Specifications

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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.
NOTE 1: The above diagram is based on a MOTOR OL setting of 100%. For lower
MOTOR OL settings, reduce the % CURRENT values by the same percentage. For
example, if MOTOR OL is set to 75%, reduce the % CURRENT values by 25%. Therefore,
the curve shifts down, but the shape of the curve remains the same.
The “non-compensated” thermal overload circuit allows 100% current continuously, and
150% current for one minute, at all speeds. In the example above, the motor operating at 10
Hz without “speed-compensated” protection would be allowed to operate continuously at 27
Amps, and could draw 40.5 Amps for one minute before tripping. Without sufficient motor
cooling, this can result in motor failure due to overheating.
The “non-compensated” circuit is selected by setting Parameter 22 - TORQUE to
CT/NOCMP. The “non-compensated” setting should only be used in applications where the
motor is properly cooled at all speeds, or the motor manufacturer has approved the motor for
full-load operation at low speeds.
NOTE 2: The operation of the motor thermal overload circuit is affected by the setting of
Parameter 34 (LOAD MLT).
18 BASE (BASE FREQUENCY)
The BASE FREQUENCY determines the V/Hz ratio by setting the frequency at which the
drive will output full voltage to the motor. For most applications the base frequency should
be set to match the motor’s rated frequency.
For example, if the drive is rated for 460 Vac output, and the BASE FREQUENCY is set to
60 Hz, the drive will maintain a constant ratio of 7.66 V/Hz (except when FX BOOST is
active, see Parameter 19) from 0 Hz to 60 Hz. This range is the region of constant torque. If
the motor speed is increased past 60 Hz, the output voltage remains constant while the
frequency increases, resulting in a reduced V/Hz ratio. This range, from 60 Hz to about 90
Hz, is the region of constant horsepower. Above 90 Hz, horsepower begins to decrease as
frequency increases. Refer to Section 6.1 - DESCRIPTION OF AC MOTOR OPERATION.