Lenze MC3000 Series - Manual (Page 53)

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17 MOTOR OL (MOTOR OVERLOAD)
The MC3000 Series is UL approved for solid state motor overload protection . Therefore,
a separate thermal overload relay is not required for single motor applications . The
MOTOR OVERLOAD circuit is used to protect the motor from overheating due to
excessive current draw .
The trip time for the MOTOR OVERLOAD is based on an “inverse I
2 t” function . This
function allows the drive to deliver 150% of the rated output current for one minute, and
even higher current levels for shorter periods of time . Once the overload circuit “times
out”, the drive will trip into an OVERLOAD fault .
The MOTOR OVERLOAD should be set to a value which is equal to the ratio (in
percentage) of the motor full load current rating to the drive output current rating . This
will result in an overload capacity of 150% of the MOTOR current rating for one minute .
If this parameter is set to 100%, the motor will be allowed to draw 150% of the DRIVE
output current rating for one minute . This distinction is important in cases where the motor
full load current rating is significantly less than the drive output current rating, such as
applications where the drive is oversized to meet torque requirements .
Example 1: A 5 Hp, 480 Vac drive is operating a 3 HP motor with a full load current
rating of 4 .8 amps . Divide the motor current rating by the drive output current
rating: 4 .8 / 7 .6 = 63% . Entering this value will allow continuous operation
at 4 .8 amps, and will also allow the motor to draw 7 .2 amps (150% of 4 .8
amps) for one minute . If the setting is left at 100%, the motor could draw
11 .4 amps (150% of 7 .6 amps) for one minute before faulting .
The MC Series drive has two options for thermal overload protection . One depends on the
speed of the drive, while the other does not . The diagram below illustrates the difference
between “speed compensated” and “non-compensated” thermal overload protection .
FREQUENCY (Hz)
M
AXIMUM C
ONTINUOUS
OUTPUT CURRENT (%)
NON-COMPENSATED
SPEED C
OMPENSA
TED
10 20 30 40 50 60
20
40
60
80
100
The “speed-compensated” thermal overload circuit offers additional protection from high
load conditions at low speeds, where motor cooling is often less effective (e .g ., motors
with shaft-mounted fans) . As seen on the diagram below, the drive reduces the allowable
continuous output current when operating at frequencies less than 30 Hz .