Specifications
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17 MOTOR OL (MOTOR OVERLOAD)
The AC400 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 faulting.
The AC400Series 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.
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.