KB Electronics KBMA-24DF - Manual (Page 13)

4 IMPORTANT APPLICATION INFORMATION
4.1 MOTOR WITH EXTERNAL FAN
COOLING – Most totally enclosed fan-
cooled (TEFC) and open ventilated 3-phase
AC induction motors will overheat if used
beyond a limited speed range at full torque.
Therefore, it is necessary to reduce motor
load as speed is decreased.
Note: Some fan-cooled motors can be
used over a wider speed range. Consult
the motor manufacturer for details.
WARNING! Some motors have low
speed characteristics which cause
overheating and winding failure under light
load or no load conditions. If the motor is
operated in this manner for an extended
period of time, it is recommended that the
unloaded motor current be checked from
2 – 15 Hz (60 – 450 RPM) to ensure motor
current does not exceed the nameplate
rating. Do not use motor if the motor
current exceeds the nameplate rating.
It is recommended that the drive
be used with Inverter Duty or
TENV motors.
Inverter duty and most totally enclosed non-ventilated (TENV) motors can provide full rated torque over an
extended speed range without overheating. See Figure 5.
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 6.
4.2 ELECTRONIC MOTOR OVERLOAD PROTECTION – The drive contains Modified I 2 t Overload Protection.*
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 200%. 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 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 con-
tinues 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. See Section 11.6, on pages 22 and 23.
*UL approved as an overload protector for motors. 13
FIGURE 5 – MAXIMUM ALLOWED MOTOR TORQUE VS . SPEED
FIGURE 6 – OPEN VENTILATED MOTOR WITH EXTERNAL FAN COOLING
Motors
and TENV
Inverter Duty
Maximum
Allo
wed
Motor
To
rque (%)
TEFC and Open Ventilated
Motors
Fan Cooled
100 60 70 80 50 30 40 10 0 20 90
Motor Speed (%)
40
0
20
60
80
100
Open Ventilated Motor
Fan or Blower
(100 CFM Min.
per HP)
Airflow