Leeson 174475, Инструкция по установке и эксплуатации, Страница: 54 / 80

16
CURRENT
(CURRENT LIMIT)
The current limit setting determines the maximum value of the drive's output current. This is usually done
to limit motor torque capability. For most applications the current limit is maintained at the maximum
setting. Depending on the setting of Parameter 0 - LINE VOLTS, the drive is capable of delivering 150%
or 180% of rated output current.
When the drive enters current limit, it will attempt to avoid an OVERLOAD fault by entering
FREQUENCY FOLDBACK, which commands the drive to decelerate in order to reduce the output
current. When the overcurrent condition passes, the drive will return to normal operation and accelerate
back to the speed setpoint. However, if the drive cannot reduce the current quickly enough by decelerating,
it will trip on an OVERLOAD fault. If the drive enters FREQUENCY FOLDBACK while accelerating a
high-inertia load, the time required to reach the speed setpoint will be longer than the ACCEL (Parameter
8) setting would indicate.
NOTE:
At 150% output current, the drive will trip on an OVERLOAD fault after one minute. At 180%
current, the drive will trip after approximately thirty seconds (see Parameter 17 - MOTOR OL below).
17
MOTOR OL
(MOTOR OVERLOAD)
The MOTOR OVERLOAD setting is used to protect the motor from overheating due to excessive current
draw. The trip time for the MOTOR OVERLOAD setting is based on what is known as 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 on 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:
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 tripping the drive.
The MICRO SERIES drive has two options for thermal overload protection. One depends on the speed
of the drive, and the other does not. The diagram on the next page illustrates the difference between "speed
compensated" and "noncompensated" thermal overload protection.
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