KB Electronics KBDF-48 - Manual (Page 8)

8
3 IMPORTANT APPLICATION INFORMATION
3.1 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. See Figure 2.
Note: Some fan-cooled motors can be used
over a wider speed range. Consult the motor
manufacturer for details.
Inverter duty and most Totally Enclosed Non-Ventilated (TENV) motors can provide full rated
torque over an extended speed range without overheating.
It is recommended that the drive be used with Inverter Duty or TENV motors.
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. If the
motor current exceeds the nameplate rating, the Boost value may have to be decreased (see
Function 3.14). Do not use motor if the motor current exceeds the nameplate rating.
3.2 ELECTRONIC MOTOR OVERLOAD PROTECTION
The drive contains Modified I
2 t Overload Protection (UL approved as an overload protector for
motors). Part of this function consists of a Current Limit (CL) circuit, which limits the drive current
to a preset level of 160% of the Motor Nameplate Rated Current setting. The factory setting for
motor nameplate current is the drive rated current, which must be set to the actual motor
nameplate current (see Important Programming Information (Section 2.2), on page 7). See
Table 3, on page 15. See Function 0.01, on page 55.
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 the Motor Nameplate Rated Current, 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.
FIGURE 2
MAXIMUM ALLOWED
MOTOR TORQUE VS. SPEED