLOAD PROTECTION
The drive contains Modified I
2
t Overload Protection (UL approved as an oveload
protector for motors). Part of this function consists of a Current Limit (CL) circuit,
which limits the drive current to a factory preset level of 160% of the rated drive
current. The CL Trimpot is used to recalibrate the drive current from 60% thru 190%.
The Power Start™ circuit provides an overshoot function that allows most motors to
develop more than 200% of starting torque and breakdown torque.
Standard I
2
t is undesirable because it causes nuisance tripping. It allows a very high
motor current to develop and will turn the drive off after a very short period of time.
KB's RMS Current Limit Circuit avoids this nuisance tripping while providing maximum
motor protection.
If the motor is overloaded to 120% of full load (75% of the CL setting), the I
2
t Timer
starts. If the motor continues to be overloaded at the 120% level, the timer will shut
down the drive after 30 minutes. If the motor is overloaded to 160% of full load, the
drive will trip in 6 seconds.
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