Lenze MC1000 Series - Manual (Page 53)

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16 CURRENT (CURRENT LIMIT)
This parameter sets the maximum allowable output current of the drive, which also
determines the torque capability of the motor. For most applications, CURRENT is
left at the maximum setting, which is 150% or 180% (of the drive’s output current
rating), depending on whether the input voltage to the drive is low or high (refer to
Parameter 0 - LINE VOLTS).
The drive will enter current limit when the current draw of the motor exceeds the
current limit setting. To respond to this condition, the drive will enter "frequency
foldback", which commands the drive to decelerate in order to reduce the output
current and regain synchronization with the motor. When the over current condition
passes, the drive will return to normal operation and reaccelerate to the speed set
point. However, if "frequency foldback" cannot correct the condition and the drive
remains in current limit for too long, it will trip into an OVERLOAD fault. If the drive
enters current limit while accelerating, the time required to reach the speed set point
will be longer than the time programmed into ACCEL (Parameter 8).
Regardless of the CURRENT setting, the drive is capable of delivering a maximum
of 150% current for one minute, and 180% current for approximately 30 seconds,
before tripping into an OVERLOAD fault. Refer to Parameter 17 – MOTOR OL.
17
MOTOR OL (MOTOR OVERLOAD)
The MC1000 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
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 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 tripping the drive.
The MC Series drive has two options for thermal overload protection. One depends on the
speed of the drive, and the other does not. The diagram below illustrates the difference
between “speed compensated” and “non-compensated” thermal overload protection.
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.