Lenze MC3000 Series - Manual (Page 54)

50 13435743_EDBM301_v14 .0
Example 2: A 480 Vac, 20 HP drive is operating a motor at 10 Hz . From the diagram, a
drive operating at 10 Hz can deliver about 75% of its output current rating
continuously . A 480 Vac, 20 HP drive’s output current rating is 27 Amps .
Therefore, the drive would be able to operate continuously at 20 Amps . The
drive would also be able to deliver 150% of that value (30 Amps) for one
minute before tripping into an OVERLOAD fault .
The “speed compensated” thermal overload is the factory default and should be used
in applications where the motor does not normally experience high loads at low speeds
for extended periods of time .
NOTE 1:
The above diagram is based on a MOTOR OL setting of 100% . For
lower MOTOR OL settings, reduce the % CURRENT values by the same
percentage . For example, if MOTOR OL is set to 75%, reduce the %
CURRENT values by 25% . Therefore, the curve shifts down, but the shape
of the curve remains the same .
The “non-compensated” thermal overload circuit allows 100% current continuously, and
150% current for one minute, at all speeds . In the example above, the motor operating at
10 Hz without “speed-compensated” protection would be allowed to operate continuously
at 27 Amps, and could draw 40 .5 Amps for one minute before tripping . Without sufficient
motor cooling, this can result in motor failure due to overheating .
The “non-compensated” circuit is selected by setting Parameter 22 - TORQUE to CT/
NOCMP . The “non-compensated” setting should only be used in applications where the
motor is properly cooled at all speeds, or the motor manufacturer has approved the motor
for full-load operation at low speeds .
NOTE 2:
The operation of the motor thermal overload circuit is affected by the setting
of Parameter 34 - LOAD MLT .
18 BASE (BASE FREQUENCY)
The BASE FREQUENCY determines the V/Hz ratio by setting the frequency at which
the drive will output full voltage to the motor . For most applications the base frequency
should be set to match the motor’s rated frequency .
For example, if the drive is rated for 460 Vac output, and the BASE FREQUENCY is set
to 60 Hz, the drive will maintain a constant ratio of 7 .66 V/Hz (except when FX BOOST
is active, see Parameter 19) from 0 Hz to 60 Hz . This range is the region of constant
torque . If the motor speed is increased past 60 Hz, the output voltage remains constant
while the frequency increases, resulting in a reduced V/Hz ratio . This range, from 60
Hz to about 90 Hz, is the region of constant horsepower . Above 90 Hz, horsepower
begins to decrease as frequency increases . Refer to Section 6 .1 - DESCRIPTION OF
AC MOTOR OPERATION .
19
FX BOOST (FIXED BOOST)
This parameter is used in applications which require high starting torque . FX BOOST
increases the output voltage at lower output frequencies (below 30 Hz for 60 Hz base
frequency), in order to boost the torque capability of the motor . Refer to the diagram
below . The factory default for FX BOOST depends on the horsepower rating of the drive .
Refer to the following table: