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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.
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.
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